KOLEJ UNIVERSITI TEKNIKAL KEBANGSAAN MALAYSIA

Investigate of Parameter Setting in Plastic Injection Molding

Thesis submitted in accordance with the requirements of the

Kolej Universiti Teknikal Kebangsaan Malaysia for

Bachelor of Manufacturing Engineering (Honours) (Manufacturing Process)

Sullyfaizura Mohd Rawi

Faculty of Manufacturing Engineering

June 2006

ABSTRAK

Pengacuaan suntikan merupakan proses yang paling meluas digunakan untuk

menghasilkan pelbagai kompenan plastik pada tahap kualiti yang tinggj. Projek ini

adalah menyiasat mengenai penetapan parameter dalam mesin pengacuan suntikan.

Mesin pengacuan suntikan &burg adalah i n d e n t yang digunakan dalam projes ini

untuk mneghasilkan "Dog bone specimen. sebagai produk. Pelbagai parameter di

dalam proses pengacuaan suntikan seperti tekanan penyuntikan, tekanan pegangan,

suhu pencairan bahan, suhu acuan, isipadu dos, and program pengapit dan parameter

proses lain seperti masapenyejukan turut dilihat. Di samping itu, projek ini juga

adalah untuk mendapatkan parameter yang optimum menght jumlah experiment

yang dijalankan berdasarkan factorial 3k yang digunakan dalam experiment

rekabentuk. Selain itu, pengujian bagi melihat keadaan mekanikal specimen yang

dihasilkan menggunakan mesin Universal Testing Machine 0 untuk menguji

kekuatan tegangan pada specimen. bacaan maximum bagi ujian kekuatan tegangan

diambil berdasarkan pengujian yang telah dijalankan.

CHAPTER 1

INTRODUCTION

1.1 BACKGROUND

Injection molding is a practical technique used in manufacturing for mass producing

plastics parts quickly and inexpensively. As plastic parts have become more popular

and critical in modem engineering applications, demand for quality has increased.

The principle of injection molding is very simple. Injection molding is process in

which a plastic material is heated until it becomes soft enough to force into a closed

mold at which point the material cools to solidify and from a specifio product. The

action that takes place is much like the filling of jelly donut. A hypodermic style

cylinder and nozzle inject the heated plastic into an opening creates in closer

container(mo1d). The material is allowed to harden, a finished part is ejected and the

cycle is represents as often as necessary to produce the total number of pieces

required.

This project conduct to investigate and optimize of setting parameter in plastic

injection molding. For the injection molding process, the parameters include ram

speed, injection pressure, barrel and nozzle temperature, mold temperature, mold

clamp force, dwell time, cooling time, and material properties. However, for this

study only five parameters (temperature of melt, injection pressure, holding pressure,

dosage volume and clamping program) were varied while the rest were held constant.

3k factorial design, a formal method of the Design of Experiments (DOE) was

applied to test these parameters in an efficient manner, using the least amount of

experiments and therefore saving resources and time. The material will be use in this

project polypropylene. We choose this material because this material have a

different properties and parameter for other material. So, finally we will find also the

accurate result for polypropylene and we can conclude of the quality of product. We

also will be do the tensile test of the product produced and analysis the result of the

test using the UTM. Processing conditions have very strong influences on properties

and performance of parts and products. Changes in processing conditions can lead to

improvements or degradation of accuracy, shape, surface finish, fracture resistance

and many other part properties and characteristics. One of the major activities of

manufacturing engineering is the assessment of the effects of changing process

parameter values on part characteristics. The primary use of process models is to

predict these effects. Often process models are inadequate for this task, usually

because the process is very complex or because accurate material behavior

descriptions at processing conditions are unavailable. So the defect of from the result

will produce should be analyze and try to improve the quality of the product.

Outcome of the defect will be define to produce the better parameter.

1.2 PROBLEM STATEMENT

Nowadays, quite a variety of different technique are employed in the fonning

polymeric material. Injection molding is the most common method for method for

fonning plastic polymer. Injection molding is the most widely used molding process

for thermoplastics. Injection molding is economical only for large production

quantities. Thus, the product of produced using injection molding have are

troubleshooting. Most of the defect of the product have a come h m not proper

parameter setting in plastic injection molding. Beside that, in this project also have to

optimize the parameter of the injection and determine the accurate value of the

parameter. Before this, the parameter is manually setting and don't have the

accurate value. The other side, the problem is to minimize of the defect of the

injection molding. A through understanding of the molding process will be help

determine the causes.

