reinforced plastics handbook || fabricating processes

Fabricating Processes

Overview

Many factors are important in making reinforced plastics (RPs) the success it has worldwide. One of these factors involves the use of the availability of different fabricating processes. All processes fit into an overall scheme that requires interaction and proper control of different operations. Factors such as good engineering product design and selecting the appropriate plastic are very important but only represent pieces of the "pie." Philosophical many different ingredients blend together to produce profitable products. Fabricating is one of the important main ingredients. In addition to fabricating in-house, there is fabricating outsourcing that also requires controls. It is also called contract manufacttt~g or professional services. This term originally was coined to mean buying rather than making parts. Now it encompasses the much broader concept of using outside organizations to replace people, including entire departments and processes, such as data processing, telemarketing, and customer services.

Different fabricating processes and materials of construction are employed to produce RP products (fibers and reinforcing additives) that represent about 20 wt% of all plastic products produced worldwide. Injection molding consumes over 75 wt% of all RP materials with practically all of it being RTPs. The processes range in fabricating pressures from zero (contact), through moderate, to relatively high pressure [2,000 to 30,000 psi (14 to 207 MPa)], at temperatures based on the TS or TP plastic's requirements that range from room temperature and higher (Figure 5.1). Equipment may be of simple construction/low cost with labor costs high to rather expensive specialized computer control sophisticated equipment with very low labor costs for the different processes. Depending on their size,

5 �9 Fabr icat ing Processes 2 5 5

equipment can process small to large parts (Figure 5.2) Each process provide capabilities such as meeting production quantity (small to large), performance requirements, proper ratio of reinforcement to matrix, fiber orientation, reliability/quality control, surface finish, and so forth versus cost (equipment, labor, utilities, etc.).

I TEMPI

BARREL TEMR

HEAT RISE AT

MOLD GATE I HEAT RISE I . ~ 1

HEAT AT l , S r ' DUE TO NOZZLE SCREW I AV l

i I

MOLD HEAT

MOLD COOL

~ ......... PLASTICIZING - ~ I*-----CURE TIME .....................

TS

TP

C C'Ej Figure 5.1 Processing temperatures for TS and TP materials

r ..... Large part over lsq ft over 5 Ibs

! !

Over250~ I thermosets

Low pressure Lamination Filament winding Compression High-pressure Lamination Post form Adhesive bond Machine Pultrusion

Product size ]

I

I Under 250~ 1 hermoplastics I

I , , , I " ! i

~ ~igh-vl~ Iu _ el

Thermoforin ! Foam I Compression Heat seal I Transfer Weld I Injection Rotoform I Lamination Blow mold I Adhesive bond I Pultrusion Structural | Foam | Rim I

I Over 250~ thermosets

I

1 Small part f~ less than lsq over 5 Ibs

I I

[Less than 250~ 1 It_herm~ lastics |

!

lL~ :v~ ! [High-volume] [Low-volume 1 ! 1 !

Casting Injection Machine Machining Blow m o l d Thermoform Low pressure Thermoform Compression Lay-up Extrusion Casting Post form Rotoform Rotoform Spray-up Rim Foam Resin transfer Adhesive bond

Figure 5,2 Guide to product size vs. process

256 Reinforced Plastics Handbook

The plastic may be either reinforced TSs (RTSs) or reinforced TPs (RTPs). The RTSs were the first major plastics to be adapted to this technology. The largest consumption of RTPs is processed by different methods such as injection molding, rotationally molding, or extruded on conventional equipment. There are even RTP sheets that can be "cold" stamped into shape using matching metal molds that form the products. It is called cold stamping because the molds are kept at or slightly above room temperature. The sheets, however, must be pre-heated.

Designing good products requires some familiarity with processing methods as summarized in Figure 5.3. Based on process to be used, different wall thickness ranges and tolerances exist. Different unreinforced plastics (URPs) and reinforced plastics (RPs) processed meet different shrinkage rates (Chapter 7). The different processes can have different processing capabilities (Chapter 9). Until the designer becomes familiar with processing, a qualified fabricator must be taken into the designer's confidence early in development. The fabricator and mold or die designer should advise the product designer on material behavior and how to simplify the design in order to simplify processing and reducing cost. Understanding only one process and in particular just a certain narrow aspect of it should not restrict the designer.

As an example, it is possible to place reinforcement precisely where it will give of its best properties. With closed molding processes, it is important to understand that the compound, whether thermoset (TS) or thermoplastic (TP), must flow inside the mold, usually under the effects of heat and pressure. While flowing, it will tend to align the reinforcement fiber in the direction of the flow, especially with granular TP molding compounds. In the original design of the product, consideration should be taken of the positioning of the material blanks in the mold. When compression molding, bulk molding compound (BMC) and sheet molding compound (SMC), stamping compounds, or injection molding RTP and RTS compounds, ensure that the material flows in such a way as to gain the optimum alignment of the fiber.

It is also important to remember that, while the compound is flowing in the mold, it is also undergoing other changes. It could be crosslinking (TSs), or simple cooling (TPs). Anything in the design or mold construction that obstructs the flow will also tend to imbalance curing or cooling, producing molded-in stress, which will usually exhibit itself as warpage. With proper process control, these type problems are eliminated or tolerated.

The pre-mixed molding materials contain randomly arranged fibers. They give properties approximately equal in all directions though with

COMPONENTS

Hydraulics. (Pum~. Va~,,N. Cyund~,. �9 P iss , )

Nozzkm Shutoff Valves

Gas InjecUon Equipment Quickmold-Claml~ng Systems

UPSTREAM AUXILIARY EQUIPMENT

Bulk Material Processing Loaders Feeders Blenders Dryers

Kneaders Magnetic Sepuators

Prslormers Pmheatars (Hlglt F ~ 4 Screw Type)

l ' 'COMPONENTS '

Process Measurement Screws

Biemstallic Barrels & Liners Instruments & Controls

Heaters & Heating Elements Motors & Drives

On-line InspeclJon Devices

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injection Molding Machimm , ,

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Printing & Stamping Equipment Assembly Equipment

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Gauge Inspection Pelletizers I Oicers Cut off Equipment

Vacuum Sizing Equipment Takeoff equipment

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Fihration Gear Pumps

Oies Feed 8locks Static Mixers

Continuous Conveyors

Figure 5.3 F low c h a r t in f a b r i c a t i n g m a c h i n e r y ( c o u r t e s y o f A d a p t i v e I n s t r u m e n t s Corp.)

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some sensitivity to the plastic melt flow direction. These are termed isotropic, usually with isotropy only in one plane (planar). Where a laminate is stacked using different types and forms of reinforcement, the directional properties of each can be harnessed to their best advantage.

In addition to using processes specifically designed for over a half century worldwide the more commonly used processes for unreinforced RPs (URPs) also process RPs. They include injection molding (IM), extrusion (EX), thermoforming (TF), foaming, calendering, coating, casting, reaction injection molding (RIM), rotational molding (RM), compression molding (CM), transfer molding (TM), reaction injection molding, rotational molding, and others that include hand lay-up, Marco process, resin transfer molding, and others are to be reviewed. Since glass fibers are extensively used, specifically during IM, the glass fibers will cause wear of metal molds during processing such as plasticating screw/barrels and molds or dies. Using appropriate metals that can provide a degree of extending their operating time can reduce this wear. Be aware that wear will occur and gradually reduce your efficiency in maximizing the performance of the equipment. Cost for replacing or repairing eroded equipment parts has always been included in the cost of operating equipment.

Popularly used basic processes each having many modifications so that there are literally hundreds of processes used. The ways in which plastics can be processed into useful products tend to be as varied as the plastics themselves. However only a few basic processes are used worldwide for most of the products produced.

If an extruder can be use to produce products it has definite operating and economical advantages compared to IM. It requires detailed process control. IM requires more sophisticated process control to fabricate many thousands of different complex and intricate products. Product markets for IM arc extensive with little to date using extruders.

While the processes differ, there arc elements common to many of them. In the majority of cases, TP compounds in the form of pellets, granules, flake, and powder, are melted by heat so they can flow. Pressure is often involved in forcing the molten plastic into a mold cavity or through a die and cooling must be provided to allow the molten plastic to harden. With TSs, heat and pressure also are most often used, only in this case, higher heat (rather than cooling) serves to cure or harden the TS plastic, under pressure, in the mold. When liquid TPs or TSs plastics incorporate certain additives, heat and/or pressure need not necessarily be used. Common features of the different

5 �9 Fabricating Processes 2 5 9

processes fit into a flow pattern to fabricate products. These features follow a pattern such as what follows:

(a) Mixing and melting: This stage takes the plastic compound and in turn produces a homogeneous melt. This is often carried cut in screw plasticators, mixer, or other compounders, where melting takes place because of heat conducted through a barrel wall and /or heat generated in the plastic by the action of shear via a screw or mixing blades. Homogeneity is called for at the end of this stage, not only in terms of material but also in respect to temperature. Equipment and RP material development continues to improve and control melts.

(b)

(c)

(a)

(c)

(f)

(g)

Tooling: When processing plastics some type of tooling is required. These tools include molds and dies for shaping and fabricating products. They have some type of female and/or male cavity into or through which a molten or melted compound moves usually under heat and pressure. They are used in processing many different materials to form desired shapes and sizes. They can comprise of many moving parts requiring high quality metals and precision machining. Some molds and dies cost more than the primary processing machinery with the usual approaching half the cost of the primary machine.

Melt transport and shaping: In a screw plasticator that melts material the next step would be to build up an adequate pressure in the plasticator so that it will produce the desired shape to be fabricated. In an injection molding process pressure is applied to force the melt into a mold that defines the product shape in three dimensions. In an extruder, the die (that initiates the shape) can vary from a simple cylindrical shape to a complex crosshead profile shape.

Forming and stamping: Processes such as compression molding and thermoforming can produce different shaped products.

Casting: With screw plasticator or other systems the melt permits castings in an open or closed mold.

Non-screw plasticating: Reactive mixing provides the melted compound, such as in reaction injection molding, to produce the plastic.

Finishing: The final stage after a process fabricates a product usually does not require secondary operations. However, there are products that may require annealing, sintering, coating, assembly, decoration, etc.


Understanding, controlling, and measuring the plastic melt flow behavior of plastics during processing is important. It relates to a plastic that can be fabricated into a useful product. The target is to provide the necessary homogeneous-uniformly-heated melt during processing to have the melt operate completely stable and working in equilibrium. Unfortunately, the perfect melt does not exit. Fortunately with the passing of time where improvements in the plastics and equipment uniformity continues to occur, melt consistency and melt flow behavior continues to improve simplifying the art of processing.

An important factor for the processor is obtaining the best processing temperature for the plastics used. A guide is obtained from experience and/or the material producer. The set-up person determines the best process control conditions (usually requires certain temperature, pressure, and time profiles) for the plastic being processed. Recognize that if the same plastic is used with a different machine (with identical operating specifications) the probability is that new control settings will be required for each machine. Reason is that, like the material, machines have variables that are controllable within certain limits that permit meeting the designed product requirements including costs.

The secondary operations fabricating methods include the broad categories of assembling, machining, cutting, sewing, sealing, and forming. Target is not to have secondary operations eliminating extra time and cost. However, there are exceptions where, due to quantity or other factors, advantages occur in time and cost. The machining techniques used are quite common to metal, wood, and other industries requiring modifications to handle conditions such as the brittleness of glass fibers and the meltable or heat sensitive RPs. Plastic shapes can be turned into products by such methods as grinding, turning on a lathe, sawing, reaming, milling, routing, drilling, and tapping.

The cutting, sewing, and sealing involve obtaining product pattern by hand, in die-cutting presses, or by automatic methods. The pieces are then put together using assembly techniques such as sewing, heat bonding, welding, high frequency vibration, or ultrasonic sealing depending on the type plastic used. Different techniques are used with certain RPs that can only use certain techniques. When a technique is used for URP, due to the heat transfer condition of glass fibers the RP may not bond or minimize bondability.

There are post-finished forming methods that can be used such as post- embossing with textures and letterpress, gravure, or screening can print them. Rigid plastic parts can be painted or they can be given a metallic surface by such techniques as mctallizing, barrel plating, or electro-

5 �9 Fabricating Processes 261

plating. Another popular method is hot stamping, in which heat, pressure, and dwell time are used to transfer color or design from a cartier film to the plastic part.

However, secondary operations can be performed during fabrication to reduce fabricating time and/or costs. Popular is the in-mold decorating that involves the incorporation of a printed foil or other material onto a plastic product during molding so that it becomes an integral part of the product; it can be located under the surface or inside the product. There are applications where the foil provides structural integrity thus reducing the more costly amount of plastic to be used in the products.

Each process provide capabilities such as meeting production quantity (small to large quantities and shapes), performance requirements, proper ratio of reinforcement to matrix, fiber orientation, reliability/ quality control, surface finish, materials used, quantity, tolerance, time schedule, and so forth versus cost (equipment, labor, utilities, etc.). There are products when only one process can be used but there can be applications where different processes can be used.

Fabricating Startup and Shutdown

Primer and secondary (Table 5.1) equipment manufacturers have procedures for startup and shutdown that provides an initial guide. The actual procedures to follow depend on a number of existing variables that include type of RP being processed and available controls. Information here provides some basic information. Machine operation takes place in three stages. The first stage covers the running of a machine and its peripheral equipment. The next involves setting processing conditions to a prescribed number of parameters for a specific RP, with a specific tool (mold, die, etc.) in a specific processing line; to meet product performance requirements. The final stage is devoted to problem solving and fine-tuning of the complete line that leads to meeting performance requirements at the lowest cost. A successful operation requires close attention to many details, such as quality and flow of feed material(s), a heat profile adequate to melt but not degrade the RP. Processors must also become familiar with troubleshooting guides that are available from equipment manufacturers.

There are fabricating turnkey operations. They are complete fabrication lines or systems with upstream and downstream equipment. Controls interface all the equipment in-line from raw material delivery to the end of the line handling the product for in-plant storage or shipment out of the plant. Target is always to reduce fabricating time in order to reduce cost. However, there are the right and wrong ways. Properly setting up


Table 5.1 Examples of auxiliary and secondary equipment

Adhesive applicator Polishing Bonding Printing/marking Chemical etching Process control for individual or all Cutting equipment Die cutting Pulverizing/grinding Dryer Quick mold change Dust-recovery Recycling system Flash removal Robotic handling Freezer/cooler Router Granulator Saw Heater Screen changer Joining Screw/barrel backup Knitting Sensor/monitor control Labeling Software Leak detector Solvent recovery Machining Solvent treatment Material handling Testing/instrumentation Metal treating Trimming Metering/feeding material Vacuum debulking Mold extractor Vacuum storage Mold heat/chiller control Oven Water-jet cutting Pelletizer/dicer Welding Plating Others

the process controls for the equipment and material being used provides the right way. Recognize that there are equipment and material variables that exist and are controllable.

Competitiveness built around an obsession for the customer should ultimately be the primary focus in a distribution or manufacturing strategy. The best approach is, therefore, to identify opportunities to improve effectiveness and reduce the costs of the manufacturing and distribution systems, both of which will benefit the customer and, thus, the manufacturer. One opportunity is to consolidate distribution centers worldwide. Another opportunity is to adopt a world-class manufacturing (WCM) approach to fabricating processes. Philosophies and methodologies such as just-in-time (lIT) fabricating to total quality management (TQM) can be applied to cut operating costs and improve quality levels, customer service, and return on manageable assets.


Reinforced Thermoplastics

Fabricating RTPs is mainly by injection molding. There is glass mat TPs molded by pre-heating and stamping, tape layering with TP preimpregnated reinforcement is carried out by a variation of filament winding, pultrusion can be used to produce TP rod/profiles with continuous filament reinforcement, and so on as reviewed in this chapter. RTPs are being used increasingly in all sectors of engineering design, for high performances such as mechanical properties, electrical insulation, lighmess in weight and good resistance to chemicals and corrosion, and/or together with excellent moldability at fast production rates such as by injection molding.

Examples of resins used include the nylon family (polyamide/PA), polypropylene (PP), polystyrene, and polyethylene. There continues to be growing applications for the higher-performance RTPs, such as polyphenylene sulphide (PPS) and polysulphone (PSU). Use is made of many other TPs (Chapter 3).

High-performance RTPs, which are increasingly used in aircraft and transportation components and in the field of general engineering products, use different molding technology to accommodate the advanced reinforcement structure. Examples include a complex notebook computer case that was one of a number of exercises produced by Composites Horizons Inc., Covina, California (CHI), in response to a request from IBM to design a lightweight mass-producible case. It would have been difficult to injection mold a component with wall thiclmess less than about 1.8 mm (0.070 in) and IBM's baseline design in aluminum was 1.27 mm (0.050 in) thick. CHI created a design using woven continuous carbon fiber in a nylon 12 matrix, at 60 vol% fiber, which gave high thermal conductivity (Table 5.2). The fiber was laid in a 0 / 9 0 ~ arrangement: the additional use of 45 ~ laid fiber was considered unnecessary, and it would have created problems with re-use of off cut waste. The wall thickness of the finished case was 0.89 mm (0.035 in) and its weight was 33% lower than the baseline aluminum design. The thinner wall also offered about 5% more internal volume for an external envelope of the same size. For mass-production of the case, CHI selected a woven carbon fabric powder impregnated with nylon (by Electrostatic Technology Inc.), press-molded in a high-speed fabricating process.

Curing Systems

The process of curing TSs is by means of a change in their molecular structure, in which, under certain circumstances, the individual


Table .5.2 Design study: a lightweight computer case press-molded in nylon 12/carbon fiber

Aluminum (baseline)

Nylon 12/ Nylon 66/ Nylon 66/ Epoxy woven short log SMC/1-in carbon c a r b o n carbon a carbon a

Flexural modulus {psi) 10 m 8 m 1.5 m 3.5 m 5 m

Specific stiffness {in] 100 m 133.3 m 30 m 70 m 83.3 m

Weight (g) 400 290 356

EMI shielding (db)

- at 200 MHz 82 55

- a t 1 GHz 95 50

- at 8 GHz 55 33

Notched Izod (ft Ib/in) - 20 2.4 9.5 18

Source: Electrostatic Technology Inc. aThese designs were considered unproducible, due to the intricacies of the part.

molecular chains can be made to link up in an irregular fashion, forming a solid infusible network. This is called curing and it will happen, in time, by normal processes (in fact, it was this phenomenon that first attracted the interest of researchers a century ago). For industrial purposes, curing (or crosslinking) is activated by means of special chemicals, heat, or irradiation.

The chemistry involved in curing is complex, and the names of the various chemicals are very complex. It will be useful, however, if the engineer or processor is at least familiar with the basic mechanism by which these materials operate, and the overall trade-off effect which often occurs, as one additive improves properties in one direction but may reduce them in another.

Standard curing systems are based on two catalyst groups. Several different terms are used in the industry to cover these curing agents: catalyst (not technically accurate, but widely used) and hardener or initiator. There are activators (also called promoters or accelerators) that are used to speed up and enhance the cure. Inhibitors (also known as retarders) perform the opposite function and are used to extend the curing time.

Curing Agents for TS Polyester Systems TS polyester resins, major and important resin matrix in the RP industry are usually cured by the addition of special chemicals that decompose to free radicals, so offering a simple technique that can easily be controlled, with regard to rate and length of cure. Polyester


resins can also be cured by heat or irradiation and the technology has been used that cure under the effect of ultra-violet.

The usual chemical curing agent is organic peroxide, which decomposes under the influence of heat. A wide range of organic peroxides is available, for various thermal stability requirements, from compounds that decompose rapidly to free radicals at ambient temperature to those active only at higher temperatures. The former are not normally used for curing unsaturated polyester resins. More often used are peroxides stable at ambient temperature and decomposing at 50-150C (122- 302F), but this is not sufficiently active at ambient to cure the resin. Reducing agents such as tertiary aromatic amines, heavy metal salts of cobalt, vanadium, iron, and similar materials are used to accelerate decomposition.

Compounds that prevent undue polymerization of the resin are called inhibitors, typically monohydric or polyhydric phenols and some quinones. They are added during fabrication of the resin to ensure storage stability. These can prolong pot life of a resin system containing peroxide and accelerator, particularly with cobalt systems. Inhibitors can also influence the ratio of cure to gel time, as they mainly prolong gel time.

Paratertiary butylcatehol extends gel time and pot life at room temperature and at elevated temperatures; 2.6-ditertiary butylparacresol is used with B PO/amine systems, to give lower peak exothcrm and gradual cure.

Use of accelerators to aid curing is the most popular method. Curing can occur at below 100C (212F) with organic peroxides combined with accelerators or pro-accelerated resins. Post curing at 80-120C (176- 248F) is normally required. The normal curing system is ketone peroxide (based on either methyl ethyl ketone, cyclohexanone or acetyl acetone) with cobalt octoate or naphthenate. Alternatively, diactyl peroxides such as benzoyl peroxide are accelerated with diethylaniline, dimethylaniline and dimethyl p-toluidcne. Amine acceleration gives a fast curing cycle but can produce tackiness in thin layers and very strong discoloration during ageing. Ketone peroxide with cobalt acceleration therefore forms the most popular curing system.

There are also: dimethylaniline with dibcnzoyl peroxide at room temperature (for normal/long gel times); dimethylparatoluidine (for very short gel times); cobalt octoatc (mainly used with ketone peroxides for polyesters at room and elevated temperatures); cobalt/amine very high reactivity with ketone peroxides for very fast cure (for example polymer concrete); and vanadium special for ketone peroxide,


hydroperoxide, and peroxy esters, giving short gel times and very high cure speed.

Curing Without Accelerators Without use of accelerators, external heat is required, making a system suitable for mechanical processes, such as hot press molding and continuous impregnation of sheet and profile. Temperatures in the range 120-160C (248-320F) are used to cure in a short cycle; accelerators offer no advantage as the rate of cure depends on thermal decomposition of the peroxide, and typical cycle times are 1-10 min.

Usually a combination of long shelf life of the uncured compound with short curing cycle is required, calling for adequate thermal and chemical stability. Organic peroxides for high temperatures are peresters and perkctals. Low initiation temperature with adequate curing performance is given by dimyristyl peroxy dicarbonate or methyl isobutyl ketone peroxides (in the latter case limiting shelf life to a few hours). Combina- tions of peroxides can be used. The more active types reduce initiation temperature while more stable types give a better degree of cure.

Selecting a Curing System A number of factors must be considered, in descending order of importance:

processing conditions: - high-output production - batch or continuous process wi th/wi thout external heat - molds closed or open to air - required shelf life of activated compound: immedia te /days /

weeks/months - possibility of using resin + peroxide and resin + accelerator as

separate - components in a two-pot system;

type and size of finished product: - thick-walled castings or thermally-insulated moldings, which can

reach a peak exotherm of over 200C (392F), producing cracks from internal stress or shrinkage, can be molded better with less- active curing systems

- surface coatings (with no exotherm and slow cure with possible air inhibition) can better use very active curing systems

- where color is important, accelerators must be kept to a minimum; amines are not suitable, ketone peroxides/metal salts are preferable.

type of resin and other ingredients: gel time is stated at 20C (68F). To achieve fast cure under practical conditions, curing temperature should be at least 20C above the critical temperature.


Table 5.3 provides information on when and where to use curing systems. It provides details on the curing systems as well as accelerators, and inhibitors. The information relates to curing processes to fabricating processes.

Table 5,3 Curing systems for TS polyesters regarding when and where to use them

Type~form Main characteristics Process

Ketone peroxides for cobalt curing at ambient temperatures

Methyl ethyl ketone peroxide: -liquid high activity

-liquid low activity

-liquid super activity

Standard for all resins including bisphenol and vinyl esters; relatively short gel times moderate heat evolution little internal stress. Versatile type for all resins particularly vinyl esters; relatively long gel times short mold release times; good for large parts and/or use in hot countries. Special for buttons/button sheets

Acetyl acetone peroxide: -liquid normal activity



Versatile type for ortho- or isophthalic- based resins; relatively short cure time strong heat evolution; suitable for thin-wall moldings. Special for thick-wall moldings; variable gel times reduced peak exotherm short mold release times. Special for very thick-wall moldings; relatively long gel times little heat acceptable mold release times.

Cyclo hexanone peroxide: -liquid normal activity

-liquid high activity

Versatile type for nearly all resins; variable gel times moderate heat little stress; suitable for large or thick-wall moldings. Versatile type for nearly all resins (also vinyl esters); variable gel times relatively short mold release times reasonable mold release factor

A B c D E f

AcDEG

a b d e f

aBCdFG

ABCDEG

abe

abCE

Benzoyl peroxides for amine curing at ambient temperatures

Dibenzoyl peroxide: - 50% powder with phthalate

- 50O/o suspension

- 40% suspension

Rapidly dissolving in all resins incl. bisphenol A and vinyl ester; variable gel times, strong heat evolution, short release times; no accelerator needed above 70-80~ (158-176~ Pourable/pumpable suspension dissolves rapidly; curing performance as above Pourable/pumable suspension dissolves rapidly; special type for easy dosing/ metering; curing performance as above

abcg

abcG

abcG


Table 5,3 continued

Type~form Main characteristics Process

Organic peroxides for curing at 60-120~ (140-248~

Dimyristyl peroxy dicarbonate: technically pure flakes

Special type for curing above 50~ but only with more thermally stable peroxides; suitable for all resin types

Methyl isobutyl ketone peroxide: liquid normal activity

Versatile type for curing above 55~ poss. with more thermally stable peroxides and/or cobalt accelerators; suitable for all resin types

Tertiary butyl peroxy 2-ethylhexanoate: technically pure liquid

Versatile type for curing above 70~ poss. with more thermally stable peroxides and/or cobalt accelerators; suitable for all resin types

Dibenzoyl peroxide: 50Olo powder with phthalate

Versatile type for curing above 70~ poss. with more thermally stable peroxides and/or amine accelerators; suitable for all resin types

Cumene hydroperoxide: 80% liquid

Special type for curing above 80~ with cobalt accelerators

1,1-Di(tert. butylperoxy) trimethyl cyclohexane: liquid high activity 50% solution in aliphatics

Versatile type for curing above 80~ Quickset in range 120-150~ for hot-press molding SMC or BMC; can be accelerated by promoters. Special for SMC/BMC at 130-160~ without accelerator; not sensitive to fillers pigments and promoters

1,1-Di(tert. butylperoxy) cyclohexane: 50% solution in aliphatics

Standard type for SMC/BMC at 130-160~ without accelerator; not sensitive to fillers pigments and promoters

Tertiary butyl peroxy benxoate: technically pure liquid

50% powder with chalk

Standard type for SM(3/BM(3 at 130-160~ can be accelerated by promoters; sensitive to some fillers and pigments (e.g. carbon black). Standard for granulated molding compounds at 130-160~ without accelerator; can easily be mixed in as free-flowing powder

Tertiary butyl cumyl peroxide: technically pure liquid

Special for SMC/BM(3 with deep flow at 130-160~ not sensitive to fillers pigments and promoters

1,3-Di(tertiary butylperoxy isopropyl) benzene: technically pure flakes

Special for granulated molding compounds at 140-170~ without accelerator; not sensitive to fillers pigments and promoters; also available as 40% powder with chalk

ef

fgh

fg

FGH gh

gH

HI

hi

5 �9 Fabricatin 9 Processes 2 6 9

Type/form Main characteristics Process

Accelerators

Cobalt octoate: -in phthalate with

1O/o cobalt

-in xylene with 6-10% cobalt

Cobalt octoate/dimethyl aniline: liquid mixture in phthalate

Dimethyl-p-toluidine: 10% solution in phthalate

Dimethyl aniline: 10% solution in phthalate

Diethyl aniline: 10% solution in phthalate

Standard for ortho or isophthalic acid resins with ketone peroxides or peresters gel and cure times vary according to peroxide 20-100~ Special for large batches or high usage can be diluted performance as for above

Special for bisphenol A or vinyl esters with ketone peroxides or peresters short gel/cure times 10-100~

For short gel/cure times with dibenzoyl peroxide suitable for all resins 10-100~

For medium gel/cure times with dibenzoyl peroxide suitable for all resins 15-100~

For long gel/cure times with dibenzoyl peroxide suitable for all resins 15-100~

A B C D e f g

A B C D e f g

a b c d E G

acG

a b c G

a b c g

Inhibitors

Di(tert. Butyl}p-cresol techn, pure powder 40O/o solution in xylene (SETA flash point ~30)

Tert. butyl catechol techn, pure powder 10O/o solution in styrene (SETA flash point ~31}

Prolongs (up to weeks/months) shelf life - gel time of resin + peroxide - at ambient temperature effect on cure times diminishes with rise in temperature efficient with many types of resin and peroxide

Prolongs (up to many hours) pot life - gel time of resin + (ketone) peroxide + (cobalt) accelerator- at ambient temperature mold release factor improved also efficient at elevated temperatures

f g H I

a c d e f g h i

Key to processes: A or a = hand lay-up; B or b = spray lay-up; C or c = injection/vacuum molding; D or d = centrifugal casting; E or e = filament winding; F or f = continuous impregnation; G or g = wet press molding; H or h = hot press molding (SMC/BMC); I or i = hot press molding {granular molding compound) [capital letter =very suitable; small letter = suitable).

Mold Release

Mold release agents are generally necessary with RTS resins. These are film-forming coatings that are applied to the mold, but there are also


internal mold release agents that can be incorporated as additives in the gel coat or in the molding material itself. Types are available for open and closed molding, TS polyester and epoxy resin transfer molding, casting slab stock, pultrusion, etc. Internal release agents are usually preferred for injection molding RTPs. During processing, the additive migrates to the surface to form a releasing film. In all cases, it is advisable to check the compatibility with the resin, to ensure that there is no detrimental effect to its properties.

Liquid mold release provides a flesh coating of wax at each application without causing wax build-up, avoiding the need to take a mold out of production for de-waxing. Paste wax uses pure canauba wax and no silicones. There are durable visible film with non-volatile organic content (such as Sealproof, from Zyvax), offering superior adhesion to gelcoat, RP, and as well as metal and wood. When coated with a proprietary release agent (such as Watershield) it provides multiple releases and reduces downtime due to cleaning.

For mold preparation and cleaning, longwearing release films can be applied. A typical system is a semi-permanent agent that polymerizes on the mold surface; grades for cure at ambient and oven temperatures are available. Flange waxes are designed for release of moldings from the f lange/edge areas of the mold; they can be applied easily (by cloth or sponge, brush or spatula) and remain smooth and wet during the molding process. Mold sealer is a basecoat for all types of release agents.

If a silicone release agent is used, be aware of certain situations. If the fabricated product is to be printed, decorated, bonded, etc. the bond or proper bond probably will not occur. It probably will interfere if electrical connections are to be made on its surface.

Processing and Patience

The starmp of fabricating lines usually requires changing equipment settings. When making processing changes, allow enough time to achieve a steady state in the complete line before collecting data. It may be important to change one processing parameter at a time. As an example with one change such as temperature zone setting, or other process control parameter, allow time to achieve a steady state prior to collecting data.

Reinforcement Patterns

To process RPs different reinforcement patterns can be used that range from chopped to long fibers, woven to nonwoven, preform to com-


pounds, and so on. In the past most of the activity in using different patterns has been with TSs and in particular with TS polyester resins. The pattern chosen is dependent on performance required by the fabricated RP product and usually also by the process to be used. Chapter 2 provides information on different types of fibers and other reinforcements used. A general introduction has been provided concerning preforms. However since preforms continue to play an important part in the RP industry since the 1940s additional information is to be presented. As an example, prcforms were used during the 1940s and 1950s with compression molding and the Marco infusion processes. It used pressure, vacuum, or a combination of pressure/vacuum systems that in turn provided proper disposal of styrene monomers and other gases.

Preform Processes

As time passed since the 1940s, significant improvements occurred processing wise, equipment wise, plastic wise, and cost wise. This is a method of making chopped fiber mats of complex shapes that are to be used as reinforcements in different RP molding fabricating processes rather than conventional fiat mats that may tear, wrinkle, or give uneven glass distribution when producing 3-D or complex shapes. Most of the reinforcement used is glass fiber rovings. They are desirable where the product to be molded is deep or very complex shapewise. Oriented patterns can be incorporated with continuous fibers in the preforms to develop required directional properties. Different methods are used with each having many different modifications. They include a plenum chamber, directed fiber, and water slurry.

Continuous rovings are fed into a cutter and after being cut to the desired lengths, fall into a plenum chamber perforated screen where the air is exhausted from under the screen. A plastic binder of usually up to 5 wt% is applied and is later cured (Figure 5.4). As the glass falls into the plenum chamber, the airflow pattern and baffles inside the screen control its distribution. Preform screen rotates and sometimes tilted to ensure maximizing uniform deposits of the roving.

With the directed fiber system strands are blown onto a rotating preform screen from a flexible hose. Roving is directed into a chopper where airflow moves it to a preform screen. Use can be made of a vertical or horizontal rotating turntable. This process requires a rather high degree of skill on the part of the operator; however, automated robots are used to provide a controlled system producing quality preforms (Figures 5.5 and 5.6).

With water slurry, chopped strands are in water (take-off used by the paper pulp industry for centuries). It produces intricate shaped preforms


Figure 5.4 Flow of glass fiber rovings traveling through a plenum machine

Figure 5,5 Schematic of the direct preform process


I

Pos 1

i

Figure 5.6 Schematic of the direct preform process spraying two molds

that are tough and self-supporting. Bonding together the preform can use cellulose fibers and/or bonding resins. Where maximum strength is not required, the cellulose content can be sufficiently high to reduce cost. The fibers can be dyed during the slurry process (Figure 5.7).

Important to being successful is manufacturing the screen. Different shapes can be used to meet different product designs. Recognize that cylindrical preforms are easier and less cosily to produce than box-like sections. In addition, it is important to recognize that during the rotation of a cylindrical part, the fibrous glass will flow uniformly onto the screen because most sections move at a uniform linear rate. With a rectangular section, it is difficult because the comers rotate in a wider circle than do the center sections and because the airflow is lowest at the comers. Contouring the box shape can improve reinforcement distribution.

Preform screens are usually made from 16-gauge perforated material with 1/8 in. holes on 3/16 in. centers. This produces about 40% open area. For some operations, a more open area is required. Perforation patterns are also used to develop specifically designed reinforcement directional properties. The screen is usually designed so that the outside contour is identical with the contour of the mating half of the mold. A screen, which is not of the correct size, will cause a great deal of difficulty in


Figure 5.7 Flow of glass fiber rovings traveling through a water slurry machine

the molding operation. If the screen is too small, the preform will tear during the molding. If too large, wrinkling and overlapping of the preform will result.

The preform is usually heavy on the flat top and light on the edges and comers. Internal baffles may be added in the preform screen to control the airflow, thus giving a uniform deposition of glass. The exact area of the baffle usually has to be worked out on a trial-and-error basis until experience is developed. Close cooperation with the preform-machine manufacturer is helpful.

When molding a product with a variable wall thickness, it is possible to vary the thickness of the preform. This is usually accomplished by baffling. Another approach that can be used is to completely block off areas where no fiber is desired. This action saves material that would otherwise be trimmed off and probably discarded. It has also proven practical to combine two or more preforms into one molded part. This technique is very useful where the thickness of the molded part prohibits the collection of the preform in one piece.

Also prepared is mat preforming. It is the usual flat fiber mat that is formed into a shape usually using a set of matched dies (often made of fiber reinforced epoxy). A mat is cut to the correct dimensions and


placed over the male half of the mold. Using an infrared device, heat is then applied to the mat and the female half of the mold is lowered. The shaped mat must be cooled before removal from the tool. The resulting preform approximates the shape of the final part to be molded.

Compression Moldings

CM is the most common method of forming TS plastic products. Until the advent of injection molding, it was the most important of plastic processes. CM is the compressing of a material into a desired shape by application of heat and pressure to the material in a mold cavity (Table 5.4). Pressure is usually at 7 to 14 MPa (1000 to 2000 psi). Some RTSs may require low pressures down to 345 kPa (50 psi) or even just contact (zero pressure). The majority of TS compounds are heated to about 150 to 200C (302 to 392F) for optimum cure; but can go as high as 650C (1200F) for the very high performing resins (Figures 5.8 and 5.9).

Table 5.4 Examples of the effect of preheating and part depth of phenolic parts on compression molding pressure (psi)

Conventional phenolic Low-pressure phenolic

Depth of Dielectric Not Dielectric Not molding (in.) preheat preheated preheat preheated

0-3/4 1,000-2,000 3,000 350 1,000 114-11/2 1,250-2,500 3,700 450 1,250

2 1,500-3,000 4,400 550 1,500 3 1,750-3,500 5,100 650 1,750 4 2,000-4,000 5,800 750 2,000

A force is also required to open the mold that is usually much less (20% of clamp) than the clamping force. One has to ensure that available opening clamping pressure is available. Usually this requirement is not a problem. Clamping predominantly use hydraulic systems. Also becoming popular are all electric drive systems and/or with hydraulic/electrical hybrid systems. The actual mechanical mechanisms range from toggle to straight ram systems. Each of these different systems has their individual advantages.

Mold cavities can be filled separately with reinforcement and resin. The reinforcement can be in loose form or as a preform. Very popular is the use of RTS sheet molding compound (SMC) or bulk molding


Figure 5.8 Various configurations of compression molding presses


Figure 5.9 Example of land locations in a split-wedge mold

compound (BMC) (Figures 5.10 and 5.11) (Chapter 4). Also used are RTP sheets and compounds. With TSs CM can use preheated material (dielectric heater, etc.) that is placed in a heated mold cavity providing a uniform heat through the compound and reducing cycle time during molding. The mold is closed under pressure causing the material to flow and completely fill the cavity. Based on how the RTS compounds are prepared they can be processed at low or high pressures. Chemical crosslinking occurs solidifying the TS molding material.

The closed mold shapes the material usually by heat and pressure. With special additives, the TS material can cure at room temperature. It would have a time limit (pot life) prior to curing and hardening. Based on the compounds preparation sufficient time is allowed to store and

Figure 5.10 Compression molding sheet molding compound (SMC)

Figure 5.11 Compression molding bulk molding compound (BMC)


Figure 5.12 A 400 ton compression press with 18 platens

handle the compound prior to its chemical reaction curing action occurs.

The mold is fastened on the platens. These platens usually include a mold-mounting pattern of bolt holes or "T" slots; standard pattern is recommended by SPI. Platens range from the usual parallel design to other configurations meeting different requirements. The parallel type can include one or more "floating" platens located between the stationary and normal moveable platens resulting in two or more daylight openings where two or more molds or flat laminates can be used simultaneously during one machine operating cycle (Figure 5.12). The other designs include shuttle (molds in which usually two, or more, are moved so that one mold is positioned to receive material and then moves to the press permitting another mold to receive material with this cycle repeating; result is to permit insert molding, reduce molding cycle, etc.), rotary or carousal system, and "book" opening or tilting press (Figure 5.13).

With certain plastic compounds mold breathing is required. This action is also called mold bumping, dwell pause, dwell, gassing, and degassing. It is a pause or repeated pauses in the application of mold pressure using plastics that gives off gases during the heating process; also to remove any entrapped air. This on-off-on pressure action occurs in parts of a second just prior to having the mold completely closed to allow the escape of gas and /or air. Application is with many TS plastics, vulcanization of reinforced TS elastomers, and any material that releases gases.

Use is made of CM charging tray; also called loading tray. It is a tray designed to charge simultaneously with material all the mold cavities of


Figure 5~13 Book opening large compression press

a multi-impression mold. The device can operate by using a tray with openings where the material is placed (manually or usually automatically) and in turn a withdrawing sliding bottom tray that initially closes the openings and slides exposing openings matching the top tray so material drops into the cavities.


Applying vacuum in a mold cavity can be very beneficial in molding plastics particularly when using low clamping pressures. Press can include a vacuum chamber around or within the mold providing removal of air and other gases from the cavity(s) and safely disposing them.

CM is a matched die molding system that can be broadly defined as a process in which the loading and/or closing of the mold causes the molding material to conform to the desired configuration in the mold cavity and in which cure (RTS) or cool (RTP) takes place while material is contained in the mold. The process includes cold press molding and resin transfer molding (RTM). The threshold of medium series production lies between 1000 and 10,000 parts per year, above which the molder may have to consider changing to injection molding or low-pressure molding, to speed up the production rate and achieve two smooth surfaces, while retaining a degree of freedom in terms of shape and size.

Changes continue to be made in molding compounds for use in compression molding. As an example is Owens Corning's future technology, dubbed SS2. It assumes that SMC will always be somewhat porous if it is manufactured using current techniques. The firm is developing a different means of placing an SMC charge in a tool so that the charge's surface stays on the surface. The reinforcing material and other additives stay in the middle of the charge after molding rather than being squeezed out, as occurs during standard compression molding.

Compression Transfer Moldings

Also called transfer molding not to be confused with the more popular resin transfer molding system to be reviewed. It is a method of compression molding principally RTSs. The plastic is first softened by heat and pressure in a transfer chamber (pot) and then forced by the chamber ram at high pressure through suitable sprues, runners, and/or gates into a closed mold to produce the molded part or parts using two or more cavities (Figures 5.14 and 5.15). Usually dielectrically preheated circular preforms are fed into the pot.

Cold Press Moldings

Cold press molding represent compression molding that involves only a moderate investment in equipment and using a lower-powered press than with large series processes. Molds can be made from RP materials rather than from high-quality steel. The reinforcement is generally mat which can be preformed if necessary; fabric can also be used. Molds can be at ambient temperature or heated to 80C (176F). Cold press


Figure .5.14 Schematic of transfer press

Figure 5.15 Screw transfer where RP travels from hopper, through screw plasticator, to pot, and into the mold cavities

molding uses a low pressure of 1-5 kg /cm 2 on the projected area of the molding and is suitable for small batches with good surface finish.

Matched male and female molds are employed, in a vertical press, giving good surfaces to both sides of the molding and a higher degree of dimensional and quality consistency than is usually obtained from hand lay-up operations. The molds are prepared and a release agent is applied. The reinforcement (usually tailored to size and shape) is then laid in the open mold and a charge of liquid catalyzed resin is poured over the lay-up. The mold is then closed, to allow cure to take place.

When cured, the mold is opened and the part removed automatically. It may well be moved to a simple jig to hold the shape while allowing the full cure to take place (meaning that de-molding can take place at an earlier stage).

Unheated molds can be used, which can be produced relatively inexpensively from special reinforced/filled epoxy resin compounds (backed if necessary with a material such as concrete). Since this technique relies on the normal exothermic curing reaction of the resin, a low-pressure press can be used, employing the power of the press only to raise and lower the mold-half, while using mechanical clamps to secure the mold halves during curing. Cold press molding is simple, low cost and effective but requires lengthy molding cycles (usually measured in hours).


Hot Press Moldings

The matched die hot press molding process identifies compression molding. It can be greatly speeded up to molding cycles measured in seconds to minutes (depending on part thickness). Use is made of heated molds produced in high grade mold-steel, on a high-pressure hydraulic press. The molds are cleaned and treated with release agent as necessary (agents that last for several cycles are available) and the reinforcement and resin are laid up as for cold press molding.

When the molding cycle is complete, the press opens and the molded part is removed and, if required, trimmed. It is probably moved to a jig for post-cure. This is normally carried out in a vertical press (compression molding), which is also convenient for loading inserts into the molding tool.

High-pressure hot press compression molding is the highest-volume method for compression molding RPs parts, usually reaching an economic output at.a level of about 10,000 parts/yea. To facilitate shop floor working, prepared combinations of resin, reinforcement, and additives (prepregs) are increasingly used, for easy handling and reduction in press-loading times. The most widely used are known as bulk molding compound (BMC) and sheet molding compound (SMC). For this matched-die compression molding, metal molds are used, which are considerably more expensive than plastic molds because of the high pressures and high mold temperature.

Flexible Plunger Moldings

This process is a take-off from compression molding that uses solid material male and female matching mold halves. This unique process uses a precision-made, solid shaped heated cavity and a flexible plunger that is usually made of hard rubber or polyurethane. This two-part system can be mounted in a press. Rather excellent product qualities are possible at fairly low production rates. The reinforcement is positioned in the cavity and the liquid TS resin is poured on it. Also used are prepregs, BMC, and SMC.

The plug is forced into the cavity and the product is cured. The plunger is somewhat deeper and narrower than the cavity. It is tapered in such a manner that contact occurs first in the lowest part of the mold. Ultimate pressure usually used are up to 400 to 700 kPa (58 to 100 psi) in the plunger causes the contact area to expand radially toward the rim of the cavity, thereby forcing the resin and air ahead of it through the reinforcement with the target of developing a void free product.


The pressure conforms to irregularities in the lay-up, permits wall thickness to be varied within reasonable limits, and makes a good surface possible only against a metal mold surface. The fact that the heat can be applied only from the cavity side leads to longer cure cycles. This factor tends to produce resin richness, and consequently greater smoothness on the side of the solid mold surface.

Flexible Bag Moldings

An air inflated-pressurized flexible-type envelope can replace the plunger. This process provides higher glass content and decreases chance of voids. Limitations include extensive trimming and only one good surface.

Laminates

This refers to many different fabricated RP processes such as contact/ low to high-pressure laminates, and continuous laminations. It usually identifies flat or curved panels using high pressure rather than contact or low pressure. It is a product made by bonding together two or more layers of laminate materials. The usual resins are TS such as epoxies, phenolics, melamines, and TS polyesters. A modification of this process uses TPs. The type of materials can be endless depending on market requirements. Included are one or more combinations of different woven and/or nonwoven fabrics, aluminum, steel, paper, plastic film, paper, etc.

High-pressure laminates generally use pre-loaded (prepreg) RP sheets in a hot mold at pressures in excess of 7 MPa (1000 psi) (Table 5.5). Compression multi platen presses are used; up to at least 30 platens producing the flat (also curved) sheets at high production rates. Laminates are molded between each platen simultaneously. Automatic systems can be used to feed material simultaneously between each platen opening and in turn after curing and the multiple platens open cured products are automatically removed. The contact or low-pressure laminates use prepregs that cure at low pressures such as TS polyester resins. Depending on the resin formulation just contact pressure is only required such as using hand-operated rollers. The usual highest pressure that identifies low-pressure laminates is at 350 kPa (50 psi).

In industry, for almost a century these laminates are used for their electrical properties, impact strength, wearing qualifies, chemical resistance, decorative panels, or other characteristics depending on laminate construction used with or without a surfacing material. They are used for printed circuit boards, electrical insulation, decorative

2 8 4 Reinforced Plastics Handbook

Table 5,5 High-pressure reinforced TS laminates using different resins

Resin type Production form

Reinforcement

E

0 ~ - ~ - ~ ~ " ~ -

m t/~

Phenol Sheet formaldehyde Tube

Rod Molded-macerated Molded-laminated

Melamine Sheet formaldehyde Tube

Rod Molded-macerated Molded-laminated

Polyester Sheet Tube Rod Molded-macerated Molded-laminated

Epoxy Sheet Tube Rod Molded-macerated Molded-laminated

Silicone Sheet Tube Rod Molded-macerated Molded-laminated

panels, mechanical paneling, etc. The major change in the process about a half century ago was making the operation completely automatic that significantly reduced labor cost.

5 • Fabricating Processes 2 8 5

Hand Lay-Ups

This is the oldest and in many ways, the simplest and most versatile process for producing RP products.

Overall, this open molding is a low cost process that has different names such as open, contact, bag, or infusion molding where slight differences or overlaps may exist between them. Different market uses at times develop different processing names. However, it is usually slow and is usually very labored intense. There are also automated systems for relatively high production runs that significantly improve lay up procedure, reduce labor lay-up, fabricating time, and cost. See information concerning "Layout of fabric reinforcement" in Chapter 6 Aerospace, All Plastic Airplanes.

The non automated process consists of hand tailoring and placing one or more layers of usually fibrous reinforcements (random oriented mat, woven roving, fabric, etc.) on a mold and followed with saturating the reinforcement layers with a liquid plastic (usually TS polyester) (Figure 5.16). Usually it is required to coat the mold cavity with a parting agent. Gel coatings with or without very thin woven or mat glass fiber scrim reinforcement are also applied to provide smooth and attractive surfaces. Molds can be made of inexpensive metal, plaster, RP, wood, etc.

Figure 5.16 Contact molding by hand lay-up

Depending on the resin preparation, the material in or around a mold can be cured with or without heat, and commonly without pressure. Curing needs include room temperature, heat sources, bag (BagM),

a"

r e-i

I l l 1-4-

I l l

- I -

",1 e l 0 "

0 0

Figure 5,17 Aut0mated-integrated RP vacuum hand lay-up process that uses TS polyester prepreg sheets


vacuum bags, pressure bags, autoclaves, etc. An alternative is to use prepreg and sheet molding compound (Chapter 4), but in this case, heat is applied with low pressure via an impermeable sheet over the material. This process can produce compact structures that meet tight thickness tolerance.

Generally, the process only requires low-cost equipment that is not automated. However, automated systems have been used. Figure 5.17 shows Grumman's automated-integrated hand lay-up system. It uses TS prepreg sheet material. Automation includes cutting and providing the layout of the cut prepreg in a mold. In turn, the designed RP assembly is delivered to a curing station such as an oven or autoclave.

This process can be recommended for prototype products, products with production runs that require thickness fight tolerance control, and molding complex products that require high strength and reliability. The size of the product that can be made is limited by the size of the curing oven. However, outdoor UV via outdoor sunlight curing or room temperature curing plastic systems permits practically unlimited product size. Alternate curing methods are used that include induction, infusion, dielectric microwave, xenon, UV, electron beam, or gamma radiation.

The general process of hand molding can be subdivided into specific molding methods such as those that follow. The terms of some of these methods as well as others reviewed here overlap the same technology; the different terms are derived from different sections of the RP industry during different periods since 1940.

Bag Moldings

Process applies an impermeable tailored flexible bag (parting film, elastomer, etc.) over an uncured thermoset RP product located in a mold cavity (male or female), sealing the edges (bagging), and introducing a vacuum and/or compressed air pressure (or water) and heat around the bag. It provides a means of evacuating air and other gases as pressure is applied. Hand operated serrated rollers arc usually used to squeeze out voids, air, etc. This high labor technique can produce compact structures that meet tight thicl~ess tolerance simulating injection molded products. This technique is also applied with other RP fabricating processes. Since the 1940s this process has been used in fabricating high performance structural parts, particularly for large parts, for military and commercial components, bridge components, containers, machine housings and covers, sports car bodies and components, and boat hulls and components.


The mold (usually female, such as a boat hull or tank) is cleaned, prepared, and sealed (Figure 5.18). Where it is of single curvature only, it may be possible to use a plastic film as a solid release agent but usually the mold is coated with a release agent and a gel coat. Surfacing tissue (of fine glass fiber reinforcement) can be applied, to produce a good smooth surface, before the main reinforcement (usually in the form of chopped strand glass mat) is laid in place, trimming the pieces to size and adding additional local reinforcement as necessary. Liquid catalyzed resin (mixed with pigments and such additives as are needed) is then carefully poured over the reinforcement. This lay out is then covered with a flexible bag and the whole assembly is worked over with hand- rollers (serrated), to ensure even distribution of the resin and (most important) effective wetting-out of the reinforcement. The objective throughout is to ensure a good bond between resin and reinforcement, eliminating all voids and air bubbles (as these will impair the physical properties of the final molding).

Figure 5.18 Glass fiber swirl mat/TS polyester RP hand lay up boat shell


Core materials such as rigid foam, balsa wood, or honeycomb may be added during the reinforcement lay up. Inserts, such as metal fittings or reinforcements can also be incorporated and virtually encapsulated in resin and reinforcement. Finally, the completed lay-up in its mold is moved to a separate area for curing, which may take several hours or may be speeded up by controlled heating, in an oven, under infra-red sources, etc.

When finally cured, the molding is removed from the mold (in many cases, the mold may itself be dismantled, to facilitate de-molding, especially where reverse curvature is involved). The molding can be trimmed, finished and fitted out as necessary. A number of moldings can be bonded to each other.

Advantages:

�9 relatively low investment cost for equipment and tooling

�9 great flexibility in part shape and laminate design

�9 wide range of physical properties, according to type of resin and amount and type of reinforcement

�9 relatively inexpensive materials.

Disadvantages:

�9 threatened by government regulations (volatiles, worker exposure, hazardous waste)

�9 difficulty of quality control, due to dependence on the skill and expertise of individual laminators

�9 relatively slow production/high labor cost

one molded controlled surface only.

Equipment required:

�9 laminating brushes

�9 brush cleaners/renovators

�9 metal rollers: paddle roller 2-6 in width x 5/8-1.75 in diameter; disc roller (for removing air from awkward areas): 3 ribs x 1.5 in diameter

The limitations improvements.

laminating rollers, with extra pile/long hair/short hair for applying resin/gelcoat.

of lay-up have led to development of many


Vacuum Bag Moldings

This process also called just bag molding, is the conventional bag molding, hand lay-up, or spray-up that is allowed to cure without the use of external pressure. For many applications, this is sufficient, but maximum consolidation may not be reached. There can be some porosity; fibers may not fit closely into internal corners with sharp radii but tend to spring back. Resin-rich and /o r resin-starved areas may occur because of draining, even with thixotropic agents. With moderate pressure, these defects or limitations can be overcome with an improvement in mechanical properties.

One way to apply such moderate pressure is to enclose the wet-liquid resin material and mold in a flexible membrane or bag, and draw a vacuum inside the enclosure (Table 5.6). Atmospheric pressure on the outside then presses the bag or membrane uniformly against the wet lay-up. An effective pressure of 10 to 14 psi (69-283 kPa) is applied to the product. Air is mechanically worked out of the lay-up by hand usually using serrated rollers. The vacuum directly helps to remove air in the wet lay-up via techniques such as using bleeder channels within the bag (using material such as jute, glass wool, etc.) to aid in the removing of air and permit drainage of any excess resin. This layup is than exposed to heat using an oven, heat lamp, or other device.

Table 5.6 Dimensions of typical disposable vacuum consolidation materials.

Width Thickness Type (mm) (mm)

Moximum Roll Surfoce operoting length mass temperature (m) (g/m 2) (~

Breather/bleed fabrics

Perforated release films

Vacuum bag film

Tacky tape

1550 5 100 150 205

10 50 340 205

(rigid) 122 0.025 183 - 160

(elastic) 0.025 183 - 260

3400 0.050 50-250 - 205

1.2 3 15 - 100

Vacuum bag film is usually a polypropylene film, modified to resist elevated temperatures. The elastic nature of the film offers high formability and resistance to puncture, also permitting the film to be stretched over complex molds without need for a large number of tucks and folds, so improving the efficiency of the vacuum. The film is suitable for use with prepreg and wet lay-up laminating systems in an autoclave.


�9 Breather~bleed fabric is a 5 mm or 10 mm thick uncompressed felt of temperature-resistant synthetic fibers, treated with mold release. The material has good drapability, allowing use with large and complicated mold patterns. It can be used also as a bleed fabric to absorb excess resin from wet lay-ups.

�9 Perforated release films are made from modified polypropylene and can be rigid (recommended for use on flat or uncomplicated mold surfaces) or elastic (thin and stretchable, for large complex molds without need for tucks and folds). The films have good mechanical properties and performance at elevated temperature and are naturally self-releasing, treated on one side to assist adhesion to the breather fabric or a protective film.

�9 Tacky tape is a butyl-based vacuum bag sealant giving high elasticity and tenacity. It has exceptional sealant properties, eliminating the risk of imperfect seals often found in the initial phases of vacuum bag application, so improving the vacuum efficiency and reducing labor. It is suitable for polyester, vinyl ester, and epoxy laminating systems.

Vacuum Bag Moldings and Pressures

To maximize properties in the product higher pressure is needed in the conventional vacuum bag system. A second envelope can be placed around the whole assemblage. Air under pressure is admitted between the inner bag and the outer envelope after the initial vacuum cycle is completed (Figure 5.19). In addition to air, application of pressure can be by steam or water that forces the bag against the product to apply pressure while the product cures. Still higher uniform pressures can be obtained by placing the vacuum assemblage in an autoclave. By this technique, an initial vacuum may or may not be employed. Result in using an autoclave is ensuring development of maximum molded product performances. Using this combination of vacuum and pressure bags results in ease of air or gas removal and higher pressures resulting in more dcnsification.

Figure 5.19 Schematic of vacuum bag molding


Autoclave Moldings

As a further improvement on the hand lay-up, vacuum bag, spray-up processes, and other processes it is possible to insert the laid-up molds (depending on size) into special bags, which are then evacuated and/or placed in an autoclave for cure under controlled (steam) heat and pressure. This vacuum consolidation method produces high-quality moldings, with complete exclusion of air bubbles and improvement to the inner surface of the molding. The controlled curing conditions also improve quality and consistency and allow high performance resin systems to be used, while opening the way to a more rapid cure with faster turn-round of molds. This technique is widely used for higher- performance moldings, such as for aircraft and aerospace applications.

Vacuum bag molding is a modification of hand lay-up, in which the lay- up is completed and placed inside a bag made of flexible film, and all edges are sealed. The bag is then evacuated, so that the pressure eliminates voids in the laminate, forcing excess air and resin from the mold. By increasing external pressure, a higher glass concentration can be obtained, as well as better adhesion between the layers/plies of laminate. Some items for the process can be disposable.

Some of the different RP processes are used in conjunction with the use of an autoclave oven (Figure 5.20). Hot air or steam pressures of 0.36 to 1380 MPa (50 to 200 psi) is used. The higher pressure will yield denser products. If still higher pressures are required (avoid this approach unless you have consider the danger of extremely high pressures), a hydroclave may be used, employing water pressures as high as 70 MPa (10,150 psi). The bag must be well sealed to prevent infiltration of high-pressure air, steam, and/or water into the molded product. In all these approaches, the fluid pressure adjusts to irregularities in the lay-up and remains effective during all phases of the resin cure, even though the resin may shrink. Use of this process includes seamless containers, tanks, pipes, etc.

~ essure line

Part

ool

Steam coils

AUTOCLAVE

Figure 5.20 Schematic of hand lay-up bag molding in an autoclave


Autoclave Press Claves

This process simulates conventional autoclave by using the platens of a press to seal the ends of open chamber. It provides both the force required to prevent loss of the pressurized medium and the heat required to cure the RP inside.

Wet Lay-Ups

This method is sometimes combined with bag molding to enhance the properties. This procedure can be called bag molding. Because it is difficult to wet out dry fibers with too little resin, initial volumetric fraction ratios of resin to fiber are seldom less than 2:1. On a weight basis the ratio is about 1:1. Liquid catalyzed resin is hand-worked or automatically worked into the fibers to ensure wet-out of fibers and reduce or eliminate entrapped air.

Glass fiber/TS polyester RPs (GRP) remains a major focus in the boat business. Crafts are still being laid up in open molds by traditional manual wet lay-up using chopped strand mat (CSM). Requiring only modest start-up investment, this accessible, entry-level technology is used for thousands of working and leisure craft annually. GRP craft range from 7 ft dinghy/tenders to plastic minehunters in service with leading navies.

As an example glass RP boats in use are visually almost indistinguishable from traditional, but more expensive, wooden craft. Slightly higher tech is the Raptor Sportier, an advanced personal watercraft (PWC) produced in North America. Although hand laid in glass fiber molds and room temperature cured, the all-glass prototype incorporates 00/90 ~ woven fabric and stitched triaxial reinforcement as well as chopped strand mat. Hull cavities are filled with two-part polyurethane foam for buoyancy and noise reduction.

Conyplex in the Netherlands, like many series production boatbuilders uses glass fiber constructions, including in its latest Contest 44 and 50 yachts, to achieve easily driven hulls of modest displacement. Perform- ance of its new-generation boats is further enhanced by wing keels, an RP wet lay-up appendage. The company has been transitioning to resin injection molding techniques.

Spray-Ups The mold is prepared as for hand lay-up but the resin and reinforcement are applied either by spray gun, which can be operated manually or by robot. This gives a more reproducible process, with greater control over the amount of each material that is deposited, opening the way to complete automation. The process has been a popular system


with RP production for over half century. Many different fabricated products have been made by spraying. Included has been using the reinforced spray-up as non-structural and structural supports to solid materials (Figure 5.21).

Figure 5.21 Thermoformed plastic, used as the female mold, is backed up with sprayed RP

With time passing significant new developments occur particularly in the spraying equipment. An air spray gun includes a roller cutter that chops usually glass fiber rovings to a controlled short length before being blown in a random pattern onto a surface of the mold (Figure 5.22). Suppliers of spray-up equipment continue to produce cleaner, reduced styrene emissions (as low as 2.2%), higher capacity, more

Figure 5.22 Contact molding by spray-up


uniform spray pattern, and more versatile. Types and performances of spray guns are many such as external or internal mixing gun, distributive/turbulent mixing gun, air atomized, airless, etc.

As the fibers leave the spray gun simultaneously, the gun sprays the usual catalyzed TS polyester plastic. The chopped fibers can be plastic coated as they exit the gun's nozzle (Figure 5.23). The resulting, rather fluffy, RP mass is consolidated with serrated rollers to squeeze out air and reduce or eliminate voids; automatic equipment is also used. A closed mold with appropriate temperature and pressure produce products.

Figure 5.23 Various methods of spraying

If required alternate layers of sprayed fibers with layers of woven roving or other fabric construction can be included during the spraying cycle.


It is possible to deposit several layers until the desired thickness is obtained. The quality of the part will depend directly on the efficiency of impregnation and removal of entrapped air. When the required thickness has been built up, the lay-up is worked over manually as with hand lay-up.

This method is used both in-plant and out of doors, for work such as on-site application of reinforced corrosion-proof coatings to chemical and similar plant. The fact that resin and reinforcement are in spray form makes it essential that the operatives use protective clothing, including face-masks, to protect them from volatile chemicals, particularly vapor from styrene (which is used as a solvent). Regulations governing emission of styrene at the workplace are being progressively tightened worldwide, by EPA and OSHA in the USA, and by national authorities in Europe.

Selection of suitable spray guns is important, to ensure that chopping and mixing of the components is effective and that operation is safe and comfortable. Although cost of this equipment is relatively low compared with costs of material, labor, and overheads, correct selection of equipment is still vital. For example, assuming a daily use of two barrels of resin (1000 lb), ten boxes of glass (500 lb) and two gallons of catalyst (15 lb), with a standard resin/glass/catalyst lay-up mix of 6 6 / 3 3 / 1 wt%, annual material costs can be calculated as US $342,372, suggesting that cutting costs on equipment is a false economy if it does not produce more effective use of materials (cost here are subject to change).

Typical spray-up systems draw the resin from the manufacturer's drum by positive displacement double-acting pump, filtering through a fine mesh screen with control devices to ensure steady even flow. Catalyst is similarly pumped, with pressurized air in the catalyst accumulator to exert back pressure against the pump. The ratio of catalyst to resin can be adjusted at the pump, typically from 0.75% to 3% _+0.1%. Glass strands are threaded through ceramic-coated eyes to the chopper and the chopped glass is dispersed evenly throughout the resin fan and carried to the mold surface, with very little trapped air, requiring little rollout.

Spray guns are available for gelcoat, chopper, saturator and flow coater, at outputs of 3.6-9 kg (8-20 lb)/min resin, 1.8 kg (41b)/min glass two-strand chopper, or 1.1 kg (2.61b)/min one-strand chopper, giving a total laminate deposition of 5.4 kg (12 lb)/min with two strands. Air consumption is 9-10 ft3/min. For higher volumes, four- and six-strand choppers are also available, dispensing 17-19 kg/min resin and 33% glass content.


Spray guns provide the following:

�9 easy start-up with accuracy and repeatability, with a precision proportioning system fitted to a 12:1 ratio positive displacement resin pump. This has a rack and pinion drive for smooth linear motion, eliminating use of unequal forces and requiring only an easy dial setting to program catalyst delivery between 0.5 and 3.5%

�9 continuous adjustability of catalyst mixing ratio between 0.16 and 2.5% with a resin roller impregnator

�9 some equipment allows the same gun to be used for spraying, wet- out, flow coaters, chopping, resin transfer molding and roller attachments, in a range of internal mix applicators with an air- operated pistol machined from 6061 aluminum stock. Attached to the gun body is a series of modular plates allowing the fabricator to select the type of nozzle outlet

�9 linear columns of resin can form the spray pattern, at significantly lower atomization levels. The nozzle can be used as a chopper as well as for simple resin wet-out, using a patented air-assist containment system to increase resin/catalyst transfer efficiency and is an external mix design.

As well as reinforcement, different spray systems have been developed to handle other materials at the same time:

�9 granite gel coat can be sprayed directly from the shipping container with a special system, with automatic recirculation modes delivering a fast and predictable spray pattern. Adjustable disperse air control gives a uniform dispersion of granules at very low pressure

�9 metal flake can be sprayed with an airless air-assisted system. The flake can be applied separately or together with a gel coat (the former gives better reflectivity). Airless air-assisted spraying reduces overspray with savings of up to 50%. Dry fillers and extenders can also be sprayed

�9 reground laminate, from recycling processes, can also be fed into some types of spray nozzles, acting as a filler with good mechanical properties in the lay-up.

Airless internal mixing A patented hydraulic injection system is claimed to provide a truly airless internal mixing method. As well as ensuring thorough blending of materials, reducing waste and contributing to strength of the laminate, it also significantly reduces styrene emissions. The system relies on a positive displacement pump and uses low-pressure hydraulic atomization to break the resin into large droplets after mixing, which improve saturation of roving by their stronger penetration than smaller droplets.


It is automated by a four-axis bridge robot, which can be set in motion just by pressing a few buttons. Graphical software is used to enter the coordinates and the four-axis design allows efficient spraying of three- dimensional molds up to 9 m (29.5 ft) wide. A further development incorporates a mechanically blended foam unit, to automate foam applications.

A low-pressure glass fiber polyester spray plant also gives low styrene emission and little mist formation. Resin and hardener are mixed and an automatic pump delivers safe exact proportions of peroxide from original containers, no peroxide being dispersed in the air. The German Technical Monitoring Service has confirmed an emission of only 5 ppm and low-pressure equipment with internal mixing also ensures that no peroxide is freely dispersed into the air.

For spray lay-up with low emission, it is claimed that external equipment can over-atomize resin and catalyst, but an internal mix system operating at lower pressure creates a 0 larger particle size which has less surface area exposed to atmosphere where styrene can flash. The flow-chop nozzle (from Venus-Gusmer) is designed to carry glass with no atomization of raw resin, catalyst or mixed material, in an overall design claimed to disperse glass throughout the resin pattern with minimal loss of glass.

Turbulent Mixing Hydraulic injection also reduces air entrapment, so that roll-out can be quicker, giving a laminate with improved properties. The units are designed for medium production rates of 1.8 kg (4 lb)/min. Systems using external mixing simply combine resin and catalyst within the spray pattern, but inclusion of a device to improve mixing (such as a turbulent mixer) improves performance of equipment. The design fits inside the gun head, to mix catalysts and resin thoroughly, using a continuous spiral groove with many cross-cuts, creating a 235 mm (9.25 in.) mixing path to ensure a complete blend. Again, airless mixing permits low-pressure atomization, which greatly reduces styrene fume emission and cuts down waste.

Distributive Mixing Distributive mixing enhances airless internal blending by pre-mixing catalyst and resin before they pass through the turbulent mixing device, giving further all-round improvements in quality and economy.

Foaming Polyester Using compressed gas instead of chemicals to foam polyester resins can reduce control problems and heat reactions. Mechanically blended foam processing systems use a non-reacting additive pre-mixed into the resin


before the gas is introduced. Compressed gas does not normally have a lasting foaming effect and the resin bubbles immediately begin to burst, but the additive sustains them until cured, also working to produce a uniform size and shape of bubble.

The process can be used to foam a wide variety of resins, with foam density dependent on the gas used. Relatively large cells provide good compressive strength. Foam layers can be sandwiched between polyester/glass laminate skins, giving better bond strength. In chop and spray applications, the fiber sprayed with foam need no roll-out, offering savings when the unit is used with robot automation.

A wheel-mounted range of pressure-feed dispensing roller machines (by applicator) blends compressed gas with a special polyester resin to create a foamed material with good impact resistance and compressive strength. Good working conditions with minimum styrene emission are claimed. No roll-out is needed after spray-up application, saving labour time and sandwich constructions can be made on one step, without waiting for layers to cure. A new dispensing unit uses patented Double Flow Technology to give an accurate catalyst dosage with good mixing with the resin. A pneumatically-operated trigger on the shaft gives individual adjustment and there is only one part in the solvent pump. The output is 1.8 to 6 kg (3.9-13 lb/min). A similar design is also used for a gel coater unit.

Ancillary Equipment for Spraying Ancillary equipment for spray-up work includes:

�9 automatic gun/robot arm mounting, with variable volume accessories

�9 wall-mounted systems

�9 flow coater nozzle (eliminating atomisation and significantly reducing over spray)

�9 air-assisted nozzle (for pattern-shaping of internally mixed catalyst and resin and optimizing the mix design)

�9 catalyst alarm (to monitor catalyst flow, rather than pressure)

�9 air purge attachment (to inject air through the mixing tube while flushing and providing extra turbulence to move thick fillers during clean out)

�9 resin roller dispenser attachments (instead of spraying, these dispense the mix internally through small holes in the roller)

�9 air motor shut-off(safety override) valve

�9 nitrogen accumulator charge system

floor and ceiling mount booms.


Bag Molding Hinterspritzen

This patented process allows virgin or recycled RPs such as PP and PC/ABS to thermally bond with the backing of multilayer PP based fabrics providing good elasticity. This one-step molding technique provides a low cost approach for in-mold fabric lamination that range from simple to complex shapes.

Contact Moldings

Also called open molding or contact (very low) pressure molding. It is a process for molding RPs in which the reinforcement and TS polyester resin are placed in a mold cavity. Depending on plastic used cure is either at room temperature using a catalyst-promoter system or by heating in an oven without pressure or using very little (contact) pressure. Contact molding gave rise to bag molding, hand lay-up or open-mold, and low- pressure molding. It has played a significant role in molding RPs. It is difficult to surpass if a few products are to be made at the lowest cost. The process relates to what was reviewed for Bag Molding.

Resin lamination should n o t be carried out at a temperature of less than 20C (68F). A catalyzed/promoted resin is heat-sensitive and catalyst levels should be adjusted to varying temperature as recommended by the manufacturer. With resin laminating, where space is limited, mold preparation such as releasing and gelcoating can be done in a tent made of polyethylene film (which can also be used as a warm area for curing resins in cold weather).

When working with TS resins, hot water, soap, and an eyewash system should always be available. Styrene vapor is heavier than air, so ventilation or extraction equipment should take off vapor from the lowest part of the mold, or the lowest level of the molding area. Air bubbles in a laminate reduce strength and may impair corrosion resistance. To minimize air bubbles, the following are recommended:

�9 Avoid violent mixing, which can mix air into the res in - but ensure that the catalyst is mixed in thoroughly.

�9 Apply resin to the mandrels first, then apply the glass and roll it into the resin. Air bubble problems are inevitable when resin is applied to dry glass.

�9 Roll the laminate from the centre, out to the edges, firmly but not too hard. Excessive pressure can fracture any existing bubbles and make them more difficult to remove.

�9 Eliminate all the bubbles from one ply before starting on the next.

�9 Thoroughly clean the rollers between uses.


Useful materials and equipment include a range of toggle mold clamps such as the following:

�9 hold-down vertical: 100-1200 lb

�9 hold-down horizontal: 60-750 lb

�9 flush mount: 300-700 lb

�9 pull action: 375 lb

�9 straight line: 300-2500 lb

�9 squeeze action: 200-700 lb.

As well as the materials and molds, resin lamination of all types needs a broad back-up of equipment for application and materials for cleaning. Many suppliers offer the full range and give useful comprehensive advice:

�9 drum carriages

�9 buckets, pots (polyethylene)

�9 knives, scissors, snips

�9 tapes/rules

�9 laminating brushes.

Styrene monomer is used as a thinner and can also be used for cleaning wax buildup from molds.

Paraffin wax solution is an organic solvent that can be added to the final layer of a laminate to prevent stickiness; up to 2% by weight can be added. Details on styrene monomer use are given in Chapter 3 Thermoset Plastics, TS Polyester Solvents.

Squeeze Moldings

This method is a take off between resin transfer molding (RTM) and hand lay-up. The reinforcement and a room temperature curing TS polyester resin are put into a mold. In turn, the mold is put into an air pressure bag where the resin is slowly forced through the reinforcement in the mold cavity at low pressures of about 30 to 75 psi (200 to 500 kPa). The RP is cured at room temperature in unheated molds. It is a slow process so one or a few products per day is usually molded.

Soluble Core Moldings

This technology is also called fusible core molding, soluble core technology (SCT), lost-wax molding, loss core molding, etc. This technique is a take off and similar to the lost wax molding process used


during the ancient Egyptian times fabricating jewelry. In this process, a core is usually molded of a low-melting-point eutectic alloy (zinc, tin, etc.), water-soluble TP, wax formation, etc. During core installation, it can be supported by the mold core pins, spiders, etc. The core is inserted in a mold (IM, CM, casting, etc.) and plastic injected or located around the core. When plastic has solidified and is removed from the mold, the core is removed by melting at a temperature below the plastic melting point through an existing opening or will require drilling a hole in the RP.

Cores of metal alloys have been used in production of automobile air intake manifolds, pump and valve housings, etc. In a typical application, lightweight RP cylinders, said to be about half the weight of aluminum equivalents, are molded on fusible cores (which have themselves been cast in GRP mold). The core molds reduce tooling costs by up to 90% and mold-making time by several days, making it viable to produce single-piece RP components. Prototype high-pressure cylinders have been produced, using the technique.

The cores are cast in a split GRP mold, using a bismuth-based alloy, formulated to flow freely and reproduce closely the contours of the mold. When the core has been cast, an uncured RP material shell is placed over it and cured. When the RP shell is cured, the alloy is melted out at 137C (278F), producing a single-piece RP cylinder, with an accurately dimensioned interior. The melted alloy is recovered and recycled.

Alloys with melting points up to 250C (480F) are available, meaning that the technique is flexible enough to be applied to a range of RPs curing and molding at relatively high temperatures. Cylinders of up to 5-liters capacity can be produced with the present technology, to withstand pressures of up to four times the design rating.

Lost-Wax Moldings

When this soluble fusible core molding technique was first used to fabricate structural supports it involved a rectangular bar of wax wrapped with RPs (such as glass fiber/TS polyester resin) (Figure 5.24). After the RP is cured (bag molding, oven, autoclave, etc.) in a restricted two part mold to keep the rectangular shape, the wax is removed at low heat by drilling a hole or removing the ends. The result is very high strength RP rectangular channel. Its shape can be rectangular, round, curved, etc. This process was used during 1944 to fabricate the first all plastic airplane. This lost wax process was used with bag molding the RP sandwich monocoque construction fuselage, wings, etc.


Figure .5.24 Lost wax process fabricated a high strength RP structural box beam

Marco Processes

During the 1940s to 1960s, this matched mold and bag molding process was used extensively to fabricate many different RP products. It was the forerunner for resin transfer molding (RTM), infusion molding, and other RP processes. Reinforcements are laid up in any desired pattern as in BagM and RTM in a mold cavity. Low cost matched molds (wood, etc.) confine the reinforcement. In this initial process, a horizontal open mold trough at the molds parting line surrounds the usual two-part mold. An opening (hole) usually in the center of the top mold halvc provides liquid catalyzed resin to flow under controlled pressure into thc mold cavity (Figurc 5.25). The plastic melt flows into the cavity to encapsulate completely and wetting the reinforcement. With proper wet-out of fibers, voids are eliminated and excellent bond of fiber to resin occurred. The resin exits into the mold's trough; the trough has controllable restrictions that aids in dispersing the resin in the cavity. In addition, vents are located in the mold located where resin is not properly covering the reinforcement or the last areas of the mold to be filled. When the mold has filled, the vents and the resin inlet(s) are closed. This mold was either not enclosed or enclosed in a sealed box, etc. When enclosed gases, etc. were released they were trapped and disposed.

This relatively closed mold method when first used was the forerunner of the resin transfer molding (RTM) and the different infusion processes. Other approaches were incorporated in the Marco process that included a vacuum applied in the opening with resin flowing from the trough (reversing the melt flow), vacuum via the trough with resin flowing from the opening entrance in the top mold half, push-pull


Figure .5.25 Use is made of vacuum, pressure, or pressure-vacuum in the Marco process

action on the resin where pressure and suction was applied in the center hole with limited and controlled vent and trough openings, and reverse of the push-pull action so it takes place from a restricted trough with limited and controlled mold opening (or openings). The Marco process was engineered with different shape and size resin drainage channeled ribs and vents to control melt flow through and around fibers. With the vacuum system, it was a takeoff of vacuum bag molding with or without pressures.

Reinforced Resin Transfer Moldings

Reinforced resin transfer molding (RRTM) is also identified as just resin transfer molding (RIM). It is a closed mold, low-pressure process in which a preplaced dry reinforcement fiber construction (such as woven and nonwoven fabric or a fiber preform) with or without decorative surface material is impregnated with a liquid plastic through an opening in the center area of a mold (Figures 5.26 and 5.27). The resin at about 100 to 200 psi (0.69 to 1.38 MPa), possible as low as 50 psi (0.3 MPa) pressure [and possibly assisted by vacuum (VARTM)])moves through the reinforcement located in the mold cavity. The air inside the cavity is displaced by the advancing resin front, and escapes through vents located at the high points or the last areas of the mold to be filled. When the mold has filled, the vents and the resin inlet(s) are closed. After curing via room temperature hardeners and/or heat, the part is removed. This process provides a rather simple approach to molding designed RP parts in relatively low-cost molds (using low pressure), and the molds are manufactured in a short time. It can also incorporate


vacuum to assist resin flow; with VARTM the process is called infusion molding. This type of action could be identified as a take-off to the Marco process.

Figure 5.26 Description of the reinforced resin transfer molding fabricating system

Figure 5.27 Cut away example of a mold used for resin transfer molding mold


RTM was described May 1957 (at the BPF Reinforced Plastics Con- ference), in a paper on work dating from 1954 at Bristol Aircraft, Filton, UK. It is basically a liquid resin version of compression transfer molding; a process in which a charge of compound is placed in a transfer pot and is injected into a closed mold, usually by a plunger system. The method allows the compound to be suitably prepared melt and then transferred fast and accurately into the mold, in which the required reinforcement has already been placed, in suitable form.

Essentially, the RTM fabricator places a preform in the mold, which is then closed. The catalyzed resin system is then pumped into the mold, impregnating the reinforcement. Heat can be applied to the mold to shorten the cure-time and the finished part is then removed. Drawbacks of the system in the past have been the manual lay-down of reinforcement, high-energy consumption, need for ventilation, and difficulty of demolding. There was no control over fiber orientation and the lay-up required separate surface veil. Past developments have gone a long way to rectify these drawbacks, but it remains vital to maintain accurate control over the process, if consistent specifications (such as acceptable surface finish) are to be achieved. Control and monitoring systems have been developed, so that RTM is an important process for molding RP parts to high levels of quality and reproduceability.

Operating at low pressure, it uses lower-cost machinery and molds than compression or compression transfer systems. It offers an economic means of medium volume production of high quality parts. For example, for production of automobile components up to 30,000 per year RTM is preferred over compression molding with SMC because of lower capital investment (about 10% and 20% of the cost of a typical SMC compression molding plant) and versatility in component design. Components of large surface area (up to 2.5 m 2) can often be more economically produced in low-pressure RTM because of the lower tooling cost and low tonnage presses (both SMC and sheet steel require higher pressures for molding/stamping).

RTM is competing over autoclave molding for production of high performance aerospace and sporting components, using advanced materials such as epoxy resins and carbon fiber reinforcement, with accurate fiber placement systems, to mold (for example) RP propeller blades. It is also frequently used in production of sandwich (such as foam core) structures, where high molding pressures could damage or dislodge the core. Novel smart core molding technology is now applied to RTM, for production of 8.5 m long rotor blades for wind power generators.

Compared with hand and spray lay-up, the RTM process also offers an


environmentally preferable option, with better quality assurance and lower process economics. It has demonstrated an ability to accommodate foam cores, precise thickness changes, inserts and different fiber styles, all integrated simultaneously. The trend is now towards perfecting catalyst injection as the preferred route, with machines metering and mixing over a wide ratio, up to 200: 1. Experience has shown (not for the first time) that it is not satisfactory to convert equipment from another process. Converted spray meter mix machines, for example, can give poor control over mix ratios and backpressure in RTM and specialized machines now favored. These give higher output with low nozzle pressure, pump speed control, precise catalyzed mix start-up, automatic predetermining of shot volume control, gel timers, automatic shutdown and clean out, recirculation systems to and from the mixing head, and process data acquisition. Mold design has also proved to be a key factor to its success. Automatic mold clamping systems with built-in manipulators and low-cost pneumatic presses have also assisted the development of RTM.

Equipment

Equipment wise five processes are available: manual, braiding, knitting, thermoforming continuous strand mat, and directed fiber preforming. The last two are most applicable to large-size industrial parts. Continuous strand mat is better for medium/small series where investment of robot preforming cannot be justified, but price differential between rovings and continuous strand mat, plus scrap rate (less than 2%) makes directed fiber a very economical way of producing preforms for high volume applications, especially when the part involves deep draw or complex geometry.

RTM is capable of many variations, and significant modifications have been introduced in recent years, testifying to the commercial attractive- ness of the process. The technology depends on: preparation of the resins to be injected, controlled injection with regulation of the cycle and a tracing/copying system by which each phase of the operation is recorded. Until recently, development has been inhibited by the lack of efficient cost-effective preforming technology and over-long cycle times. The latest developments have centered on mixing, resin flow, process control, feeding, etc.

Mixing Technologies

The basic mixing head technology developed for polyurethane components can be applied also to RTM for other reactive chemicals. A typical metering system today can handle TS polyesters, epoxies and


acrylics, using impingement high-pressure mixing in a self-cleaning head not requiring solvents for rinsing, giving an output of 100--400 cm3/s at ratios from 100:1 to 100:2.5, for an injected weight of up to 40 kg (88 lb). Cylinder metering is used (with advantages in processing filled and abrasive resins); with common oil tank feeding two pumps for resin cylinder and mixing head accumulator and catalyst cylinder. Two- component reactive resin blending is possible with metering to +_2%, at 25-2000 cc/min, in a volumetric ratio range of 1:1 to 40:1 (Table 5.7).

Table 5.7 Equipment selection chart for resin transfer molding

Medium to large size parts

High volume hydrajectors

5:1 Power No or medium ratio filler loading

10:1 Higher filler Power ratio loading

4:1 Power ratio

7:1 Power ratio

8:1 Power ratio

Polyester Epoxy

Small to medium size parts

Standard volume hydrajectors

No or medium

filler loading

Higher filler loading

Low flow

Advanced composite

Re:~in transfer

ram

Two component resin/hardner

11-4.7:1 mix ratio

2:1-11:1 mix ratio

1 1-8:1 mix ratio

Higher fiberglass content

5-50 Ib/min

I EPo21

I EPo, I

A Wolfangel design runs four components, at 40 kg/min output, metering and mixing resin, peroxide, accelerator and inhibitor on-line, so making it possible to fill a large tool in one minute. Compared with a pre-mix arrangement, an on-line mixing system also guarantees fresh resin, which gives better flow through the glass fiber pack. A user is claimed to have achieved a timesaving of nearly 50% in molding large vacuum-assisted components.


Improvement of Resin Flow and Injection

Irregular flow and pressure during the mold-filling sequence can cause waviness, dry patches and overloading. Since many advanced RTM resins arc solid or highly viscous at room temperature (and therefore all machine hardware must be thoroughly heated), a compact injection machine needs to have custom-fitted heated mantles on the reservoirs, pumps, fittings and mixers to eliminate any cold spots.

The heating capability is from ambient to 150C (300F) and insulation ensures that, even when operating at over 100C (212F), the exterior is only warm to the touch. A modern machine is kept to 760 x 1000 mm (2.5 x 3.30 ft) floor space. One or two components can be injected, maintaining pre-programmed pressures of between 140 and 2050 MPa: the ratios are from 1:1 to 150:1, flow rates up to 1500 cm3/min and flow rate and injection pressure are both controlled.

A dosing unit suitable for all flowable two-component materials is available, adjustable to any required mixing ratio, with pneumatic drive operated by push-button. Further development for RTM makes it possible to produce high quality moldings at high volume: designed to overcome problems with resin flow and wetting out of reinforcements, all injection parameters can be controlled by programmable logic control (PLC), allowing optimization both of flow and pressure within the actual mold.

A low-pressure injection system, with 4:1 ratio pump at 9.5 liters/min output, is available, with a stainless steel catalyst delivery pump offering rates of 0.5 to 4.5%. The injection gun has a single moving valve, which shifts when the gun is activated. The system includes complete loop recirculation, air logic maintaining consistent static and dynamic pressure, and a stroke counter located at the injection gun.

Low-cost injection systems have been developed for RTM systems. The Spartan and VR-2 units, respectively, inject TS polyester and vinyl ester systems with a catalyst content ranging from 0.5-4.5% and can handle closer ratio mixes, as for epoxy or hybrid systems. The two can be combined in a single unit (VR-3).

Improved Process Controls

Available is a programmable logic-controlled machine for a one-stroke machine with no pulsing of resin flow and full control of all filling parameters. It uses hydraulically driven and computer-controlled delivery cylinders, with the catalyst cylinder mechanically connected to the material cylinder, for accurate and complete dosage of the components. The system


allows for pre-setting of all mold-filling parameters and control of all aspects of injection.

A catalyst flow monitoring and alarm system demonstrates outstanding sensitivity over the whole range of flows in meter mix machines, for RTM and also for spray and dispensing machines. Various options offered include a miniaturized light-emitting diode bar display repeater, for remote mixing head mounting, and an enlarged wall-molded catalyst flow indicator. The unit is designed to fit to any meter mix machine and is an inexpensive solution to the problem of accurate monitoring of catalyst flow.

Another monitoring system, which can be retrofitted to a range of makes of machine, can measure catalyst flow rates down to 6 ml /min (equivalent to 0.5% catalyst) pumped with the resin at an output lower than 1 liter/rain. It has two electronic flow indicators, 100 mm scale meter, and 10-element light-emitting diode (LED) display.

With a controlled flow-pressure module, an accurate RTM unit (from Magnum Industries, USA) allows the user to select exact fluid pressures, for high flow rates eliminating pressure differentials and surges during pumping. A minimum of moving parts gives quick easy flushing at low and high pressures. The catalyst level can be held to an accuracy of within 0.1%.

The Megaject Mark II injection system, from Plastech TT, offers facilities such as pre-setting the amount of catalyzed resin to be injected (pre-determining counter), pre-setting safe nozzle pressure (mold pressure guard), catalyst flow sensor and auto-stop (Catal), and automatic valve and controller (autosprue).

Feeding and Cleaning

An industry-standard programmable logic control has been added to a RTM valve, which allows a single pump to feed ten stations and can give remote flushing without removing the gun from the mold. The control works in concert with the computer-controlled process monitor, operated by remote keypad and, in combination, greatly improves and facilitates the RTM process.

A connector developed by Wolfangel leaves a clean injection port every time, avoiding the need to clean or clear the port after every molding. The system uses pneumatic tubes similar to fire hoses in size and construction to apply a uniform clamping force over the whole molding tool (which is a continual problem with conventional clamping systems), while permitting rapid opening and closing.


Preform Systems

The Swedish company Fiberteknik AB has developed VARIM, using Aplicator's P-4 robot preforming system and RI-10 injection machine to give a fast and economic net preform production, using a pair of perforated screens of the same shape as the molding tool. A computer controlled robot with a chopper forms a veil by spraying glass fiber filaments onto the screen and air is sucked through to hold the glass in place. Chopped or continuous strands are then sprayed onto the veil, either randomly distributed or oriented to meet performance specifications and a powder binder is sprayed with the glass.

The lower part of the screen is then moved into the press and the upper part is applied to compress the preform, using hot air to melt the binder. The cycle time depends on the maneuverability of the robot, preforms area, part complexity, and required glass load. The maximum capacity for the chopper is in the range of 3 kg/min but, on complicated shapes and narrow corners, the robot must work more slowly to deposit the glass evenly. Time required to consolidate the fibers is very short (less than 30 s), irrespective of surface area and glass load.

Automations

Added to an RTM line is automation that gives advantages in production, reduction of materials wastage, and increase in part consistency. Ongoing R&D programs by several manufacturers are converting it from a slow labor-intensive process into an industrial system for production of high-volume parts for the automotive industry. Among the first results are: a fully-automated RTM system, from preforming to injection stage; two new preforming processes and development of a new one-stroke pump, and mold inlet valve and sealing/filling system.

The automated system is based on a robot with several glass fiber delivery systems, including glass fiber choppers. Typically, with three glass fiber delivery systems, the first delivers glass fibers for the surface veil (a low-tex direct roving opened up into single filaments, chopped and transported pneumatically to the nozzle). The main chopper produces 25-125 mm strand lengths and a powdered binder is combined with the glass fiber in pneumatic tubing (a device attached to the nozzle also permits orientation of the fibers in the desired direction). The third glass fiber system delivers continuous fibers, metered and delivered through a pipe by a speed-controlled motor; an electronically controlled knife cuts the roving when required at two speeds, the higher for making loops of continuous fibers, the lower for straight


oriented fibers robot placed in desired direction. A mold carrier with preform screens completes the system.