1.2 OBJECTIVE OF PROJECT

Objective of this project is :

1. To optimize of the parameter in plastic injection molding.

2. To determine the maximum tensile strength value of polypropylene.

1.4 SCOPE OF THE PROJECT

This project to investigate the parameter setting in plastic injection molding, so for

the started to optimize the parameter, the sequence of the process is:

i. Material selection for polypropylene. Pure polypropylene have been

used in to investigate the parameter setting in injection molding

ii. Produce the specimen using Plastic injection Molding. The

parameter should be setting based on the parameter selection.

iii. Tensile test for the produced specimen using Universal Testing

Machine (UTM) to find maximum tensile strength of value for each

specimen based on different parameter setting.

iv. Analyze the result and find the optimum result for this investigation

for parameter setting of Plastic Injection Molding.

CHAPTER2

LITERATURE REVIEW

2.1 INTRODUCTION OF PLASTIC INJECTION MOLDING

Injection molding, the polyiner analogue of die casting for metal is the widely used

technique for fabricating thermoplastic material. It also is perhaps the most common

and versatile method of forming plastic into plastics. Injection molding process is a

process in which polymer is heated to highly plastic state and forced to flow under

high pressure into mold cavity, where it solidifies. The molded part called is

molding, is then removed fiom the cavity. The process produces discrete components

that are almost always net shape. Complex and intricate shapes are possible with

injection molding the limitation being the ability to fabricate a mold whose cavity is

the same geometry as the part. In additional, the mold must provided for part

removal. Injection molding is the most widely used molding process for

thermoplastic. Some thermoset and elastomers are injection molded., with

modification in equipment and the operating parameters are allow for cross-linking

of these material. In this project, I use polypropylene as material to produce the

product based on the optimum parameter will be setting in plastic injection molding.

Injection molding is cyclical process, where each cycle produces a part or parts. The

cycle of the machine starts with of closing of the mold, the machine then fills the

molds, the part solidifies, the mold opens, the part is extracted and the mold close

again. Generally injection molding have four stage starting the filling, packing,

cooling and finally ejection. According A. Tolga Bozdana, Omer Eyerci'ogSlu are

said injection molding process is a cyclic process. Four significant stages of the

process are filling, packing, cooling and ejection. The first stage is the "filling

stage" in which the mould cavity is filled with hot polymer melt at injection

temperature. After the cavity is filled, in the "packing stage", additional polymer

melt is packed into the cavity at a higher pressure to compensate the expected

shrinkage as the polymer solidifies. Next, the mould is cooled until the part is

sufficiently rigid to be ejected, and this stage is the "cooling stage". The last one is

the "ejection stage" in which the mould is opened and the part is ejected, after

which the mould is closed again to begin the next cycle. For thermoplastic materials,

the injection molding machine converts granular or pelleted raw plastic material into

final molded parts via melting, injection, packing, cooling and ejection cycle.

2.2 ELEMENT OF PLASTIC INJECTION MOLDING

The injection molding machine itself consists of the clamp unit, the injection unit, the

control unit and a hopper. The hopper becomes modified to include a loader, dryer,

and the same cases an additive feeder.

onveyor r l

I Loaded

Dryer

AF

Figure 2.1 Element Of Plastic Injection Molding Bernie A. Olmsted and Martin E.

Davis, (200 1)

Clamp Mold

Injection Unit

I I Control Unit Plastic

Temp controller Grinder - -

The injection unit heats, melts, pumps and injects the plastic into the mold then

mold "closed" . the control unit monitors and as the name implies, control the

functioning of the injection unit and the clamp unit. The mold is mounted within the

clamp unit and this unit opens the mold allow plastic parts to be ejected and holds the

mold closed when melted plastic is being injected. The mold is purchased from mold

maker, whose capabilities may include computer- aided-design (CAD) and

computer-numerically-controlled (CNC) milling machine that help automate the

manufactures of complex mold. The mold consists of two halves, the core half (or

male part shape) and the cavity half (or female part shape). Because the core is made

to be a little cavity represents the part. This area between the core and the cooled and

ejected fiom the mold to become the plastic part. In order to solidify the plastic part

in the mold so that it can be removed, it is usually necessary to cool the mold. The

cooling is accomplish by circulating cool water through cooling channel that are

machined into the mold itself. The water is cooled by chiller, which can either be fi-ee

standing unit nearly the press or by a part of the temperature controller system that

may be serve several molds in several injection molding machines. Temperature

controller may takes the from chiller as describe in the parameter of the plastic

injection molding.