A production line package from Matrasur (the RTM Concept), which is designed to optimize throughput and maximize control, with guaranteed quality, allowing precise costings to be set and ensuring adherence to environmental constraints. It features a volumetric injection system with double action constant flow and low pressure. Technology is used for a uniform mix and for manipulating and closing molds. A gun permanently fixed to the mold gives automatic injection and flushing. To reduce costs, a glass mat can be laid in the molds instead of using a preform. The system is able to produce two to three parts per hour per mold, with gelcoat (approximately 24 parts per day), regardless of size. Without gelcoat the rate increases to 5-6 parts per hour.

Magnum Venus Products (MVP) simplified the RTM process with its comprehensive line of systems and services-PrecisionTECH TM RTM. This concept covers the entire process, from initial consultation, through to training of staff. To start with, MVP offers consultation on the design of the customer's specific system, even designing a turnkey line for larger operations. With the design completed, MVP will build the part-specific TransPRO TM Preform and TechLock TM Clamp system. For automation, the OptiLogic TM Control and Uniport TM Injection Sprue can be incorporated. A diverse range of pumping systems is available, from basic to high-volume systems. Once the system is built, MVP offers training on the process and equipment to the customer's staff.

A fully-automated RTM plant, for high consistency and reduced costs, is producing at 20,000 parts/year at Sisteme Compositi SpA, Frosinone, Italy. A smart control system comprises a mathematic model simulating the rheology and kinetics of the resin system, a sensor for temperature, pressure and degree of polymerization, data acquisition, and software for real-time comparison between predicted and experi- mental data and servomechanisms. Resin and gelcoat systems are stored in separate rooms in thermostatic tanks under pressure to prevent solvent evaporation, stirred continuously by pneumatic motors and with levels controlled by load cells. Resin is transferred by hydraulic pump and ge 1 coat by pneumatic pump giving a feed to a specific robot. The pump control is based on precise and controlled pressure balance between resin/catalyst and gelcoat/catalyst ratios.

A resin injection gun was developed to interface with the injection gate valves, with an anti-dripping system. Molds are opened automatically when the sensors measure a preset degree of polymerization, reducing cycle times. Five hydraulic hoists on the five parallel molding lines


instead of molding presses control tool movements, simplifying operations.

A comparative cost study between hand lay-up, SMC, and smart RTM for production of class A Euroclass 380 bus body panels, showed a specific mono-skin structure body panel weighing 5 kg showed material costs were similar for hand lay-up and RTM and about 30% lower than SMC. Although a single hand lay-up tool was 85% lower than an RTM tool, this was offset by the need for many more tools to match a life cycle of nearly 3000 parts, and RTM tooling costs were 50% lower than for SMC. Labor time per cycle was three minutes both for RTM and SMC, compared with 60 minutes for hand lay-up. The investment amortization time was fixed at seven years.

RTM Melt Resin Filling Monitoring

Monitoring the flow of resin during the RTM process is important to producing consistent quality moldings. A SMART weave sensing system developed by the US Army Research Laboratory can provide mold-filling data by making in process measurements at multiple locations inside the mold, to provide a map of the mold filling process. The technology has been licensed to Micromet Instruments Inc, to develop a commercial product. It depends on the fact that the presence of resin at any point in a mold can be detected by measuring the dielectric properties of the medium between two electrodes at that point. If no resin is present, conductance will be very low, rising as resin is introduced. Multiple sets of electrodes throughout the mold can therefore present an accurate picture of how the resin is flowing.

Dielectric measurements have been used since the 1940s to monitor the cure of TS resins that used imbedded sensors, etc. The equipment (parallel plate or interdigitated comb electrodes) could not be used in the numbers required for all-over flow monitoring. The SMART weave system is based on a sensor grid of electrically conductive filaments crossing in non-intersecting planes to produce a sensing gap between them. Typically, this can be produced by placing the filaments on opposite sides of layers of preform, or by weaving them into the preform. The filaments can be low in cost (such as metal-clad aramid and graphite tow fibers, or bare metal wires, glass fiber insulated thermocouple wire or copper tape) and they offer a large number of sensing nodes for a relatively low number of connections. The user, as required, can configure the geometry of the grid. Since the grid forms a permanent part of the molding, there are also opportunities for monitoring the health of the structure (such as gross damage detection and moisture


absorption) throughout its service life. An electronic package and a computer running a Lab VIEW software program monitor the signals from the sensing nodes.

This system has been used in development on the lower hull and crew capsule of the US Army's Composite Armored Vehicle and the Northrop-Grumman Advanced Technology Transit Bus. Research is continuing at the US Army Research Laboratory and the University of Delaware Centre for Composite Materials.

Bladder Molding with RRTM

Developed by the Institute of Plastics Processing (IKV), Aachen, Germany is combining bladder molding with RRTM. It makes possible to produce hollow components with complex geometry. The normal bladder process involves winding prepregs around an inflatable bladder, which is then placed in a heated mold cavity. The mold is then closed and the bladder is inflated. To overcome the disadvantage of the stickiness and poor drapability of the prepregs, the process can be combined with RRTM (which does not rely on prepregs). Dry textiles as necessary are placed around the inflatable bladder and preformed in the mold, prior to injection of the resin to impregnate the reinforcement. A tubular sprue system has proved its worth for molding hollow components. Key process parameters are" resin temperature during injection, mold temperature, injection pressure or resin volume flow, bladder pressure during injection, and bladder pressure during curing (Figure 5.28).

Figure 5.28 Bladder molding combined with RTM for fabrication of hollow structural fiber reinforced components (courtesy of IKV, Aachen, Germany)

5 �9 Fabricating Processes 31 5

Different materials can be used for bladders, giving larger, equal, or smaller diameter than the largest diameter of the hollow component. These can be fixed (polyolefins) or flexible (silicones) and an auxiliary core can be used to pre-drapc the reinforcement. It is possible to match the possible variations to each specific molding project. IKV tested a number of options in transparent (polymethyl methacrylate) molds, looking also at lost core systems, in which the bladder or core remains in the hollow molding, and possibilities of automation.

As opposed to the relatively thick silicone bladders that have to be removed from the component, thinner bladders may remain inside without negative effect on mechanical properties. Thin unstretched tubular polyamide films appear to offer best drapability and, at an air pressure of 0.5 MPa, they could expand into the edges of the component. Good results were obtained with these with braided glass fiber textile tubes, with the following process parameters:

�9 preforming with auxiliary cores (such as plastic foams)

�9 inflating the bladder at 0.7 MP A air pressure, for draping the textile

�9 resin injection at 0.2 MPa pressure, reinforcement

for impregnation of the

increasing air pressure to 1.1 MPa after closing the venting channel, for consolidation of the impregnated reinforcement.

The system can be automated by designing a pneumatic piston-based bladder-sealing unit to replace the time-consuming manual system used at present. Reduction in manual work (mainly in inserting, fixing and sealing the bladder) can save up to 30% of the total cycle time, and pneumatic coupling of both process steps, closing the pneumatic pistons and inflating the bladder, can produce a further reduction. Beyond this, IKV concludes that the most effective way of further reducing cycle time is to use quicker-acting resin matrix systems (since curing accounts for about 45% of the cycle time at present).

Advanced RTM

Dow-United Technologies Composite Products Inc (Dow-UT), Wallingford, Connecticut, developed an advanced resin transfer molding (AdvRTM) process for production of complex and flight- critical airframe and engine structures in RPs. The process has been further developed to improve the quality of RP aerospace components substantially at the point where two or more sections are molded together. A patented technique of shaped unidirectional fiber preforms

31 6 Reinforced Plastics Handbook

involves molding unidirectional carbon fiber pre-treated with resin into the three-dimensional shapes required to fill the gaps between sections of RP components. Previously the sections could not be molded as a single piece without problems such as risk of excessive resin build-up at the junction, making the joint unable to withstand high stress loads.

RTM Molding with Phenolics

What has been reviewed principally involves TS polyesters. RTM with phenolic resin has also been used by companies such as Kobe Steel. These resins generally offer better fire/smoke/toxicity properties and reduced weight, but arc more brittle and are more difficult to mold by RTM to give acceptable color and surface finish (Table 5.8). An RP mold, made of glass fiber/polyester with a special vinyl ester gelcoat for prolonged service life, heated to 50C during molding was used. It was possible to mold a shell within 15 min floor-to-floor cycle time with the aid of a tool allowing faster prcforming.

Table 5,8 Mechanical properties of RTM reinforced plastics (railway carriage seat shells)in phenolic and acrylic resin

Properties Phenolic Acrylic a

Flexural modulus (GPa) 23.57 (0.48) 22.37 (0.62) Flexural strength (MPa) 588.50 (3.60) 491.30 (27.20) Tensile modulus (GPa) 14.81 (0.80) 14.91 (0.51) Tensile strength (MPa) 254.90 (28.80) 292.35 (11.3) Notched Izod impact (kJ/m 2) 242.5 (32.5) 268.8 (29.5) Charpy-flat impact (kJ/m 2) 47.54 (4.68) 50.26 (4.73) Barcol hardness 35 43 NFF- 16-101 M 1/F1 M2/F1 UL 94 V-O V-O Viscosity (mPa s @ 30~ 300 550 Injection pressure (bar) 3 3 Mold fill time (sec) 23 43 Mold temperature (~ 60 40 Post cure 4 h @ 60~ - Porosity (%) 4 2.5 Vf(%) 30 32 Specific gravity 1.57 1.81

a 70 phr FST filler loading Source: Kobe Steel


RTM Molding with Epoxies

Artificial limbs have been an application of RTM, demonstrating higher performance capabilities. For lower weight and better fatigue performance, carbon fiber-reinforced epoxy was used in preference to cast aluminum in the manufacture of the artificial intelligent prosthesis frame, housing a microprocessor-controlled pneumatic and hydraulic module for a lower-limb artificial prosthesis manufactured by NABCO, Japan, and Blatchford, UK. The frame has a weight limit of 200 g and had to pass a number of tests, maintaining its performance at temperatures from -40 to 100C.

The RP leg module flame was designed with computer-aided systems. Finite element analysis was used to determine the reinforcement pattern, and a special epoxy with amine curing was formulated. The reinforcement fabric was wrapped on a sacrificial mandrel/core and placed in an aluminum mold at 80C (176F) and the epoxy resin mixture, preheated to 45C (113F), was injected using minimal pressure to avoid displacing the fiber/mandrel insert. The part was demolded in 15 min, followed by removal of the mandrel and deflashing. Manufacturing cost was almost 50% of the cost using autoclaving.

Autoclave to VARTM

Autoclave curing is important to the RP industry, particularly for the aerospace RP industry. Very few RP processes can match the consistent part quality and attainable high fiber volume of prepreg laminates cured in an autoclave. However, hand lay-up processes dominates autoclave including those with automated cutting tools and laser placement systems driven by computer-aided design (CAD) programs minimize part-to-part variations and reduce labor costs. Fiber placement brings the autoclave process even closer to complete automation.

However, autoclave processing remains expensive, especially for medium- to large-size production runs with low cycle times compared to most other processes. Capital cost is high, fiber placement equipment is usually even more expensive, maintenance, and operating expenses tend to be higher than for ovens, presses, and similar equipment.

With military budgets under increasing pressure, engineers have looked for alternative processing methods that can reduce costs while maintaining the high performance of autoclave-cured components. Within the last few years, liquid composite molding (LCM) technologies have advanced to the point where they can provide that alternative. LCM processes involve the injection of a liquid resin into a dry fiber preform,


and include resin transfer molding (RTM) and vacuum-assisted RTM (VARTM).

As reviewed in conventional RTM, the preform is placed into a closed, matched tool and resin is injected under pressure about 100 to 200 psi (0.69 to 1.38 MPa). Early RTM processes lacked the consistency needed for aerospace components, in both dimensional tolerances and mechanical properties. Fiber volume fractions were significantly lower than the 60 to 65 wt% typical of prepregs. Problems with predicting flow fronts as well as flaws that were introduced into the preform when closing the matched metal molds often led to high void contents and dry spots.

Improvements in both materials and processes have made RTM a viable option for aerospace manufacturing. However, it normally takes 10 to 15 years for a new technology to become accepted in the aerospace industry. Use of RTM began about 1998 when Lockheed Martin (Fort Worth, Texas, U.S.A.) selected RTM for many of the F /A-22 Raptor's structural components. RPs comprise approximately 27 wt% of the F /A-22 ' s structural weight (24% TS and 3% TP). RTM accounts for more than 400 parts, made with epoxy resins. The wing's sine wave spars were probably the first structural application of RTM composites in an aircraft. For a vertical tail on another Lockheed Martin aircraft, the RTM process reduced the part count from 13 to one, eliminated almost 1,000 fasteners, and reduced manufacturing costs by more than 60%.

Unfortunately a general lack of material properties databases has slowed the adoption of RTM in the aerospace industry. Most large companies spend several years and tens or even hundreds of thousands of dollars qualifying RPs for structural applications. Using prequalified materials produces a significant savings in development cost and time. RP manufacturers like VSC, though, are seeing a willingness to develop databases for RTM materials.

Since the RTM process is more complex than autoclave curing, it is more difficult to develop a general qualification methodology. With prepregs, the material manufacturer mixes the resin and impregnates the tape or fabric under highly controlled conditions. Once a material is qualified, the end user just has to demonstrate site equivalency of its manufacturing process. With RTM, however, both the resin mix and the resin content are more variable. In particular, the final resin content depends on maintaining a good flow front. Still, it is possible to develop general allowables for RTM systems. As of October 2001, a general method for qualifying braided RTM systems was prepared. The methodology has been used to qualify RTM parts made with a PR250


resin (Cytec Engineered Materials Inc., Tempe, Ariz., U.S.A.) and an AS4 carbon fiber (Hexcel Corp., Dublin, Calif., U.S.A.) braid produced by A&P Technology Inc. (Cincinnati, Ohio, U.S.A.).

High cost of tooling has limited the adoption of RTM. Although the price is competitive with autoclave tools, autoclave programs can usually get by with a single set of tooling for both development and production. With RTM the resin flow front (and hence the part quality) is highly dependent on the tooling geometry. Often it is necessary to build one or more sets of prototype tools, to develop and test the process, before the production tooling can be built. Although prototype tooling is less expensive than production tooling, it is not that inexpensive that it can be considered expendable.

Case Histories

Resin/glass spoilers for about 30% of the Ford Fiesta models sold in Europe have been molded by RTM resulting in an output of 1000 per day by molder Sotira, France. The highest volume from the company previously was 500 parts/day for the Citroen XM spoiler. A three-cavity mold is used to produce the spoiler, which weighs 1.3 kg (about half the weight of the same part on the previous model), and has a Class A surface finish matching the body color. The production uses Sotira's patented Injection Compression System, which is a modification of standard RTM, on a production line, which cost FFr 5 million. The spoiler is molded on a low density (110 k g / m 3) polyurethane foam core, sanded to give good adhesion and fitted with aluminum plates for mounting inserts. This is encased in two pieces of Vetrotex U750-375 glass mat and two layers of surface veil in a preform mold, and the preforms are loaded into the three cavities of the chromed steel mold, with the addition of a second surface veil. A special Matrasur injection machine at the mold-parting line, at about 200 tonnes pressure, injects a low-profile zero-shrink polyester resin sequentially.

Where conventional tools would use vents to exhaust air during injection, the Sotira process incorporates a compression chamber for air expansion, ensuring that all air is evacuated from the cavities, in turn eliminating porosity. Injection takes 8 s per cavity, curing takes 2.8 rain and total cycle time is less than 4 min.

Bonnets for mini excavators manufactured by Kobe Steel, Japan, are molded by RTM using a modified acrylic resin and glass fiber fabric. The 850 mm wide x 420 mm high component has double curvature and is molded with a glass fiber strand preform with surface mat on the outer surface, preformed with an organic binder. The resin system contains 100


phr calcium carbonate, pigment, and flow modifying agents to achieve a good surface finish with a thin gelcoat layer. The resin formulation was modified to reduce the molding cycle to less than 30 min, improving the mechanical and thermal performance and surface finish.

The mold was a nickel shell heated to 60C, with the mold halves mounted on a hydraulic frame, and the resin was injected using a Venus- Gusmer on-line mixing RTM machine at 3 bar injection pressure. The fill time and resin cure time were 2 and 10 min, respectively.

Railway carriage interior components molded by RTM have been developed by Kobe Steel Europe with Transintech, UK, and Compin, France. A lightweight (5 kg) seat back shell with high static load and absorption capacity is molded in a modified acrylic resin (from Ashland) with a filler combination to achieve low fire, smoke and toxicity (FST) properties (which is easy to mold by RTM, with little effect on mechanical properties). A combination of glass fiber-based fabrics of _+45~ non-crimp (936 g/m 2) with unidirectional reinforcement and continuous filament mat (450 g / m 2) is used.

Infusion Molding

This is a take off from vacuum associated RP processes. Infusion molding is the relative modern technology used to identify and streamline different RP processes such as vacuum bag molding technologies. Infusion refers to the introduction of a media into a substance identifying the application of a vacuum that moves a plastic melt through a fibrous construction. Examples of processes are Seemen Composites Resin Infusion Manufacturing Process (SCRIMP), resin transfer molding (RTM) vacuum-assisted resin injection (VARI), vacuum-assisted resin transfer molding (VARTM), resin infusion under flexible tooling (RIFT), old Marco molding, and others. One way or another vacuum infusion has been used since at least the late 1940s continually providing improvements in the fabricating processes.

They involve using a mold surface or cavity that is covered with reinforcements and sealed. A vacuum is drawn on the space between the mold and the seal containing the reinforcements, and a liquid resin is allowed to infiltrate the reinforcements directly or via channels to obtain wet out of all fibers. The resin flows through the reinforcements and cures/solidifies to form the finished RP. Low-cost RP tooling can be used and environmental emissions arc controlled within this closed process. Any fumes that develop during the fabrication process can be


contained, disposed quickly, and safe. Large high reinforcement content structural RP parts can be produced to make parts such as small to large boat hulls, bridge structures, housings, and windmill blades.

Savings occur due to the use of less materials, improved and desired smooth finish, etc. Very active in using infusion molding is the boat industry worldwide since they have contaminating processes such as wet lay-ups. Boat builders want lower weight, faster vessels, and the processes that produce them to be more efficient, and cleaner. These issues are leading a growing interest in closed molding techniques such as compression moldings.

Faced by the thrust of current legislation (MACT, OSHA etc) which logically disapproves of the wet lay-up/open mold technology that has served the industry for over a half a century, boat builders are exposed between various alternatives (Chapter 3). These include prepregs and its semi-preg resin film infusion (RFI) derivative; or one of several resin infusion processes. Compression or injection molding provides molding operations into closed molds that have include enclosures providing vacuum infusion.

Present day developments include Lotus for production of car bodies, the female mold half is first coated with a gelcoat and dry reinforcement (mainly continuous filament mat and some woven roving) and PUR foam formers are placed on it. The foam inserts have previously been wrapped in continuous filament mat to create a stiff torsion box on each side. The inner male mold, with an airtight peripheral seal, is then placed over the loaded mold and bolted in place and resin is then drawn evenly into the mold by vacuum.

The entire process is complete in several minutes for small panels and approximately one hour for upper/lower body shell halves. Released from the mold, the halves are joined with epoxy structural adhesive on an overlapping joint along the waistline of the car. The joint is reinforced in certain areas by over bonding with glass fiber cloth. The body, a one-piece molded floor-pan and the chassis are then bolted together, creating structure with almost double the torsional stiffness of the basic backbone chassis.

Warship builder Vosper Thornycroft, initially a SCRIMP licensee, subsequently developed its own method for single-shot infusion of up to 30 laminate plies, including heavy shipbuilding fabrics, and complex structures with inserts, cores, stiffeners and fasteners. Since vacuum bags are used for consolidation rather than precision matched tooling, set-up costs are modest and labor is said to be halved compared with hand lay-up.


Large spars are fabricated. An 86 ft unstayed carbon fiber mast built by Composites Engineering for Ocean Planet, a yacht built by Schooner Creek Boat Works for an American owner, utilizes an infusion resin and hardener specialty formulated by MAS Epoxies.

For the best in high volume fraction, low-void quality, autoclaved prepreg construction is used. New Zealand builder Southern Ocean Marine utilized both prepreg and wet preg in building Mike Golding's latest Owen/Clarke designed Open 60 in carbon/Nomex sandwich. The rotating carbon wing mast was also manufactured using carbon prepreg.

Prepregs based, for example, on Hexcel Composites M10 and SP's SE84 resin system, both of which can be cured in the mid 80C, Hexcel's 75C curing M34 and other similar products, are making prepreg technology more accessible.

A lead adopter, at least among production boat builders, of new resin film infusion materials is the French company Poncin Yachts, which offers hulls laid up in SP Systems Resin Infusion Technology (SPRINT) material. The company could equally have chosen Hexcel Composites HexFit, or another of the several semi- preg products now available on the market.

An interesting experiment was carried out by Julian Spooner, technical manager at the Advanced Composites Manufacturing Centre in Plymouth, UK, compared infusion and prepreg manufacture directly by molding the two hulls of a catamaran using the two methods. The first hull was manufactured from SE84 carbon prepreg and Nomex honeycomb core. Outer and inner sldns and core were laid up separately and the laminate was vacuum consolidated and cured in ovens built around the mold prior to each cure cycle. The second hull was produced by single-shot resin infusion of carbon/aramid inner and outer skins over a Baltek SuperLite balsa core. The resin used was Sicomin SR 8100 epoxy. The laminate was cured at room temperature, and then post- cured at an average 100C after the hull was &molded.

The hulls are part of a full-scale comparative manufacturing study. Structural stiffness, weight, manufacturing time, and production cost will be assessed for each produced item. A laboratory study of specific structural performance for the different laminates will complete the comparison.

Resin infusion is likely to emerge as offering a combination advantages attractive to marine fabricators (and others) contemplating a transition from open molding. Early, unofficial, indications suggest that resin infusion is likely to emerge as offering a combination of structural,


manufacturing, and cost advantages attractive to marine fabricators contemplating a transition route from conventional open molding.

At the upper end of the glass technology scale, GRP is the basis for Sandown class mine hunters built by Vosper Thornycroft for the UK Royal Navy using a proprietary infusion process. A number of navies have selected FRP for the structures of mine countermeasures vessels since it provides immunity to magnetic influence mines.

SCRIMP Proeess

The Secman Composites Resin Infusion Process (SCRIMP) is a gas- assist resin transfer molding patented process. This resin infusion molding process is a vacuum resin molding technique patented by Seeman Composites, USA, arising from an attempt to develop an environmentally friendly alternative to open molding. The technique is based on use of a distribution medium (knitted mesh fabric incorporating a network of resin distribution channels) which is incorporated in the laminate lay-up, which is molded by the vacuum bag method. The medium is placed on both sides of the fiber pack (on one side only, for thin laminates), to ensure that thorough wetting out is achieved quickly. A resin-permeable peel ply is placed between the distribution layer and the part being molded, so that the mesh, together with excess resin, can be removed and disposed of afterwards. The system has proved itself for thick, large-area laminated structures, from flat to highly contoured. Refractory and metal/organic fabrics can also be laid into the laminate, enabling it to resist passage of fire.

Some organizations, however, challenge the basic novelty of the system, citing other techniques for incorporating resin distribution and questioning the need to remove and dispose of the distributor. There was the Marco process of the 1940s-1960s that included laying down glass fiber reinforcement constructions with resin channels in a vacuum bag and in turn applying a vacuum placing TS polyester in the reinforcement followed with its cure. Bofors, Sweden, uses penetrations in the fiber cloth to speed up resin flow and Fibrelite, UK, injects resin through gaps and channels in the cores of sandwich RPs, to produce manhole covers. Scott Bader and DSM Resins have developed a texturized embossed film to act as a resin distributor.

The process allows panels measuring several meters in each dimension to be infused with resin quickly and thoroughly, to give more rapid and consistent production. It can be controlled more closely than open- mold wet lay-up, giving parts of higher consistency and reproducibility. Structures, which are both thick and complex, can be impregnated and

324 Rein%reed Plastics Handbook

(if the resin flow is controlled so that the advancing wave front penetrates all parts at the same rate) structures incorporating cores, inserts, stiffeners, and fasteners can be infused in the one shot.

Generally, the larger the part, the better the economics. Set-up costs are said to be modest and labor savings can be up to 509/0 of hand lay-up costs. There appears a consensus that it has a good future in styrene- free volume production of moldings with large relatively planar surfaces.

From the technical viewpoint, users comment that low viscosity resins are preferred, with longer than normal working time and low cxotherm. A dry fiber pack basis for a boat hull can be infused in about 30 rain and cured at ambient temperature. Diluents in the resin may have some effect on quality. It is unlikely that the mechanical properties will compare with those of a prepreg curing at 60C (140F) and above, meaning that it will be hard to achieve the strength requirements of (for example) the aircraft industry, but they may well meet the requirements of boat builders who do not need the level offered by prepregs.

The process has been licensed to some 30 organizations worldwide. The UK shipbuilder Vosper Thornycroft uses it for molding large bulkhead panels for minehunters and believes that laminates can be developed to meet stringent new regulations, such as the latest International Marine Organization (IMO) code (which some have feared would spell the end of TS polyesters in fabrication of primary marine structures).

SCRIMP has been shown to be able to produce consistently high quality parts, including high-end military minesweepers, pleasure boats, windmill blades, marine fenders and piling, a 14.6 m (48 ft) all-RP flatbed trailer, utility poles for transmission lines, small temporary bridges. Lockheed has also used it for carbon fiber laminates. A noteworthy application was an insulated railcar, molded in two parts, with a 5-tonne body (fabricated from Dow's Derakane vinyl ester with Vetrotex E-glass fiber and Dow polyurethane (PUR) foam core for insulation, molded on a segmented FRP tool).

In USA, SCRIMP fabricated large RP products such as a transportation bus weighs about 10,000 kg (22,000 lb) that is 3200 kg (7000 lb) lighter than steel units are. It fabricates a trial 75 ft high integrated mast structure for an amphibious warfare vessel. The RP structure, made up of infused panels, encloses and supports the forest of antennas visible on a normal warship, in a weatherproof, electro-magnetically tuned mast / housing.


Canada's West Bay Son Ship Yachts recently transitioned its RP production from open-mold spray-up processes to a vacuum infusion processes. Infusion with glass offered a clean, low emissions way to make better parts and saving on labor and materials. Two years of research into different methods and materials preceded the decision to change from open molded isophthalic polyester/glass to vacuum bagged vinyl ester/glass for its 58-107 ft RP yachts. With growing confidence, infusion progressed from small panels and stringers through bulkheads to full hulls (the first was infused during 2003), and then to decks. Stronger parts with a higher glass-to-resin ratio developed. It is estimated that weight savings of up to 30% occurs for the boat hulls that in turn permits higher speeds.

Injection Moldings

The process of IM is used for reinforced TPs (RTPs); however, it is principally used for (URTPs) and some TSs. IM machines (IMMs) represent over half of all the primary plastic fabricating machines in the world processing all types of plastics and plastic compounds; representing just a USA multibillion-dollar business. Figure 5.29 is a simplified version of IM machine (IMM) where plastic flows from hopper, through plasticator (screw-barrel), to mold cavity.

The IMMs used for molding RTPs are a take off the same system as in molding UTPs. Temperatures/pressures differ, as does the design of the screw. Unlike UTPs that just melt in the plasticator and solidify in the cooled mold, the TSs melt in the plasticator and cure to a harden state in the mold that operates at a higher temperature than in the plasticator. Over 50 wt% of all glass fiber RTPs go through IMMs. The RP TS compounds that are thick and pasty (BMC, etc.) are processed through ram IMMs as well as screw IMMs (Figures 5.30 and 5.31).

Both short and long glass and other fibers are injection molded. They require special designed screws in their plasticators. They generally use reciprocating screw IMMs. Also used is two-stage IMMs design that includes replacing a preloader with a single or twin-screw extruder.

This widely used process in the plastic industry is essentially a process in which a charge of material is fed from a hopper into a cylinder, from which it is injected under pressure into a mold, through a nozzle on the machine, a sprue and (usually) runner system in the mold. From the runner it goes through a gate, into the cavity of the mold. It is ideally suited to processing of TP compounds but, because of the efficiency of


Figure 5 .29 Schematic of IMM with reciprocating screw melting system (hydraulic operation shown/also used is electrical or hybrid systems)

Figure 5.30 Schematics of ram and screw injection molding machine with a preloader usually providing heat to the RP compound


Figure 5,31 Examples of different injection molding machine plasticators

the process and its ability to produce large numbers of three- dimensional moldings to a high (in some cases very high) degree of precision, TS compounds are also processed.

IMMs with the proper screw designs process short or long fiber reinforced compounds. As an example with a suitable machine and


tooling, designated ZMC (low viscosity molding compound), for many years has been used in the production of complete rear doors for some European hatchback designs of passenger car. These are molded as an inner and an outer shell, which are then bonded together with a resin adhesive.

Compared with compression molding, IM offers a number of advantages. It is a natural closed molding system, producing parts with good finishes on both sides, and is automatic and highly cost-efficient. It produces reproducible moldings, to constant weight and identical properties, and will produce parts not prone to porosity (which can be further guaranteed by introducing a vacuum during the injection stage in the mold cavity, to evacuate gases and air occlusions). This action also speeds up the injection speed, giving in turn a better surface and shorter cycle time.

In the past, the main criticism of IM of RTPs or RTSs has been the relatively low mechanical strength of the molded product, compared with the theoretical strength of the components. This was due to reducing fiber length. Screws cut fibers during its plastication in the plasticator (identifies the screw in the barrel), which damages the strength-producing fibrous reinforcement. Older injection molding technology (up to 50 years ago) only employed a plunger method that did not damage fibers, but the main line of development has long been to use screws to move the compound along the barrel of the machine because they improved the melting action resulting in a more uniform melt and shorter cycle time.

As experience of the materials has developed, design of the screw is of paramount importance and it has been possible to adapt screw designs to more efficient processing of glass fiber RTP and RTS compounds. This allows the machinery manufacturers to offer (in principle) one basic machine body which can be adapted to processing RTPs and RTSs as well as the more popular unreinforced TP, simply by changing the design of screw. Nevertheless, plasticizing glass fiber-reinforced compounds has a strong abrasive effect on the screw, and various grades and treatments have been developed to give this component a longer life.

Alongside the economic importance of injection molding have grown up sub-industries concerned with more efficient control of the whole molding process (by use of microprocessor systems) and robot external handling of parts, including placing of inserts before molding and removal of parts after molding.

The advent of robots for part-removal has many implications, since it is relatively simple to have the robot present the part to other equipment

5. Fabricating Processes 329

for, when required, deflashing, trimming, addition of inserts and printing, as well as to optical and weighing inspection systems, for control of quality (dimensions, weight, etc.) at the machine.

Molding Reinforced Thermoplastics

Different TPs are IM. Low-pressure [17-140 bars (250-2000 psi)] can be used making it easy to fabricate small to relatively large moldings (limited based on size of IMMs). The optimum pressure is usually 70 bar (1000 psi). Total cycle time is most significantly affected by section thickness. The following conditions are examples of the molding cycle: mold temperature 120-160C (248-320F), barrel temperature 20-60C (68-140F), injection pressure (as noted above), and injection time 0.5-6.0 s. Details on IM TPs are provided in the literature.

RTPs can be IM with virtually the same equipment as is standard for other forms of UTPs injection molding. The exception is that, although the reinforcement (such as glass fiber) has been treated with special lubricants prior to compounding, it may still have some abrasive effect on the critical metal parts of the injection system, particularly the screw and barrel (plasticator). For molding operations involving continual or frequent use of reinforced compounds, it may well be better to use dedicated machines. In any case, care should be taken to ensure that the screw and barrel are suitably hardened (and attention is paid to wear during the normal maintenance schedule): With continuous use, (24 h every day) plasticator could wear in six months if special plasticators are not used and would require maintenance, etc. to maximize fabricating performance.

It is also important to ensure that the compound is stored according to manufacturers' recommendations, and to check the moisture content before injection. In operation, compounds fed into a hopper on the machine, from which it is drawn by gravity into the heated barrel, where they are melted (plasticized) by a rotating screw. The screw also has an intermittent reciprocating movement, so that it acts as a plunger, to inject the melt into a closed mold. The whole molding cycle is precisely timed, with accurate metering of the shot-weight, exact control of movement and a programmed profile of injection and hold-on pressures.

As well as continuous development of machine design and especially machine control systems, there has been considerable development of molds. These may be quite simple or very sophisticated, with multiple cavities, moving parts. Since the molding of RTPs is essentially a question of the control of heat transfer, advanced temperature control systems are frequently used, which can repay their additional cost by


reducing the molding cycle. At the production rates of injection molding, a few second reductions from the cycle can make a large difference in monthly or annual production costs.

Particularly sophisticated tooling is used for major application for RTPs such as automobile air intake manifolds. These large (1.5-3 kg) moldings, in glass fiber/reinforced nylon, are IM on fusible metal cores, which are subsequently melted out, or are molded as two mirror images, which are then ultrasonically welded together.

Injection-Compression Moldings

Also called ICM, coining, and injection stamping. ICM is a variant of injection molding. The essential difference lies in the manner in which the RTP thermal contraction in the mold cavity that occurs during cooling (shrinkage) is compensated. With conventional injection molding, the reduction in material volume in the cavity due to thermal contraction is compensated by forcing in more melt during the pressure- holding phase.

By contrast, with ICM, a compression mold design is used where male plug fits into a female cavity rather than the usual flat surface parting line mold halves for 1M (Figure 5.32). The melt is injected into the cavity as a short shot thereby not filling the cavity. The melt in the cavity is literally stress-free; it is litcrally poured into the cavity. Prior to receiving melt, the mold is slightly opened so that a closed cavity exists; the male and female parts are engaged so the cavity is closed. After the melt is injected, the mold automatically closes producing a relatively even melt flow. Upon controlled closing, a uniform pressure is applied to the melt. Sufficient pressure is applied to provide a molded product without stresses easily meeting and repeating very close tolerance measurements.

Vacuum-Assisted Resin Injection Moldings

This process has been used since the 1940s to provide better control of the molding cycle to fabricate parts that are more precise. Those that used it were Bell Laboratories (USA) and D. V. Rosato. This technology is to combine IM of the compound with a complete or partial vacuum in the mold cavity, to facilitate the impregnation of the reinforcement. The vacuum injection process requires a perfect seal around the edge of the mold. Development included applying vacuum from the hopper, through the plasticator, and into the mold. It has very little interest.


Figure 5.32 Example of mold action during injection-compression (courtesy of Plastic FALLO)

Overmoldings Also called two-shot injection molding, in-mold assembly, two-color rotary or two-color shuttle. Two materials are molded so that the first molded shot is over-molded by the second molded shot; first molded part is positioned so the second material can be molded around, over, sections, or through it. The two materials can be the same or different and they can be molded to bond together or not bond together. If materials are not compatible, the materials will not bond so that a product such as a universal or ball-and-socket joint can be molded in one operation. If they are compatible, controlling the processing temperature can eliminate bonding. A temperature drop at the contact surfaces can occur in relation to the second hot melt shot to prevent the bond.

An example is shown in Figure 5.33. This three IM part universal joint is all molded from the same RP (glass fiber/nylon 6 / 6 molding


compound) producing extremely flat and snug fit mating surfaces that do not bond since mold temperature control was properly set:

(a) assembled universal joint with inserted bearings and part of mold

(b) prototype mold for center part, and

(c) view of the complete universal join being removed from mold cavity; the center cube with bearings was molded in its own mold, it was put into a different mold that molded one U arm, and the cube with its one arm was put into this prototype mold resulting in the complete universal joint.

Figure 5.33 Precision IM of universal joint; RP molded of glass fiber/nylon 6/6 molding compound

In addition to universal joints, other examples of this type product using this technique include many such as inner-door panels for automobiles where woven or nonwoven textiles are placed in a mold and the melt is injected. In-mold labeling is another application that goes beyond just a printed message. Individual labels or continuous film can be indexed in the mold at the beginning of each cycle. Besides printing on the film, the film can serve many other functions (increasing impact, toughen plastic, etc.), or the film can contain additives and stabilizers to protect the surface of the molded product.


Also molded are unreinforced or RP to metal hybrids. Plastic-metal hybrids are replacing all-steel structures in automotive front end modules at an accelerated rate. Technical approaches to hybrids are multiplying as more resin suppliers develop alternatives to the over- molding method first established by Bayer. Tier One automotive part suppliers, while tight-lipped on their plans, are also working on proprietary hybrid concepts.

Hybrid moldings, which combine thin-wall steel stampings and glass fiber RTPs into integrated load-bearing parts, have appeared on a dozen new car and truck platforms in 2004, doubling North American usage. This action opens the floodgates for other load-bearing automotive parts reports Paul Platte, Bayer Polymers' director of automotive marketing and industry innovation. As examples, he cites instrument panel and bumper crossbeams, door modules, and tailgates. Non-automotive applications, from appliance housings to bicycle frames, are also emerging.

Hybrid conversion in front-end modules has been led by Bayer, whose patented injection over-molding method has been used by Volkswagen, Audi, Nissan, and Ford since 1996. Ten new platforms, including several light trucks, are used in North America through mid-2004. Mso aboard are alternative hybrid systems developed by Rhodia Engineering Polymers, Dow Automotive, and BASF Performance Polymers.

It is reported that hybrid systems face stiff competition from direct- compounded, long-fiber polypropylene RPs in semi-structural load- bearing automotive uses. Ml-plastic RPs have greater potential for weight and cost reduction than do hybrids, while both approaches increase parts consolidation and foster functional integration.

As alI-RP approaches continue to improve, hybrids seem to be carving out a niche in higher performance applications. Hybrid solutions are potentially aided by more stringent side-impact regulations in USA and a mind-set among automotive engineers favoring metal inclusion in load-bearing subassemblies.

Bayer's hybrid system exploits the fact that increased side support in open section, V-shaped steel stampings significantly boosts their load- bearing strength. Metal inserts with flared through-holes are stamped, put in an injection mold and over-molded with 30 wt% short-glass reinforced nylon 6 to create a cross-ribbed supporting structure. The metal and nylon are joined by nylon melt penetrating through-holes to form rivets that provide mechanical interlocks. Because an injection press opens in one direction, Bayer's system initially limited cross-rib geometry to just two dimensions. Bayer says tooling side actions now open the way for multi-directional ribbing designs.


Bayer's hybrid structures have an open section, yet the flexural, axial, and torsional strengths match those of many closed-section, box-like structures. Their approach can thin-wall and lightweight metal stampings 40% to 60% and yet deliver excellent load-bearing strength. Payoffs are evident in the Audi A6 front end module and Ford Focus grille-opening reinforcement, which boast weight and cost savings of at least 10%.

A recent refinement is in-mold assembly of hybrids. Two or more metal stampings are robotically placed in the mold with holes aligned, and then they are over-molded into one piece with nylon. This approach will reduce the assembly costs of front ends, door modules, and window regulators.

Certain applications of over-molding are not restricted to a low pressure. Two-shot molding is an example where a plastic is molded into a shape, then placed in another cavity before a second plastic is injected. The first plastic injected serves as a solid mold wall for the second cavity. Keys for a computer keyboard, knobs, and other items are often molded this way to provide information that does not fade or wear away with use. Many variations exist, including molding an elastomer over a rigid plastic, and molding a frame around a lens or optical window. Each step in over-molding is essentially the standard molding process even though the integral structure might resemble a product molded by coinjection molding.

D-LIFT Extruder/Injection Processes

A hybrid of twin-screw compounding extruder and injection molding is proving its commercial viability for molding structural automotive parts of direct-compounded long fiber thermoplastic (D-LFT) composites. The latest proof is the front-end cartier of the 2003 Volkswagen Golf V, injection molded of 30 wt% long glass filled polypropylene by RKT Kunststoffe, part of Aksys Group, in Kongen, Germany. This 7.5 lb part is the third commercial application of the IMC (injection molding compounding) process of machine builder Krauss-Maffei (USA office is in Florence, KY). This system piggybacks a twin-screw compounding extruder on top of an injection machine with a. special accumulator and a plunger injector. The accumulator allows the extruder to run continuously to produce more homogeneous melt quality than with conventional start-and-stop plasticating.

The extruder takes in continuous glass roving and c u t s it while wetting it with already molten resin. As compared with conventional injection molding of long fiber pellets, this process saves one heat history on the


resin and subjects the glass fiber to less shear, preserving fiber length. IMC is also more suitable for heat-sensitive materials such as natural fibers. The machine system includes material handling and gravimetric feeding systems. It has a plasticating capacity of more than 5000 lb /h r of PP or HDPE. The largest press in commercial operation is 2000 metric tons, though KM has a 2700 ton in its own laboratory.

Krauss-Maffei has delivered 10 of these IMC systems to European molders and two more to European university laboratories. Two more are on order for RKT plants in Germany and Mexico. One system is used in the USA for a non-automotive application. The VW front-end carrier is molded on a 1300-ton press. The first commercial use of IMC was to produce the front-end carrier for the Citroen C3, introduced in late 2001. The second application came early last year, a similar part for the Peugeot 307 (tel: ????6.06-283-0200, www.kraussmaffei.com).

Pushtrusion/Injection Processes

The patented Pushtrusion direct inline process for molding small to large products is accomplished by compounding and injection molding (IM) in a single operation long and/or short fiber from continuous fiber reinforcements. Inventor Ronald Hawley in 2001 patented the Pushtrusion process that can pull glass fiber from supply creels at rates as high as 600 feet per minute.

Equipment maker Milacron Inc. is manufacturing the machine and retrofitting IM machines under a nonexclusive equipment license from PlastiComp Inc. of LaCreseent, MN, USA. Dow Automotive of Auburn Hills, MI is a nonexclusive resin supplier with development capability. They will use the process to complement Dow technology being transitioned from Europe. Owens Corning of Toledo, OH will provide product development expertise and will serve as the exclusive supplier of glass fiber rovings to Pushtrusion molders.

The modular Pushtrusion process pulls standard, continuous length fiber from supply creels at rates of 400-600 feet per minute, embeds the fibers into molten resin under high pressure, uses a chopper for cutting and maintains material temperature through an entrainment die. This pliable mixture moves through a nozzle directly into the IM machine's (IMM's) plasticator where the screw action, under pressure, completes the melting action providing proper bond of fibers to resin. This approach of a single melt minimizes fiber degradation; process eliminates usual wear on screws and barrels from the resin-glass fiber mix. The Pushtrusion process can be applied in other technology. See in this chapter Pushtrusion/Extrusion Processes.


Injection Molding ZMC

Injection molding is most suitable for large series production of RTS compounds because of the possibility of automating the process and attaining high production rates, and in particular ZMC (low viscosity molding compound) compounds that provide developing a total manufacturing system (molding material/press/mold/finishing line), for parts with properties superior to conventional bulk molding compounds (BMCs) and a high quality surface finish. Application has been for the two-part rear door of the Citroen BX saloon. The molding temperature is about 160C (310F) and injection pressure 150-200 bars. Molds are made from high compressive strength steel.

IMMs for TS polyester molding compounds can have either a flighted screw or a solid injection piston and are of modular design, allowing quick and effective change of injection unit from screw to piston. A unique aspect of one design is an automatic feed unit in three capacities of 100, 350 and 650 liters.

Liquid Injection Moldings

Also could be called LIM, liquid molding, or reaction injection molding (RIM). This LIM process, which has been in use since at least the 1940s, involves proportioning, mixing, and dispensing two liquid TS plastic formulations via two plungers; one contains the basic plastic with or without reinforcements (fibers, flakes, additives, etc.) and the other a catalyst system to active the plastic. This compound is directly injected into a closed mold. It can be used for encapsulating electrical and electronic devices, decorative ornaments, medical devices, auto parts, etc. It differs to reaction injection molding (RIM) where it uses a mechanical mixing rather than a high-pressure impingement mixer. Flushing the mix at the end of a run is easily handled automatically (Figures 5.34 and 5.35).

Plastics used include silicones, acrylics, etc. To avoid liquid injection hardware from becoming plugged with plastics, consider using a spring- loaded pin type nozzle. The spring loading allows you to set the pressure so that it is higher than the pressure inside the extruder barrel, thus keeping the port clean and open.

Development at the Advanced Materials Intelligent Processing Centre (AMIPC) of the University of Delaware, set up in the USA with funding from the Office of Naval Research is a project called liquid molding technologies. It potentially offers lower costs for molding URPs and RPs. The Centre is working on virtual manufacturing,

Figure 5.34 Example of a more accurate mixing of components for liquid injection casting via a moving wedge technique

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resin-hardener mixing chamber

mixing motor drive

ram injector

casting in mold cavity

clamp

resin and hardner proportioning chamber

Figure 5.35 Example of a liquid injection molding casting process

laboratory-scale evaluation and prototype development. Virtual manufacturing uses computer simulations to identify ideal processing parameters based on liquid injection molding simulation (LIMS) that is a computer-aided mold design tool developed by a team at the University of Delaware. The information gained from this will be used to develop a laboratory scale validation unit at the University of Delaware's Centre for Composite Materials, including evaluation and integration of on-line sensors. Previous research has shown that thermo- graphy techniques can be used to detect voids and other inconsistencies in RPs, and thermal imaging is seen as having high potential as an external sensor in RTM. In the third phase, the laboratory-scale intelligent manufacturing cell will be scaled up for production of an actual prototype at Boeing's facility at St. Louis.

Pulsed Moldings

Various processes are reported to provide a means to pulsate the IM melt to improve performance of the molded parts.

Scol/'i l4~

Scorirn Moldings British Technology Group tradename Scorirn is for the process of creating dynamics to molten plastics inside a mold cavity (or in extrusion from a die) to improve melt flow. Purpose of the


Scorim process is to eliminate what they report as problems for IM RTPs. There is the tendency of reinforcing fibers to align with the direction of flow, as the molten plastic passes at high pressure through a gate and then fills the mold cavity. There is the difficulty of molding thick sections without also producing sink marks caused by differential shrinkage. Subjecting the melt to pressure pulsing during the injection and hold-on stages can produce better results under both situations.

A shear-controlled orientation system of molding (and extrusion), which gives much greater control over the distribution and alignment of fibers in processing RTPs, has been patented. It is believed to be the only system that offers complete control and management of the melt during the entire freezing phase.

Fitted to standard plasticizing units, Scorim provides multiple feeds for the melt flow and independently controlled pistons in the melt channels. Once the cavities have been filled in the normal manner, the system is activated at the hold-on pressure stage. Pulsing of the pistons generates shear in the melt as it solidifies, optimizing homogeneity and eliminating internal welds. Where fibrous reinforcements and fillers are involved, the shear also tends to orientate the fiber and resin matrix, further improving the properties of the finished molding.

Liquid Crystal Polymers A pulsed molding process for LCP was developed by Klockner Ferromatik, using two opposed injection units, either in tandem or in sequence, to produce distinct orientation of the opposing melt streams in the cavity, while also allowing a flow-through and eliminating weld lines. The maximum attainable tensile strength is influenced not only by the type of sprue system but also by the number of pulsing cycles, the cooling gradient of the melt in the mold and the length of the stroke. Tensile strength of the LCP was increased from about 228 N / m m 2 to 298.5 N / m m 2 (or about 150% of the 207 N / m m 2 claimed by the manufacturer of the plastic). At the same time, up to 250% increase in notched impact strength was achieved, due to orientations in the core layer running parallel to the flow direction.

It was used commercially for the production of window flames for the European Airbus A-340, following a study that indicated that IM parts were increasingly important in reducing unnecessary weight in aircraft. However, using most types of IM TPs introduced also the question of flame and smoke generation. This led to interest in LCPs as an effective combination of high performance mechanical strength and fire resistance and replacement of the aluminum die-formed part on the Airbus with moldings in LCP saved 50 kg.


Pultrusions

This is a process that is dedicated to just continuously processing RPs that usually have a constant cross sectional shape from profiles to flat panels (I-, U-, H-, flat panel, corrugated panel, and other shapes). The reinforcing fibers are pulled through a plastic (usually TS however, TP are also used) liquid impregnation bath through rollers, etc. and then through a shaping die followed with a curing action (Figure 5.36). There are also systems where no plastic bath is used and the plastic is impregnated in the die that is a take-off in extruding wire and cable providing a very controlled impregnation.

Figure 5.36 Schematic examples of the pultrusion process

This process can produces products that meet very high structural requirements, high weight-to-strength performances, electrical requirements, etc. The material most commonly used is TS polyester with glass fiber. Other TS plastics, such as epoxy and polyurethane are used where their improved properties are needed. When required fiber material in mat or woven form is added for cross-ply properties.


In contrast to extrusion, in this process a combination of liquid resin and continuous fibers (or combined with short fibers) is pulled continuously through a heated die of the shape required for continuous profiles. Extruder pushes the material. Glass content typically ranges from 25 to 75 wt% for sheet and shapes, and at least 75% for rods.

In this process, continuous fiber reinforcement usually in the form of roving or mat/roving is drawn through a resin bath to coat each fiber with specially formulated resin mixture (Chapter 2). The resin- impregnated fibers are then assembled by a forming guide and pulled through a heated die in which shaping and compacting them and initiating the cure. The rate of reaction is controlled by the catalyst in the mix and by the arrangement of heating and cooling zones in the die. Curing is at 120-150C (250-300F), and mirror-polish machined steel, generally chromium-plated, is used for the die. High frequency (HF) or ultra- high frequency (UHF) pre-heating of the reinforcement may also be used, linked with the curing by the heated die.

The profile is pulled through the die at a constant speed, for continuous production. The cured product is rigid, and the downstream equipment will include a cutting saw and stacking device. The process is suitable for complex hollow or solid profiles with high mechanical properties. Profiles have a high ratio of reinforcement to resin, and the fiber is orientated in the length direction, giving units of very high tensile strength.

Typical applications of pultruded products are extensive and serve many different industries:

�9 electrical: lift-truck booms, ladder components, third rail cover, cable trays, lighting poles, pole line hardware

�9 anti-corrosion: grating, mist eliminators, sucker rods, troughs, pipe, sub-structure beams

�9 construction: all-composite buildings, frames, window frames, roof supports, patio doors, trim, roof deck panels

�9 transport: roll-doors, bus panel components, interior trim, display panels, drive shafts, luggage rack, dunnage bars.

Development in equipment continues. As an example, a line of low-cost pultrusion machines with electromechanical controls, eliminating high maintenance costs and minimizing noise, has been developed by Composites Machines Co, Salt Lake City, USA. The advanced technology pultrusion (A TP) line comprises three series:

�9 Model a up to 50 mm x 25 mm profiles, 450 kg pull force, with simple electromechanical control and formed wheel pullers


�9 Model /3 up to 200 mm x 100 mm profiles, 2268 kg pull force, caterpillar pullers for higher pulling forces and larger profiles

�9 Model 6 up to 610 mm x 250 mm, 2268-9072 kg pulling force, reciprocating pullers with linear roller beating ways, ball screw drive and brushless servo motors giving high precision in closed-loop pulling and reciprocating synchronization.

MMFG, the largest pultruder in the USA, is progressively replacing its open bath wet-out with multi-stage pre-die injectors. The direct die injectors that are currently available, while significantly reducing styrene emissions, often give inadequate wet-out of roving, producing parts with poor mechanical properties. The MMFG pre-die concept is claimed to reduce emissions by over 90% without and loss of properties. Line speed increases of 30-40% are also being sought. Special machines include one for pultruding plate up to 1.2 m wide. Another machine can produce 610 mm I-beams, and foam-filled building panels are produced on a Pulstar reciprocating machine with the foam functioning as the mandrel. A current research and development project is production of large glass and carbon fiber reinforced phenolic I-beam.

Continuous Laminations

Continuous fabricating between layers of film is used for continuous production of sheet, both colored and translucent, in flat or profiled (such as corrugated) configuration. Catalyzed polyester resin is metered onto a carrier film using a doctor blade and glass fiber (now usually 25 mm length) is cut from rovings and deposited on the resin. An alternative (but more expensive) material is chopped strand mat. A 28 wt% glass content is typical. After impregnation, optimized by pressure rollers or flaps, a second film is applied to the upper face and the whole lay-up is passed through a nip roller. Figure 5.37 is a schematic of a continuous production of

Figure 5.37 Continuous production of profiled sheet line


profiled sheet, forming the profile transversely with twin forming belts inside the curing oven. An alternative method is to form longitudinal profiles in the green stage, before the sheet enters the oven.

The laminate usually passes over a heated surface (typically 60C) to introduce heat at an early stage, maximize line speed and reduce internal distorting stresses, also reducing resin viscosity and so aiding wetting-out. The sandwich then passes over preformers, molds as necessary, to produce corrugations or other profiles, and passes through a curing oven at 80-120C. As the continuous sheet leaves the cure chamber it is edge-trimmed and cut to desired length. As well as acting as a conveyor and release agent, the two layers of film also ensures that both surfaces are, smooth.

Choice of cartier film offers two alternatives: a relatively high-cost film, which can be re-used several times, or a lower-cost film that can be left adhering to the surface of the sheet, as a protection in transit. The process can also be used for production of simple open profiles without sharp angles.

A gelcoat can be incorporated as necessary, applied 100 pm thick to the carrier film. Thin gauge film (19 lam) is used once; thicker gauge (38 pm) is reused (some manufacturers are known to reuse 5-6 times). The line speed at an average product weight of 1 kg /m 2 x 0.5 mm thick is up to 24 m/rain: for 4 k g / m 2 x 3.0 mm the speed is 5-10 m/min.

Some techniques for manufacture of heavy-duty sheet for packaging and tarpaulins for construction, agriculture and transport, have involved pultruding (also laminating for long production runs). They can include sandwiching a reinforcing fiber web between layers of sheet. Reinforced PVC sheet is produced similarly by pultrusion (also calendering for long production runs). Reinforced hose is produced by braiding the fiber reinforcement construction through a pultrusion die (also extrusion for long production runs).

Other techniques

Different TP pultrusion processes are used. As an example Thermoplastic Pultrusion Technologies (TPT), Yorktown, VA, USA, uses a hot-melt injection process for pultruding RP thermoplastic. Unlike TS pultruded profiles, TP profiles can be postformed and reshaped. Higher continuous use temperatures are possible with some TP matrices, and line speeds are faster with raw materials usually costing less.

The TPT process and apparatus consist of a creel, fiber heating unit, resin feeder (which may also include an extruder), impregnation


chamber with resin flow control mechanism, resin metering and profile die, cooling mechanism, and pulling mechanism.

The process can combine conventional glass fiber roving, aramid, or carbon fiber tows with TPs, most commonly polyethylene terephthalate (PET) and nylon (polyamide/PA). Other plastics used include polyphenylene sulphide (PPS), styrene-maleic anhydride (SMA), high- density polyethylene (HDPE), and polypropylene (PP). The TPs can take the form of pellets, chips, chunks, or shreds, and as the process uses hot-melt injection, no solvents or two-part systems are involved. Additives such as colorants and fillers can be used as required.

The process achieves excellent fiber wet-out and allows the use of recycled TPs as well as virgin plastics. Fiber wetting is often a major problem with hot-melt TP pultrusions because TPs have a much higher viscosity than TSs at typical processing temperatures. Outside coatings of most TPs can be applied in-line while pultruding, by using an extruder.

Many applications exist including round and flat profiles for tension cables, high strength rivets, bolts, bicycle spokes, reinforcement cables for erosion control revetments, and straps for transporting heavy construction materials. Production of prepreg, rods, and ribbons, reinforcement bars for concrete, tool handles and high fiber weight long fiber pellets are fabricated.

Dow has developed a pultrusion simulation modeling (PSM) service designed to help fabricators achieve higher levels of productivity and reliability. Process variables such as pull speed, part and die temperature, heater output and pulling force can affect the quality of pultruded components. The PSM tool allows fabricators to predict processing performance for specific applications, and is accurate to within 10% of actual performance. The tool has been validated in customer trials and allows the pultrusion process to be optimized quickly.

Without effective modeling, it could take several weeks to a month to optimize a process, not to mention all of the accompanying scrap or waste material. Using this state-of-the-art and unique modeling technology, one can optimize the process within a week and have a lot of confidence in the processing performance that one gets before ever running a pultruder. A technical service engineer can run PSM tool from a laptop or any standard computer platform on site. Dow is using the tool as part of a technical support service and to develop new pultrusion resins that will be faster and easier to process.


Extrusions

While injection molding is the largest process in the plastics industry in terms of number of plastic fabricating machines operating, extrusion of TPs is by far the largest in terms of volume of material processed. The vast majority of this material, however, is film, sheet, and profile such as pipe, practically all of which is unreinforced. However, there are film, sheet and profiles, such as thermoplastic polyurethanes (TPU) as thin as 5 mills, that are bonded in layers with glass, polycarbonate, fiber glass fabrics, paper, and other substrates to produce high-strength laminates with anti-ballistic properties, together with the ability to allow for new architectural features (backing for automotive foam-in-place seating, athletic equipment, computer keyboard covers, building panels, etc.). Other thermoplastics are used such as thermoplastic polyesters, polyvinyl chlorides, copolyamides, and polyolefins with each plastic composite providing different requirements ranging from high impact resistance to reducing costs.

The extruder, that offers the advantages of a completely versatile processing technique, is unsurpassed in economic importance by any other process. It is to be used whenever the opportunity exists such as fabricating RP lumber. This continuously operating process, with its relatively low cost of operation, is predominant in the manufacture of shapes such as profiles, sheets, tapes, filaments, pipes, wire and cable coatings, rods, in-line postforming, and others. The basic processing concept is similar to that of injection molding (IM) in that material passes from a hopper into a plasticating cylinder in which it is melted and pushed forward by the movement of a screw. The screw com- presses, melts, and homogenizes the material. When the melt reaches the end of the cylinder it precedes through a die that has an opening, the shape of the product to be produced. Practically only TPs go through extruders with some of TP containing reinforcement such as glass fiber. A limited amount of unreinforced or reinforced TSs is extruded.

In extrusion, plastic material is first loaded into hopper using upstream equipment that provides the proper mix of ingredients to be processes. This mixture is fed into a long heating chamber (plasticator) through which it is moved by the action of a continuously revolving screw. At the end of this plasticator the molten plastic is forced/pushed through an orifice (opening) in a die with the relative shape desired in the finished product. As the extrudate (plastic melt) exits the die, it is fed downstream onto a pulling and cooling device such as multiple rotating rolls, conveyor belt with air blower, or water tank with puller.


Important to the RP industry is the use of multi-screw extruders (two or more screws in a barrel). Multi-screw extruders are primarily used for compounding plastic materials. A major, large market for RP molding compounds prepared by extruders is RTPs. Different designs of multi- screw extruders are used to produce compounds based on the plastic being processed and the products to be fabricated. At times, their benefits can overlap, so the type to be used would depend on cost factors, such as cost to produce a quality product, cost of equipment, cost of maintenance, etc.

Latest in extruder design has 12-screws. Worldwide being used for in-line mixing/compounding (also reactive extrusion, devolatization, and fabricating specialty products such as fuel cells) is at present 14 extruders that use 12-screws. They are called ring extruders (REs), because 12 screws are set up in a circle around a fixed core. Each screw has a 30 mm diameter with length-to-diameter (L /D) ratio is 32-1. Its design passes the plastic from one screw to the next, around the circle providing less shear for gentle melt processing of heat- and shear-sensitive materials like thermoplastic, elastomers, or highly filled materials.

Machine was developed by ExtriCom GabH of Lauffen, Germany (company used to be called BIach Verfahrenstechnik GmbH). Century, based in Traverse City, Mich., has the rights to manufacture, sell, and service the RE machines in North and South America. Century introduced the 12-screw machine to the USA at The 2000 NPE show. Its first extruder was sold to an undisclosed USA customer that is running a highly filled plastic (85wt% filled) that requires a high level of devolatization and dispersion (Asmut Kahns, Century business advisor).

Eventually more markets will develop using RTPs so forward planning recognizes some basics in their operations. Size of the die orifice initially controls the thickness, width, and shape of any extruded product dimension. It is usually oversized to allow for the drawing and shrinkage that occur during conveyor pulling and cooling operations. The rate of takeoff also has significant influences on dimensions and shapes. This action, called drawdown, can also influence keeping the melt extrudate straight and properly shaped, as well as permitting size adjustments. Drawdown ratio is the ratio of orifice die size at the exit to the final product size. With RPs when compared to URPs the drawdown ratio is significantly reduced if not eliminated.

A major difference between extrusion and IM is that the extruder processes plastics at a lower pressure and operates continuously. Its pressure usually ranges from 200 to 1,500 psi (1.4 to 10.3 MPa) and could go 5,000 or possibly 10,000 psi (34.5 or 69 MPa). In IM, pressures go from 2,000 to 30,000 psi (13.8 to 206.7 MPa). However,


the most important difference is that the IM melt is not continuous; it experiences repeatable abrupt changes when the melt is forced into a mold cavity; during injection the screw melting action is usually not operating. With these significant differences, it is actually easier to theorize about the extrusion melt behavior, as many more are required in IM. In turn more controls are required when IM.

Good-quality plastic extrusions require homogeneity in terms of the melt-heat profile and mix, accurate and sustained flow rates, good die design, and accurately controlled downstream equipment for cooling and handling the product. Four principal factors determine a good die design: internal flow length, streamlining, construction materials, and heat control profile. Heat profiles are preset via tight controls that incorporate cooling systems in addition to electric heater bands. Barrels external surfaces can include the use of forced air and/or water jackets to aid in controlling the melt temperature. In some machines, a water bubbler channel is located within the screw.

Pushtrusion/Extrusion Processes

The Pushtrusion technology is applied to extrusion. It can be apply to different fabricating processes providing the capability of processing/ compounding long fibers (glass, etc.). As reviewed in this chapter's section under the heading Pushtrusion/Injection Molding Processes, it is applied to preparing compounding long glass fibers and other size and type fibers. PlastiComp Inc., LaCreseent, MN provides its Pushtrusion brand technology into the profile extrusion market through an agreement with GaMra Composites Inc.

This technology, that provides in-line compounding of glass RP materials, provides GaMra with cost savings of 50% on RP polypropylene profiles. Previous efforts to use highly loaded glass fiber fillings in profile extrusion had only limited success because of the cost involved. Pushtrusion is giving the fabricator a real benefit in the savings obtained from doing the compounding right in the extrusion line. The process now is drawing extrusion interest not only because of cost savings, but also because of its ability to reduce the thermal expansion rates of products such as lineals by as much as 90%.

Pushtrusion is being used to make products such as window components and railing systems for the building and construction market. Initially, it had been used with PP, ABS. PlastiComp is under way to adapt it to PVC and higher performing engineering resins.

In the long fiber RP markets applying Pushtrusion to profile extrusion, as in injection molding requires only a little modification. Technically,


profile extrusion is an easier process for Pushtrusion because it is a continuous process, unlike molding, which is a sequential process.

Pulsed Melts

Various processes are reported to provide a means to pulsate the melt to improve performance of the extruded product. An example is the Scorim process reviewed in the Injection Molding section in this chapter. A variation in the IM process has been applied to production of reinforced extruded TP pipe. This has been a center of interest, on the argument that a predictable orientation of fiber would considerably increase the pressure resistance of the pipe, without the need to increase wall- thickness (on the analogy of winding a TS resin pipe with continuous filament). The Scorim process combines the extrusion of a fiber- reinforced compound with pressure pulsing around the periphery of the die, which appears to have the effect of orientating the reinforcement.

Thermoformings

Thermoforming consists of uniformly heating TP sheet to its softening temperature that is usually URP. However, RPs can be used; they have limited use. The heated flexible TP sheet or film is forced by vacuum and/or pressure against the contours of a mold. Force is applied by mechanical means (tools, plugs, solid molds, etc.) or by pneumatic means (differentials in air pressure created by pulling a vacuum between plastic and mold or using the pressures of compressed air to force the sheet against the mold).

Almost any TP can be thermoformed. However, certain types make it easier to meet certain forming requirements such as deeper draws without tearing or excessive thinning in areas such as corners, and/or stabilizing of uniaxial or biaxial deformation stresses. Ease of thermo- orming depends on stock material's thickness tolerance and forming characteristics. This ease of forming is influenced by factors such as to minimize the variation of the sheet thickness so that a uniform heat occurs in the sheet material thicknesswise, ability of the plastic to retain uniform and specific heat gradients across its surface and thickness, elimination or minimizing pinholes in the plastic, and stabilizing of uniaxial or biaxial deformation.

Many forming techniques are used. Each has different capabilities depending on factors such as formed product size, thickness, shape, type plastic, and/or quantity (Figure 5.38). Mold geometry with their


Figure 5.38 Examples of thermoforming methods

different complex shapes vs. type of plastic material being processed will influence choice of process.

Thermoforming is a fast, low-cost method of producing essentially shell-shapes, but suffers from the need to have a relatively large trim area around the forming. Little has been done to date to utilize this process for reinforced materials, because of the lack of suitably reinforced sheet and the problems of fiber thinning that must necessarily arise when the two-dimensional sheet is drawn over the three-dimensional mold. A form of thermoforming is used; however, with high performance TP/fabric prepreg sheet (especially where the forming is required in one plane only, or where there is relatively shallow depth of draw, as with some dish-shaped panels used on aircraft.

A combination of matched molding and thermoforming has been found successful with some high-performance parts. The material (fabric-reinforced PPS laminate) is heated to 316C (600P) in an infrared oven and then rapidly transferred to the mold. A seal is made


between the laminate and the periphery of the mold and vacuum is applied, drawing the hot laminate to conform to the mold surface.

The mold can be heated or used at ambient temperature, depending on mold design and final application. Parts up to 660-950 mm (2-3 feet) x 1.22 mm (0.048 in) thick can be formed in a matter of seconds. Thermoforming can also be used, to some extent, for preforming reinforced TP foam cores for laminating with TS resins. The Advance RP process (developed by Advance USA, East Haddam, CT, USA) is an adaptation of thermoforming for production of RPs.

In the recent pass various organizations thermoformed b-stage RTS sheet (sheet molding compound/Chapter 3). Different shapes were formed that include boat hulls.

Reinforced Reaction Injection Moldings

There is reinforced reaction injection molding (RRIM) and reaction injection molding (RIM) (Figure 5.39). These process predominantly uses TS polyurethane (PUR) plastics both reinforced and unreinforced types. Others include nylon, TS polyester, and epoxy that are reinforced and unreinforced types. PUPs offer a large range of product performance properties. As an example PUR has a modulus of elasticity in bending of 200 to 1400 MPa (29,000 to 203,000 psi) and heat resistance in the range of 90 to 200C (122 to 392F). The higher values are obtained when glass fiber reinforces the PUR (also with nylon, etc.). In addition to RRIM there is structural RIM (SKIM) that identifies fabricating higher strength structural parts. Very large and very thick RRIM or SKIM products can be molded with or without reinforcements using fast cycles.

When compared to injection molding (IM) that processes a plastic compound, RIM uses two liquid PUR chemical monomer components (polyol and isocyanate) that are mixed to produce the polymer (plastic). Additives such as catalysts, surfactants, fillers, reinforcements, and/or blowing agents are also incorporated in the reactive system that produces the basic polymer. Their purpose is to propagate the reaction and form a finished product possessing the desired properties (Table 5.9).

Mixing is by a rapid impingement in a chamber (under high pressure in a specially designed mixing head) at relatively low temperatures before being injected into a closed mold cavity at low pressure. An exothermic chemical reaction occurs during mixing and in the cavity requiring less energy than the conventional IM system. Polymerization of the

5 �9 Fabricatin 9 Processes 351

Figure 5.:39 Example of typical polyurethane RIM processes (courtesy of Bayer]

monomer mixture in the mold allows for the custom formulation of material properties and kinetics to suit a particular product application.

RIM Infusion Technology

The reinforcement can be included during mixing of the two liquid monomer components or put into the mold cavity followed with injecting the polymer mix in the closed mold. This system provides another infusion technology as explained earlier in this chapter.

RRIM is a process to consider at least for molding large and/or thick products. With RRIM technology cycle times of 2 min and less have been achieved in production for molding large and thick [10 cm (3.9 in.)] products. It is less competitive for small products. Capital requirements for RIM processing equipment are rather low when compared with injection molding equipment (includes mold) that would be necessary to mold products of similar large size.


Table 5.9 RRIM and injection molding processing conditions compared for large surface production parts

PUR-RIM Injection molding

Plastic temperature, ~ (~ 40-60 (50-140) 200-300 {392-572}

Plastic viscosity, Pa, s 0.5-1.5 100-1,000

Injection pressure, bar {psi} 100-200 {1,450-2,900} 700-800 (10,100-11,600}

Injection time, s 05.-1.5 5-8

Mold cavity pressure, bar {psi} 10-30 (145-430} 300-700 (4,400-10,200}

Gates 1 2-10

Clamping force, t 80-400 2,500-10,000

Mold temperature, ~ {~ 50-70 {122-158) 50-80 (122-176)

Time in mold, s 20-30 30-80

Annealing 30 min. @ 120~ (248~ Rarely Wall/thickness ratio 1/0.8 1/0.3

Part thickness, typcal maximum, cm {in.} 10 (3.9} 1 (0.04)

Shrinkage, %

Unreinforced 1.30-1.60 0.75-2.00

Reinforce-glass parallel to fiber 0.25 0.20

vertical to fiber 1.20 0.40

Inserts Easy Costly

Sink marks around metal inserts Practically none Distinct

Mold prototype, months 3-5 {epoxy) 9-12 (steel)

Mold alterations Cost-effective Costly

Polyurethane Processes

Details on reinforced reaction injection molding (RRIM) using TS polyurethane (PUR). Two-component PUR mixtures, in which a polyol and isocyanate are brought together and a fast reaction takes place to produce a solid compound with useful properties lend themselves well to molding with reinforcement. This can be a flat or preformed mat placed in the mold, or a chopped reinforcement introduced into the stream of components in the mixing head.

The two components (polyol and isocyanate) which are reacted together to produce a PUR are brought together in a mixing head, into which it is also possible to add other components, such as mineral fillers (or ground-up recycled resin/glass moldings, acting as a filler). Because of the fast reaction of the polyurethane system components, the ingredients are mixed very rapidly and then injected under pressure into a closed mold, in which a preform or other fiber lay up has been placed.


Depending on the size and geometry of the part, it is also possible to feed fiber reinforcement in continuous roving form into the mixing head, where it is chopped and mixed with the liquid reacting components and the whole mix is then injected (Figure 5.40).

Figure 5.40 Schematic of reinforced RIM with (a) continuous reinforcement chopped in the mixing head (RRIM] and (b] mat reinforcement placed in the mold (SRIM)


A combination of chopped roving and preform may also be used. The use of PUR gives a wide range of properties, extending towards elastomeric (for parts such as automobile fenders or bumper beams) and introduces cellular/foam forms.

SRIM is a reinforced PUR molding process in which the fiber reinforcement is in the form of a mat, which is usually preformed and laid into the mold before the polyurethane mix is injected from the mixing heads. Since it uses the reinforcement in the form of a mat, it offers better structural properties, but also suffers from the disadvantage of requiring a preform. Experience has demonstrated that this stage can be integrated into the molding process and is not a serious drawback to commercial production (Table 5.10).

Table 5.10 RTM, SRIM and thermoplastic injection molding processes compared

Materiels R TM S-RIM TIM a

Temperature (oC)

Materials Mainly room temp - 40 Room temp - 80

Mold Material viscosity (Pa s) 0.1-1 Pot life/cream life (s) 120-1000 Molding pressure (bar) 0.9-6.0 Mold clamping (tonnes) per 1 m 2 surface area 20 Equipment cost (US $ x 103) tens

40-60 180-400

70-100 50- 200 0.1-1 102-105

2-20 100-200 300-1500

100 3000 hundreds thousands

aThermoplastic injection molding. Source: Reinforced Plastics.

Development of the process has been rapid and in many different directions, aiming primarily at improving the cycle time, introducing internal release agents (to eliminate lengthy mold cleaning between cycles), and allowing other materials to be included in the mix. Computer control has made a major difference, because of the need for split-second control over mixing time and ratios, with the possibility of integrating with peripheral systems such as robot demolding, finishing and take-off, on the lines of the technology now developed for injection molding of TPs. Important developments have also been made in the ability to introduce finely ground-recycled material into the mixing head and use it as filler with positive properties.

Low density reinforced reaction molded polyurethane (LD-RRIM) is claimed to simplify production of interior automobile components,


especially complex parts such as door panels. Dow's Spectrim RL system is used for many of the interior parts of the Dodge Viper GTS Coupe, by Chrysler and Prince Co. Different textures and materials can readily be incorporated, allowing for deep draw designs, while retaining one-piece construction. The technology was developed with Strapazzini Auto, Italy.

Long Fiber Technology As with all RPs, the length of reinforcing fiber in PUR systems plays a significant role in the strength of the molded part. Typically, fibers are hammer-milled and are up to 400 pm long, enabling them to pass through a mixing head and be poured into the mold. Even where high volume fractions of 20% or more are achieved, the additional strength gained from fiber which is, in practical terms, little more than a powder, is only limited. For high strength, longer fibers are needed, and therefore other methods of incorporating them have been developed.

The first such method was structural reaction injection molding (SRIM), as described above, in which the fiber reinforcement is in the form of a mat, which is usually preformed and laid into the mold before the polyurethane mix is injected from the mixing heads. This, however, introduces the additional operation of preforming the reinforcement, so adding to cost.

Long Fiber Injection Processes Alternative technologies have been developed for introducing long fibers (in the range 15-100 mm length) into the injection stream at some stage and avoiding the cost and delay of preforming:

�9 Long fiber injection (LFI) of PUR, developed by the German companies KraussMaffei (machinery) and Elastogran (processing): the fiber is introduced inside the mixing head

�9 Fipur-Tec, developed by Bayer: chopped glass fibers are sprayed into the PU resin just downstream of the mixing head.

For long fiber injection (LFI), an existing high-pressure head is modified to include a cutter and fiber delivery nozzle, to cut and meter the glass fiber directly at the mixing head. Glass fiber is used in the less- expensive form of rope (roving), which is fed straight from a spool to a cutter. The cut fibers are fed at high speed into the hollow within the annular liquid pipe formed by the reaction mix issuing from the mixer discharge. The kinetic energy promotes effective wetting out, enabling fibers up to 100 mm long to be processed. The fiber is introduced into the resin stream as late as possible, to minimize abrasion wear and need for frequent replacement of components. The polyol component can contain a significant proportion of recycled material, or up to 80%


natural raw material. The mix is sprayed into a metal mold that is then closed. High pressure exerted on the reacting mix produces very good resin/fiber bonding.

The process can be controlled by automatic closed-loop systems: for example, concentration of fiber is varied by controlling the speed of the cutter. Integrated cutter, mix, and spray heads, controlled by robot, can inject rapidly into the mold and so contribute to short molding cycles.

Hydraulic piston-based metering pumps are used in preference to rotary pumps, which can be prone to clogging. After each shot, a separate cleaning piston removes the last traces of the reaction mix from the head. Data for the LFI process suggest that high-strength components can be produced, offering lower weight and higher dimensional stability than SRIM moldings. With the fight fiber concentration, moldings as thin as 1.5 mm can be produced, giving a strength equivalent to 3 mm thick SRIM, while being substantially lighter. Savings in materials and weight are claimed to be up to 40% less than with SRIM.

Since it is possible with this process to distribute the fibers more evenly across the whole cross section of the part, and to wet them out more thoroughly, it is possible to achieve high strength with relatively thin sections. Moldings are also almost free from warpage and remain dimensionally stable over a wide range of temperatures. Basic parts are lighter and more dimensionally stable than SRIM equivalents, with only slightly lower mechanical properties. At higher densities (such as 1000 g/m3), the mechanical properties are equal or better.

Trials (mainly with automobile components manufacturers) have sug- gested that, compared with SRIM, LFI can give 20% lower glass fiber cost, with glass waste of 3-5% (compared with 30% when a preform is used) and 2-3 mm wall thickness (compared with 3-4 mm). On a pilot application, a PU metering unit with a robot controlled mixing head was able to fill three or more average molds in succession. With a demolding time of about 3 min, two operators could produce 45 or more parts per hour, including time for cleaning molds and handling demolded parts.

Calculations suggest that the faster cycles and lower glass cost could reduce unit cost by 15-20%. Standard SRIM metering equipment and mold tools can be used. The investment for a complete plant, however, would be in the region of DM 1 million. On the other hand, with large investment in SKIM, the older process will be hard to displace, while many designers believe that, with directed fiber preforms and very high volume fractions, SRIM may still have the edge where high mechanical performance is required.


Developed by Bayer, the Fipur-Tec system claims much the same advantages as the LFI process, but with the advantage that a special glass cutter shoots fibers into the PUR stream just outside and downstream of the mixing head. This avoids possible mechanical damage to the fiber, so that the full reinforcing properties of the fiber can be delivered to the molded part. It also avoids possible clogging problems. Krauss-Maffei is said to be studying a similar modification to its LFI process. The Hennecke MX head produces a wide (60 mm) rectangular PUR stream, giving a consistent distribution of random oriented fibers.

Rotational Moldings

This method, like blow molding, is used to make hollow one-piece parts. RM consists of charging a measured amount of TP without or with chopped fiber reinforcements (RRM) into a warm mold cavity that is rotated in an oven about two axes. In the oven, the heat penetrates the mold, causing the plastic, if it is in powder form, to become tacky and adhere to the mold female cavity surface, or if it is in liquid form, to start to gel on the mold cavity surface. Since the molds continue to rotate during the heating cycle, the plastic will gradually become distributed on the mold cavity walls through gravitational force. As the cycle continues, the plastic melts completely, forming a homogeneous layer of molten plastic. After cooling, the molds are opened and the parts removed (Figures 5.41-5.43).

RRM can produce uniform wall thicknesses even when the product has a deep draw from the parting line or small radii. The liquid or powdered plastic used in this process flows freely into corners or other deep draws upon the mold being rotated and is melted/fused by heat passing through the mold's wall.

This process is particularly cost-effective for small production runs and very large product sizes. Figures 5.44 and 5.45 provide examples of reinforced and unreinforced products that range from polyethylene small beach ball to chopped glass fiber cross-linked polyethylene (XLPE) 22,500 gallon large tank. This size tank with 11/2 in. wall thickness uses a triple XLPE charge with the first about a 2,500 lb charge, and the following two each at 1,500 lb.

The molds are not subjected to pressure during molding, so they can be made relatively inexpensively out of thin sheet metal. The molds may also be made from lightweight cast aluminum and electroformed nickel,


Figure 5.41 Rotational molding's four basic stations (courtesy of The Queen's University, Belfast)

Figure 5 ,42 Rotational molding of the two axes is at 7:1 for this product (courtesy of Plastics FALLO)

Figure 5.43 Rotational molding of a three station/two axis carousel RM machine


Figure 5.44 Examples of RM recreational products


Figure 5.45 Example of large tank that is RM

both of them light in weight and low in cost. Large rotational machines can be built economically because they use inexpensive gas-fired or hot air ovens with the lightweight mold-rotating equipment.

There are rock 'n' roll rotational machines. They are designed for long length to small diameter ratio. An example of a machine is one designed for large-diameter; heavy technical parts located in the Molding Co. plant in Farmington, MO. It has a loading station and an unloading station. Operators stand on a platform about 5 feet above the floor. The mold swings back and forth with the oven while the mold turns.

This machine built by Ferry Industries Inc., Stow, Ohio is also called a rocking oven machine, the rockin' press. It pivots at the center and swings 45 degrees in both directions, oven, molds and all. Traditional carousel rotomolders mount the mold on arms that cycle through a fixed oven. But to make very long, cylindrical shapes, such as a kayak, the rock, 'n' roll style is preferred. Making that shape of parts with a fixed oven would require a huge oven, and lots of wasted space. Most kayaks are made at dedicated kayak factories, so it is unusual for a custom molder like Molding Co. to get into rock 'n' roll molding. However, the big Ferry press can make industrial parts and things a lot bigger and heavier than a kayak.

The Rotospeed rocking oven machine can make parts measuring as large as 24 feet by 10 feet. It is robust enough to carry 9,000 pounds of combined mold and part weight. Molding Co. bought the machine to


make parts measuring 23 feet long, 4 feet and 8 feet wide. To fit the big machine into the plant, the company had to bring in heavy equipment and dig a pit 18 feet deep by 13 feet wide. The machine is a sophisticated piece of equipment. All the different motions on this machine are computer controlled. You can stop any of the variables at this machine, at any time, and control speeds. Every motion is adjustable.

The operator can freeze the swinging motion at any point and keep rotating the molds, to build up thicker walls in strategic areas of the part. Different colors can be added during the operation. The amount of control gives a huge flexibility as far as the type of parts and the uniqueness of parts that can be molded.

Blow Moldings

BM can be divided into three major processing categories:

extruded BM (EBM) with continuous or intermittent melt (called a parison) from an extruder and which principally uses an unsupported parison (Figure 5.46),

Figure 5,46 Schematic of the extrusion BM process

injection BM (IBM) with noncontinuous melt (called a preform) from an injection molding machine and which principally uses a preform supported by a metal core pin (Figure 5.47), and


Figure 5.47 Schematic of the injection BM process

stretched/oriented EBM and IBM to obtain bioriented products providing significantly improved performance-to-cost advantages. These BM processes offer different advantages in producing different types of products based on the plastics to be used, performance requirements, production quantity, and costs. Practically all the plastic used is unreinforced TPs (Figure 5.48). Some parts have been blown using milled and short glass fiber reinforcements (Figure 5.49).

Figure 5.48 Examples of complex BM products


Figure 5.49 Milled glass fiber/ polyethylene blow molded water floatation wheels

The BM lines have an extruder with a die or an injection mold to form the parison or preform, respectively. In turn, the hot parison or preform is located in a mold. Air pressure through a pin-type device will expand the parison or preform to fit snugly inside their respective mold cavities. Blow molded products are cooled via the water cooling systems within mold channels. After cooling, the parts arc removed from their respective molds.

Increased attention has been paid to the technology influencing the use of reinforcement in the BM compounds. Elongational flow is one of the main processing criteria and plastics which in their clongational viscosity exhibit strong strain-hardening thus tend to have good process ability by B M.

A team at Yamagata University, Japan has studied the effect of glass spheres in a high-density polyethylene compound for BM (Chapter 2 Glass Characteristics, Glass sphere regarding melt flow). A high molecular weight HDPE (as used for automobile fuel tanks, and many other applications) was examined, both unfilled and reinforced with glass spheres of 18 mm diameter and 2.6 specific gravity.

Storage modulus and loss modulus increased with glass bead content at both low and high frequencies and it was shown that Trouton's law (that, for homogeneous plastics, elongational viscosity in the strain rate independent region is very close to three times the shear viscosity) holds good for RP systems as well as for the virgin plastics.

Strain hardening has an anomalous dependency on strain rate, and is more marked at lower strain rate. In RPs, strain rate dependence of


strain hardening is similar to that of virgin HDPE. The hardening phenomenon appears at large strain and is generally believed to be caused by elastic behavior of elongated polymer chains. The glass beads suppress the large deformation of matrix polymer chains around them (which may possibly be one of the causes of the suppression of chain hardening by the glass beads).

In B M, it is important for the parison to be able to resist draw down and, since this occurs slowly, it can be expected that a compound with a strong strain hardening at low rates of strain (as exhibited by the RPs tested) will have good BM processability.

Foams

A generic term for flexible to rigid materials containing many cells (open, closed, or both) dispersed throughout the material. Foamed products, whether TPs or TSs with and without reinforcements, have been a large part and special category within the plastics industry. They are known by different names such as cellular plastics, expanded plastic foams, structural plastic foams, low-pressure foams injection molding, high-pressure foams injection molding, and just plastic foams. It is a plastic whose apparent density is decreased by the presence of numerous empty cells throughout the mass.

The manufacture of foam plastic products cuts across most of the processing techniques fabricating RP parts. The foams can include milled, to short and long cut glass fiber reinforcements in their mix. Foams can be fabricated during extrusion, injection molding (Figure 5.50), blow molding, casting, calendering, coating, rotational molding, spraying (Figure 5.51), etc. Typical requirements in such instances will include blowing agents in the plastic.

Blowing agents, also called foaming agents are used for the production of plastic foams. Depending on the basic plastic and process, different blowing agents arc used to produce gas and thus to generate cells or gas pockets in the plastics. Various controls to accommodate the foaming action are used. They are divided into the two broad groups of physical blowing agents (PBAs) and chemical blowing agents (CBAs). The compressed gases often used are nitrogen or carbon dioxide. These gases are included into a plastic melt to form a cellular structure. The volatile liquids arc usually aliphatic hydrocarbons, which may be halogcnated, and include materials such as carbon dioxide, pcntane, hexane, methyl chloride, etc. Polychlorofluorocarbons were formerly

Figure 5.50 Schematic of foam reciprocating injection molding machine for low pressure

Figure 5,51 Liquid (left), froth (center), and spray polyurethane foaming processes

l J1

-rl o" m , r a,1 m o

0 t'3 q J l

t'1)

(.~ 03 Cn


used but they have now environmental problems.

been phased out due to the reported

Different types are available because certain plastics can only use a specific type. There are some techniques unique to foamed plastics. It is possible to use spray guns or mixing metering machines to mix the liquid ingredients with additives and reinforcements together and direct them into a mold cavity. The mixed ingredients with their chemical blowing reaction start to foam after leaving the dispensing equipment.

There is a unique technology of molding structural foam, foams with integral solid skins, and a cellular core resulting in a high strength-to- weight ratio. When processing structural foams, several techniques are used with most related to injection molding and extrusion. Uses range from water floatation devices to high performance structural components.

Foamed Reservoir Moldings

Also known as elastic reservoir molding, this process creates a sandwich of plastic-impregnated, open-celled, flexible polyurethane foam between the face layers of fibrous reinforcements. When this plastic RP is placed in a heated mold and squeezed, the foam is compressed, forcing the plastic and air outward and into the reinforcement. The elastic foam exerts sufficient pressure to force the plastic-impregnated reinforcement into contact with the mold surface (Figure 5.52).

Figure 5.52 Foamed reservoir molding


Syntactic Cellular Plastics

Also called RP syntactic foam or syntactic foam. An RP compound made by mixing hollow microspheres of glass, epoxy, phenolic, etc. into a fluid TS plastic with its additives and curing agents. It forms a moldable, curable, lightweight mass, as opposed to foamed plastics in which its cells are formed by gas bubbles, etc. Use includes water floatation apparatus.

Centrifugal Moldings

Centrifugal fabrication has been in use since the 1940s. It is suited to the production of hollow parts such as pipes with two smooth faces. Fiber reinforcement (usually roving chopped during the process) and TS or TP resin are sprayed onto the inside of a mandrel rotating at high speed and the centrifugal force pushes the resin outwards, impregnating the reinforcement (Figure 5.53). Predominately used is TS plastics. The process is generally used to produce cylindrical parts (such as pipes, tanks, vats and silos) but it is increasingly used to produce slightly tapered poles (such as telegraph and street lighting posts). A device to prevent air entrapment needs to be incorporated into the mechanism.

Figure 5.53 Schematic of centrifugal casting of reinforced plastics

Eneapsulations

Also called conformal coating. It encloses a product in a closed envelope of plastic unreinforced or reinforced by immersing the product (solenoids, ornament, sensors, motor components, integrated


circuits, and other articles.) in an unheated or heated plastic. Different processes can be used that range from casting to injection molding.

Casting permits applying different techniques. As an example, half or part of the casting can gel. The product such as an ornament is placed on the gelled plastic followed with the final pouring of the plastic without or with chopped fiber reinforcements.

The typical TP RP encapsulation process is an insert injection molding or liquid injection molding operation. The insert, a coil or an integrated circuit, for example, is placed in a mold equipped with either fixed spider type supports, retractable pins, or other features to support it when molten material is injected.

This technique with insert molding is a clean, repeatable process that lends itself to automation and cellular manufacturing, and fits well with total quality management (TQM). With off-the-shelf process controls and systematic production methods, manufacturers can deliver repeatable, high-quality products that come out of the tool ready for assembly. The products generally do not require costly trimming or deflashing, as do many TS encapsulated products when using processes such as compression molding.

Although horizontal clamp injection molding equipment can be used for encapsulation, vertical-clamp machines allow easier insert placement and greater insert stability during mold clamping movement. For high- volume production, a vertical machine with a shuttle or rotary table is highly efficient. For example, a two-station table fitted with two lower mold halves allows molding at one station while an operator or robot unloads finished products and loads inserts at the other shuttle station.

Castings

Some TPs and TSs begin as liquids that can be cast and polymerized into solids. In the process, various ornamental or utilitarian objects can be embedded in the plastic with out or with fiber reinforcements as reviewed for the encapsulation process. By definition, casting applies to the formation of an object by pouring a fluid plastic solution into an open mold where it completes its solidification. Casting can also lead to the formation of sheet, made by pouring the liquid compound onto a moving belt or by precipitation in a chemical bath. Casting differs from many of the other techniques described in this book in that it generally does not involve pressure or vacuum casting, although certain materials and complex products may require one or the other.