2.3 CATEGORIZING THE PARAMETER

There are so many parameter to control, they can be detailed within the confines of

the major categories. Parameter is important to create the great result and good

product. Philip Mit Chell are said many parameter affect the injection molding

process. A practical approach to understanding these parameter is appropriate, and

those parameter that have the greatest effect on the quality and cost effectiveness of

the molded product are targeted. Figure 2.2 shown that the parameter involve one or

more of four basic categories: temperature, pressure, time and distance.

Figure 2.2 : Categories Of Parameter (Douglas M. Bryce, 1997)

23.1 TEMPERATURE

2.3.1.1 TEMPERATURE OF THE MATERIAL

Temperature of the material. The primary temperature of concern is the

temperature to which to which the plastic material must heated before it is injected

into a mold. All material have range of temperature within which they are most

efficiently injected while still maintaining maximum physical properties. Philip Mit

Chell(1996) also said melt temperature or temperature of material is that

temperature at which the plastic material is maintained throughout the flow path.

This path begin when the material is transferred h m the machine hopper into the

heating cylinder of the injection unit. It is then augured through the heating cylinder

and into the machine nozzle. From the material is injected into the mold where it

travels along a runner system, through gates and into cavity. Control the melt

temperature is essential all along that path. For amorphous materials(those that

soften-when not melt heat is applied) this range is rather broad: with aystalline(those

that actually melt when is applied) it is fairly narrow. With both types of material,

however there is a temperature point at which point at which the flow the easiest and

still maintains proper physical properties. This is called the ideal melting point and

must be attained through educated guesses and trial-and-error. While this seem

primitive it only required as a fine-tuning adjustment once a specific production run

is initiated and is finalized specification for specific product. The guessing process

actually begins by setting the temperature of heating cylinder such that the material

being injected is a temperature recommended for that generic material. The plastic

temperature is measured as it leaves the heating cylinder to make sure it is within the

proper range and then adjusted up or down depending on cycle time, required

pressures, mold temperature and variety of other parameters. These adjustment are

made are made during a pilot run of the process and until acceptable parts are

produced. When parts meet specification a setup sheet is created listing the values

for all parameter of concern. The sof€ening(or melting) of plastic is achieved by

applying heat to the plastic material causing the individual molecules to go into

motion. To a point the more heat that is applied the faster the molecules move. So,

more heat is applied to degrade of the plastic material. The heater bands, which

resemble hinged bracelets are assembled to control the temperature. There are three

basic temperature zones for the heating cylinder; rear, center, and h n t .

2.3.2 PRESSURE

Pressure is required for variety of reasons in injection molding process. The areas of

the injection machine require pressure and pressure control: the injection unit and the

clamp unit. We will focus on injection pressure, holding pressure and clamping

pressure. The two closely related in that the clamp unit must develop enough

pressure overcome the pressure developed by the by injection unit during the

molding process.

23.2.1 INJECTION PRESSURE

Injection pressure is a primary pressure used for the injection molding process. It

can defined as the amount of pressure required to produce the initial filling of the

mold cavity. The cavity image is the opening in the mold that will be filled with

plastic to form the product being molded. Initial filling represents approximately

95% of the total filling of the cavity image. This is applied to molten plastic.

Normally pressure is depend

2.3.2.2 HOLDING PRESSURE

Holding pressure is applied at the very end of the primary injection stoke and used

for the final 5% filling of the cavity image. This pressure have to completes the final

mold filling and maintains pressure against the plastic that was injected so that it can

modify while staying dense and packed. It is called holding pressure because the

holds pressure against the cooling plastic in the cavity image while that plastic

solidifies. This helps to ensure a dense part, molded with uniform pressure and

controlled shrinkage. Holding pressure are usually in the range of 50% of the

primary injection pressure. Herman F. Mark (2003) also said the secondary pressure

is half (or less) of the initial injection pressure.