Stampings In the stamping process, usually a reinforced TP sheet material is precut to the required sizes. The precut sheet is preheated in an oven, the heat depending on the TP used (such as PP or nylon, where the heat can range upward from 270 to 315C (520 to 600F). Dielectric heat is usually used to ensure that the heat is quick and, most important, to provide uniform heating through the thickness and across the sheet. After heating, the sheet is quickly formed into the desired shape in cooler matched-metal dies, that can use conventional stamping presses or SMC type compression presses.

Reinforced TS plastic B-stage sheet material can be used with its required heating cycle (Chapter 3). However, the most popular is to use TP sheets.

Stamping is potentially a highly productive process capable of forming complex shapes with the retention of the fiber orientation in particular locations as required (Figure 5.54). Figure 5.55 compares stamping of Azdel (Chapter 4 Glass Mat Thermoplastics) with other processes. The process can he adapted to a wide variety of configurations, from small components to large box-shaped housings and from fiat panels to thick, heavily ribbed parts.

Figure 5.54 Processing sequence for compression stamping glass fiber reinforced thermoplastic sheets


150

125 -

1 O0 --

7 5 -

5 0 -

2 5 -

Parts per hour

at 80% lob efficiency '

I Thermoplast ic s tamping

ection molding

~, Thermoset compression ~ . molding (SMC)

Structural foam

1 I I ' " I ~" I I ' 1 20 40 60 80 100 120 140 160

Part cycle time. seconds

Figure 5.55 Stamping compared to other processes

300

_ 250

- 200

- 1 5 0

- 1 0 0

._ . ._ . . ._-50

Parts per shift per year, thousands

Cold Formings

This process is similar to the hot-forming stamping process. It is a process of changing the shape of a TS or TP sheet or billet in the solid phase through plastic (permanent) deformation with the use of pressure dies. The deformation usually occurs with the material at room temperature. However, it also includes forming at a higher temperature or warm forming, but much below the plastic melt temperature, and lower than those used in thermoforming or hot stamping (Figure 5.56).

Figure 5.56 Cold forming


Different forms of glass fiber/TS plastics are used with or without special surface coatings such as gel coatings. Materials are compounded with controlled pot life so that they start their cure reaction after being placed in the mold cavity. For room temperature cure, cure occurs by an exothermic chemical reaction that heats the RP. Pressures are moderate at about 20 to 50 psi (140 to 350 kPa). Molds can be made of inexpensive metal, plaster, RP, wood, etc.

Comoform Cold Moldings

This is another version of cold forming by utilizing a thermoformed plastic skin too impart an excellent surface and other characteristics (for weather resistance, etc.) to a cold-molded thermoset RP. For example, a TP sheet is placed in a matched mold cavity with an RP uncured material placed against the sheet. The mold is closed and the fast, room temperature curing resin system hardens. The finished product has a smooth TP-formed sheet backed-up with RP.

Filament Windings

Also called FW. Produces high strength and lightweight products that consist of two RP ingredients that are the reinforcement and a plastic matrix. The plastic is usually a TS material. The process uses a continuous reinforcement (glass, carbon, graphite, PP, wire, and other materials in filament, yarn, tape, etc. forms) either previously impregnated (prepreg) or impregnated at the machine with a plastic matrix that is placed on a revolving (removable) mandrel followed with curing. Reinforcements have set pattern lay-ups to meet performance requirements; target is to have them stressed based on performance requirements of the molded part (Figure 5.57).

Figure 5.57 Schematic of the filament winding process

,m,,

a"

D,1 I l l r-I,,

:!::

0 "

0 0

Figure 5,58 Early 20th century tape wrapping patent of a tube-making machine by H0ganas-Billesh01ms A. B., Sweden


The use of circumferential wrappings to increase the bursting strength of certain structures is not new. Historically, wire and tape (Figure 5.58) wrappings have been used to prevent bursting of cannon barrels and to wrap two-part wooden pipes both to increase the bursting strength and to hold the two parts together so that a leakproof cylinder is formed. Use of filamentary structures for applications requiring ultimate structural performance is rather recent (1940s) and unique (Tables 5.11 and 5.12).

Table 5,11 Filament wound structures for commercial and industrial applications

Railway tank cars Storage tanks: acids, alkalies, water, oil, salts, etc. High-voltage switch gears Electrical containers Propellers High-pressure bottles Decorative building supports Containers for engines, batteries, etc. Buoys Valves Aircraft tanks Aircraft under-carriage Aircraft structures Fishing rods Round nose boat Boat masts Lamp poles Golf clubs Race track railing Auto bodies Drive shafts Air brake cylinder Heating ducts Acid filters Recoil-less rifle barrel Pontoons Motor housing Computer housings Marker buoys Laundry tubs Ventilator housings Rifle barrel Dairy cases Auto and truck springs Circuit breaker rupture pots Cartop boats Electroplating jigs

Irrigation pipes Salt water disposal pipes Underground water pipe Oil well tubes Ladders Extension arms for telephone trucks Textile bobbins Weather rockets Gas bottle-mines Structural tubing Insulating tubes Electrical conduit Chemical pipe Pulp and paper mill pipe Water heating tanks Pipe fittings and elbows Truck-mounted booms Highway stanchions Capacitor jackets and spacers Coil forms Electronic waveguides Printed circuit forms Electric motor rotors, binding bands Circuit breaker housing High-voltage insulators Rectifier spacers Antenna/dishes Rotating armatures-DC motors DC commutator Fan housing High voltage fuse tubes Floating ducts Automotive parts Tank trucks Light poles Brassiere supports Looms


Table 5.12 Filament wound structures for aerospace, hydrospace, and military applications

Rocket motor cases Rocket motor insulators Solid propellent motor liners Nose cones for space fairings Rocket nose cones (2) Rocket nozzle liners Jato motor APU turbine cases High-pressure bottles {gas or liquid] Vacuum cylinders Torpedo launching tubes Rocket launcher tubes Flame thrower tubes Missile landing spikes Deep space satellite structures Radomes Igniter baskets Wing dip tanks Helicopter rotor blades Thermistors Missile shipping cylinders Boat ventilator cowlings

Liquid rocket thrust chamber Rocket exit cones Chemical rockets Chemical tanks Sounding rocket tubes Tactical bombardment rockets Tent poles Heat shields Artillery shell shipping grommet Artillery round-protective cones Submarine fluid pipes Submarine tanks and containers Submarine ventilation pipes Submarine hulls Underwater buoys Cryogenic vessels Electronic packages Submarine fairwaters Sonar domes Engine cowlings Fuse cases Torpedo cases and launchers

Frequently used is some form of glass: continuous filaments roving, yarn, or tape. The glass filaments, in what ever form, are encased in a plastic matrix, either wetted out immediately before winding (wet process) or impregnated ahead of time (preimpregnated process). The plastic fundamentally contains the reinforcement, holding it in place, sealing it from mechanical damage, and protecting it from environmental deterio- ration, Reinforcement-matrix combination is wound continuously on a form or mandrel whose shape corresponds to the inner structure of the part being fabricated. After curing of the matrix, the form may be discarded or it may be used as an integral part of the structural item.

Reinforcements have set pattern lay-ups to mcct performance requirements (Table 5.13 and Figures 5.59 and 5.62). Target is to have them uniformly stressed. Figures 5.61 provides the relationship of RP density vs. percent glass fiber by weight or volume that can be related to the compacting action that occurs when FW.

In winding cylindrical pressure vessels, tanks, or rocket motors, two winding angles are generally used (Figure 5.63). One angle is determined by the problem of winding the dome integrally with the cylinder. Its magnitude is a function of the geometry of the dome. These

Table 5.13 Different filament winding patterns meet different performance requirements

Type of winding Considerations Machinery required

Hoop or circumferential High winding angle. Complete coverage of mandrel each pass of carriage. Reversal of carriage can be made at any time without affecting pattern.

Simple equipment. Even a lathe will suffice.

Helix with wide ribbon Complete coverage of mandrel each pass of carriage. Reversal of carriage can be made at any time without affecting pattern.

Simple equipment with provision for wide selection of accurate ratios of carriage-to-mandrel speeds. Powerful machine and many spools of fiber required for large mandrel.

Helix with narrow ribbon and medium or high angle

Multiple passes of carriage necessary to cover mandrel. Programmed relationship between carriage motion and mandrel rotation necessary. Reversal of carriage must be timed precisely with mandrel rotation. Dwell at each end of carriage stroke may be necessary to correctly position fibers and prevent slippage.

Precise helical winding machine required. Ratio of carriage motion to mandrel rotation must be adjustable in very small increments. Relationship of carriage to mandrel positions must be held in selected program without error through carriage reversals and dwells. Relationship between carriage position and mandrel rotation must be progressive so that pattern will progress. t.n

Helix with low winding angle Fibers positioned around end of mandrel close to support shaft. Characteristics of "helix with narrow ribbon" apply. Fibers tend to go slack and loop on reversal of carriage. Fibers tend to group from ribbon into rope during carriage reversal. Mandrel turns so slowly that extremely long delay occurs at each end of carriage stroke and speed-up of mandrel at each end of carriage stroke is highly desirable to shorten winding time.

Similar machinery required as for "helix with narrow ribbon." Take-up device for slack fibers is necessary if cross-feed on carriage is not used. Cross-feed on carriage is required for very low winding angles. Programmed rotating eye can be used to keep ribbon in fiat band at carriage reversal. Mandrel speed-up device must be programmed exactly with carriage motion or pattern will be lost. Polar wrap machine can be used for narrow ribbons with winding angle below about 15 ~ without take-up device or mandrel speed-up being

required, continued

"TI

o"

o

LII

td~

Table 5.13 continued 0")

Type of winding Considerations Machinery required m ,

Zero or longitudinal

. ~

Mandrel must remain motionless during pass of carriage and then rotate a precise amount near 180 ~ while carriage dwells. Fibers must be held close to support shaft during mandrel motion or fibers will slip.

Precise mandrel indexing required. Simple two-position cross feed on carriage sufficient. Vertical mandrel machine and pressure follower for ribbon sometimes required to preserve ribbon integrity.

m ~

-I- Polar wrap Low angle wrap. Fibers may be placed at different

distances from centers at each end when geodesic (non- slipping) path does not have to be followed.

Polar wrap machine with swinging fiber delivery arm desirable for high-speed winding. Helical machine with programmed cross-feed will wind polar wraps more slowly.

o - o o

Cone General considerations same as for helical winding except that carriage motion is not uniform.

Programmed non-linear carriage motion required. Other machine requirements same as for helical winding.

Simple spherical Planar windings at a particular angle result in a heavy build-up of fibers at ends of wrap. For more uniform strength, successive windings at higher angles are required.

Sine wave motion of carriage is required for carriage with no cross-feed. At low angles of wind, cross-feed is necessary because carriage travel becomes excessive. Polar wrap machine may be used if range of axis inclination is large enough.

Simple ovaloid Similar to simple spherical winding but with different carriage or cross-feed motion.

Helical machine with programmed carriage or cross-fee. Polar wrap machine can be used where geodesic (non- slipping) path is in a plane.

True spherical Path of fibers programmed to give uniform wall thickness and strength to all areas on sphere.

Special machine best approach. Otherwise complex programming of all motions of helical machine required.

Miscellaneous For successful filament winding, it must be possible to hand-wind with no sideways slipping of fibers on mandels surface.

Machine to reproduce motions of hand winding. Programmed motions in several axes may be required.

O" m ,

t'3

t~3

o

t'D

t~


Figure 5.59 Schematic of an RP composite S-2 glass fiber/burn resistant phenolic adhesive prepreg and aluminum metal lay-up (adhesive by Cytee, formerly American Cyanamid, Havre de Grace, MD)includes use on the Airbus A380 fuselage providing directional properties

l Winding position ~1 Shift right

[•1 1

3

I Winding position #2 Shift left

I

2 i -

Winding position #3 Shift right

E 3

Rotate ~-

I

1

:f Winding position #1,

start second cycle

Figure 5.60 Box winding machine with position changes of clamp tooling


0.09

0.08

d ~

u

0.07

c

0.06

0.05

0.04

Practical

range

/ /

/ / /

/ / / /

/ /

,4 / /

/ / / /

J7 / /

/ / J

/ /

/

I I I I I, I I I I 20 40 60 80 100 Per cent glass by weight or volume

Figure 5,61 Filament winding density vs. percent glass by weight or volume

Figure 5,62 Fiber arrangements and property behavior (courtesy of Plastics FALLO)

Figure 5.63 Helical filament winding

windings also pick up the longitudinal stresses. The other windings are circumferential or 90 ~ to the axes of the case and provide hoop strength for the cylindrical section.

It is possible to wind domes with a single polar port integrally with a cylinder comparatively easily without the necessity of cutting filaments. Cutting is obviously not desirable, since it interrupts the continuity of


the orthotropic material. The usual procedure in winding multiported domes is to add interlaminate reinforcements during the winding operation where the ports are to be located.

It is possible to wind integrally most of the bodies of revolution, such as spheres, oblate spheres, and torroids. Each application, however, requires a study to insure that the winding geometry satisfies the membrane forces induced by the configuration being wound.

FW can be carried out on specially designed automatic machines. Precise control of the winding pattern and direction of the filaments are required for maximum strength, which can be achieved only with controlled machine operation. The equipment in use permits the fabrication of parts in accordance with properly designed parameters so that the reinforced filamentous wetting system is in complete balance and optimal strength is obtained. The maximum strength is achieved when filaments in tension carry all major stresses. Under proper design and controlled fabrication, hoop tensile strengths of filament wound items can be achieved of over 3,500 MPa (508,000 psi), although strength of 1,500 MPa (218,000 psi) is more frequently achieved.

Since this fabrication technique allows production of strong, lightweight parts, it has proved particularly useful for components of structures of commercial and industrial usefulness and for aerospace, hydrospace, and military applications. Both the reinforcement and the matrix can be tailor-made to satisfy almost any property demand. This aid in widening the applicability of FW to the production of almost any item wherein the strength to weight ratio is important. FW is used in different shapes such as the usual circular and elliptical shape to produce rectangular shapes.

FW structures present certain problems because of the lack of ductility in the glass reinforcement. These can be partially solved by proper design and fabrication procedures. Reinforcements other than glass can be used to obtain good ductility, but some of these have lower temperature strength and characteristics. Proper construction constitutes a well-proved means of utilizing an intrinsically nonductile reinforcement to obtain a high degree of confidence in the structural integrity of the end product. Since glass has high strength and is a relatively low-cost product, glass filaments arc still the major reinforcing materials. Other filaments for applications requiring properties such as higher temperatures or greater stiffness include quartz, carbon, graphite, ceramics, and metals alone or in combinations that include glass fibers.

A further difficulty with the basic materials is that they do not lend themselves readily to simple concepts and to simple comparisons. The


matrix components are essentially the same plastics as those used for conventional RP laminates. Epoxy plastics are more widely used than others are, although phenolics and silicones give structures with higher temperature properties. TS polyesters are used for many commercial structures in which cost is a problem and high temperatures do not prevail.

For certain FW vessels, the low modulus of elasticity of the glass-plastic material is a serious disadvantage. Only moderate improvements in modulus of elasticity by modifications in glass composition or in processing tend to be feasible. Any significant improvement in modulus of elasticity requires changes in the glass composition. There are effective additives to the glass to increase its modulus without proportional increase in density such as beryllium oxide.

Interlaminar shear constitutes possible limitations on FW parts. Mthough the absence of interweaving (such as fabrics) boosts tensile strength by eliminating cross flaying, shear strength is limited by the bonding of the reinforcement to the plastic. In conventional woven cloth laminates, the high points of one layer tend to interlock with the low points of adjacent layers. This results in strengthening of the RP against shear failure. Compared to other plastics or matrices epoxy gives better interlaminar shear because of its inherently better bonding. By proper design, the low values of interlaminar shear can be minimized.

FW structures have lower ultimate bearing strengths than conventional laminates, for they are more rigid and less ductile. Accordingly, they have less ability to absorb stress concentrations around holes and "cut- outs." The original higher tensile strength permits allowable design stresses under these conditions. Since cutting, drilling, or grooving for attachments or access openings reduces the high mechanical strength of filament wound structures, proper design is necessary. Damaging machining operations are to be avoided after final curing of the part. Destructive "cut-outs" or attachment holes are to be eliminated by incorporating the use of premolded plastic or metal inserts into the designs.

Techniques cannot be used for every structural element. The shape of the part must permit removal of the winding mandrel after final curing. Reversed curvatures should be eliminated whenever possible, since it is difficult to wind them and hold the filaments under tension. In order to meet this problem, fusible, expandable, and multiparty mandrels are often required.

The cost of FW parts is low only when volume production is achievable. Manufacturing processes should be mechanized and completely


automated to obtain, by extensive and careful tooling, the close tolerances which are required in filament wound structures to meet high- strength but low-cost objectives (Figure 5.64). Precision winders with carefully selected mandrels and speed controls, special curing ovens, and matched grinders are required. It takes time to develop this equipment, and a high initial investment is necessary. Once the original tooling cost has been amortized, the unit cost of individual filament wound parts becomes relatively low, since the basic materials have a low cost.

Figure 5.64 Schematics of "racetrack" filament winding machines. Top view shows a schematic of a machine built to fabricate 150,000 gal rocket motor tanks; other view is machine in action


This racetrack filament wound tank was fabricated by the Rucker Co. for Aerojet-Gencral (1966). It measured 6.7 m (22 ft) high, 18.3 m (60 ft) wide, 38.1 m (125 ft) long that weighed 32 ton, of all RP. Just the mandrel for this FW machine weighed 100 ton all of metal. Total weight of the steel-constructed machine was 200 ton. The tank contained about 251 million km (158 million miles) of glass fiber, used 8 ton of textile creel containing 60 spools of glass fiber moving up to 7.24 k m / h (41/2 mph), and took three weeks to manufacture the epoxy- glass fiber RP tank in the Todd shipyard in Los Angeles, California.

Tape Windings

Tape windings or layering is a technique developed for production of high-performance laminates, using either TS or TP RPs. A preimpreg- natcd (prepreg) material, in which resin and reinforcement have been combined under factory conditions (to give more reliable mix ratios), is used (Chapter 4). It is in the form of a continuous tape, which is applied to the surface of the mold or former. With TS resin matrices, this can be consolidated by roller during its heat cure cycle. With TP resin matrices, heat is also applied, fusing the matrix as the tape is applied. New equipment has made tape placement within reach of small shops.

This process offers a method of producing more predictable laminated structures, for high performance applications. Prepreg tapes are mainly made of the higher performance materials, such as epoxy/carbon or PEEK/carbon. This system can also be used in filament winding.

For TP tapes, a TP welding head has been developed, in conjunction with a hot gas torch. This uses the latter as a heat source to melt the prepreg tow prior to lying down and consolidation with the pressure roller. It does not rely on tension and can therefore be used for concave surfaces. Winding speeds of more than 0.15 m (6 in.)/s has been achieved with graphite/PEEK prepregs.

Fabricating RP Tanks

Classical stress analysis proves that hoop stress (stress trying to push out the ends of the tank) is twice that of longitudinal stress. To build a tank of conventional materials (steel, aluminum, etc.) requires the designer to use sufficient materials to resist the hoop stresses that result in unused strength in the longitudinal direction. In RP, however, the designer specifies a laminate that has twice as many fibers in the hoop direction as in the longitudinal direction.


Consider a tank 0.9 m (3 ft) in diameter and 1.8 m (6 ft) long with semi-spherical ends. Such a tank's stress calculations (excluding the weight of both the products contained in it and the support for the tank) are represented by the formulas:

s = p d / 2 t for the hoop stress

and:

s = p d / 4 t for the end and longitudinal stresses

where s = stress, p = pressure, d= diameter, and t = thickness.

Tensile stresses are critical in tank design. The designer assumes the pressure in this application will not exceed 100 psi (700 Pa) and selects a safety factor of 5. The stress must be known so that the thickness can be determined. The stress or the strength of the final laminate is derived from the makeup and proportions of the resin, rnat, and continuous fibers in the RP material.

Representative panels must be made and tested, with the developed tensile stress values then used in the formula Thus, the calculated tank thickness and method of lay-up or construction can be determined based on:

1 0 0 x 3 x 12 th = 20 x 103 = 0.450 in.

2 x 5

tt = 1/2 th = 0.225 in. {or the same; thickness with half the load or stress) th = hoop thickness t t = longi tudinal thickness

s h = hoop stress

st = longitudinal stress 5 h = 20 x 103 psi (140 MPa)

safety factor = 5

p = 100 psi (700 Pa) d = 3ft (0.9m)

If the stress values had been developed from a laminate of alternating plies of woven roving and mat, the lay-up plan would include sufficient plies to make 1 cm (0.40 in.) or about four plies of woven roving and three plies of 460 g / m 2 (11/2 oz) mat. However, the laminate would be too strong axially. To achieve a laminate with 2 to 1 hoop to axial strength, one would have to carefully specify the fibers in those two right angle directions, or filament wind the tank so that the vector sum of the helical wraps would give a value of 2 (hoop) and 1 (axial), or wrap of approximately 54 ~ from the axial.

t h = pd /25 h or th = pd/45h

where:


Another alternative would be to select a special fabric whose weave is 2 to 1, wrap to fill, and circumferentially wrap the cylindrical sections to the proper thickness, thus getting the required hoop and axial strengths with no extra, unnecessary strength in the axial (longitudinal) direction, as would inevitably be the case with a homogeneous metal tank.

As can be seen from the above, the design of RP products, while essentially similar to conventional design, does differ in that the materials are combined when the product is manufactured. The RP designer must consider how the load-bearing fibers are placed and ensure that they stay in the proper position during fabrication.

Processing, Equipment, Products

Filament winding is a technique used to produce high performance hollow symmetrical products. A mandrel of the required shape is rotated on its axis (usually horizontally, but depending on the size and shape of the object to be produced). When fully wound with resin- wetted or prepreg reinforcement, the TS resin lay-up is cured, on or off the mandrel. An equivalent process for TPs uses a form of TP prepreg (usually in tape form) which is similarly wound, but is consolidated by heat during the winding process without the need for curing.

The first recorded use of filament winding was for lightweight RP hoops for the Manhattan Project in 1954 (which later became the basis for Naval Ordnance Laboratory (NOL) rings used for tension and shear tests). Dick Young of M W Kellogg Co had explored its potential in the mid-1940s and built the first dedicated machine; Dick prepared the first patents on FW; D. V. Rosato performed some work with Dick on rocket motor cases. The first filament wound product as such was the nozzle for the X248 rocket motor unit, produced in 1948, using the E-glass with an epoxy resin system for higher temperature capability.

The first commercial filament winding machines were designed and made by McClean Anderson in 1960s. (Brandt) Goldsworthy Engineering designed and built the first six-axis machine for US Army Aviation Systems Command in 1965, which was delivered to Boeing Vertol, Pennsylvania, USA, for production of helicopter blades. The 1980 Beech Stars hip had a filament-wound fuselage. In the early 1980s, automobile drive shafts looked to have a big future, but the growth of this application was largely negated by the development of front wheel drive.

Filament winding has potential for compressed natural gas (CNG) bottles as an alternate fuel for automotive truck and bus. Bottles in production measure 3 m x 250-300 mm diameter and are certified by US Dept of Transportation. Lampposts up to 46 m long have also been


made. D. V. Rosato during 1947-1951 conducted an R&D project for a large CNG company to determine the feasibility of replacing steel tanks with RP FW tanks. Result was that technically FW was superior but cost was excessively high. By the 1970s, FW tanks containing oxygen were approved and used by fire fighters replacing their steel tanks used for breathing; major advantage was the FW lighter weight.

The process is capable of great flexibility in type, mix, density and direction of winding, and is used in particular for production of vessels required to withstand high pressures. The pattern of winding also lends itself to computerization. In many cases the reinforcement consists of a band of several rovings; the band may be positioned perpendicular to the axis of the mandrel (circumferential winding) or inclined at an angle relative to the axis (helical winding). High glass contents (60-75%) are obtained, producing parts with very good mechanical properties, for applications demanding rigidity. Pre-impregnated roving facilitates production of parts with very high glass content (80%). FW parts will have only one smooth face.

Almost any continuous reinforcement can be used. The most commonly used is glass, both E and S, but carbon and aramid fibers are also used, and quartz, boron, ceramics and metal wire and strip have all been successfully applied. Latest developments include adaptation of the process for asymmetrical products. Suitable resins include TS polyesters, epoxies, bismaleimides, polyimides, silicones, phenolics and thermoplastics.

Fibers are usually wound either encapsulated in a B-staged resin (prepreg winding) or with dry fiber which is impregnated with resin during the winding process (wet winding). In some cases, the part is wound dry and then the entire part is impregnated under pressure. Wet winding has the advantage of using the lowest cost materials, with long storage life and less frequent compaction cycles during winding, using a low viscosity resin to coat the fiber completely. The prepreg systems tend to produce parts with more consistent resin content and can often be wound faster because the fiber is already wetted-out. Prepreg fibers also minimize slippage during winding because of their tacky nature and require less consolidating time after winding, and allow a wide range of resin systems to be used.

On-site TP winding requires heat and pressure to consolidate the RE; heat sources include lasers, infrared and quartz lamps, induction heating shoes and hot gas torches. The consolidation pressure can be applied either by fiber tension or by mechanical devices. On-site winding is being used in the USA for reinforcement and refurbishment of structures such as


concrete supports for highways, to increase their resistance to earth- quake shocks.

Proper design of the mandrel or tooling is essential to produce high quality parts. It must maintain dimensional accuracy and avoid excessive residual stresses during the cure stage, and it can be very long, inviting problems of sag. If it is to be removed, it must be designed to minimize fiber damage during extraction of the part. Some mandrel types include plaster, sand/polyvinyl alcohol, sand/sodium silicate, water-soluble salts, eutectic salts, low melt metals, collapsible and solid metals, composites, inflatables, and molded TPs.

The primary classes of FW are hoop, polar, and helical. The simplest is hoop or circumferential winding, in which fibers are wound approximately normal to the mandrel axis of rotation with the fiber payout head advancing one band width for each revolution of the spindle. Hoop winding is usually combined with helical winding in more complex parts. Polar or tumble machines are used for parts wound using a planar winding path (such as for a short closed-end pressure vessel). These machines normally have the mandrel mounted vertically, over which a rotating arm wraps fiber onto the mandrel.

For smaller parts, tumble machines can be used to rotate the part about a fixed arm. Helical winding machines give greatest flexibility and are most common. There is a wide variety of machinery and control systems and these machines can be used to wind from simplest to most complex shapes.

Mechanically controlled helical winders are the simplest and least expensive, winding parts that require sinusoidal motion of the fiber payout eye at both ends of the part with constant carriage lead through the part. Programming is usually by changing the gear ratio between spindle and carriage drive, and the horizontal carriage drive chain length. Servo-controlled helical winders provide greatest versatility, with one or more axes under servo control. The number of axes used and the method of controlling them depends on the economics and needs of the process, ranging from simple two-axis to machinery with eight or more servo controlled axes.

Computerized servo control gives the greatest versatility with easy development of program and machine set-up. Very complex parts can also be wound. Better control can be obtained by defining the entire process, which will then be automatically executed by the machine, making for a more reproducible product. Automatic logging can be used to document the process and monitor quality.


Auxiliary process control allows external devices to be controlled during winding, allowing further automation. The winding pattern can turn simple devices on and off or activate smart devices that can interact with the winding process. Fiber tension can be controlled and varied automatically during winding.

Application software allows most winding patterns to be generated numerically and then fine-tuned if necessary, using the robotic teaching capabilities of the machine. Analysis capabilities allow finite element models to be created from the winding patterns, and the process can be simulated and the projected RP analyzed early in the design stage, without having to do the initial winding.

Typical equipment is computer-controlled and offers facility for both chop-hoop and helical winding. Helical winding is preferred where additional axial strength is required (such as, for example, suspended pipes or tanks) without using unidirectional tape as used in chop-hoop winding. The latter method is, however, a very efficient and economical means of producing large-diameter pipes and tanks, and for pipes that are to be joined with an O-ring type of bell-and-spigot joint.

The systems can include many options: surface veil, chopped liners, sprayed resin systems, hoop winding strands, helical winding strands, resin bath, unidirectional tape, woven roving tape, pigmented resin surfaces, wax-resin surfaces, gel coats, B PO catalyst, epoxy resins, vinyl ester resins, filled resin systems, abrasive resin systems, polyester foam, syntactic foam, and charting film.

Racetrack and Other Winders Because of the great flexibility of the process, it is possible to offer a great many options in machine dimensions (the size and sheer bulk of the product being the limiting factor). For example, very large diameter ducting for fume scrubbers now being fitted to electrical power stations is FW (in some cases, with a temporary production unit constructed on site), while other machines can produce tanks up to 7.3 m diameter and 15 m length (24 x 50 ft). A wide variety of machinery is available, including simple lathe-like hoop winders, polar and tumble machines, ring winders, racetrack winders, robotic arm helical winders, vertical and horizontal helical and special purpose machines. Sizes range from small tabletop machines, for laboratory R&D work, to giants (Figure 5.64).

FW technology is also being used for production of blanks for high performance products outside the hollow cylinder geometry. Springs for motor vehicle suspension elements are produced by filament winding and then cut, while a carbon/epoxy I-beam has also been produced on a small machine, producing a tube on a cylindrical mandrel


which was then loaded in the uncured state into a die and press- molded. A considerable amount of development of engineering structures can be expected in these directions, where the FW machine is used as a means of producing a high density/high-precision lay-up.

Developments in FW include large vehicle bodies (by Schindler Waggon) and the company is investigating whether its technology can be used for economic production of other large structures such as bus bodies, insulated or double-shell containers, passenger bridges for airports or shelters. Internacional de Composites SA, Toledo, Spain is also investigating buses. The design for the interior is a series of filament wound rings, joined together with further filament wound elements. Different core sections, open or closed can be used, with diverse exterior geometry. Any element not containing concave geometry can be wound, it is claimed. During the 1950s-1970s various USA organizations (commercial and military) built FW box shaped low cost housings.

A CNC control system with brushless motors has been used by specialist processors to re-engineer a filament tube-winding machine for fast, accurate and consistent performance. The control made the unit capable of distributing resin-impregnated fibers evenly around the rotating mandrel at programmable feed rates of 60 m / m i n irrespective of the pitch of the filament. The vertical machine produce tubes up to 3 m finished length and 760 mm inside diameter.

Usually the mandrel must be spun uniformly and the FW around it, placing limitations on the design, and even symmetrical RP structures generally have to be joined by the same mechanical fasteners as are used for metal. Another system, however, attaches to any commercially available robot arm and is based on a unique fixed mandrel system. An integrated computer graphics simulation package allows complex winding processes to be tried and proved at the workstation and downloaded when proved. Speed and repeatability offers production quantities. The material payout is a direct function of the speed and position of the winder (not the mandrel), giving precise tension, so that it is actually possible to tie structural components together into integrated assemblies.

A collapsible mandrel for FW is used to make a 9.8 m long x 1880 mm diameter glass-reinforced graphite/epoxy rocket motor case. A unique collapsing design curls in on itself to allow easy part removal without use of rams or winches that may damage the product. Mandrels are constructed over a series of rolled tings, each ring individually finished by a special grinding machine producing close tolerances. After grinding, the mandrel is skinned with sheet metal and painted.


Filament Winding Terms

ABL bottle The ABL (Allegeny Ballistic Laboratory) is an internal pressure vessel used to determine the quality and properties of the filament winding material in a vessel.

Angle Winding angle is the angular measured in degrees between the direction parallel to the filaments and an established reference. It is usually the centerline through the polar bosses, that is, the axis of rotation.

Axial Filament parallel or at a small angle to the rotational axis (0 helix angle).

Balanced Winding pattern so designed that the stresses in all fibers/filaments are equal.

Biaxial Winding in which the helical band is laid in sequence, side by side, with cross-over of the fibers eliminated.

Bladder/liner An elastomeric (barrier) lining for the containment of hydroproof and hydroburst pressurization medium during curing. When a protective inside liner is required, the properly bonded bladder remains in the FW structure.

Bleedout The excess liquid plastic that migrates to the surface of a winding.

But t wrap Tape wrapped around the mandrel in an edge-to-edge condition.

Cake forming The collection (package) of glass fiber strands on a mandrel during the forming or winding operation.

Circuit One complete traverse of the fiber feed mechanism of a filament winding machine.

Circumferential Filaments are essentially perpendicular to the axis of rotation.

Closure In filament winding, it is the complete coverage of a mandrel with fiber. When the last tape circuit that completes the mandrel coverage is laid down adjacent to the first without gaps or overlaps, the winding pattern is said to have closed.

Directional proper ty See Chapter 7 Directional Properties

Displacement angle The advancement distance of the winding reinforcement on the equator after one complete circuit.

D o f f The act of removing a full package such as a roving ball from a winding machine.


Doily Planar RP applied to a local area between windings to provide extra strength where a cutout such as a port opening is to be included.

Dome The spherical or elliptical shell ends of a FW container.

Doub le r A local area with extra reinforcement, wound generally with the part, or wound separately and fastened to the part.

Dry wind ing A term used to describe FW using impregnated roving as differentiated from wet winding.

Dwell In filament winding, the time that the transverse mechanism is stationary while the mandrel continues to rotate to the appropriate point for the traverse to begin a new pass.

Equa to r junct ion Also called the tangent line or point. It is the line in a tank that describes the junction of the cylindrical portion with the end of the dome.

Fi lament wind ing radius Radius of a cylindrical surface of a mandrel that meets the inside surface of the bend during bending. With free or semi-guided bends to 180 ~ in which a shim or block is used, the radius of bend is one-half the thickness of the shim or block.

Fi lament wind ing tape Unidirectional prepreg tapes are used for laying or around a mold in filament winding.

Fi lament wind ing tape laying Tape is laid side by side or overlapped to form a structure.

Fi lament winding, tension The amount of tension on the reinforcement as it makes contact with the mandrel; target is to have the required tension uniformly applied to all reinforcements.

Fi lament wind ing test A parallel filament wound tensile hoop test specimen of a specific diameter such as 15 cm (developed by Bob Bennett at Naval Ordnance Laboratory during the 1950s) provides a simple means to conduct mechanical tests.

Fi lament winding, wet wind ing A term used to describe the process of winding unimpregnated roving directed toward a mandrel where the reinforcements are impregnated with plastic just prior to contacting the mandrel.

Gap 1 It is the space between successive windings in which they are usually intended to lay flat next to each other.

2 It is the separations between fibers within a filament winding band.

3 Distance between adjacent plies in lay-up of unidirectional tape materials.


Geodesic isotensoid It has a constant stress level in any given surface at all points in its path. Filament wound plastic structures are extensively used.

Geodesic-isotensoid contour Dome contour on a pressure vessel in which the filaments are placed on geodesic paths so that they exhibit uniform tensions throughout their lengths under pressure loading; this design produces a pressure container with the combination of providing the highest pressure loading for the lightest weight. The term geodesic is the shortest distance between two points on a surface; major principal use is in designing load-bearing structures that includes the use of RPs.

Geodesic-ovaloid Contour for end domes where the fibers form a geodesic line; the shortest distance between two points on a surface of revolution. The forces exerted by the filaments are proportional to meet hoop and mechanical stresses at any point.

Helical path Filament band advances along a helical path, but not necessarily at a constant angle except when winding a cylinder.

Knuckle area Also called Y-joint. It is the area of transition between sections of different geometry such as where the skirt joins the cylinder of a pressure vessel.

Lap The amount of overlap between successive windings usually intended to minimize gapping.

Lattice pat tern It is a pattern with a fixed arrangement of open voids.

Longs Low-angle or longitudinal windings.

Loop s t rength tenacity The tenacity or loop strength value obtained by pulling two loops against each other that can cause the fibrous (particularly glass) material to be cut a n d / o r crushed.

Mandrel In filament winding, the form that is usually cylindrical onto which preimpregnated reinforcements are wound.

Muticircui t Requires more than one wrapping circuit before the band repeats by laying adjacent to the first band.

Net t ing analysis The analysis assumes that all stresses induced in the RP structure are carried entirely by the filaments and the strength of the plastic is neglected. Also assumes that the filaments possess no bending or shearing stiffness and carry only the axial tensile loads.

Pa t te rn Different patterns are used to meet different performance requirements. Names of patterns include hoop or circumferential, helix narrow or wide ribbon, helix low angle, zero or longitudinal, polar wrap, simple or true spherical, and ovaloid.


Planar The winding path lies on a plane that intersects the winding surface.

Planar helix FW domes where the filament path lies on a plane that intersects the dome while a helical path over the cylindrical section is connected to the dome paths.

Polar Winding in which the filament path passes tangent to the polar opening at one end of the chamber and tangent to the opposite side of the polar opening at the other end. It is a one-circuit pattern that is inherent in the system.

Pole piece Basically a winding in which the filaments do not lie in an even pattern. The supporting part of the mandrel is usually on one side of the axes of rotation.

Prepreg and bag molding Used to obtain special high performance RP products requiring special fiber patterns and/or high fiber volume content such as 65%. These prepregs can be cut to the required shape and fitted in a mold.

Random pat tern Winding with no fixed pattern. If a large number of circuits are required for the pattern to repeat, a random pattern approached can be used.

Reverse helical pat tern As the fiber delivery arm traverses one circuit, a continuous helix is laid down, reversing direction at the polar ends, in contrast to biaxial, compact, or sequential winding. The fibers cross each other at definite equators, the number depending on the helix angle. The minimum region of crossover is three.

Roving The term roving is used to designate a collection of bundles of continuous filaments/fibers, usually glass fibers, either untwisted strands or twisted yarns. Rovings can be lightly twisted; their degree of twisting and format depends on their use. As an example for filament winding they are generally wound as bands or tapes with little twist as possible.

Roving ball The supply package offered to the filament winder (and others) consisting of a number of ends or strands wound onto a length of cardboard tube to a given outside diameter. Usually designated by either fiber weight or length in yards. Spool is sometimes used to identify the roving ball, however, the preferred term is roving ball.

Roving ball doff To remove a finished package (roving ball, twister tube, forming cake, etc.) from a spindle.

Roving band A collection of strands or ends which act together as a band or ribbon.


Roving catenary A measure of difference in length of the strands in a specified length of roving caused by unequal tension. The tendency with some strands in a taut, horizontal roving to sag more than others which in turn can effect the properties of the fabricated part.

Roving cloth A coarse textile fabric woven from rovings.

Roving, collimated Made using a process permitting parallel winding so that the strands are more parallel than in standard roving.

Roving cord It is the central member of an assembly. Includes all types threads, twine, and rope produced by twisting fibers together.

Roving end A strand of roving consisting of a given number of filaments gathered together.

Roving end count An exact number of ends supplied on a ball or roving.

Roving fiber tension even Process whereby each end of roving is kept in the same degree of tension as the other ends making up a ball of roving.

Roving fuzz A measure of broken filaments in a strand or roving.

Roving integrity Degree of bond between strands in a roving.

Roving knuckle The point at the end of a way-wound roving ball where the roving reverses its axial direction.

Roving open top package A term used to describe a roving package in a carton which has no top.

Roving r ibbonization A phenomenon occurring in a finished roving on which the individual strands have been "blocked" or bonded together to give a ribbon of strands; describes the degree of bonding together of the strands of roving which make up the roving band.

Roving, spun A heavy low-cost glass, aramid, etc. fiber strand consisting of filaments that are continuous but doubled back on themselves.

Roving strand count The number of warp fiber/yarn (ends) and filling fiber/yarn (picks) per inch. Cross section or thickness of fiber, yarn or roving expressed as denier or decitex.

Roving, textile A form of fibrous glass having less twist than is present in a yarn. As a fibrous glass reinforcement, it means strands of continuous fibers wound into a cylindrical spool. Usually 60 strands, or ends are used. For staple fibers, roving is used to designate one or more slivers with a very small amount of twist and thus indicates an intermediate stage between liver and yarn.


Roving tow The precursor of staple fibers is tow, which consists of large numbers of roughly parallel, continuous filaments. They are converted by cutting or breaking into staple fibers or directly into a slivers, intermediate stages between staple fibers and yarns. In the latter case, the filaments remain parallel.

Roving twist, balanced An arrangement of twists in a combination of two or more strands that do not kink or twist when the yarn produced is held in the form of an open loop. For example, single S twist fiber plied with a Z fiber results in "balancing" the fibers. The amount of twist with plying provides many different combinations useful in different applications. Filament yarns can exist in an almost twistless form, but this is not the case for staple fiber yarns.

Single-circuit pattern Filament path makes a complete traverse of the chamber/mandrel, after which the following traverse lies immediately adjacent to the previous one.

Slip angle pattern Angle at which a tensioned fiber will slide off the filament wound dome. If the difference between the wind angle and the geodesic angle is less than the slip angle, the fiber will not slide off the dome. Slip angles for different fiber-plastic systems vary and must be determined experimentally.

Winding pat tern The total number of individual circuits required for a winding path to begin repeating by laying down immediately adjacent to the initial circuit.

Calendering The calendering process is used in the production of plastic products. It converts plastic into a melt and then passes the paste like mass through roll nips of a series of heated and rotating speed-controlled rolls into webs of specific thickness and width. The web may be polished or embossed, either rigid or flexible. At the low cost side these lines can start a $ million (USA). A line, probably the largest in the world processing PVC sheet, built by Kleinewefers Kunststoffanlagen GmbH, Munich, Germany, cost $33 million (1999). It is a 5-roll using L-type configuration. They have 3500 mm roll-face widths and 770 mm diameters with an output rate at 4000 kg/h.

Calendering in the manufacture and surface finishing of plastic products, such as URP sheets and films, plastic impregnated nonwovens, and woven fabrics sheets and films, requires roll systems to meet stringent control of their nip pressure requirements. In this respect, products of


uniform quality and thickness, with defined properties, call for an adjustable nip and /or controllability of the nip pressure. Control across the full roll width is achieved by various methods such as: suitable compensation of the deflection of a pair of rolls, mechanical-geo- metrical compensation such as roll bending, axis crossing and crowning of the rolls, and hydraulic compensation systems. Bowl deflection can occur. It is the distortion suffered by calender rolls resulting from the pressure of the plastic running between them. If not corrected the deflection produces a sheet or film thicker in the middle than the edges.

The calender was developed over a century ago to produce natural rubber products. With the developments of TPs, these multimillion- dollar extremely heavy calender lines started using TPs and more recently process principally much more TP materials. The calender consists essentially of a system of large diameter heated precision rolls whose function is to convert high viscosity plastic melt into film, sheet, or coating substrates. The equipment can be arranged in a number of ways with different combinations available to provide different specific advantages to meet different product requirements. Automatic thickness profile process control is used via computer, microprocessor control.

The calendering configuration of rolls may consist of two to at least seven rolls. The number of rolls and their arrangement characterizes them. Examples of the layout of the rolls are the true "L", conventional inverted "L", reverse fed inverted "L", "I", "Z", and so on. The most popular are the four-roll inverted "L" and "Z" rolls. The "Z" calenders have the advantage of lower heat loss in the sheet or film because of the melts shorter travel and the machines simpler construction. They are simpler to construct because they need less compensation for roll bending. This compensation occurs because there are no more than two rolls in any vertical direction as opposed to three rolls in a four roll inverted "L" calender and so on.

The nip is the radial distance or "V" formed between rolls on a line of centers. In-going safety devices in the nip areas are built into these machines. They protect the hands of operators. An emergency stop device is placed in an accessible location on the upstream side. If a problem develops, the machines immediately stop.

Variations in these multimillion-dollar calender lines are dictated by the very high forces exerted on the rolls to squeeze the plastic melt into thin film or sheet web constructions. High forces at least up to 6000 psi (41 MPa) could (if rolls were not properly designed and installed) bend or deflect the rolls, producing gauge variations such as a web thicker in


the middle than at the edges. During calendering, particularly film, roll- separating forces in the final nip may be as high as 6000 psi. This potential problem is counteracted by different methods that include the following:

crowned rolls, which have a greater diameter in the middle than the edged

crossing the rolls slightly (rather than having them truly parallel), thus increasing the nip opening at both ends of the roll; and

roll bending, where a bending moment is applied to the end of each roll by having a second beating on each roll neck, which is then loaded by a hydraulic cylinder. Controls are used to perform any roll bending and crossing of the rolls.

Powder Metallurgy

There are plastics that are essentially nonfusible and difficult to fabricate by conventional shaping processes. Parts molded from these plastics are fabricated by techniques ranging from powder metallurgy methods to modifications of conventional injection, transfer, compression, and extrusion that include ram instead of screw plastication. An example is polyimide (PI) plastic. Since PIs are essentially nonfusible and difficult to fabricate by conventional shaping processes. As an example DuPont with its Kapton TM has employed special processes, including a high temperature-pressure procedure similar to that used in powder metallurgy, to fabricate its PI into finished parts (called VespelTM). This process is useful for producing parts for low friction, high temperature applications.

Powder metallurgy involves the atomization of liquid metals that is compacted in a mold by the sintering process to produce a solid part. Sintering identifies the forming in a mold of parts from fusible metal or plastic powders. The process involves holding the pressed powder (such as PTFE, UHMWPE, and nylon) at a temperature just below its melting point for a prescribed period based on the plastic used. Powdered particles are fused (sintered) together, but the mass as a whole does not melt. This solid-state diffusion results in the absence of a separate bonding phase. After being withdrawn, it is heated to a higher temperature to completely fuse the sintered material. This process is accompanied by increased properties such as strength, ductility, and density.


There is the isotactic molding system, also called isotactic pressing or hot isotactic pressing (HIP). It is the compressing or pressing of powder material (plastic, etc.) under a gas or liquid so the pressure is transmitted equally in all directions. Examples include autoclave, sintering, injection-compression molding, elastomeric mold using hydrostatic pressure, and underwater, sintering.

Processing Fundamentals

While the processes differ, there are elements common to many of them. In the majority of cases, TP are melted by heat so they can flow. Pressure is often involved in forcing the molten plastic in a mold cavity or through a die and cooling must be provided to allow the molten plastic to harden. With TSs, heat and pressure also are most often used, only in this case, higher heat (rather than cooling) serves to cure or solidify the TS plastic, usually under pressure.

Melt Flow Analysis

Measuring melt flow is important for two reasons. First, it provides a means for determining whether a plastic can be formed into a useful product such as completely fill a mold cavity, a usable extruded extrudate, provide mixing action in a screw, meet product thickness requirements, etc. Second, the flow is an indication of whether its final properties will be consistent with those required by the product. The target is to provide the necessary homogeneous melt during processing to have the melt operate completely stable and working in equilibrium.

In practice, even though with the developments that has occurred in the past and continues this perfect homogeneous stable situation is never achieved and there are variables that continually affect the output. If the process is analyzed one can decide that two types of variables affect the quality and output rate. They can be identified as:

1 the variables of the machine's design and manufacture and

the operating or dynamic variables, which control how the machine is run.

Software provides simulation of the desired process and comparison with reality. The purpose of flow analysis is to gain a comprehensive understanding of the melt flow filling process based on process controls. The most sophisticated computer models provide detailed information concerning the influence of filling conditions on the


distribution of flow patterns as well as flow vectors, shear stresses, frozen skin, temperatures and pressures, and other variables. The less sophisticated programs that model fewer variables are also available. From these data, conclusions can be drawn regarding tolerances, as well as part quality in terms of factors such as strength and appearance.

Processing and Thermal Interface

Different plastic characteristics influence processing and properties of plastic products. Important are glass transition temperature (Tg) and melt temperature (Tm) (Chapter 3). The Tg relates to temperature characteristics of plastics that influence the plastic's processability. It is the reversible change in phase of a plastic from a viscous or rubbery state to a brittle glassy state. Below Tg thermoplastic behaves like glass and is very strong and rigid. Above this temperature, it is not as strong or rigid as glass, nor is it brittle as glass. At and above Tg the plastic's volume or length increases more rapidly and rigidity and strength decrease. Most noticeable is a reduction that can occur by a factor of 1,000 in stiffness. The amorphous TPs have a more definite Tg when compared to crystalline TPs. Even with variation, it is usually reported as a single value. The Tg generally occurs over a relatively narrow temperature range.

Crystalline plastics have specific melt temperatures (Tm) or melting points. Amorphous plastics do not. They have softening ranges that are small in volume when solidification of the melt occurs or when the solid softens and becomes a fluid type melt. They start softening as soon as the heat cycle starts. Regardless, a melting temperature is reported usually representing the average in the softening range (Chapter 3).

The T m is dependent on the processing pressure and the time under heat, particularly during a slow temperature change for relatively thick melts during processing. In addition, if the T m is t o o low, the melt's viscosity will be high and more costly power required for processing it. If the viscosity is too high, degradation will occur. There is the correct processing window used for the different plastics.

Process Control

In the past because melts have different properties and there were many ways to control processes, compared to what has happened, it was difficult to interrelate them. Detailed factual predictions of final output were rather difficult to arrive prior to prototyping, fabricating, or


having prior experience. Research and hands-on operation have been directed mainly at explaining the behavior of melts like with other materials (steel, glass, and so on). Modern equipment and process controls (PCs) continue to overcome some of this unpredictability. Pro- cesses and equipment are designed to take advantage of the novel properties of plastics rather than to overcome them. Figures 5.65 and 5.66 provide examples of process controls (PCs) used on injection molding machines (IMMs) and molds. Extensive R&D has been performed producing programmers for IMMs because they represent over 50% of all machines sold worldwide. To date other processes have available PCs that usually monitor and control just a few parameters when compared to IMM PCs. When required, PC automation improves process efficiency, product quality, and reduces fabricating cost.

Regardless of the PC available or used, the molder setting up the process uses a systematic approach that should be outlined. Once the system is operating, the processor methodically makes one change at a time to set up the most efficient operation.

Many devices are available for PC, such as pressure and temperature sensors, actuators, or computer programs, which can be used by the molder. These devices can be connected with the automation apparatus and integrated into a procedure.

To the designer, a model is a quantitative abstraction of a physical process in which the description of the process is represented by the solution to a set of mathematical equations. The model equations represent the behavior of the real process to the extent that the equations embody an accurate description of the underlying physical and chemical phenomena of processing. The mathematical formulation enables the model to be used for a variety of purposes, including design, control, and exploration of operating strategies. The effect of changes in process variables can be inferred from the model response without excessive experimentation.

Mathematical models are used for these purposes in the plastic/ chemical, petrochemical, and other industries. Computer-aided design (CAD), computer-aided manufacturing (CAM) are use in product designs and product manufacturing operations. The essential elements of any model of a physical process are threefold: the geometry, the relevant laws of physical conservation (momentum, mass, and energy), and the specific constitutive relations (see the Software section in Chapter 9).

The manufacturing processes offer great flexibility for creating a wide range of RP products. Recent years have seen a growth in PCs software

I I

, \

Cycle Time

Heat

Fill Time

Ram Stroke

Plasticate Time

Peak Hydraulic Pressure

Peak Plastic Pressure

Average Hold Pressure

Average Back Pressure(

Figure 5,65 Examples of basic process controls for injection molding machines

Virgin ~ Fillers

Material Regrind Nozzle Melt Temperature . . ~ S c r e w n Reinforcements Mold Temperature ~~~Back Pressure Runner ~ B a r r e l Heats Temperature

Melt Viscosity' Melt Compressibility

Shot Size Distance Pull Back Distance

Material

Boost Pressure Boost Flow Back Pressure Back Pressure Build Up Time Back Pressure Build Up Rate

Hold Pressure Hold Pressure Build up Time Hold Pressure Build up Rate

~ Screw RPM Screw Condition Screw Tip Final Fill Speed Overall Fill Speed Machine Friction

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Clamp Closing Time �9 o Clamp Open Time -41 Hold Timer Setting

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Figure 5 .66 Example of mold operation controls


programs to serve the needs of processors. These tools provide a contribution to the past rules of thumb with analyses based on sound theoretical principles, and combine the benefits of ease of use with the speed of the computer. This approach results in cost-effectiveness when applied to a large number of problems. This type of development is on going so improved methods are always forthcoming.

In general, these analysis tools fall under the domain of computer-aided engineering (CAE). However, analysis tools in no way replace skill or education in the basics of RP materials properties, mold design, or processing. Analyses can only supplement knowledge and improve productivity and accuracy.

In CAE, a design or process is proposed as the first step. The designer or engineer then constructs a model for the specific design using a prescribed method. The computer rapidly evaluates the results of both the input conditions and the model. The output conditions are listed by the computer, and the designer or engineer evaluates the consistency of the results with experience and determines modifications to meet performance requirements and in turn can provide a guide to setting up the PC.

Processing Window

Regardless of the type of controls available, the processor setting up a machine uses a systematic approach based on experience or that should be outlined in the machine and /or control manuals. It is a defined area or volume in a processing system's PC pattern. This window for a specific plastic part can vary significantly if changes are made in its design and the fabricating equipment used. Note that a major cause for problems with any process is not of poor product design but instead that the processes operated outside of their required operating window.

Once the machine is operating, the processor methodically makes one change at a time, to determine the result for each change. It provides a range of processing conditions such as melt temperature, pressure, shear rate, etc. within which a specific plastic can be fabricated providing acceptable and optimum properties. Windows such as a molding area diagram (MAD) and molding volume diagram (MVD) can be used during injection molding. The same approach can be applied to the other processes (compression molding, resin transfer molding, filament winding, extrusion, blow molding, etc.).

By plotting at least injection pressure (ram pressure) with mold temperature, a molding area diagram (MAD) will provide the best combination of pressure and mold temperature necessary to produce


quality parts (Figure 5.67). Developing this 2-D MAD approach ends up with a dramatic and easily comprehensive visual aid in analyzing these variables. Within the diagram area, all parts meet performance requirements, however rejects could occur at the edges since material and machine capability are not perfect; variability exist (Chapter 9). Other controllable parameters can be added to target for improved quality such as melt temperatures (in the plasticator, nozzle, and in the cavity), rate of injection, etc.

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E E n-

T Short shot area

Mold temperature

Figure 5.67 Molding area diagram processing window concept

After a 3-D molding volume diagram (MVD) is constructed, it can be analyzed to find the best process settings of three combinations evaluated during start-up such as melt temperature, mold temperature, and injection or ram pressure (Figure 5.68).

This type of procedure can be used in setting up, as an example, a complicated molded IM product. As shown in Figure 5.69 IMM control settings involve melt temperature, mold cavity filling speed, shot size, melt pressure packing, etc. With proper control, high quality parts are fabricated.

The term PC is often used when machine control is actually performed. As the knowledge base of the fundamentals of the fabricating process continues to grow and understand, the control approach is moving


Figure 5.68 Molding volume diagram processing window concept

PROCESS ANALYSIS

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F O R M I N G

PROCESS CLOSED-LOOP

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the mold cavity

away from press control and closer to real process control where material response is monitored and then moderated or even managed.


The fabricator should note that changes in process parameters, such as injection rate, could have dramatic effects on moldings, especially mechanical properties, meeting tolerances, and surface properties.

Process Control and Patience

As reviewed when making processing changes, have patience by allowing enough time to achieve a steady state in the complete fabricating line before collecting data. It may be important to change one processing parameter at a time. As an example with one change such as extruder screw speed, temperature zone setting, cooling roll speed, blown film internal air pressure, or another parameter, allow four time constants to achieve a steady state prior to collecting data.

Processing and Moisture

Recognize that properties of designed products can vary, in fact can be destructive, with improper processing control such as melt temperature profile, pressure profile, and time in the melted stage (Figures 5.70- 5.72). An important condition that influences properties is moisture contamination in the plastic to be processed. There arc the hygroscopic plastics (PET, etc.) that are capable of retaining absorbed and adsorbed atmospheric moisture within the plastics. The non-hygroscopic plastics (PS, etc.) absorb moisture only on the surface. In the past when troubleshooting, plastic's reduced performance was 90% of the time due to the damaging effect of moisture because it was improperly dried prior to processing. Now it could be at 50%.

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Figure 5.70 Example of moisture effect on glass fiber/PET RPs

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Strain, %

Figure 5,71 Effect of moisture on tensile

behavior of reinforced nylon 6 (33 wt% glass fiber) at elevated temperature


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300

250

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Moisture, %

Figure 5.72 Impact of temperature and moisture on tensile strength reinforced nylon 6 (33 wtO/o glass fiber)

All plastics, to some degree, are influenced by the amount of moisture or water they contain before processing. With minimal amounts in many plastics, mechanical, physical, electrical, aesthetic, and other properties may be affected, or may be of no consequence. However, there are certain plastics that, when compounded with certain additives such as color, could have devastating results. Day-to-night temperature changes are an example of how moisture contamination can be a source of problems if not adequately eliminated when plastic materials are exposed to the air. Moisture contamination can have an accumulative effect. The critical moisture content that is the average material moisture content at the end of the constant-rate drying period is a function of material properties, the constant-rate of drying, and particle size.

Although it is sometimes possible to select a suitable drying method simply by evaluating variables such as humidifies and temperatures when removing unbound moisture, many plastic drying processes do not involve removal of bound moisture retained in capillaries among fine particles or moisture actually dissolved in the plastic. Measuring drying-rate behavior under control conditions best identifies these mechanisms. A change in material handling method or any operating variable, such as heating rate, may affect mass transfer.

Drying Operations

When drying at ambient temperature and 50% relative humidity, the vapor pressure of water outside a plastic is greater than within. Moisture migrates into the plastic, increasing its moisture content until a state of


equilibrium exists inside and outside the plastic. However, conditions are very different inside a drying hopper (etc.) with controlled environment. At a temperature of 170C (350F) and -40C (-40F) dew point, the vapor pressure of the water inside the plastic is much greater than the vapor pressure of the water in the surrounding area. Result is moisture migrates out of the plastic and into the surrounding air stream, where it is carried away to the desiccant bed of the dryer or some other device.

Target is to keep moisture content at a designated low level, particularly for hygroscopic plastics where moisture is collected internal. They have to be carefully dried prior to processing. Usually the moisture content is >0.02 wt%. In practice, a drying heat 30C below the softening heat has proved successful in preventing caking of the plastic in a dryer. Drying time varies in the range of 2 to 6 h, depending on moisture content. As a rule of thumb, the drying air should have a dew point o f - 3 4 C (-30F) and the capability of being heated up to 121C (250F). It takes about 1 ft 3 min -1 of plastic processed when using a desiccant dryer.

The non-hygroscopic plastics collect moisture only on the surface. Drying this surface moisture can be accomplished by simply passing warm air over the material. Moisture leaves the plastic in favor of the warm air resulting in dry air. The amount of water is limited or processing can be destructive.

Determine from the material supplier and /or experience the plastic's moisture content limit. Also important is to determine which procedure will be used in determining water content. They include equipment such as weighing, drying, and /or re-weighing. These procedures have definite limitations based on the plastic to be dried. Fast automatic analyzers, suitable for use with a wide variety of plastic systems, are available that provide quick and accurate data for obtaining the in-plant moisture control of plastics.

Machines Not Alike

Just like people, not all machines may be created equal. Recognize that identical machine models, including auxiliary equipment, built and delivered with consecutive serial numbers to the same site can perform so differently as to make some completely unacceptable by the customer, assuming they were installed properly.


Plasticator Melting Operation

The RP processing methods that process well over 50wt% of all plastics is injection molding. This process as well as a few others uses a plasticator to melt plastics. It is a very important component in a melting process with its usual barrel and screw (or screws). If factors such as the proper screw design and /o r barrel heat profile are not used correctly fabricated products may not meet or maximize their performance and very important not provide for low cost parts.

Plasticators have a wide operating range to meet different performance requirements of all the different plastic compounds processed. Its rotating drive system can be via a hydraulic and /o r electrical motor. Electrical motors tend to increase melt processing efficiency that in turn increases production rate. It has a wide operating range to meet different performance requirements of all the different plastics processed. Important is to obtain maximum throughput with as close to a perfect melt quality. It is an endless target due to the limits and /o r variabilities that exist of the plastics, machines, and controls. Since the start of using screw plasticators and with time passing definite improvements have been occurring in the melt quality. This action continues because advancements tend to be endless in applying advanced screw designs and the changing melting characteristics of plastic compounds.

Screw

Figure 5.73 provides an introduction to the performance of a plasticator where the screw usually has a 17.6 degree flight helical angle. There are the following three main parts (zones) of a screw:

Figure ,5.73 Nomenclature of an injection screw (courtesy of Spirex Corp.)


1 Feed Zone: This is the part of the screw that picks up the plastic compound at the feed opening (throat) plus an additional portion downstream. Many screws, particularly for extruders, have an initial constant lead and depth section, all of which is considered the feed section. This section can be welded onto the barrel or a separate part bolted onto the upstream end of the barrel. The feed section is usually jacked for fluid heating and /o r cooling.

2 Transition~Melting Zone: It is .- the section, also called the compression zone, of a screw between the feed zone and metering zone in which the flight depth decreases in the direction of discharge. In this zone the plastics starts in both solid and molten state with target to have all molten upon leaving this zone.

3 Metering Zone: This section is a relatively shallow portion of the screw at the discharge end with a constant depth and lead usually having the melt moves 3 or 4 runs of the flight length.

Many different screw designs are available to meet the desired performance for the different plastics and RP being processed. The features common to all screw plasticators are screw(s) with matching barrel(s) that have at least one hopper/feeder (usually two hopper/feeder for RPs) in-take entrance for plastics/reinforcements, and one discharge por t /exi t ing of the melt. The essential factor in their "pumping" process is the interaction between the rotating flights of the screw and the stationary barrel wall. If the plastic compound is to be mixed and conveyed at all, its friction must be low at the screw surface but high at the barrel wall. If this basic criterion is not met the material may rotate with the screw without moving at all in the axial direction and out through the mold/die . The clearance between the screw and barrel is usually extremely small. The difference in the diameters of the screw and barrel bore (diametrical clearance) are more commonly one-half the diametrical clearance that is referred to as the radial clearance.

In the output zone, both screw and barrel surfaces are usually covered with the melt, and external forces between the melt and the screw- channel walls has no influence except when processing extremely high viscosity materials such as rigid PVC (polyvinyl chloride) and UHMWPE (ultra high molecular weight polyethylene). The flow of the melt in the output section is affected by the coefficient of internal friction (viscosity) particularly when the mold/die offers a high resistance to the flow of the melt. The constantly turning screw augers the plastic compound through the heated barrel where it is heated to a proper temperature profile and blended into a homogeneous melt. The rotation causes forward transport. It is the major contributor to heating the plastic compound


via the plastic's shearing action once the initial barrel heat startup occurs.

The primary purpose for using a screw is to take advantage of its mixing action. Theoretically speaking, the motion of the screw should keep any difference in melt temperature to a minimum. It should also permit materials and colors to be blended better with the result that a more uniform melt is delivered to the mold/die .

The designs of the screw is important for obtaining the desired mixing and melt properties as well as output rate and temperature tolerance on melt. Generally, most machines use a single, constant-pitch, metering- type screw for handling the majority of plastic compounds. A straight compression-type screw or metering screws with special tips (heads) is used to process heat-sensitive TPs or most TPs.

The helix angle affects the conveying and the amount of mixing in the channel. Experience has shown that a helix that advances one turn per nominal screw diameter gives excellent results. This corresponds to an angle of 17.8 degree that has been universally adopted. The land width is usually 10% of the diameter. The radial flight clearance is the clearance between the screw flight and the barrel. It is specified considering the following effects:

�9 Amount of leakage flow over the flights

�9 Temperature rise in the clearance. The heat is generated in shearing the plastic. The amount of heat generated is related to the screw speed, the design of screw and the material

�9 The scraping ability of the flights in cleaning the barrel

�9 The eccentricity of the screw and the barrel

�9 Manufacturing costs.

The length of the screw is the axial length of the r ighted section. An important criterion of a screw design is the ratio of the length over the diameter of the barrel (L /D) . As reviewed, a screw has three sections: feed, melting (transition), and metering. The feed section that is at the back end of the screw can occupy from zero to 75% of the screw length. Its length essentially depends upon how much heat has to be added to the plastic compound in order to melt it. The pellet or powder are generally fed by gravity into this section and are conveyed some distance down the barrel, during which time they become soft. Both conduction and mechanical friction accomplish heating.

The melting (transition) section is the area where the softened plastic compound is transformed into a continuous melt. It can occupy


anywhere from 5% to 50% of the screw length. This compression zone has to be sufficiently long to make sure that the entire plastic compound is melted. The straight compression-type screw is one having no feed or metering sections.

In the metering section, the plastic compound is smeared and sheared to give a melt having a relatively uniform composition and temperature for delivery to the mold/die. As high shear action will tend to increase the melt's temperature, the length of the metering zone is dependent upon the plastics heat sensitivity and the amount of mixing required. For heat-sensitive materials, practically no metering zone can be tolerated. For other plastics, it averages out at about 20 to 25% of the total screw length. Both the feed and the metering sections have a constant cross section. However, the depth of the flight for the feed zone is greater than that in the metering zone.

The screw compression ratio or C / R relates to the compression that occurs on the plastic compound in the transition (or compression) section; it is the ratio of the volume at the start of the feed section divided by the volume in the metering section (determined by dividing the screw feed depth by the screw metering depth). The C / R should be high enough to compress the low bulk unmelted plastic compound into a solid melt without air pockets. A low ratio will tend to entrap air pockets. High percentages of regrinds, powders, and other low bulk materials are usually processed by a high C/R. A high C / R can over pump the metering section.

A common misconception about C / R is that engineering and heat sensitive plastics should use a low C/R. This is true only if it is decreased by deepening the metering section, and not having a more shallow feed section. The problem of overheating is more related to channel depths and shear rates than to C/R. As an example, a high C / R in polyolcfins can cause melt blocks in the transition section, leading to rapid wear of the screw and/or barrel. For TSs the C / R is usually one so that accidental over heating does not occur and cause the plastic compound to solidify in the barrel. Their barrels are usually heated using a liquid medium so that very accurate control of the melt occurs with no overriding the maximum melt heat. With overheating TS melt solidifies. If it solidifies, the C / R of one also permits ease of removal by just "unscrewing" it from the screw. C / R ratio of one's is also used for TPs when the rheology so requires.

Fixed screw speed, pitch, diameter, and depth of the channels relate to output. A deep-channel screw is much more sensitive to pressure changes than a shallow channel. In the lower pressure range, a deep


channel will mean more output; however, the reverse is true at high pressures. Shallower channels tend to give better mixing and flow patterns.

The flow pattern in the screw flights changes with the backpressure. The flow of a particle in the flights with open discharge and in the blocked flow there is a similar circulatory motion between the flights, and no forward motion because the open end is closed. There is the greatest mixing when the flow is blocked. The importance of this flow concept is that it shows that the more blocked the flow, the better the mixing in the screw. The higher the pressure the greater the pressure flow and the lower the output. In injection molding, this pressure corresponds to the backpressure setting of the machine. This is the reason that color dispersion is improved and homogeneity increased by raising the backpressure. Often warpage and shrinkage problems can be overcome in this manner.

In our high technological world, the art of screw design is still domi- nated by experienced trial and error approaches providing the exact capabilities of the screws for a particular plastic compound operating under specific conditions. Screw design technology is considered to be empirical and /or secretive, however scientific approaches to screw designs based on an analytical melting model can be used. Available are computer models that play a very important role that are based on proper data input and, very important, experience of the person with a set up similar to the one being studied. When new materials are developed or improvements in old materials are required, one must go to the laboratory to obtain rheological and thermal properties before computer-modeling mixing can be performed effectively.

Production rate of an acceptable melt from a screw is its most important function. It is often limited by its melting capacity. The melting capacity of the screw depends on the plastic compound properties, the processing conditions, and the particular geometry of the screw. Once the melting capacity is predicted, the screw can be designed to match the melting capacity.

Mixing

The action of mixing plastic compound can be distributive and/or dispersive. They are not physically separated. In dispersive mixing, there will always be distributive mixing. However, the reverse is not always true. In distributive mixing, there can be dispersive mixing only if there is a component exhibiting a yield stress and if the stresses acting on this component exceed the yield stress.


In order for a dispersive mixing device to be efficient, the mixing section should have a region where the plastic compound is subjected to high stresses. It also has a high stress region that should be deigned so that exposure to high stresses occurs only for a short time, and all fluid elements should experience the same high stress level to accomplish uniform mixing. In addition, they should follow the general rules for mixing of minimum pressure drop in the mixing section, streamline flow, complete barrel surface wiping action, and easy to manufacturing the mixing section.

As reviewed concerning screw types, these dynamic mixers are used to improve screw performance. Static mixers are sometimes also inserted at the end of the plasticator. Proof of their success is shown by their extensive use worldwide. Each type of mixer offers its own advantages and limitations. Where practical they should not be located in a region where the melt viscosity is not too low. With some of these installations because they may have to operate at a lower speed to avoid problems such as surging, independently driven mixers can be used so machines can operate at optimum speed. Other benefits of independently driven mixers involving feeding capability and performance occur. For example, metering pumps can inject with precision liquid additives directly into the mixer.

Screw Wear

When injection molding, extruding, blow molding, etc. short or long glass fiber reinforced compounds there will be steel wearing of the plasticator screw/barrel, particularly the screw, usually within six months operating 24 hours per day. Steel wear also occurs in the molds runner and gate systems and cavities. Wear is associated with reducing the performance of molded products and /o r increasing cost to mold. Special screw designs and materials of construction have been used to process glass fiber RTPs and RTSs most efficiently.

Wear Resistant Barrel

As it is easier to replace a screw than a barrel, the barrel is made harder than the screw. Barrel and screw assembly operates in an aggressive environment that can cause gradually or severe wear problems. To improve the wear resistance of the barrel, it may be modified, or lined. Modification may be by nitriding or ion implantation but these treatments are not as good as lining. Lining is done with a wear resistant alloy. This wear resistant layer may be cast in during barrel manufacture or the liner may be inserted subsequently or for rebuilding a worn


barrel. These bimetallic barrels are usually used when abrasive compounds are being processed.

The increasing use of plastics with abrasive fillers and reinforcements created a demand for an even more abrasion resistant barrel than the standard i ron /boron type. The use of glass fiber reinforced compounds for injection molding has been the single most important factor since a fabricator would be lucky if they could reach 6 months of continuous operation. This need has been successfully answered by the development of liner materials containing metallic carbides such as tungsten carbide and titanium carbide extending their life.

Barrel Heating and Cooling Method

Heat to soften the plastic compound is supplied in two ways: by external barrel heating and internal frictional forces brought about on the plastic compound due to the action of the metal screw in the metal barrel. The amount of such frictional heat supplied in the plasticator is appreciable. In many extrusion operations, it represents most of the total heat supplied to the plastic.

To provide temperature control, the barrel is divided into zones. Each of these zones is fitted with its own external heating/cooling system. The smallest machine will have three zones and larger machines may have twelve. A temperature sensor and associated equipment, usually a microprocessor-based controller for each of the zones developing a temperature profile to provide the best melt. Target for these controllers is to measure the melt temperatures that are important rather than the cylinder temperatures.

There are three principal methods of barrel heating: electrical, fluid, and steam. Electrical heating can cover a much larger temperature range. It is clean, relatively inexpensive, and efficient. The heaters are generally placed around and along the barrel, grouped in zones. Each zone is usually controlled independently, so the desired temperature profile can be maintained along the barrel. Electrical heating is generally preferred because it is the most convenient, responds rapidly, easiest to adjust, easy to clean, requires a minimum of maintenance, covers a much larger temperature range, and is generally the least expensive in terms of initial investments.

Fluid heating, such as the use of heated oil, allows an even temperature over the entire heat-transfer area, avoiding local overheating. If the same heat-transfer fluid is used for cooling, an even reduction in temperature can be achieved. The maximum operating temperature of most fluids is relatively low for processing TPs, generally below 250C


(482F). With its even temperature, the required fluid heating is desirable with TS plastics so that no accidental overheating occurs to chemically react and solidify in the barrel.

Steam heating was used in the past, particularly when processing natural rubber. Now it is rarely used. Steam is a good heat-transfer fluid because of its high specific heat capacity, but it is difficult to obtain steam to the temperatures required for TP processing of 200C (392F) or greater.

The cooling of barrels is an important aspect. The target is to minimize any cooling and, where practical, to eliminate it. In a sense, cooling is a waste of money. Any amount of cooling reduces the energy efficiency of the process, because cooling directly translates into lost energy; it contributes to machine's power requirement. If a machine requires a substantial amount of cooling, when compared to other machines, it is usually a strong indication of overheating the plastic, improper process control, improper screw design, excessive L / D , and /or incorrect choice of plasticator.

Cooling is usually required with forced-air blowers mounted under- neath the barrel. The external surface of the heaters or the spacers between the heaters is often made with cooling ribs to increase the heat-transfer area. The design using fibbed surfaces will have a larger cooling area than flat surface, result is significantly increases cooling efficiency. Forced air is not required with small diameter extruders because their barrel surface area is rather large compared to the channel/rib volume, providing a relatively large amount of radiant heat losses.

Fluid cooling is used when substantial or intensive cooling is required. Mr-cooling is rather gentle because its heat-transfer rates are rather small compared to water-cooling. However, it does have the advantage in that, when the air-cooling is turned on, the change in temperature occurs gradually. Water cooling produces rapid and steep change; a requirement in certain operations for processing certain RPs. This faster action requires much more accurate control and is more difficult to handle without proper control equipment.

The larger barrels are often liquid cooled, using cored channels to circulate the cooling medium because they require intense cooling action (feed-throats also use water-cooling). However if not properly controlled, problems could develop. If the water temperature exceeds its boiling point, evaporation can occur resulting in poor cooling control. The water system is an effective way to extract heat, but can cause a sudden increase in cooling rate resulting in a nonlinear control problem; resulting in more difficulty to regulate temperature. Nevertheless, the


water cooling approach is used very successfully with adequate installation and adequate control and startup procedures.

IMMs for processing TS resins and rubbers (that are TSs) control the barrel temperature most of the time indirectly with an external heat exchanger. It is operated by a liquid heat-transfer medium such as oil or brine. Curing of the plastic or rubber compound occurs in the mold cavity(s) by the application of higher heat than what exists in their barrel melt. A chemical crosslinking action occurs with the additional heat resulting in the solidification of the TS materials.

Depending on the IMM operation capability as well as type plastic compound being processed, the melt passing from the plasticator through the nozzle may also require heat control. This action is usually required when processing certain heat sensitive plastics and/or if a long nozzle is used.

Purging

Purging is an important tool to permit color changes, remove con- taminants such as black specks, and plastic compound adhering to screws and barrels. At the end of a production run, the plasticator may have to be cleared of all its plastics in the barrel/screw to eliminate barrel/screw corrosion. This action consumes substantial nonproductive amounts of plastics, labor, and machine time. It is sometimes necessary to run hundreds of pounds of plastic compound to clean out the last traces of a dark color before changing to a lighter one; if a choice exists, process the light color first. Sometimes there is no choice but to pull the screw for a thorough cleaning.

Purging material include the use of certain plastics to chemical purging compounds. Popular is the use of ground/cracked cast acrylic and PE- based (typically bottle grade HDPE) plastics. Others are used for certain plastics and machines. Cast acrylic, which does not melt completely, is suitable for virtually any plastic. PE-based compounds containing abrasive and release agents have been used to purge the softer plastics such as other polyolefins, polystyrenes, and certain PVCs. These types of purging agents' function by mechanically pushing and scouring residue out of the plasticator (Table 5.14).

The chemical purging compounds are generally used when major processing problems develop. However to eliminate the major problems with their associated machinery downtimes, regularly scheduled purgings prevent problems and can yield operational benefits. With the proper use of these purging agents' helps to reduce reject rates significantly. The schedule depends on factors such as plastic compound

41 8 Reinforced Plastics Handbook

Table 5.14 Examples of purging agents when changing plastic compound in a plasticator

Material to be purged Recommended purging agent

ABS Nylon PBT polyester PET polyester Polycarbonate

Acetal Engineering resins Fluoropolymers Polyphenylene sulfide Polysulfone Polysulfone/ABS PPO Thermoset polyester Filled and reinforced materials Flame-retardant compounds

Polyolefins Polystyrene PVC

Cast acrylic, polystyrene Polystyrene, low-melt-index HDPE, cast acrylic Next material to be run Polystyrene, low-melt-index HDPE, cast acrylic Cast acrylic or polycarbonate regrind; follow with

polycarbonate regrind; do not purge with ABS or nylon Polystyrene; avoid any contact with PVC Polystyrene, low-melt-index, HDPE, cast acrylic Cast acrylic, followed by polyethylene Cast acrylic, followed by polyethylene Reground polycarbonate, extrusion-grade PP Reground polycarbonate, extrusion-grade PP General-purpose polystyrene, cast acrylic Material of similar composition without catalyst Cast acrylic Immediate purging with natural, non-flame-retardant

resin, mixed with 1% sodium stearate HDPE Cast acrylic Polystyrene, general-purpose, ABS, cast acrylic

being processed, size and plasticator operational settings with its time schedule that it is in use. Repeated equipment shutdowns and startups are the most common cause of degraded plastic build-up. Purging compound producers can recommend the time schedule to be used in order to minimize down time and increase profits.

Tools

Overview

Tools include molds, dies, mandrel, jigs, fixtures, punch dies, etc. for fabricating and shaping parts. These terms are virtually synonymous in the sense that they have female or negative cavity through which a molten plastic compound moves usually under heat and pressure or they are used in other operations such cutting dies or stamping plastic sheet dies, etc. Tool is the term that identifies all these devices particularly used to identify molds (Table 5.15). Molds represent an

Table 5,15 Guide for tools used in different processes

I I Injection

I I Hi vol lo vol

I I Steel Cast al

(machine I hobbed ) Mach. AI electro I form Et Kirksite back-up I

Filled epoxy

I R m l

I I I Extrusi~ I IB'ow molding I

I Hi vol ] Steel

I I ~ol Hi vol Lo vol Hi

I I I AI AI Steel I I (mach

BeCu P las te r hobbed)

I I Electroform Epoxy- Et back-up fiberglass

I Filled epoxy

Steel

Cast a I

Mehanite

Filled epoxy

I Tool to be made I I

I Processes I I

I I Comp. e{ trans I

I Lo vol

I AI

I Electroform ~t back-up

Filled lepoxy

I IT,hermoforming I

I I Hi Lo

I I AI Wood

I Plaster

Foam process {not struct)

I Plaster

I AI

I Plastic

I Electroform

I Sprayed metal

I IRotoform I I I

Hi vol Lo vol

I I Cast al Sheet

I metal Electroform

I ICaslingl Electroformed

I Spraylmetal

Dipped metal

I Silicone

I Plastic

I Elastomer

Rein/f plastics

I Plastic

I AI

I Wood

I Steel

I Electroform

I Sprayed metal

I Elastomer

I En~p I

Dip metal

Plastic (tp or ts)

I Elastomer

"1"1 o-

u l

o


important part in fabricating the different RP products using the different fabricating processes.

Molds arc used in many RP processes with many of the molds having common assembly and operating parts with the target to have tool's cavity designed to form desired final shapes and sizes. A mold can be an unsophisticated low cost to highly sophisticated expensive piece of machinery. It can comprise of a single to many parts requiring high quality metals and precision machining. To capitalize on its advantages, the mold may incorporate many cavities, adding further to its complexity. Many molds, particularly for injection molding and compression molding, have been preengineered as standardized products that can be used to include cavities, different runner systems, cooling lines, unscrewing mechanisms, etc.

A die is a device, usually of steel, having an orifice (opening) with a specific shape or design geometry that it imparts to an RP melt such as the extrudate pushed from a pultruder or extrudate pumped form an extruder. The function of the die is to control the shape of the extrudate. The important word is control. In order to do this, the extruder must deliver melted plastic to the die targeted to be an ideal mix at a constant rate, temperature, and pressure. Measurement of these variables is desired and usually careful performed.

A mold, particularly for injection molding and compression molding, or die, particularly for pultrusion and extrusion, is a controllable complex mechanical device that must also be an efficient heat exchanger. If not properly designed, handled, and maintained, it will not be an efficient operating device. Hot melt, under pressure, moves rapidly through them. In order to solidify the hot melt, water or some other media circulates in the mold or die to remove heat from RTPs or heat is used with RTSs. All kinds of actions can be used to operate the mold such as sliders and unscrewing mechanisms or die with melt pressure and directional channels. CAD and CAE programs are available that can aid in mold and die design and in setting up the complete fabricating process. These programs include melt flow to part solidification and the meeting of performance requirements.

There are variable conditions during processing that influence part performance. Of paramount importance in injection molds is gate location(s) and controlling the cavity(s) fill rate or pattern. The proper fill helps eliminate part warpage, shrinkage, and other problems. In the practical world of mold design, there arc many instances where trade- offs must be made in order to achieve a successful overall design. As an example, while naturally balanced runner system are certainly desirable,


they may lead to problems in mold cooling or increased cost due to excessive runner-to-part weights. Available are software flow analysis guides allowing successful designs of runners to balance for pressure, temperature, rate of flow, etc.

Design of a die includes:

1 minimize head and tooling interior volumes to limit stagnation areas and residence time;

2 streamline flow through the die, with low approach angles in tapered transition sections; and

3 polish and plate interior surfaces for minimum drag and optimum surface finish on the extrudate.

Basically, the die provides the means to "spread" the plastic being processed/plasticated under pressure to the desired width and thickness in a controllable, uniform manner. In turn, this extrudate is delivered from the die (targeted with uniform velocity and uniform density lengthwise and crosswise) to takeoff equipment in order to produce a shaped product (rods, sheet, pipe, profile, etc.).

Depending on material, molding process, and the anticipated number of parts-off, molds and tools for RPs can be made of (in ascending order of production numbers) RPs themselves, alloys, aluminum, or chromed steel (Tables 5.16-5.18). The general characteristics and use of each are as follows:

1 Steel molds made from machined steel are especially suitable for mass-produced parts. Their high cost is written off over long production runs giving a piece-part which can be very economical, both in injection molding of RTPs and RTSs and high-pressure compression molding of RTSs. Commonly used is P20 steel, a high grade of forged tool steel relatively free of defects and is a prehardened steel. It can be textured or polished to almost any desired finish and is a tough mold material. H-13 is usually the next most popular mold steel used. Stainless steel, such as 420 SS, is the best choice for optimum polishing and corrosion resistance. The mold can be equipped with moving parts and elaborate automated ejection systems, which can make the relatively large capital investment more attractive by further reducing the molding cycle and producing parts that require little or no finishing. Steel molds may also be required to counter the abrasive effect of glass fiber.

2 Aluminum offers lighter weight, better heat conductivity than steel and lower machining costs and is a mold-making material for low-

Table 5,16 Example of tool materials arranged in order of hardness

Suitable for Material Class Suitable for

Thermoplastics glass-filled

Prototype injection molds TP resins

Low-pressure thermosets

SMC BMC

Structural f foams

High-pressure thermosets

Phenolics Ureas Diallylls Melamines Alkyds

Casting

Carbides Steel, nitriding Steel, carburizing Steel, water-hardening Steel, oil-hardening Steel, air-hardening Nickel, cobalt alloy Steel, prehardened 44 Rc Beryllium, copper Steel, prehardened 28 Rc Aluminum bronze Steel, low alloy ~t carbon Kirksite (zinc alloy) Aluminum, alloy Brass Sprayed metal Epoxy, metal-filled Silicone, rubber

Thermo- plastics unfilled

Blow molds

Vacuum- forming sheets TP resins

m

I l l

m ,

I l l

"1"

:3

O" 0 0 ~--

Table 5,17 Performances of plastic molds vs. type of molding

Molding Materials Flexural modulus Impact Heat resistance x 103 kg/mm 3 strength (HDT 18.6)~

Paintability by baking [74o- 75ooc)

Weight ratio 1 [Equiflextural modulus) Moldability

Cost 2 (Mold cost) 3

Compression molding

Filament winding

Injection molding

- - Hot press

Cold press - - ~

- - Stamping

I RLM - A~x

Polyester + GF (SMC) 1 o ~ A >200

Polyester + GF (BMC) 1.1 1̀ 1'

Polyester + GF 1.6~4.2 @ 1̀

(High-strength SMC)

Polyester + GF 0.8 O 150~200

(Resin injection)

Polyester + GF (Hand lay-up)

PP + GF or sawdust. paper pulp 0.6 @ 160 (AZDEL, etc.)

Nylon + GFEF 0.8 @ 215

(ST)(, etc.)

Epoxy + CF (CFRP) 15 O >200

PP + GF, talc 0.6~0.4 1̀ 120~105

(EPDM) AS + GF

PBT or nylon + GF 1.2~1.4 1̀ 205~215

Foamed styrene 2.4~2.5 O~A 80 (100) or ABS (+ GF)

Urethane + GF (RRIM) 0.1~0.2 @

@ 0.65 O~A

@ 0.6 f

0.4~0.5

A~x 0.62 A

- 0 . 5 @

O ? @

- 0 .2 A

- 0 .5 O

@~O 0.5 ?

0.4,,,0.6

O~A

O (O)

t

O (O) @ (@)

A~O (O)

A (O)

A,,,x (O) @ (-)

A (O) O (O)

A (@~O)

t.n

-I'i

i ,

o

Note: I. Ratio based on sheet metal weight as I" 2. Relative comparison for 400-500 kg; 3. Mold cost for sheet metal. Symbols: @ Excellent; O Good; A Fair; x No good .p ,

(,o


Table 5.18 Examples of machining

Machining method Purpose of machining operation

Cutting with a single-point tool with a multiple-point tool

Cutting off with a saw by the aid of abrasives

shearing by the aid of heat

Finishing by the aid of abrasives

Turning, planing, shaping Milling, drilling, reaming, threading, engraving

Hack sawing, band sawing, circular sawing Bonding abrasives: abrasive cutting off, diamond cutting off Loose abrasives: blasting: ultrasonic cutting off Shearing, nibbling Friction cutting off, electrical heated wire cutting off

Bonded abrasives: grinding, abrasive belt grinding Loose abrasives: barreling, blasting, buffing

Types of parts Kinds of machining methods used

Bearing, roller Button Cam Dial and scale Gear Liner and brake lining Pipe and rod Plate (ceiling, panel) Tape (mainly for PTFE)

Turning, milling, drilling, shaping Turning, drilling Turning, copy turning Engraving, sand blasting Turning, milling, gear shaving, broaching Cutting off, shaping, planing, milling Cutting off, turning, threading Cutting off, drilling, tapping Peeling

Processing method Purpose of machining operation

Types of machining used

Compression, transfer, injection and blow molding

Extrusion Laminating

Vacuum forming

Degating deflashing, polishing

Cut lengths of extrudate Cut sheets to size, deflashing edges Polish cut edges, trim parts to size

Cutting off, buffing, tumbling, filing, sanding Cutting off Cutting off

Cutting off, sanding, filing


pressure work. It may not be suitable for long runs or some RP systems.

Zinc alloy (kirksite) high-quality molds can be made from cast zinc alloys, offering good non-porous surfaces but are relatively heavy, with lower heat conductivity than aluminum, requiring closely spaced cooling channels.

Nickel shells are particularly good for high-quality surface repro- duction, with good hardness and good release properties. For structural rigidity, shells will normally require backing with a steel or aluminum frame, or a suitable casting material. Cooling lines can be attached or plated on the rear of the shell before backing.

Epoxy/reinforced can be used for molds for open (contact), and low-pressure cold-press molding. Epoxy molds have poor temperature control and tend to be fragile. Compounding with a metal filling improves heat conductivity but, in general, epoxy molds should only be used for short runs or for prototype parts, when quality is not the key criterion, but cost and flexibility are paramount.

Contact Molds

Molds for hand lay-up (as well as others such as spray-up, contact molding, thermoforming, and casting) are usually made of TS polyester or epoxy shell set in a cradle made of a material such as steel angle. They can usually be made in-house, on a model of the product that could be made from an inexpensive material that can be shaped or sculpted, such as plaster, balsa wood, or expanded polystyrene, sealed and coated with a release agent.

Low-cost molds for contact and low-pressure molding can be made from reinforced TS polyester or epoxy compounds with glass fiber and / or mineral filler reinforcement. These constructions have also been used to make prototype molds for compression and injection molding, with the advantages that they can be produced quicldy and at low cost (often in-house) and can readily be modified. Latest technology uses blocks of resin-based compounds that can be machined by computerized instructions, for production of prototype molds.

Epoxy prepreg tapes, with carbon, glass, or aramid reinforcement, have been used for production of tooling. They require an initial cure at 20-80C (68-176F) and offer a maximum service temperature of at least 200C (392F) in air. A low-temperature cure means very low residual stress levels. Low-temperature master models can be used directly without


intermediate molding stages, giving improved accuracy in tooling and high quality/long-life tools.

High-quality cast aluminum tooling as an alternative to all-RP tooling is also used, without the high cost of electroforming.

A ceramic matrix material offers improvements in mold-building, eliminating print-through, enabling a mold to be built without steel reinforcement and reducing weight by up to 75%. A patented mold closure system, using plates built on the mold on which wedges slide to close the halves, knocked into place and knocked out by hammer, as needed, is also available.