2.3.2.3 CLAMP PRESSURE

Clamp pressure can be defined as the amount of pressure required to hold the mold

closed against injection pressure. The clamp unit of molding machine can be

mechanically or hydraulically activated and this pressure is applied against the mold

that forms the plastic product. The clamp force or clamp pressure must be equal the

injection force. The degree of the applied must be at least equal to the amount of the

pressure applied by the injection unit. If the lOOOOpsi injection pressure is used then

at least lOOOOpsi clamp pressure must be used. In fact, a precautionary measure of

additional equal to approximately 10% should be used to ensure that the clamp stays

closed in the event that in injection pressure drifts upward slightly. If the clamp

pressure is too low, the mold will blow open during injection. Flash occurs and the

cavity image will not fill with plastic. If clamp pressure is too great, the mold may

collapse from the total force being applied.

Pressure and temperature distribution within the mould be as uniform as possible but

is impossible to achieve with injection molding. The pressure will drop when the

mould is being filled due to flow resistance. Temperature difference also occur as the

fill takes a finite time (even through very short). The objective is to achieve the most

uniform state possible in the fill the process. The flow resistance during mould fill is

a crucial factor here. A low flow resistance ensures a faster mould fill and a

reduction in local pressure difference.

23.3 TIME

During the injection molding process, many internal activities take places. Some

occur while others are active(paralle1) and some must wait until others are

completed. The most important activity at this point is the overall cycle time.

2.3.3.1 INJECTION TIME

Injection Time is related to injection rate (ccJsec.) and injection rate should be high

enough to avoid freezing of melt during filling phase. Higher injection rate does not

affect the thermally stable commodity polymers like PP, PS ect. The higher injection

rate can be limited on account of sensitivity of polymer to shearing while passing

through narrow passages (especially for engineering polymers). Freezing time is

proportional to cube of minimum wall thickness. Generally injection time is also

proportional to square of wall thickness.

233.2 COOLING TIME

Cooling Time it can be observed that the largest portion of cycle time is woling time

which is proportional to square of maximum wall thickness and also efficiency of

woling set up. Therefore for faster production, wall thickness has to be low and

efficiency of cooling system in mould as high as possible.

23.4 DISTANCE

The final parameter is that of distance. Although it's the last item on the list of

parameter priorities, control of distance is critical to producing highquality products

at reasonable cost. This is primarily due to the fact that excessive distance requires

excessive time as started earlier time is money. Because distance is so closely related

to time the various functions involving requires are basically the same as those

related to time plus a few other.

While selecting injection molding machine the following specifications are required

to be evaluated.

Table 2.1 Specification Are Required In Plastic Injection Molding (Edward S.

Wilks,, 2001)

INJECTION UNIT I Maximum swept volume cc 1 max. shot weight

I @-

Maximum metering stroke in rnm.

i Maximum injection speed g/s or cds i I

1 I Maximum injection pressure Kg/cm2

Maximum Injection Power .Kgcm/sec.

Plasticizing rate g/s or Kglhr.

1 CLAMPING UNIT I

Clearance between Tie bars and platen size mm x I

To understand

calculation of max. shot weight for a

material.

dependence of quality of melt for

consistency of molding.

how it ensues melt to spread through out

when in fluid condition- before it fieezes.

Its relationship with freezing time.

how it overcomes resistance to flow

during filling and; pressure phase on

account of flow ratio and; viscosity.

how it takes care of difficulty in filing for

thinner walled and high flow ratio parts.

how it influences cycle time.

To understand

how it accommodates mould. I

Maximum daylight mm and mould open strokc

mm

Minimum mould height

i Clamping force

I Torque Kgm and rpm I I DlWE POWER I Power supply frequency 60 or 50 Hz.

Pump-motor rating. Kw

NO LOAD CYCLE TIME

its significance for ejection of deep parts.

its relationship with mould open stroke

and daylight.

its dependence on cavity pressure and

method to compute cavity pressure.

To understand

torque requirement for -viscosity of- melt.

its influence on speeds.

to match the application - usage of

machine.

Conventional 1 Proportional and

Cartridge valve Hydraulic

Controls

Electrical I solid state 1

microprocessor controls

Open loop or closed loop controls

[t indicates the time for non processing

?art of the cycle time

2.4 UNDERSTANDING DEFECT

The complexity the injection molding process and the inter-dependence of many

variable involved, means that any molding defect may have several different causes.