For contact molding, only one mold-half is needed, which can be either male or female, depending on which face is required to be smooth (Figure 5.74). Almost any material of sufficient rigidity can be used such as those reviewed for making the mold with the most common method being glass fiber-reinforced TS polyester.

Figure 5.74 Construction of plaster pattern mold for contact molding

A good mold made of RP will produce hundreds of moldings, with a minimum amount of maintenance. It consists of a shell, reinforced as necessary, often mounted on a light timber or metal flame. Once a suitable master pattern has been prepared, it is possible to produce many RP molds from it, easily and at low cost, using the contact molding process (but in reverse). Patterns can be made from timber or metal, or using plaster on a timber framework. They must be accurately


finished and well polished, to give a smooth mold surface. Plaster and other porous materials must be sealed with a solution of shellac or cellulose acetate, before waxing and polishing.

Gel coats should be about 0.6 mm thick; thicker than is usual for a finished molding, but allowing for any rubbing down which may be necessary during the lifetime of the mold.

If it is intended to use the mold many times, over a long period, it is a good precaution to make it of a heat-resistant resin, to give a harder tougher surface, with better overall stability.

Molds can be reinforced with external ribbing, using cores on an open metal profile, plastic piping, or foamed plastics (Figure 5.75). These should be added when the laminate itself is sufficiently cured, to avoid the danger that contraction of the resin around the ribs may distort the laminate itself, and so leave an impression on the mold surface.

Figure 5 .75 Design and lamination of ribs to reinforce a contact mold

With larger contact molds, it may also be necessary to design to split the mold, to facilitate demolding of the part (Figure 5.76). This calls for inclusion of flanges, which are reinforced with continuous roving on the sharp corners, where areas of unreinforced resin are particularly prone to damage. When the resin is fully cured, the flanges should be trimmed to about 75 mm (approximately 3 in).

To avoid damage and ensure long life, the flanges should be about 50% thicker than the mold shell itself and metal plates should be incorp-


Figure 5.76 Construction of the flange for split molds

orated along their length to spread the load of the mold-clamping bolts which should be placed at intervals of about 150 mm (6.25 in).

An automated pattern construction for the marine industry pioneered by Mollicam, Florida, USA, uses five-axis computer controlled milling machines to generate exact complex shapes for all sizes of glass fiber RPs tooling. The equipment can machine up to 9.75 x 3 x 1.8 m (32 x 10 x 6 feet) as a single unit, with easy combination of multiple units for larger sizes. Recent projects have exceeded 18.3 m (60 feet) in length. Advantages claimed include:

time saving: a typical 6 m (20 ft) hull can be CNC machined in less than 40 hours and, with full five-axis machining, hand finishing is minimal

�9 accuracy: tolerances ofless than 0.4 mm (1 /64 in) can be achieved relative to the computer model

�9 symmetry: tool paths are developed for one side of the hull and a mirror image is computer generated.

New and unique methods have also been developed to produce a superior plug, machined with an outer seamless shell of high-density plastic foam, which is strong but light for easy handling, and evenly absorbs heat generated in the mold-making process.

High-temperature processing of high performance resins, at up to 475C (800F) temperature and up to 2.76 MPa (400 psi) pressure is provided by CareMold, a system developed by Composites Horizons Inc., USA. It uses proprietary expendable mandrel material and permits


casting at room temperature or high temperature breakaway tooling for complex stiffeners and shapes.

Autoclave Molds

Molds for autoclaving must be more substantial and can carry more detail. Depending on molding requirements and conditions, they can be produced by electroforming nickel. To cure large parts size restrictions and availability of autoclaves and ovens can occur. Additional heat is obtained by incorporating electrical heating elements in the molds.

Cold Press Molds (low pressure)

Soft tools will usually comprise a shell of reinforced epoxy tooling compound, backed or reinforced (depending on size) with concrete or other dimensionally stable material. They take about 6-8 weeks to make and have a life of about 2000 components. Costs, amortized over this number, usually amount to around 2% of product value. As an alternative, electroformed nickel tools can be used, costing about three to five times more, but giving a lifetime output of about ten times (20,000 components).

Resin Transfer Molds

Mold design has proved to be a key factor in successful development of resin transfer molding (RTM). The guidelines given for mold design are in general valid, but advice should also be sought from suppliers. For RTM, the accuracy of cavity thickness dimensions is critical. Good mold seals are essential. A fight 45 ~ pinch-off is inadequate, and usually it is better to use a double seal configuration, to allow an optional vacuum-assist system during injection. Automatic mold clamping systems with built-in manipulators and low-cost pneumatic presses have all assisted RTM development.

A mold-making system is used for the RTM process using TS polyester- based materials. Molds have already been used for three years and are reported to show good surface despite 5000-6000 demoldings, while involving minimum capital outlay and giving a shortened lead-time.

Electroformed nickel shell tooling can be useful for high quality/ medium volume (such as RTM). FET Engineering, Kentucky, USA, has a die spot press allowing both halves of the tool to be placed outside the press with both cavities in the up position, allowing work on the tool while registration of both halves is maintained. Helisys rapid prototype machine and SNK digitizer have been installed. Vapor


deposition development allows production of uniform nickel tools much faster than currently allowed by electroform technology.

RP tooling has been developed over the past ten years, less costly than nickel shell tooling, allowing RTM to be used for smaller production runs but, with rapid turn-round and high temperatures, conventional gel coats and resin systems can quickly fail. The Durabuild range (from Hawkeye, USA) was initially based on air-cure TS polyester putties, primers, and high-gloss coatings used for repair and resurfacing, including high-gloss, heat distortion temperature up to 150C (302F), low porosity, and good impact resistant surface. If the mold is structurally stable, resurfacing will last as long as the original. Molds have been known to give good production for several years.

Wolfangel GmbH, Germany, has a process that can make a mold in three days (compared with usual two weeks). First, laminating layers are laid on the tooling gel applied to the normal pattern, using virtually zero-shrinkage TS polyester, three or four layers of hand-lay TS polyester/glass fiber. The gel/laminate is then allowed to cure (one day). On the second day, a heavy deposit of chopped glass/polyester is sprayed on the back of the laminate and consolidated using a special resin allowing build-up to 15 mm (0.6 in) without exotherm problems. Finally, a lightweight plastic concrete is spray-applied, building the tool and adding additional rigidity, up to 50 mm (2 in) thick non-sagging.

The TS polyester-based mix is based on a Reichhold tooling resin, mixed with equal parts of alumina trihydrate, with low exotherm chemistry, allowing a high build in a single shot. It also incorporates a core material such as end-grain balsa, to simplify the shape of the back of the mold and further increase stiffness without a weight penalty. A further spray/chop layer is then consolidated to the final back of the tool and once the mold is cured, the steel supporting frame can be attached to fittings laminated into the tool. Wolfangel developed this system for its own RTM production. The largest part so far reported is a 100 m 2 mold made up as follows:

�9 mold gel coat

�9 styrene-resistant buffer 3 mm

�9 tooling resin spray/chop laminate 5 mm

�9 balsa wood core 10 mm

�9 resin spray/chop laminate 5 mm

�9 balsa wood core 10 mm

�9 resin spray/chop laminate 7 mm.

5 �9 Fabricatincj Processes 431

Because of the reduced pressure, molds for low-pressure molding can be either larger or made with less expensive materials. USA molder Modern Tooling, with back up from Alpha Owens Corning, has adapted metal-working technology from the aerospace industry to reduce machining costs in mold-making, eliminating many of the steps in the process. A large mold can be made in half the usual time.

Filament Winding Molds

The mandrel (mold) on which filament-wound products are produced is essentially fight, but solid, in construction and (usually) near cylindrical. The most common product is pipe, when the mandrel can be made simply of aluminum sheet on a lightweight framework. RP can also be used. An important aspect of the mandrel is that it must be possible to extract it easily from the wound lay up, after curing. This may mean use of a tapered or collapsible mandrel (Figure 5.77), or even a sacrificial mandrel (made possibly of plaster) which is broken up inside the cured product.

Figure 5,77 Basic configuration of a mandrel for filament winding, and the principle of a collapsing mandrel

Cylindrical mandrels are available from specialist suppliers, to order, across a complete range of sizes from typically 610 mm (2 ft) diameter

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Figure 5.78 Examples of injection molding mold layouts, configurations, and actions


to 3600 mm (12 ft), constructed over a series of roller tings, each individually ground to produce a perfect surface. After grinding, the mandrel is skinned with sheet metal and then painted. It is also possible to obtain mandrels constructed with special infrared curing elements along the surface, to encourage efficient heat-penetration of the laminate and economical use of power.

Injection and Compression Molds

Mechanically, the tools for molding TSs and TPs have some similarity. Historically, TSs have been compression molded, using vertical presses, while TPs have been injection molded, predominantly using a horizontal press configuration (Figure 5.78) with molds viewed alone in a vertical position (Figures 5.79 and 5.80). Since at least the 1950s, the injection molding process has been molding TSs. A vertical press lay out is sometimes used for molding TPs, especially where metal inserts are involved (as in electrical/electronics work). In both cases, the molding tool comprises two matching halves (male and female), with guide pins to ensure exact mating and ejector pins to ease removal of the molded part (Tables 5.19 and 5.20).

Figure 5.79 Sequence of mold operations

Whereas in compression molding, the charge of material is loaded into the open mold, an injection-molding tool incorporates an enclosed system for feeding the material into the tool and distributing it evenly. This is usually done through a gate, which is positioned to give the best balance of feed to all parts of the mold cavity. For larger or more


1. Back plate Z 17 20 25 8 7:6 "~.S "~ .~21 t, 23 3,:~. ~2 1:,~ 9 2. Backplate ( [ ' / " I ' ~ % ' ~ " / / / / '/'i ~I/' ~ ' " ' ' / ' : / / / / ~ ~ . ~ ] ' ~ I~ I~ 5.4, 3. SupportC~176176

: . _ : _ . . . . . . . . ' ~ ~ ,, ........ ~ ........ ;J...~ �9 8. Ejector back plate ~ . " x . ~ ~ ; . �9 t; i xl~\\: 9. Locating ring

�9 ~\~.. ~4-'--.--_-~- ' ~.\"~ ~ ~ . ~ 10. Locating ring "~ , .. - ' - \ \ . ~ ~ 11 Locating guide pillar

12. Stripper rod �9 - -[-. _L _ 13. Ejector coupling rod :;i : - ,t I' " -: ~ ~ ~ ~ - - 14. Locating guide bush ~ ~ ' ~ ] "15. Heated nozzle

- - - - - - -~-~-~- ,-7~..- 16. Stripper rod guide bush 17: Ejector bush 18. Thermal insulating plate

/ / / ~ - - i - _ . ~ . ~ ~ ~ ! 19. Electrical service connector box f O r h e a t e d nozzle

/ / / t ~ 11 i' / t / / / / " i / / ,.~~ ~ " 21.20" Centering dowel i .i 22. Cap screw

23. Cap screw 1] 10 30 38 36 . . . . 32 ~6 29 "3S ~1 ~0 t~ !,1 2~, 28 .~ ~8 31 24. Cap screw

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25. Cap screw 26. Cap screw 27. Cap screw 28. Cap screw 29. Cap screw 30. Cap screw 31. Cap screw 32. Cap screw 33. Ejector coupling rod connector 34. Ball catch 35. Ejector pin 36. Ejector stop button 37. Support pillar 38. Spring washer 39. Spring washer 40. O-ring seal 41. Water channel service coupling 42. Water channel sealing plug 43. Cooling water spiral core 44. O-ring seal 45. O-ring seal 46. Pressure sensor 47. Core insert block 48. Cavity insert block 49. Stripper bar 50. Side action 51. Cam

F i g u r e 5 , 8 0 Description of mold shown in previous figure

complex molds, there may be a number of gates, which are linked together in the mold by runners.

Figure 5.81 is a schematic of a compression mold. Figure 5.82 shows positive, flash, and scmipositive molds. They provide different methods of processing RPs to meet different part performance requirements. Air is entrapped and/or gases are formed during processing requiting vents at the parting lines; an example is shown in Figure 5.83.

The essential difference between tools for TSs and TPs is that, whereas the TS compound must be made to cure/solidify in the mold (and so the mold is heated), the TP compound is already molten and must be "frozen" (and so the mold is cooled).


Table 5ol 9 Mold quotat ion guide prepared by the Society of the Plastics Industry

__...... . �9 . . . . . . . . .

L ~ THE MOt, DMAKERS OIVtSlON

THE SOCIETY OF THE PLASTICS INDUSTRY, INC. �9 3150 Des P|lines Avwnutl (Rivet Rolidl, O~ls Pi l lnel . lU. 80018. Tlittahoml: 3t2t297,6150

TO . . . . . . FROM QUOTE NO. _ DATE _

. . . . DELIVERY REQ Gentlemen:

Please submit your quotation for a mold as per following specifications and drawings: COMPANY NAME Name 1. BIP No. Rev. No, No, C av.

of 2. . . . . BtP No. Ray. No .............. No, Cav, Partts 3. _ _ .., BIP No . . . . . Rev. No . . . . No, Cav.

No. of Cavities: Design Charges: Prk~: I~llvery:

Type of Mold: O Injection Mold Construction O Standard D 3 PlBte 0 0 Stripper 0 [3 Hot Runner (3 0 Insulated Runner 0 [3 Other (Specify) _ []

O Mold.Bead Steel 0 0 gl D 0#2 0 0r O

Hardness CavitlN " Cores 0 Hardened O 0 Pre.Hard O O Other (Specify) _ _

ElatiOn Cavities . . . . . . . Coles O K.O. Pins 0 E] Blade ~0 , 0 0 Steers 0 0 Stdpper 0 0 Air 0 [3 .Special Lifts 0 0 Unscrewing (Auto) 0 0 Removable Inserts (Hand) 0 0 Other (specify)

Design.by:.. 0 Moldmaker 0 Customer Type of Design: 0 DelaI!ed Design. [3 Layout Only Ltmlt Switches: O Supplied by Engraving: 0 Yes 0 NO ApproxlmMe Mold $1m=

O Compression O Transfer O Other (specify) Material _

Cavities Cores [3 Tool Steel D O Beryl. Copper D O Steel Stnktngs O O Olher (Specify)

Special Features O Leader Pins & Bushings in K,O. Bar

Spring Loaded K.O. Bar Inserts Molded in Place Spring Loaded Plate Knockout Bar on Statlona~ Side Accelerated' K.O, Positive K.O. Return Hyd, Operated K.O. Bar Parting Line Locks Double Ejection Other :(Specify) . . . . . . . .

Finish Cavities Cores [3 SPF.JSPI O O MiLch. Finish O O Chrome Plate O O Texture O O Other (Specify) , ..

Side Action Cavities . . . . Cores

Angle Pin C ~] Hydraulic Cyl. t::)

Air Cyl. O O Positive Lock O O Cam O D K.O. Activated Spring Ld, O O Other (Specify) [ ]

Pleas Clamp Tons Make/Model

Cooling Cavities: - Core O Inserts O

Retainer Plates O O Other Plates O O Bubblers O O Other(Specify) _

O Edge O Center Spree O SutPGete O Pin Point O O t h e r ( S p e c i f y ) _

O Mounted by Moldmaker

He| lo l l b i l l I ~ 0 Moldmaker O Customer DupllclllinO O u l i |)~ OMoldmaker [:)Customer Mold Function Try.Out .By:. O Moldmaker O Customer Tooling Mochdts or Muter/s.By: O Moldmaker O Customer Trj-Out.Matedal Supplied.By: O Moldmaker L'] Customer

Terms subject to Purchase Agfeemerd. This quotation holds for 30 days.

Special Instructions: _ ~ ............

T-he PriCes quoted me on the buiS'O! Piece part print, models or designs submitted or suPPiled..~10uld there-be shy change In the final design, prices are subject to change.

By . . . . . . . . . Title . . . . Ohlldl~llor Use o! l i l t | 3 part form is rer as IOItows: !) White lind yellow �9 Iron! with fmlml| t tO quotql.

PiNk �9 fltlilni,,thed in active file. ~ WhiNi ot i0 tMI , return4d with quolalion Tallow �9 rattier.;4 In MG~,.:kSr~ lcl ive fllIL ....... i, . . . . . . . . . . . . . . . . . . . . . . . .

Table 5 , 2 0 Time guide to manufacture a mold" number of columns represent weeks

Stage I Quotation

Elapsed time varies

Stage 2 Design and build

i ~ , 3 , , , ~ , ~ , , , 8 91011 i~ 13 I , i , i~ i , 1819 ~0 ~I ~ ~3 ~ , ~ ~ i ' ' ' l l I I I

I Design product Design Decide on quantity Preliminary mold design

i Decide number of cavities Approve preliminary

I Select molding machine Order steel Set mold specifications ~ Detail design

- o

Screen candidate vendors ~ Review and approve

I Issue quote request g

Review quotes 15 Review mold concepts r

Finalize product drawing c- Place order

Release drawing place order

Mochining Core and cavity Slide and inserts Core pins, etc. Mold base

Benching Detailing Fitting Polishing Assembly

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o

Stage 3: Development

ii~13141~i~j~,819110111,,,,,, I~13

Vendor try-out (first) Mold functioning Major dimensions Corrections

Vendor tryout (additional) Q.C. detailed inspection Corrections: touch-up

IIIII1,111 In-plant try-out(s) Q. C. inspection(s) Touch-up(s) Process standards Q.C. release Engineering release Release for production

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I Key: A top plate B core pin M C male half D guide pin E push-back rods ' ~ 1 (to return ejection system) L

F0u,0e0us. I II G bottom plate H risers or parallels J ejector pin K ejector or knock-out bar L female half M pressure pads

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Tool top force

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Figure 5.81 Cross sectional view of a multi-cavity semi-positive mold used for compression molding

Figure 5.82 Example of mold types (a) positive compression mold, (b) flash compression mold, and (c) semipositive compression mold

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Figure 5,83 Example of vent locations in a compression mold processing reinforced thermoset plastics

An RP system for mold-making for compression, resin transfer molding (RTM), resin injection molding (RIM), thermoforming, and contact molding has been developed by Lenox Polymers, USA, using high- performance polymers with metals and fibers, producing a material which can withstand temperatures of over 300C (570F). It is claimed to reduce automobile tooting costs by at least 50%. A major supplier is reported to have funded a $100,000 program to manufacture automobile door interior panels in plastics, first parts from which have been delivered.

A few considerations are listed to help understand what is needed in designing molds for injection molding. Even though there are many factors to be considered that include its operation, designing the mold is not complex. What is needed is a thorough understanding of the requirements of the literature.

mold backing plate A plate used to support cavity bushings, and similar mold parts.

blocks, guide pins,

mold, balanced A mold is laid out/designed with runner(s) and cavity(s) spaced and sized for uniform melt flow, fill, and packing pressure throughout the system.

mold base Assembly of, usually steel, plates that holds or retains the cavity(s).

mold bottom plate Part of the mold contains the heel radius and push ups (ejection mechanism). It is used to join the lower section of the mold to the platen.

mold cam bar The stationary angled bar or rod used to mechanically operate the slides on a mold for side action core pulls.

mold cavity Also called die or tool. The space between matched molds that encloses the molded part. It is the depression in the mold that


forms the outer surface of the molded part. There can be single or multiple cavities in one mold.

mold cavity chase Enclosure of any shape, used to shrink-fit parts of a mold cavity in place, to prevent spreading or distortion in hobbing. In addition, to enclose an assembly of two or more parts of a split cavity block.

mold cavity, compression The male cavity is designed as a plug that fits into the female cavity so that the mold action during closing provides a hydraulic pressure loading. The tight fitting male plug literally acts as a hydraulic ram.

mold cavity debossed Depressed or indented lettering or designs in the cavity producing bossed impressions on the molded part.

mold cavity draft Also called draft in the direction of the mold. On most molded parts, there are features that must be cut into the surface of the mold perpendicular to the molding parting line. To properly release the part from the tool, parts usually include a taper. The amount of mold draft required will depend on factors such as type plastic being processed, processing conditions, surface finish, etc. As an example, a highly polished surface will require less than an unpolished mold. Any surface texture will increase the draft at least 1 ~ per side for every 0.001in. (0.003 cm) depth of texture. Special mold cavity surface action can be used. With elastomeric material, it is possible with its rubber condition, that ejection does not require the required draft.

mold cavity, duplicate plate Removable plate that retains cavities; used where two-plate operation is necessary for loading inserts.

mold cavity etched To treat the surface with an acid, leaving relief to form the desired design texture on the molded part.

mold cavity ejector Different mechanical means are used to eject or remove the molded part from the cavity.

mold cavity, female The indented half of a mold designed to receive the male half.

mold cavitygrit blasting Steel grit or sand are blown onto the wall cavity to produce a rough surface. This surface treatment may be required to permit air in leaving the mold during molding an d /o r provide a desired surface finish on the part.

mold cavity hobbing Forming single or multiple mold cavities by forcing a hob into relatively soft steel blank. Hobbing is a technique where a master model in hardened steel is used to sink the shape of the cavity into a heated mild steel such as beryllium copper. The hob is


larger than the finished plastic molded part because after hobbing, the metal shrinks during cooling.

mold cavity honing Using a fine grained whetstone or equivalent to obtain precise accuracy to the surface finish.

mold cavity, male Also called plunger. The extended half of a mold designed to match the female half.

mold cavity register Angle faces on the mold that match when the mold halves are closed, to ensure their correct alignment.

mold cavity retainer plate They hold the inserted cavities in a mold. These plates are at the mold parting line and usually contain the guide pins and bushings that line up the two halves of the mold.

mold cavity, split Cavity made in sections.

mold cavity, split-ring A mold in which a split cavity block is assembled in a chase to permit the forming of undercuts in a molded part. The part along with the molded part(s) is ejected from the mold and then separated.

mold cavity surface The surface of the mold cavity that faces and reproduces its surface condition on a molded part. A significant advantage of the molding processes is the fact that surface polish and textures arc molded into the part. No secondary surface-finishing operations are required unless special finishes are required such as plating, hot stamping, etc. High gloss finishes, dull, matte, textured, etc. (as well as their combinations) surfaces on parts are feasible.

mold cavity unit Cavity insert(s) designed for quick interchangeability with other cavity insert(s).

mold cavity venting Basically shallow channel(s) or minute hole(s) in the cavity and/or in the mold parting line to allow air and other gases that may form during processing escape.

mold chase An enclosure of any shape used to: (1) shrink fit of a mold cavity in place, (2) prevent spreading or distortion in hobbing, or (3) enclose an assembly of two or more parts of a split block.

mold design Keep the design as simple as possible (Figure 5.84).

molding cold slug The first plastic melt to enter an injection molding machine cold runner mold; so called because in passing through the spruc orifice it is cooled below the effective molding temperature. Usually a well in the runner system is used to unload this cold slug.

molding cold slug well The space or cut-out in the runner system (such as opposite the sprue travel of the melt in the mold) to trap the cold slug so that it does enter the cavity.


OPENING

PARTING LINE

SIDE VIEW OF PART

. . . . . . . . . . ~ PARTING LINE

LINE

PARTING LINE

Figure 5.84 Examples of simplifying mold construction to produce openings without side

action movements.

molding, dished A term used to describe a depression in a molded surface.

molding, double-shot A method for producing 2-color or 2-different plastics in a part using an IMM with two plasticators. The part molded first becomes an insert for the second shot. Other processes can be used such as injection blow molding and compression molding.

molding dwell Time between when the injection screw ram action is fully forward holding pressure on the plastic in the cavity and the time the ram action retracts.

molding, film insert FIM starts with a cut film that is decorated and /o r labeling, thermoformed to shape, and then insert in the mold.

molding flash line A raised line evident on the surface of a molding and formed at the junction of the mold faces such as at the parting line after the removal of the excess flash. It is usually removed by high- speed buffing or grinding.

molding pressure Pressure maintained on the melt after the cavity is filled until the gate freeze-off allowing the complete transformation to a solid state.


molding pressure pad A metallic reinforcing device designed to absorb pressure on the land areas of the mold when the mold is closed.

molding pressure required It is the unit pressure applied to the molding material in the mold cavity such as during injection molding. Mold material of construction is to support this pressure without the mold cavity moved. The area is calculated from the projected area taken at fight angles under pressure during complete closing of the mold, including areas of runners that solidify. The unit pressure is calculated by dividing the total force applied by this projected area. It is expressed in psi (Pa). To determine pressure required for a specific material the melt pressure used is based on experience and/or from the material supplier. The pressure is multiplied by the projected area. Result is the total clamping pressure required. To ensure proper pressure is applied, consider using a safety factor (SF) of having available another 10% more pressure. With experience, this SF can be reduced or even eliminated.

molding, rotary Also called rotary press. Refers to a type of injection molding, blow molding, compression molding, etc. utilizing a plurality of mold cavities mounted on a rotating platen or table. This process is not to be confused with rotational molding.

molding shrinkage It is the difference in dimensions between a plastic molding and the mold cavity in which it was molded, both being at room temperature when measured, expressed in./ in. (cm/cm). Shrinkage usually occurs in the mold while it is solidifying or curing; however, certain plastics may take up to 24 hours before it has completed its shrinkage. In designing a product and its mold, it is extremely important to make allowance for shrinkage.

mold knife edge Describes a projection from the mold surface that has a narrow included angle. They are undesirable because they are susceptible to breakage under molding pressures.

mold land Describes the area of those faces of a closed mold which come into contact with one another.

mold leader pin and bushing Also called guide pins. Pins, usually four, maintain the proper alignment of the male plug and female cavity as the mold closes. One of the pins is not symmetrical placed so that the mold halves can only be aligned one way eliminating misalign- ment. Hardened steel pins fit closely into hardened steel bushings.

mold life For any mold, the term mold life refers to the number of acceptable parts that can be produced in a particular mold. There are molds that run a few hundreds to many millions. Design and construction that relates to cost of a mold depends on the lifetime required.


mold locating ring Also called register ring. It serves to align the nozzle of an injection cylinder with the entrance of the mold's sprue bushing.

mold locking ring A slotted plate which locks the parts of a mold together while the material is being injected or placed.

mold, loose punch Male part of the mold when it functions in such a way that it remains attached to the molding when the press opens and molding removed. It is commonly used for moldings possessing threads or undercuts, when the punch cannot be removed from the molding merely by opening the press.

mold manifold It is a runner system in a mold that has its own heating and /o r cooling insulated section to control the melt and be ready for injection into the cavity.

mold parting line Also called cutoff or spew. A line established on a 3-D model from which a mold is to be prepared, to indicate where the mold is to be split into two halves (sections) representing where they meet on closing (Figure 5.85).

Figure 5.85 Example of molding with or without parting line on threads


mold pillar support The general construction of a mold base usually incorporates an ejection housing. If the span in the housing is long, the forces during molding can cause a sizable deflection in the plates that are supported by the ejector housing causing flashing, etc. To overcome this problem, pillar supports are included so that deflection does not occur.

mold pin Different pins are used that include dowel pin, ejector pin, leader pin, return pin, side draw pin, and sprue draw pin.

mold, preengineered Standardized mold components have been available at least since 1943. They provide for exceptional quality control on materials used, quick delivery, interchangeability, and lower cost. These available preengineered molds and mold parts provide high quality manufacturing techniques that result in consistent quality and reduced mold cost. The different manufacturers of these preengineered mold bases and components provide similar but also different products. The variations can provide unique and different approaches to meeting complex product designs. A major advantage to the molder is saving time and money should a component ever need replacement. Most often, these components serve the function of the mold and are not designed for use as plastic-forming mold members.

mold pressure pad Reinforcement of hardened steel distributed around the dead/open area in the faces of a mold to help the mold land absorb the final pressure of closing without collapsing.

mold runner A mold manifold runner system involves all the sprues, runners, and gates through which melt flows from the nozzle of an injection molding machine (the pot of a transfer molding machine, etc.) through the mold and into the mold cavity(s). There are primary, secondary, and tertiary (sometimes more) runners to provide melt flow into one or more cavities. Their diameters are based on the melt flow requirements of the plastic being processed that are easy to determine.

mold runner, balanced Exists in a multicavity mold when the runners linear distances of the melt flow from the sprue to the cavity gates is the same.

mold runner, cold (for thermoplastic) Mold in which the melt within the mold [sprue to gate(s)] solidifies by the cooling action of the mold requiring their removal and usually recycled.

mold runner, cold (for thermoset plastic) Mold in which the melt within the mold [sprue to gate(s)] is cooled in the mold maintaining its free melt flowing characteristic so that the next shot starts from the


gate(s) rather than the nozzle. The cavity and core plates are heated to solidify the plastic but the runner system is kept insulated from the cooler manifold section. This action eliminates TS scrap that is similar to a hot runner system for TPs.

mold runner, hot (for thermoplastic) Mold in which the melt within the mold [sprue to gate(s)] are insulated from the chilled cavity(s) and core(s). They remain hot producing no scrap and the next shot starts from the gate(s) rather than the nozzle.

mold runner, hot (for thermoset plastic) Mold in which the melt within the mold [sprue to gate(s)] are hot as in the cavity(s) and core(s); all solidify by the heating action. The solid sprue to gate(s) can be recycled at least as filler.

mold runner, insulated Mold has oversized runner passages formed in a conventional cold runner for certain TPs. The passages in the heated mold runner system are of sufficient diameter that, under conditions of operation, an insulated surface occurs on the plastic melt runner wall with hot melt flowing in the center of the runner(s). The next shot starts from the gate(s).

mold, runnerless Identifies a TP hot runner or a TS cold runner even though runners are used.

mold side action Mold operates at an angle to the normal open-closed action permit t ing the removal of a part that would not clear a cavity or core; may have a pin to core a hole that has to be withdrawn prior to opening the mold.

mold, single impression A mold with only one cavity.

mold, split-ring Also called split mold. Mold in which a split-cavity block is assembled in a chase to permit forming of undercuts in a molded part. These parts are ejected from the mold and then separated from the part.

mold sprue Also called stalk. Feed opening in a mold that is directing melt into the mold from an I M M nozzle.

mold sprue bushing A part of the mold which provides an interface between the injection molding machine nozzle and runner system in the mold.

mold spewgroove The groove in a mold that permits the escape of excess or surplus plastics.

mold, stack Also called three-plate mold. Rather than the usual two- plate to handle a single mold, there is a third or intermediate movable plate. It makes possible center or offset gating of each cavities on two levels. Thus, it is a two level mold or two sets of


cavities stacked one on top of the other for molding more parts per cycle. These molds generally use a hot runner manifold located in the center plate (platen). There are also four-stack molds in use.

mold standard and practice The SPI continually updates its publication on designing plastic molded parts entitled Standards and Practices of Plastics Molders. It is useful to designers, purchasing agents, custom molders, processors, etc. Details presented include engineering and technical guidelines commonly used by molders for injection, compression, and transfer molding processes; lists tolerance specifications for plastic materials in metric and English units; and a glossary of terms. It reviews important commercial and adminis- trative practices for purchasers to consider when specifying and purchasing molded parts. These customs of the trade include mold type, safety considerations, maintenance requirements, contract obligations, charges and costs, inspection limitations, storage, dis- posals, proper packing and shipping, and claims for defects.

mold stripper-plate Plate that strips a molded part(s) from a cavity with or without air support.

mold undercut Reverse or negative draft such as a protuberance or indentation in a mold molding a rigid plastic, necessitating inserts or a split mold for removal of the part; if a flexible mold can be used, it will provide for the rigid part ejection. Molding a flexible plastic with a slight undercut usually can be ejected intact.

mold venting, water transfer This technique is based on negative pressure coolant technology. Mold coolant is being pulled; not the more conventional way of pushing under water pressure but having a negative pressure. This system permits venting into the water via the mold knockout pins, difficult locations in a cavity (such as long, thin cores) that entraps air during molding, etc. They require that the pin or cavity (through a porous metal media) run through the water line. Coolant does not leak into the cavity because it is under atmospheric pressure. In an emergency, it could eliminate water leak in a cracked mold that extends into the water line.

mold water channel Channels, through which water circulates to cold the melt in the cavity, are designed to properly extract heat.

mold yoke In a large single cavity mold, the entire cavity and core plates usually form the mold cavity. In a smaller and multicavity mold, core and cavity blocks (inserts) are mounted on or in the various plates of the mold base. When various components are mounted in the plates, the plates are called a yoke.


Mold Temperature Controls Correct control of mold temperature is fundamental to producing good quality moldings and achieving economic molding cycles. Reinforced compounds have a faster rate of cooling than unreinforced and the mold must allow for adequate cooling to take advantage of this facility. Poor cooling results in rising mold temperatures and longer cycle times, but inadequate heating can produce voids, shorts, and poor surface finish. Cooling and heating channels should also be located directly in mold inserts and cores, if the design permits it.

Good temperature control over all areas of the tool is recommended and correctly placed cartridge heaters used in conjunction with thermocouples and efficient controllers give an ideal system. Mold heating can be by any of the conventional methods, but direct conduction from platens is probably the easiest and most versatile. Conventional methods of production of heat are by oil or electricity.

It is standard operating procedure to heat the female tool about 10C higher than the male half, as an added precaution against tool lock-up. This also facilitates removal of the part from the tool.

Hardening~Platings In all cases, tools (especially the cavity areas that form the molded product) have to withstand high working temperatures without distortion and resist the abrasive characteristics of the molding compound (and general abuse in handling), while giving an expected life of up to several hundred thousand moldings. For strength, toughness and hard-wearing surface, tool cavities should be made from a good grade of tool steel. The exact composition depends on several factors, such as tool size, tool life expectancy, and surface finish requirements.

For steel when correctly hardened (typically at 800C) and tempered at 200C, a hardness of Rockwell C52-58 would be expected. Where additional skin hardness is needed (as on sliding surfaces of the tool) it is possible to harden by flame or vacuum treatment, but any subsequent grinding (which may be carried on final assembly of the tool) will easily remove the surface and expose the softer substrate, which will be very prone to wear.

It is normal toolroom practice to polish the tool cavities and, when samples have been accepted by the customer, to chromium plate all cavity and flash surfaces of the tool. Chromium plating, which normally has a thickness of 0.005 ram, is recommended because:

�9 high gloss is imparted to the molded part


�9 ejection and extraction of the molding is facilitated, especially when molding deep-drawn parts

�9 core pins and thread formers (which are subject to considerable wear) can be replated as soon as any appreciable wear is detected.

Ion implantation can also be used as an alternative to (or in addition to) chromium plating, to increase tool life.

For sheet molding compound (SMC) auto body parts, surface finish is critical. A quality of 1200 SPI (international polishing standard) is demanded, with no undulations, orange peel, punctures, stains, or holes. Optical quality control is implemented using light beams. Rough and semi-finished machining is done with three-dimensional milling, finishing by spark erosion and chrome plating. Thermoregulation is critical: no temperature difference greater than 5C is acceptable on any of the molding surfaces. Steel molds are usually employed for high volume applications. RPs can be used for medium/low volume.

Standard Mold Parts By rationalizing design and production requirements, it is possible to make use of the very wide range of ready-made standard mold components on the market. These include standard die sets, guide pins, bushes, ejector pins, cartridge heaters, and runner systems, so that in many cases it may be possible to build up the entire tool from standard components, with only the cavity/cavities produced to order. This simplifies and speeds up tool production, while keeping costs to a minimum; also ensures use of the best steels, etc. With multi-cavity tools, it may also be possible to advance the finishing of only one cavity, for proving the tool and production of sample moldings, before incurring the cost of the other cavities.

There has been some development work (in Japan) on using or blanking off individual cavities to adjust to varying production requirements, and combining different but balanced cavities in the same tool, for completely flexible production.

Clearances TS polyester molding compounds require very close pinch-off clearances, so it is possible to develop high hydrostatic pressures (which is desirable in all moldings). The locating or guide strips should be plentiful and substantial, or the tool halves may become misaligned. The material must be under compression up to the final stop, so nip lines must be long and with very small clearance. Shear edges should be avoided and tools designed with a very thin vertical flash. Where the design cannot accommodate this type of flash, semi-horizontal pinch- off must be used.


Electroformed Molds Electroformed molds are produced by a process derived from standard electroplating. The metal (usually nickel) is first dissolved and then re- assembled electrolytically around a model. By this means a very dense non-porous metal shell is formed, exactly conforming to the three- dimensional contours of the model and, at the same time, able to reproduce fine surface detail, such as textures or engraving. After forming, the shell (which may be up to 10 mm thick) is removed from the model and engineered into a finished mold by various methods, according to the molding process in which it will be used.

Electroformed molds are subject to interaction of temperature and pressure they can be used in the following cases:

�9 any process where combined working temperatures and pressures do not exceed 6 bars and 100C (212F)

�9 where no cavity pressure is involved, in any process involving temperatures up to 300C (570F)

�9 at ambient temperature, in any process with pressure up to 60 bars.

Other advantages include:

�9 large size: a typical maximum is 12.2 m x 1.8 m x 3.05 m

�9 significantly lower cost, compared with machined tools

�9 surface qualities: satin, polished, textured, leather grained, without large additional cost;

�9 non-porous hard surface: 200--400 VPN, on request

�9 good thermal conductivity: 0.22 cal/cm s/C, giving rapid warm-up and fast curing.

For RPs, electroformed molds are mainly useful for autoclave molding and for closed matched die molding (cold press, low-pressure hot press, resin injection, resin transfer, low-pressure phenolic molding, and resin casting) as well as for reinforced reaction injection molding (RRIM). Prototyping can also use this technique for mold-making.

Mold~Platen Insulations Since injection molding and compression molding machines operates with heated molds, efficient thermal insulation on the mold and platen can make an important contribution to reduction of operating costs, by improved energy efficiency and temperature control. A number of materials are in use that includes specially engineered glass fiber- reinforced TS polyester RP, which gives a good balance of high resistance to heat, high compressive strength, and low thermal conductivity, with


low moisture absorption, resistance to oils and other fluids, durability and good machinability.

With possibly a considerable investment in the cost of a mold, it is important to maintain it properly. This means (obviously) storage where it will not be exposed to extremes of temperature or humidity, or to physical damage. Wear and tear on the tool during its lifetime will normally be all-too visible in terms of moldings with poorer finish or reduced tolerances. Not so visible is the wear on components such as guide pins. At regular intervals during the lifetime of the tool, it is particularly important to check guide pins for wear and platens for parallelism, as accurate location of the male half into the female half is clearly of paramount importance.

Heat Transfer Fluids Many of the molding processes for RPs employ heat transfer fluids to control the heating of molds. When a continuous supply of fresh air comes into intimate contact with the heat transfer fluid (as when there is constant tool changeover), significant oxidation occurs. Chemically, the result is that some molecules in the fluid are converted to organic acids. In practical terms, the fluid becomes thicker, darker, and more odorous, while heat transfer capability drops dramatically.

There are heat transfer fluids (such as Paratherm OR) that resist oxidation and also offer increased thermal efficiency, significantly higher flash and fire points and longer service life. Precise uniform temperature control to 316C (600F) is provided, in dosed-loop systems where the heat transfer fluids are more than occasionally exposed to air. Typical performance characteristics are:

�9 optimum use range: 49-316C (150-600F)

�9 maximum recommended film temperature: 338C (650F)

�9 flash point (coc: ASTM D 92): 190C (370F)

�9 fire point (coc: ASTM D 92): 210C (410F)

�9 auto ignition (ASTM D 2155): 373C (710F).

Mold Cost/Maintenance Molds in general are very expensive with the major cost principally in machine building labor. The proper choice of materials of construction for the cavity, core, and other components is paramount to quality, performance, and longevity (number of parts to be processed) of a mold. Add good machinability of component metal parts, material that will accept the desired finish (polished, textured, etc.), ability to transfer heat rapidly and evenly, capability of sustained production without constant maintenance, etc. Using low cost material to meet high


performance requirements will compromise mold integrity. As an example, the cost of the cavity and core materials, for more than 90% of the molds, is less than 5% of the total mold cost. Thus, it does not make sense to compromise mold integrity to save a few dollars; use the best material for the application.

Molds require very careful handling when in use and storage. Any protruding parts should be protected against damage in transfer. The mold surfaces, especially cavities and cores, should be covered with a protective, easy to remove, coating against surface corrosion when the mold is not operating. For special protection, vacuum containers are used after the mold is properly dried. Records should be kept to ensure required maintenance is accomplished on a regular time schedule.

Mold Design for RRIM

Edge gating is recommended for molds for RIM, except with structural RP, which should be center gated to prevent the glass mat from moving (Figure 5.86). The mixture should enter the mold cavity as a laminar stream. Air entrapment, which happens with incorrect mold-filling, is the largest single cause of defective moldings. For RRIM materials, where back pressure is most likely, the most simple and cheapest type of gating is direct fill, center-gated. Flow lengths are minimized and there is uniform flow in all directions. The mixing head attaches directly to the mold wall, creating an airtight seal and minimizing leakage. A self- cleaning mixing head mounted flush in the cavity wall also minimizes waste material from the gating (Figure 5.87).

Disadvantages of direct fill are the possibility of causing blemishes opposite the entry point, because the material makes a 90 ~ turn over a sharp edge (which can cause bubbles or scarring). The mixture will be free of bubbles only if the wall thickness at the entry point is less than one-eighth of the diameter of the entry area. If the mixing head cannot be flush-mounted, the sprue should be as short as possible, to allow mold release to be sprayed into the sprue cavity.

Short fiber-reinforced systems are particularly vulnerable to formation of weld lines because the fibers tend to align with the direction of the flow. When flow fronts join, there is no fiber crossover or homogeneity, and the problem is intensified with faster reaction and gelling times. To minimize this, consider locating the gate closest to the largest obstruction to the flow. Computerized moldflow analysis can help predict where problems may occur.

In structural molding, the glass mat should fill the entire mold cavity leaving no empty areas between mat and mold wall. The mat therefore

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Figure 5.86 Examples of RIM mold configuration (courtesy of Bayer)


Figure 5.87 Gating and runner systems demonstrating melt flow in RIM molds (courtesy of Bayer)

should be slightly oversized and the mold should have a (renewable) steel shear edge to cut away any glass fiber overhang, a pocket to hold the excess and a mold seal external to the shear edge. Glass fiber can erode softer metal, so it is advisable to use steel to make molds for structural RIM SRIM). Typical molding pressures are 100 psi for rigid solid RIM systems and 200 psi for SRIM.

Asse m b ly/Joi n i ng/Fi n ish i ng

Different methods arc used for assembling, joining, or finishing RP to RP products as well as RP to other materials. It is important to both


designer and end-user that the techniques, advantages, and limitations of these methods be understood so that intelligent choices can be made. As an example, different materials that include RP-to-RP and RP-to- metal could have different thermal expansions and could cause failure of the assembly. Parts to be assembled for RTP include for high volume production solvent bonding, adhesive bonding, ultrasonic welding, hot tool welding, electromagnetic and induction bonding, and dielectric heat welding; RTP include for low volume production gas welding, adhesive bonding, ultrasonic tool welding, hot tool welding, and spin welding. RTSs include for high volume production molded-in inserts, mechanical fasteners, adhesive bonds, and electromagnetic and induction heating of adhesives; RTSs include for low volume production adhesive bonding and mechanical fastening. To complete the secondary finishing work on certain parts after they arc fabricated such as decorating, deflashing, buffing, tapping, degating, machining, etc.

RP moldings can be finished with all the techniques familiar in other branches of plastics. Due to the reinforcement content and the high flow of the resin, TS moldings often require trimming after they come off the mold. Depending on the process and materials, this can be achieved simply with a sharp knife, rasp, and abrasive paper or it may call for mechanized systems (Figure 5.88). Sensible investment in tool design and manufacture, and good maintenance should (ideally) remove the need to trim compression and injection molded parts. However, this is not always the case, and flash removal systems can be employed in which the moldings are tumbled with mildly abrasive materials such as nutshells. Reground TS material may itself be used effectively in such systems for finishing/buffing moldings.

After curing and removal from the mold, TS RPs continues to mature and will benefit from being kept for about two weeks at normal ambient temperature. This maturing process can be speeded up by a period of post-cure at a higher temperature. Three hours at 80C (176F) is ideal, but this may not be practicable for larger moldings, when a lower temperature over a longer time will be suitable, such as overnight at about 40-60C (104-140F).

For best results, the molding should then be allowed to stabilize at room temperature for a day or two after post-curing and, to prevent warping during this period, it may be a good policy to place a large molding in a simple jig while it fully cures. When working with TS resins, it is essential that the resin be fully cured before any finishing operations are carried out.

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Large and complex TP moldings should be rested on jigs for a time after they come out of the press. It is good practice to think in terms of robot aided demolding for such parts, for safety, speed, and quality. This may also permit moldings to be demolded before they have cooled (so shortening the molding cycle).

Certain TPs such as nylon moldings may require annealing after molding, to achieve their full mechanical strength. Consult the resin supplier for detailed advice.

Practically any TS RP molding has to be trimmed, machined (Table 5.21), and finished in some way, before it is finally put into service. Most of these operations are standard common sense and should not present difficulty to the fabricator, but some points differ from normal workshop practice.

Considerable time can be saved if the RP can be trimmed while the resin is still in a green stage; when it has not yet fully hardened and is still in the mold. This can be done with a sharp trimming knife held at right angles to the RP, or with scissors, but taking great care not to disturb or distort the lay-up. It is often possible to design the edge of the mold to serve also as a trimming guide, suitably reinforcing it for this purpose. After trimming, the molding can be left in the mold to develop its full cure and, when fully cured, the trimmed edge can be finally finished with a fine file, glass-paper or wet and dry abrasive paper.

TP moldings should not require trimming but, because of the fiber content in the molding compound, it may have been necessary to design the mold with a relatively large gate, so that moldings must be demolded with sprues and runners still attached. These can easily be removed with a sharp knife and should be placed in a separate bin, for granulating and feeding back into the machine hopper. The mold can be designed to remove gates during the molding cycle.

Stamped glass mat thermoplastic (GMT) parts will also require trimming, which (due to volume of production) can usually be automated by robot or similar system.

Saws, hand, jig-saw, or circular saws may be needed for trimming thicker moldings, both in TSs and TPs, where the trim is more than 1 mm thick. Precautions must be taken to protect workers against dust, with adequate extraction systems and wearing of masks.

Robot or robot-assisted systems are being increasingly introduced into RP molding, at all levels of production, assembly and finishing. Where some processors prefer to specify such equipment separately, and adapt it themselves to their own production needs. Primary machinery

Table 5.21 Guide to machining

Sawing

Speed Blade (ft/minute) (teeth/in.)

Turning

Cutting Speed

(ft/minute) Feed

(in./rev)

Front Clearance Top

(o) Rake

Drilling

Speed Speed Speed 1/8 in. diem. 1/2 in. diem. 1 in. diem.

(rev/minute) (rev/minute) (rev/minute)

Routing Speed

(rev/minute)

Milling Speed

(ft/minute)

Phenolic laminates 1500 to 2 to 11 (skip

7,500 tooth)

Molded thermosets 2,000 to 8 to 20

6,000

Glass filled laminates 300 to 3 to 10

400

Polyethylene, PTFE 2,000 to 4 to 9 (skip

6,000 tooth)

Acetal, Polypropylene, 3,000 to 4 to 9 (skip

Nylon 6,000 tooth)

Rigid PVC 4,000 to 4 to 11

11,000

Acrylics 5,000 to 4 to 12

10,000

Polystyrene 2,500 to 10 to 24

400 to

600

250 to

800

300to

500

300to

500

500 to

1,000

300to

1,000

100 to

500

300to

0.005 to 12 to 15 Oto30

0.010

0.005 to 15 0 to 15

0.010

0.004 to 0 to 15 15 to -5

0.008

0.004 to 18 to 30 Oto-5

0.010

0.004 to 18 to 30 Oto-10

0.010

0.008 to 15 to 22 Oto-5

0.024

0.005 to 15 to 20 Oto-5

0.010

0.002 to 15 0

8,000 2,000 700

8,000 1,500 600

1,000 500 300

1,000 5OO 300

1,200 800 500

5,000 1,000 500

6,000 1,000 600

1,500 700

4,000 1,000 0.008

18,000 to

24,000

18,000to

24,000

18,00 to

26,000

15,000 to

24,000

15,000 to

24,000

15,000 to

24,000

15,000 to

24,000

300

24,000

4,000 to

6,000

4,000 to

6,000

150 to

40O

800 to

1,500

800 to

1,500

600 to

900

700 to

1,200

12,000 to

1,000

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o

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manufacturers integrate robots complete production system.

into their machinery, offering a

Robotic systems for loading and demolding presses are in widespread use, to optimize production efficiency. Machines are available either from the molding press manufacturers, or direct from the equipment manufacturers (for processors who prefer to assemble their own production systems). Essentially robot systems operate on three or more axes, using electrical, servo, or pneumatic drives, for fast positive action and need to be specified according to the load to be carried and distance to be traveled. Systems use robot control with learning ability, by which the device is programmed by 'walking it through' the sequence of operations. Control of the device can usually be integrated with the control system of the press.

Design of the gripper system (by which the robot device holds the molding) depends on the configuration of the molding itself (and it may be a good idea to incorporate in the product design features to aid robot handling). In many cases, the gripper arrangement can be customized in-plant by the processor.

Fully cured RP are not easy materials to cut or machine, since glass fiber tends to blunt most ordinary steel tools. An additional complication is the size of many RPs, making it difficult to take the work to the machine. Portable hand tools are often used for this work, and portable reciprocating electric saws have proved themselves for trimming and shaping, especially where high-grade blades are used. Where possible, abrasive discs or wheels are recommended for cutting. Carbide-tipped steel tools should be employed for all other machining operations. Available have been the proper tools for trimming, shaping, drilling, cutting, etc. Workers operating such equipment should be provided with suitable respiratory protection, as recommended by manufacturers.

RPs are not homogeneous, and can be vulnerable to delamination. The work should, therefore, be firmly supported or clamped as close as possible to the cutting line. Where a large run of moldings is to be produced, it will save time and effort if cutting/trimming jigs are also produced. Water is not recommended as a lubricant or coolant, except where there are facilities for thoroughly drying the laminate afterwards. Overheating must be avoided. Waterjet cutting systems, which can readily be robotized, are used for cutting and trimming parts such as panels, on a production line basis.

Holes of up to 10 mm (0.4 in) diameter can be drilled with carbide- tipped twist drills. Above this size, cutters are recommended rather than drills. Cracking can be avoided by using as little pressure as possible and


by starting the drilling on the good (gelcoat) side. When drilling, they should be backed with hardwood to prevent the drill breaking through and producing ragged edges.

Where the production justifies it, investment in CNC-controlled routing machines makes it possible to machine very large moldings and reduces the time required to deftash and finish a typical component (such as going from 40 min to less than seven). The machines have an in-built teach-and-learn capability.

Two basic types of abrasive paper are normally used for fairing and finishing. The more coarse type of sanding uses aluminum oxide production paper, supplied on the roll and cut to length for use with boards or mechanical sanders. Wet or dry silicon carbide paper, which can be used wet (with water as lubricant) or dry, is always used for final finishing of coatings. The paper is usually cut and used with a rubber hand-held sanding pad. Table 5.22 is a guide to recommended grades of abrasive paper for use in different stages of fairing (note that the lower the grade number, the larger the grit particle size and the lower the density of the cutting particles).

Table 5.22 Grades and types of abrasive paper for fairing and finishing

Surface Type of sanding Production Wet or dry paper grade grade

Expoxy filler or wood

Polyester gelcoat High build epoxy surfacer/undercoat Polyurethane undercoats Polyurethane finish coats

General rough 40-60 Not used fairing 80-100 Rarely used Fine fairing 120-180 180-220 Finishing (wood only) Surface preparation 120-180 180 Fine surface fairing 80-120 120-180

Very fine surface fairing 180 180-220 Keying for finish coats Not used 280-320 Very fine surface fairing Not used 280-320 and keying

Joining, Fastening

Mechanical joints to assemble RP moldings to each other or to other components usually employ metal inserts or fixtures (also used are RP and URP types), which are molded into the product. With open / contact molding, this can readily be organized within the scope of the molding cycle. For press molding of both TSs and TPs, it can often be a


time-consuming and costly operation (as, for example, in molding electrical or electronics components, where many fine contacts must be inserted into a molding tool). For insert work, a vertical configuration press is usually employed, and it is increasingly possible (and cost effective) to robotize this operation. Where inserts must be loaded by hand, however, it is important to pay close attention to safety precautions for working with an open mold and working with hot metals.

In some cases with TPs it is possible to insert metal fittings, such as bushes, into the molded component, using friction or ultrasonic methods of insertion, and fusing the TP around the metal insert. With RTP compounds, this may create undesirable stresses, however, and advice should be sought from the material supplier.

Other useful techniques with TPs include hot staking, where the plastic is deformed around a metal part (such as a plate), to secure it in place. Methods of applying range from electrically heated irons to hot air systems, and will depend on the degree of complexity of the part, the number of hot stakes to produce and the required speed of production.

TP components can also be welded for assembly, using hot-plate or ultrasonic systems, or by spinning two matching circular parts to weld the surfaces by friction. The technique employed will, largely, be dictated by the type of TP involved.

Information, as a guide, follows where a welding process is followed by (a) equipment cost, (b) tooling cost, (c) typical output rates, (d) normal economic production quantities, and (e) general remarks:

Hot-gay (a) very high, (b) low (holding fixture only), (c) 0.3 to 1.5 m (12 to 60 in) of weld seam per minute, (d) very low, and (e) manual operation.

Hot-plate: (a) moderately low to high, (b) moderate to high, (c) about 120 parts/h/fixture cavity, (d) medium to high, and (e) setup time 1 h or less.

Induction: (a) low to moderate, (b) low, (c) about 900 parts/h, manually loaded, (d) high, and (e) setup time 1 h or less.

Spin: (a) moderate, (b) moderate, (c) about 640 parts/h, manually loaded, (d) high, and (e) setup time 1/2 h, mechanization possible.

Ultrasonic: (a) moderately low to high, (b) moderate to high, (c) about 1000 parts/h, manually loaded, (d) high, and (e) automatic operation desired.

Vibration: (a) moderate, (b) moderate, (c) about 240 parts/h from single cavity, manually loaded, (d) medium to high, and (e) setup time 10 min., multiple cavities and mechanized loading possible.


In this, and in other aspects of secondary assembly and finishing, it is well worthwhile to take into account the requirements for handling and manipulating in the original design of the parts.

Adhesive Bonding

An adhesive is a substance made principally from TP and TS plastics (also vegetable, animal by-products, silicates, etc.) which applied, as an intermediate is capable of holding material together by surface attachment. Mechanism of adhesion (adherence) is the phenomenon in which interfacial forces hold surfaces together. Adhesion may be by molecular attraction, mechanical, electrostatic, or solvent depending upon whether it results from interlocking action, from the attraction of electrical charges, from valence forces, or solvent action, respectively.

Advances in the use of TP and TS plastic adhesives have made possible the adhesive bonding of RP structural and nonstructural parts in appliances, automobile, aircraft, medical devices, and so on. Adhesives with strengths higher than some metals are used (epoxy, etc.). The wealth of adhesive technologies that are available could make adhesive selection a task if one does use the proper approach such as determining specifically what performance requirements are needed (as with any selection procedure). The best adhesive for an application MI1 depend on processing considerations and meeting the performance requirements. Tables 5.23 and 5.24 provide information on types and use of adhesives.

Table 5.23 General comparison of adhesives used with plastics and coatings

Bond properties Standard Two-part Methcrylates Sil icones Cyano- epoxies polyurethanes a [Plexus} acrylates

Shear strength Poor-fair Good-excellent Excellent

Peel strength and Poor-fair Good-excellent Excellent flexibility

Impact resistance Poor-fair Good-excellent Excellent

Poor-fair Excellent

Good- Fair-good excellent

Good- Poor-fair excellent

Temperature Fair-good Fair-good Excellent Good- Fair-good resistance excellent

Moisture resistance Poor-fair Good-excellent Excellent

Gap filling Fair-good Good-excellent Excellent

Cure speed Fair-good Poor-fair Excellent

Fair-good

Excellent Poor-fair

Poor-fair Excellent

aUsually requires a primer. Source: ITW Plexus


Table 5 .24 Basic types of adhesives used with reinforced plastics

Adhesive Curing temp Adhesion Resistance Gap system to water filling

Main uses

Urea- Room temp formaldehyde

Fair Poor Fair

Resorcinol Room temp Good Good Fair

Phenolic Usually Good Good Fair elevated temp

Polyester Room temp Fair: will Fair/good Excellent bond well only to part-cured polyester

Epoxy Room temp Excellent Excel lent Excellent with most substrates

Used mainly for interior grades of plywood

Used for large laminated structures where high clamping pressures can be achieved

Used for marine-grade plywood; similar to resorcinol {has good heat and fire performance]

Used for laminating with fiber (FRP) and for bonding 'green' FRP moldings

Used for high- performance lamination and for bonding to wood and aluminum

Two-part methacrylate adhesives offer good structural bonding properties with TS laminates and composites, including SMC/BMC, phenolic and flame retardant resins, vinyl esters, and epoxies (Table 5.25). A typical range can be mixed and dispensed using hand-held cartridge dispensers, or with automated meter mix equipment. There are adhesives having good gap-filling characteristics and thixotropic formulations that can be used on vertical or inverted surfaces without sagging, curing on their own at room temperature.

Robotized dispensing of a high-quality bead of two-component structural adhesives (epoxy, polyurethane, acrylic) is possible with servo-powered equipment. Positive displacement pumps control the mix ratio and flow of low to high viscosity adhesives and sealants.

There are solvent adhesive, also called solvent fusion. They provide a method of joining two TP types by application of a solvent to soften the part surfaces. Types have to be those that solvent will attack.


Table 5 .25 Two-part methacrylate adhesives designed for structural bonding

Type Description Color

Times (rain) at 22~

Working Fixture

MA300 All purpose adhesive Cream

MA310 Difficult to bond plastics Cream

MA320 General purpose Off-white

MA3940 Excellent low temperature Off-white

MA3940LH Faster setting version of MA 3940 Blue

A0420 All purpose adhesive 0ff-white

AO420FS All purpose adhesive - very fast curing Blue

MA425 All purpose adhesive Blue MA550 White, UV stable White

MA920 Low odor Blue

MA922 Low odor Blue

MA925 Low odor Blue

4-6 12-15

15-18 3O-35

8-12 25-30

12-15 25-3O

4-5 8-10

4-6 15-18

1-2 3-4

30-35 80-90 40-45 70-75

4-6 15-18

17-24 35-40

30-35 80-90

Source: ITW Plexus.

Softening the plastic increases the movement of the plastic chains, allowing them to intermingle at the joint interface. Adhesion occurs after solvent evaporation. Solvent application must be carefully controlled for optimal joint strength and to avoid damage to the part. With time the solvent can penetrate the plastic with damage occurring immediately or latter when in service. Solvent solutions that attack TPs are also used to determine the amount of undesirable "frozen stress" existing in parts.

There are different solventless adhesive systems. An example from Liquid Control LTD., UK is their successful Liquid Control Compact Twinflow | meter, mix, and dispensing machine. This compact, variable ratio meter, mix, and dispense machine consistently delivers a supply of two part adhesives. This machine processing, as an example, polyurethane laminating adhesive to their laminator is capable of supporting line speeds in excess of 1000 f t /min on laminating webs of up to 50 in. wide. The Compact Twinflow is gravity fed from a four-drum rack eliminating the need for on board reservoirs and the transfer pumps required to keep them replenished. Level sensors between the top and bottom drums provide an "empty" signal when it is time to change to a fresh drum. In the meantime, the supply of material is uninterrupted since the bottom drum maintains a supply of material during the changeover.

Additional confidence in the quality of product is delivered through the EnGarde ratio monitoring and flow rate system provided as an


accessory by Liquid Control. In line flow meters constantly monitor the fluid streams and report not only ratio but provide material consumption information for each production run. Phasing lights enable the operator to detect and adjust the equipment for any lead/lag conditions that may occur due to variations in flow characteristics of the two materials.

Joints and Adhesives

The basic types of joint can be used with either bonded or mechanical fastening, but there are special requirements when using mechanical fasteners with tapered thickness plates. The simplest design (unsupported single lap) is the weakest; the most complex (stepped-lap joint) is the strongest (Figures 5.89 and 5.90).

�9 . ~ _ . - 1 , , , r . . . . .

Double i . . . . . . . . , ]

i

Unsupported single lap joint

, _ ' i ~ l i , , i 3 | , , , l

Single strap or butt joint . . . .

,, , I

Tapered single lap joint , ,

, , I , , ,

l : ' i J " , ' , J

Double lap joint

~ - ' ' , , ' , ' i

I r . .. u '~ . . . . . i J I

Double strap joint

. , u . . . . . . ~'~

T a p e r e d strap joint , , , , ,

Stepped lap joint

1_.. ' ,,,, / Scarf joint

Figure 5 . 8 9 Basic types of joint

The purpose of a joint is to transfer load, which will create differential stress between the components as well as in the joining medium, whether fasteners or adhesive. Figure 5.91 has diagrams that illustrate the basic mechanisms that identify (a) single cover butt joint, (b) adhesive in shear (c) fastener in shear, and (d) bending of plates at ends of a joint.

In many RPs, the most convenient joint will be a bonded type (possibly produced during the lay-up process), and the geometry is therefore very influential. The weakest joints are those where failure is limited by inter-laminar failure of the adherend, or peel of the adhesive. Next strongest are those where the load is limited by the shear strength of the adhesive. The strongest designs will fail outside the joint area, at a load equivalent to the strength of the adherend.

Selection of adhesives depends on many requirements, particularly the nature of the materials to be bonded, the function of the bond,


Good

I I I I

Bad

I 1

I

I____ I

Figure 5.90 Examples of good and bad joint configurations

production conditions, and the expected performance of the finished product.

Consolidations

Onc-piccc RP components replace multiple parts previously made both from RPs and from metals or other materials including unreinforced plastics. The motivations for the design and the challenges for fabricating


(a)

(b)

y

B x,,,,~ , c [ ,r /x, , A i , ., I I----~

I I I y x y

,! ..... '5'.,'.~.i..,:'., ~ ............................ �9 ....... J 4 - - ~ ~- ...... " . . . . . . . . ' " ] I , ~ - ' - ~

(c)

(d)

, , < 7 " . . . . . . , .

x x

x

Figure 5.91 Basic mechan i sms o f l oad - t r ans fe r in j o in t s

can differ as much as do the end products. A common incentive for part consolidation is cost. RPs frequently earn their way onto a project not just for their properties, but because reduction of part count makes it possible to mold and assemble what would otherwise be a much more expensive multi-part structure at or near the cost expected for parts made with competing materials. Yet a great deal of design and engineering innovation may be required before those benefits can be reaped.

Painting, Surface Finishing

Where the molding is to be painted, it is a good precaution to avoid using waxes or silicone release agents in the first place, or use these only as a primary release agent.

Primers with particularly good adhesion to RP surfaces are offered by several paint manufacturers and, for a durable surface, it is advisable to use them. They can be applied without previous abrading, if the surface is clean and dry. When normal paint primers are used, it is advisable, however, to rub down the surface with a fine abrasive, to obtain good keying.

Most paint systems can be used on RPs. For stoving finishes, the molding should be post-cured at 80C before the finish is applied: air- drying finishes can usually be applied without post-curing. Most cellulose finishes are also suitable, but with these, it is particularly important to ensure that the resin has fully cured, to prevent the solvents attacking any uncured resin.

5 �9 F a b r i c a t i n g Processes 4 6 7

In the early 1980s, the automotive industry in the USA began using in- mold coating (IMC) technology for SMC bodywork panels, using primers to eliminate pinholes, improve the surface finish and improve the grit resistance of the paint range (Table 5.26). The IMC process applies a coating of primer lacquer of about 0.1 mm thickness to the relatively rough surface of a molding during pressing. Taking advantage of the need to open the press slightly during curing to allow venting, a special lacquer is injected at this point and the mold is re-closed with electronic/hydraulic control of the parallel motion.

Table 5 . 2 6 Guide for coatings vs. properties

E

E E 0

~ ~ ~ . ~ ~ ~ ~ ~ ~ ~ ~ ~ ' ~ ' ~ ' ~

Hardness

Adhesion

Flexibility

Mar resistance

Gloss (85 units

plus 60 ~

2 1 2 2 2 3 1 2 2 2 2 2 2 2 2 2 1

2 1 2 1 1 2 1 1 1 2 2 2 2 2 2 1 1

3 2 2 1 1 1 2 2 2 3 3 2 1 1 1 1 1

2 2 2 2 3 3 1 2 1 2 2 2 3 3 2 2

1 1 2 2 3 4 1 2 1 1 1 1 4 4 4 4 2

Fabricability after aging 4 1 2 1 1 1 3 2 2 3 3 2 1 1 1 1 1

Humidity resistance 2 1 1 1 1 1 1 1 1 2 2 1 1 1 1 1 1

Grease and oil 2 2 2 1 1 1 1 2 2 2 2 2 1 1 1 1 1

resistance

General chemical 3 2 3 2 1 1 1 2 2 2 2 2 1 1 1 1 1

resistance

General corrosion 2 2 2 2 1 1 1 2 2 2 2 2 1 1 1 1 1

resistance (industrial

atmospheres)

Exterior durabil ity 2 2 3 2 2 2 4 4 2 1 1 1 1 1 1 2 1

(pigment)

Exterior durabil ity 3 3 3 3 3 3 4 4 2 2 2 2 1 1 1 4 1

(clear films)

Ratings" 1 = excellent, 2 = good, 3 = fair, 4 = poor


While the process was found to offer significant advantages, such as manufacturing versatility, fast color-change, and use of near-zero volatile organic compounds (VOC), there were also some drawbacks. These are particularly entrapment of air, need for a sophisticated system to adjust parallelism on the mold halves, difficulty of coating vertical surfaces, and increased cycle time and limitation of possible locations for the ejector system.

In 1985, a USA paint manufacturer introduced a high-pressure injection molding IMC process (HPIP). It was based on injection of the coating into a closed mold at 300--400 bars pressure, before the SMC was fully polymerized. At this stage, the compound is still compressible, allowing the coating to flow between it and the mold. The technology overcomes the objections to conventional IMC and is used today worldwide (Table 5.27).

Table 5.27 Typical physical properties of a decorative in-mold topcoat

Property Test Specification Almond White

Gloss 600 ASTM D523

Pencil hardness ASTM D3363

Barcol hardness ASTM D2583

Color fastness ANSi Z124.1 dE 1.0

200 h Xenon arc

Stain resistance ANSI Z124.1 50

Wear/cleanability ANSI Z124.1 First cleaning 50/0

10,000 cycles 2nd cleaning 20/0

Chemical resistance ANSI Z124.1 repairable

Water resistance: ANSI Z124.1 dE = 1.5

100 h @ 150oF no blister

100 h @ 212~ dE = 1.5

no blister

Point impact 20 in ANSI Z124.1 No cracks,

no chips

87 91

3H 3H

94 95

dE = 0.75 dE = 1.0

37 34

1.8%, 1.2% 0.9% 0.1%

repairable repairable

dE = 1.0 dE = 1.5

no blister no blister

dE = 1.5 dE = 1.5

no blister* no blister*

No cracks, No cracks,

no chips no chips

Note: with certain SMCs there are occasionally a few small blisters.

The IMC must flow and polymerize at the same temperature as the SMC (150C) or BMC (up to 170C), in 15-60 s. The formulation is an unsaturated (TS) polyester resin requiring addition of an organic peroxide before use. At ambient temperature, it has a pot life of about five days.

IMCs used by the automotive industry are usually employed as conductive or non-conductive primers, for subsequent painting with a


conventional topcoat. The sanitary, electrical, garden furniture, leisure, and other industries also use IMCs as finishing coats.

Using formulations based on TS polyesters or polyurethanes, the high- pressure closed mold system can be used with SMCs/BMCs, RIM and RRIM polyurethanes, RTM, and also injection molded TP parts. The basic concept has been extended to injection molded TPs and has been developed jointly by a coatings manufacturer, machinery manufacturer, and an automotive group. It involves phased injection into the mold of two components, the molding material itself, and a special pigmented coating formulation based on powder coating technology, using computer-aided mold design with existing two-component molding technology to produce a molding with automotive class A finish direct from the mold. The coating meets the growing demand for more compatible materials for recycling.

Even the best quality isophthalic gelcoats can be subject to loss of gloss and yellowing, eventually exhibiting a chalky and stained surface, particularly on boat decks. Osmosis is a further problem. Water ingresses between gelcoat and RP creating pressure and the unreinforced gelcoat layer gives slightly, showing as blisters. A novel in-mold coating, Crystic Protec, from Scott Bader, uses a new polymer backbone. The gelcoat is formulated to resist sunlight and yellowing and, to counter osmosis, it uses the concept of chemically matching the gelcoat and RP resin, giving a measured water uptake 24% less than the best isophthalic gelcoat.

For producing a high-quality finish on compression molded parts, Ferro developed a premold coating in powder form which melts rapidly on coming into contact with the heated ld surface [130-160C (266-320F)], but the inner surface remains unreacted until contact with the molding compound, when it bonds chemically to form an integral laminate. Post-finishing is unnecessary and release agents are not required, even with low profile SMC. Chemical and physical properties are excellent and the coating has a high Barcol hardness, resisting damage during handling, even when hot. It can be applied by conventional manual or automatic powder spray equipment. A range of colors and decorative effects is available. In can be used particularly in sanitary ware, automotive and exterior applications.

When a colored gel coat has been used and subsequent painting is not required, the release agent should first be thoroughly washed off. The molding can then be buffed or polished with any of the normal cutting compounds. If the product it is not to be painted, extra care should be taken to ensure that all trace of release agent is removed. Release agents


based on polyvinyl alcohol can easily be washed off with detergent and plenty of warm water. There may be difficulty in removing any wax or silicone release agent still adhering, and it may be necessary to use wet or dry abrasive paper to remove these release agents.

Washing Equipment

Equipment for washing RP tools and equipment takes a variety of forms, depending on size and volume requirements. A typical system is based on manual washing stations, with or without solvent bath, for small-scale washing of brushes, rollers and guns, which can be located close to the workplace and connected to existing extraction systems or to portable activated carbon filter units. A closed chamber can be loaded via a foot-operated flip-up lid and solvent fed by gravity from holding tanks, draining to a waste tank below, which can be transferred to a solvent recovery unit. For safe efficient cleaning of larger items there are washing booths with fume hoods, which can be connected to extraction systems and drain solvent to portable containers for recovery.

The washing process can be speeded up by a rotary machine or fully automated, with a washing tunnel. Rotary machines, using a horizontally rotating basket in which the components are loaded, will handle items from 38 to 78 cm diameter x 50-58 cm height x 25-40 kg weight. Tunnels (which are suited mainly to washing printing ink from silk screens and litho/flexo ink trays) employ a traveling longitudinal spray nozzle, directed at a 45 ~ angle to the object being washed automatically reversing after completing a pass.

Solvent Recovery Systems

Recovery of expensive solvents is an attractive proposition, and there is a range of equipment suited to various requirements. Details are presented in Chapter 3 Polyesters, TS, TS Polyester Solvents.

Troubleshooting

Processing of RP is an art of detail. The more you pay attention to details, the fewer problems/faults develop in the process. If it has been running, it will continue running well unless a change occurs. Correct the problem and do not compensate. It may not be an easy task, but understanding what you have equipment wise, material wise, processing wise, environment wise, and/or people wise can help. In order to understand potential problems/faults and solutions of fabrication, it is

5 �9 F a b r i c a t i n 9 Processes 4 7 1

helpful to consider the relationships that are directly related to machine capabilities and variables, plastics processing variables, and product performance. The following Tables 5.28-5.31 list typical problems/ faults and solution/corrective measures. Much more information is available in the literature usually associated with specific processes (see Bibliography).

Table 5.28 Troubleshooting thermoset RPs

Problem Possible cause Solution

Nonfills Air entrapment

Excessive thickness variation

Gel and/or resin time too short

Improper clamping and/ or layup

Blistering Demolded too soon Improper catalytic action

Extended curing cycle Improper catalytic action

Additional air vents and/or vacuum required

Adjust resin mix to lengthen time cycle

Check weight and layup and/or check clamping mechanisms such as alignment of platens, etc.

Extend molding cycle Check resin mix for accurate catalyst content and dispersion

Check equipment, if used, for proper catalyst metering

Remix resin and contents; agitate mix to provide even dispersion

Table 5 .29 Problem/fault vs. solution/correction of injection molded glass fiberlTPs

Fault Priority order for remedies

a

Blisters 7

Brittleness 27

Excessive flash 30

Gas burns 30

Oversized part 30

Poor surface finish 17

Poor weld lines 14

Short shots 8

Silver streaking 7

Sink marks 14

Undersized part 12

Voids 14

Warping 5

b c d e f g h

27 30 34 3 4

26 34 4 25 7 17 32

10 29 2 6 7 28 27

3 27 21 2 7

29 28 27 17 33 23

14 15 23 9 2 7 12

13 17 8 3 21 30 1

14 15 21 23 9 17 18

15 27 4 34 18 26 30

13 30 18 19 20 22 8

13 31 23 16 9 8 15

13 30 18 19 20 22 17

14 17 31 32 1 27 28

i j k I

30

15

19

31

17

31

11

23

20

11

18

8

24

12

19

7

16

3

Table 5 . 3 0 Problem/fault vs. solution/correction of glass fiber/TS polyester reinforced plastic el)

Fault Appearance Cause

Wrinkling

Gel coat cures too slowly

Pinholing

Attack on the gelcoat by solvent from monomer in the laminating resin due to undercure of gelcoat. Can be corrected by ensuring that the resin formulation is correct the gelcoat not too thin and temperature and humidity controlled keeping the mold away from moving air (especially warm air}. If the workshop has hot air blowers they should be directed away from the molds.

Temperatures below 16~ (commonly due to storage of containers outdoors overnight) will drastically affect gel time; high humidity moisture condensation and damp release film also retard cure also check catalyst concentration and grade of polyether ketone peroxide (activity may vary)

Small bubbles of air trapped in the gelcoat before gelling also when resin is too viscous or too- high filler content or when gelcoat wets release agent imperfectly.

m ,

I l l

,-r

0 "

0 0

Poor adhesion of gelcoat Flaking of gelcoat in handling blister local surface undulations when viewed obliquely

Can be caused by imperfect consolidation of the laminate contamination of gelcoat before glass fiber is laid up or (more frequently} gelcoat left too long to cure.

Spotting Small spots all over gelcoat surface

Usually due to inaccurate dispensing of one ingredient of the resin formulation.

Striations Color waves over surface Flotation of color paste most common when color used is a mixture of more than one paste; remedied by thorough mixing or using different paste

Fiber pattern Pattern of fiber reinforcement is visible through gelcoat or prominent on surface

Usually when gelcoat is too thin or reinforcement is laid up and rolled before gelcoat has hardened sufficiently or molding demolded too early. Does not usually affect performance but can be unsightly.

'Fish eyes' Patches of pale color (usually up to 6 mm diameter)

Can occur on very highly polished mold especially when using silicone-modified waxes gelcoat 'de-wets' from certain areas leaving thin spots. Can also occur in long straight lines following brush application strokes. Rarely encountered when liquid release agent is correctly applied.

Table 5 , 3 0 continued

Fault

Blisters

Crazing

Star cracking

Internal dry patches

Poor wetting of mat

Leaching

Yellowing

Appearance Cause

Usually indicates delamination/trapped air or solvent; over large area can also indicate undercured resin {and may take six months after molding to show). Can also form when subjected to excessive radiant heat during cure. Often caused by using liquid rather than paste catalyst. Below-surface blister probably due to imperfect wetting of fiber from allowing insufficient time for mat to absorb resin {can usually be detected by inspection on demolding).

Hair-cracks in surface; poor surface gloss

Can happen immediately after manufacture or may take some months to develop. Generally associated with resin-rich areas due to unsuitable resin or formulation in gelcoat; common cause is adding styrene monomer to gelcoat. Or gelcoat may be too hard relative to its thickness (harder the gelcoat more resilient must be the resin). When occurring some months after exposure to weathering crazing can be due to undercure too much filler or too flexible resin.

Over-thick gelcoat occurs when laminate receives reverse impact" gelcoat should never be more than 0.4 mm (0.016 in} thick.

Trying to impregnate more than one layer of mat at a time; presence of dry patches can be confirmed by tapping surface with a coin.

Reverse side has unglazed appearance

Not enough resin used in laying-up or lay-up not properly consolidated reverse side will appear 'glazed' if fibers are correctly coated.

Loss of resin in weathering leaving fibers exposed

This is a serious fault. Either the resin has not been properly cured or an unsuitable resin has been used.

Slight yellowing after exposure to sunlight

Can be marked with white or translucent laminates. Surface phenomenon due to absorption of ultraviolet (UV) radiation; does not affect mechanical properties. Most resins contain UV stabilizers to reduce rate of yellowing; high resin content discolors less rapidly than high glass content (75% resin in sheet will make yellowing negligible).

(,n

-i-1

o -

m, , ,

" 0

0

I l l

w


Table 5.31 Examples of computer software information generated and typical problems it can solve

Cooling Analysis

Mold surface Temperature Freeze time Coolant temperature and flow rate Metal temperature

Part quality: even cooling prevents distortion

Minimizes cycle time. Optimizes coolant: can eliminate need to chill coolant. Cooling efficiency: ensures optimum circuit design.

Warpage Analysis

Warped shape Single variant Warpage shape

Tendency to warp. Indicates fundamental causes of warpage.

FIow Analysis

Fill pattern

Pressure distribution

Temperature

Shear stress distribution

Shear rate cooling time

Flow-angle packing pressure volumetric shrinkage

Weld line position. Air trap position. Position of vents. Overpacking: excessive costly part weight. Overpacking: warpage due to differential shrinkage. Underflow: structural weaknesses. Clamp force required. Overpacking: ribs, etc. sticking in mold. Poor surface finish. Weak weld lines. Distortion due to differential cooling. Quality of part: tendency to distort. Quality of part: tendency to crack. Cycle time: low stresses permit hotter demold

temperature. Avoids degradation of material. Shows tendency to distort due to uneven cooling. Quality of part: molecular orientation. Under/overpack to poor packing. Dimensional variations due to poor packing.

Fabricating products involves conversion processes that may be described as an art. Like all arts they have a basis in science and one of the short routes to processing improvement is a study of the relevant sciences (as reviewed throughout this book that range from the different plastic melt behaviors to fabricating all size and shape products to meet different performance requirements.


Repairs

RP may need to be repaired to rectify faults occurring during production, or to repair damage in service. Most fabricating faults can readily be dealt with at the trimming/finishing stage, where materials and equipment are available. When repair involves RTPs the fact that TPs can easily be heated and melted can simplify the procedure. Simple patchwork is performed with ease. If there is a catastrophic failure of the reinforcement, repair is probably difficult or impossible. Examples of repair procedures are highlighted in Figure 5.92.

Damaged area Damaged area repaired Edges chamfered . cut out

I~'";';"~"-".:,E;';:.'.';'.".7 ~ J l.':':.,_':.'.,:'.~'.'.":.'.::;.'..i

~ Undamaged J laminate /

Temporary mould

Rib laminated over repair

~,~,~ ~,,-'~~;.-;~-.;;-.;;;~;--.~..--~"~ '~ "~'i' l . ' .v ':::- ':i:: '-:". ' .T.- ' , , , ' . l

Laminate Temporary mould

Figure 5.92 Two methods for repairing damaged fiber reinforced plastics

With RTSs, any loose resin and reinforcement should be removed and the affected area cleaned and dried. It may be useful to roughen surrounding areas, to obtain better adhesion. For superficial damage (to gelcoat only), apply catalyzed/activated resin to the damaged area and allow to set/solidify. Film, such as transparent cellulose, tape or TP polyester, may be used to keep the resin in position and impart a smooth surface finish. Apply a thicker film of resin than usual, to allow for shrinkage. When the repair patch has fully hardened, dress the resin back to the correct contour of the molding.

Where damage goes beyond the surface, resin and reinforcement should be laid up overlapping the edges to give good adhesion over a wide area. If the laminate is fractured, the whole of the damaged area


can be cut away and the inside edge of the aperture chamfered to make it larger on the gelcoat side than on the reverse. The surrounding area should be roughened to obtain good adhesion. Where the surface area is large, a temporary mold should be built up on the exterior surface, release agent applied and left to dry. For smaller holes, a piece of cellulose or polyester film can be fixed over the hole with adhesive tape, to act both as mold and release agent.

The literature provides repair techniques ideas for RPs. As an example with very extensive damage, the best solution, if feasible, is to consider replacing the molding in its original mold and repair it in the mold. Where a strengthening rib can be added, a different repair method can be used. The hole is chamfered to be larger on the inside than on the gelcoat side and the actual repair is carried out as described above, but it need be no thicker than the laminate. One or more reinforcing ribs are then laminated over the area, overlapping as far as possible.

Energy

When examining energy consumed or lost, the equipment used in the complete production line as well as the plastic is involved. Regarding electric energy, 1998 was the year that the USA government-subsidy stopped. Cost of energy started to doubled and tripled setting market forces in motion that makes more energy-efficient all-electric equipment desirable. As an example, an injection molder facing a mandatory energy cutback realizes 2 or 3 all electric IMMs could run on the same power as it takes to run one.hydraulic machine of the same size. As the electric injection machines become larger, more energy advantage over hydraulics exists.

Here is one of many examples where RP provides for energy savings. Table 5.32 illustrates the total energy saving and increase in fuel economy obtained by designing a single component front-end grille opening panel of SMC to replace a multi-component metal assembly on a production car. It replaced many metal stamping, machining, and fastening operations, as well as the associated dies and fixtures that eliminated the replacement of 16 steel and die cast parts with a single RP molding. The data in the table assume a vehicle life of 9.2 years.

When discussing energy the target is usually to reduce consumption and thus reduce cost of fabricating. Regardless of action, being taken it may appear that the cost of energy continually increases as any savings occur in the workplace, at home, etc. In the mean time energy shortages

Table 5.32 Energy savings and fuel economy


Front-end panel Metol RP

Zinc die cast and stamped steel, weight, Ib (kg) BTU/finished part (hp-hours/finished part] Fuel economy increase, % Fuel saving, gal (1)

Source: Ford Motor Co.

20 (9.07) 400,000 (157.24)

9 (4.08) 200,000 (78.62) 0.2

13 (49.205)

continually occur. Interesting with all the political movements worldwide, their appears insufficient action to meet the International Energy Agency report that the global population presently at 6.3 billion will increase requiting worldwide energy demand to increase 25% by 2015.

Upgrading Plant

When a plastic fabricator considers updating a fabricating facility with a state-of-the-art operation the usual operating factors already in use require reviews and up dates such as material handling and services (electric power, water cooling, etc.) to machine safety operations. Estimating cost and site location are two initial pitfalls that must be avoided. One can over-estimate difficulties or underestimate challenges with results ranging from expensive too disastrous financial situations. However, these problems can be avoided by assembling a qualified high-quality team that includes an architect, facility contractor, and if needed a consulting engineer that has experience with plastics manufacturing plants.

Regarding choosing the correct site is often the most critical decision in the process. This action contains various variables such as make sure there is adequate access to power and water. Consider what combination of highway and rail access will work best for receiving raw materials and shipping products. Check local zoning laws such as permitting silos or cooling towers. Determine if the local labor supply is adequate for the type of people required. Select a site that permits future expansion. Design the building so that expansion can be accomplished without interrupting production. Wiring and piping systems should be designed with expansion possibilities. More loading dock space should be planned. Parking area must be easy to enlarge. New venting and air conditioning technology can help reduce operating costs significantly.


All types of RP molding should be carried out in a controlled environment. This is essential where TS polyester resins are being processed and there may be risk of over-exposure to styrene emissions, and is advisable also when working with TPs, because of heat build-up from machines, the effect of sudden drops in temperature (as, for example, when doors are left open), and the removal of fumes.

With the more mechanized molding processes, adequate space is obviously necessary, and special attention must be paid to safety precautions and guarding. The apparent simplicity of hand lay-up or spray- up should not lead one into thinking that generous space here is not equally essential. The ideal for all work is a large, airy, solid factory building, free from draughts, and with a controlled temperature of 20-25C.

Precise machine alignment reduces wear and tear, minimizes downtime, and reduces flashing such as for alignment of injection molding machines.

Cleanliness and good housekeeping are, similarly, the watchwords for all types of plastics molding. It is good practice to introduce a flow-line layout: firstly, because it is logical, and second because it separates the various stages of production (material preparation/molding/assembly/ finishing), preventing dust and contamination from one affecting another. Working with TS resins, this is particularly important.

It is also worth remembering that, as work progresses from one stage to the next, so also does the value of the product increase. Damage/failure at a late stage is much more costly. Effective quality control should be interposed between critical stages, where it is practicable to introduce feedback loops to correct any rejects.

Examples of plant layouts for various RP operations are shown in Figures 5.93 and 5.94.

No RP plant in the world can rival Boeing's 22,000 m 2 (237,000 ft 2) Composites Center of Excellence at Seattle, Washington, USA, which makes parts for military and civil aircraft, including the B-2 stealth bomber, which is the world's largest application of s in aerospace engineering to date. The upper and lower skin panels of the wing are thought to be the largest single-piece aircraft RP structure yet made. The outboard wing sections are effectively flying fuel tanks, each 20 m (65.6 ft) long, with a total wingspan of 52 m (170 ft).

Included in the equipment in Seattle are five computerized tape laminating machines capable of producing parts up to 36.5 m (120 ft) long and 6 m (20 ft) wide from coordinates taken direct from production data. A channel laminator is used for very long reinforced plastics


Figure 5.93 Casting production line for filled TS polyester molding

Figure 5.94 A resin transfer molding (RTM) plant

stiffeners and high-speed water-jet cutting systems and precise five-axis milling machines refine parts to microscopic tolerances. Also installed are the world's two largest autoclaves, 27.4 m (90 ft) long x 7.6 m (25 ft) in diameter.

Another interesting operation, in addition to many worldwide is a fully- automated RTM plant, for high consistency and reduced costs, is


producing at 20,000 parts/year at Sisteme Compositi SpA, Frosinone, Italy. Details have been provided in above section entitled Reinforced Resin Transfer Moldings, Automations.

FALLO Approach

Conditions that are important in making plastic products the success it has worldwide are summarized in Figure 5.95. All designs, processes, and materials fit into the overall FALLO (Follow ALL Opportunities) approach flow chart that produces products meeting required performance and cost requirements.

Designers and processors to produce qualified products at the lowest cost eliminating or significantly reducing troubleshooting fabricated products have used the basic concept of the FALLO approach. This approach makes one aware that many steps are involved to be successful, all of which must be coordinated and interrelated. It starts with the design that involves specifying the plastic and specifying the manufacturing process. The specific process (compression molding, resin transfer molding, injection molding, and so forth) is an important part of the overall scheme. The FALLO approach diagram consists of:

Designing a product to meet performance and manufacturing requirements at the lowest cost;

2 Specifying the proper plastic material that meets product performance requirements after being processed;

3 Specifying the complete equipment line by:

(a)

(b)

(c)

(d)

designing the tool (die, mold) "around" the product,

putting the "proper performing" fabricating process "around" the tool,

setting up auxiliary equipment (up-stream to down-stream) to "match" the operation of the complete line,

setting up the required "complete controls" (such as testing, quality control, troubleshooting, maintenance, data recording, etc.) to target in meeting "zero defects";

Using this type of approach leads to maximizing product's profitability. If processing is to be contracted, ensure that the proper equipment is

Purchasing and properly warehousing plastic materials and maintaining equipment.

THE COMPLETE PROCESSING OPERATION ... THE FALLO APPROACH

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PRODUCT CONCEPT

Select PLASTIC material

use VALUE ANALYSIS

approach to meet performance to

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SELECT PROCESS

FALLO Follow ALL Opportunities

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MANUFACTURING OPERATION Integrate all individual operations that produce parts

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operator, conveyor, robot, etc.

Secondary operation packaging,

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MOLDED PRODUCT ready for delivery

Set up PREVENTATIVE MAINTENANCE

Set up TESTING/QUALITY CONTROL- Characterize properties: mechanical, physical, chemical, thermal, etc.

Set up practical, useful TROUBLESHOOTING GUIDE based on 'causes ~ remedies' of potential 'faults'

I GOOD MANUFACTURING PRACTICE

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Figure 5.95 FALLO approach includes going from material to fabricated product (courtesy of Plastics FALLO)

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SOFTWARE OPERATION

Immediately after the product is in production take the next important step. Reevaluate and target the product to be produced at a lower cost.

Use the FALLO approach by reexamining the parameters going from the product design through production. Examples of potential cost reductions include: (1) redesign product with thinner walls to reduce production cost, etc. (2) reduce cost by using less plastic, change to a more expensive plastic that reduces processing cost etc. (3) modify process control to reduce production costs, etc, and, (4] other parameters reviewed in this publication

IF YOU DO NOT TAKE THIS ACTION - someone else WILL TAKE THE ACTION

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available and used. This interrelationship is different from that of most other materials, where the designer is usually limited to using specific prefabricated forms that are bonded, welded, bent, and so on.

Summary of Figure 5.95 is that acceptable products will be produced. It highlights the flow pattern to be successful and profitable. Recognize that this action provides meeting the phrase of first to market with a new product capture 80% of market share.

reinforced plastics handbook || fabricating processes

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