Of which more than one may be present at any given time. Consequently a remedy

that curves one fault may engender another. The conclusion is that injection molding

trouble shooting is a job for the expert. Provided these limitations are understood, the

trouble shooting chart will provide a usehl guide for the problem solving.

Table 2.2 Trouble shooting of plastic injection molding ( Bernie A. Olmsted and

Martin E. Davis, 2001)

Problem

Short Shots

Sink marks

Possible Cause

Insufficient feed

Insufficient pressure

Melt Temperature too low

Injection time too short

Nozzle cold on the start-up

Mold cold

Feed system too small

Air trapped in mold

Plasticizing capacity inadequate

Unbalanced cavity in multi-cavity

mold

Excessively thin region

Melt temperature

Suggested remedy

Increase

Increase

Lengthen cycle

Increase temperature gradually

Increase screw speed and back

pressure

Increase

Fit nozzle heater

Reduce coolant flow

Fit mold temperature controller

Enlarge sprue or runner or gate

Add or clean vents

Increase cycle time

Use a larger machine

Adjust runner or gate size

Redesign part

Reduce barrel temperatures

Voids

Surfaces defects

near gates

Flash

Insufficient material injected

Insufficient dwell time

Premature gate freezing

Sharp variations in wall thickness

Wrong gate location

Part ejected too hot

Cavity pressure too low

Volatiles from overheated material

Consideration on granules

Premature freezing of flow path to

thick

Mold too cold

Mold too hot

Excessive injection pressure

Excessive melt temperature

Mold parting face faulty

Insufficient clamp force

Increase feed

Raised barrel temperature

Increase mold temperature

Enlarge gates

Increase

Enlarge gate

Increase

Redesign part

Relocate

Increase cooling time

Use nucleated grade

Increase

Raise barrel temperature

Increase mold temperature

Enlarge gate

Reduce heating

pre-dry

Improve storage

Increase pressure

Increase mold temperature

Use nucleated grade

Enlarge gates

Increase mold temperature

Increase pressure

Increase injection speed

Cold mold near gates

Reduce pressure

Reduce runner

Reduce heating

Repair mold

Increase

Flows marks

Weld lines

Bad surface finish

Brittleness

Warping

Foreign matter on the mold parting

face

Flow restriction in one or more

cavities of multi-cavity mold

Melt temperature too low

Incorrect gate location

Incorrect gate type

Injection pressure too low

Inadequate venting

old cavity soiled

Mold temperature too low

Flow length too great

Excessive use of mold lubricant

Melt temperature

Mold too cold

Melt degraded by excessive

heating

Material contaminated

Incorrect part design

Excessive use of regrind

Melt temperature too low

Incorrect part design

Over packing near gate

Sharp variations in wall thickness

Flow length too great

Use a larger machine

Clean mold

Identify and remove

Increase heating

Relocate

Adjust

Increase

Vent cavity

Clean mold

Increase

Relocate gate

Increase number of gates

Mold lubricants not recommended

Increase heating

Increase mold temperature

Decrease heating

Clean hopper and barrel

Redesign part

Reduce proportion of regrind

Increase heating

Redesign part

Reduce shot volume

Reduce injection pressure

Reduce injection time

Reduce heating

Check runner and gates sizes

Redesign part

Relocate gates

r

Wrapping

Silver streaks

Nozzle drool

Bum marks

Part sticking

Unbalanced multiple gates

Part ejected too hot

Inadequate or badly located

ejectors

Temperature variations between

the mold halves

Melt temperature too low

Melt too cold

Condensation on mold

Entrapped volatiles

Excessive nozzle temperature

Excessive melt temperature

Incorrect filling pattern

Molding too hot

Insufficient draft on side walls

Excessive injection pressure

Cavity finish poor

Cores misaligned by injection

pressure

Increase number of gates

Relocate gates

Balances feed system

Increase cooling time

Use nuclected grade

Modify mold

Adjust cooling circuits

Modify mold

Increase heating

Increase mold temperature

Dry mold

Increase mold temperature

Predry material

Improve storage

Vent mold

Reduce heating

Reduce heating

Purge barrel

Reduce gate

Improve venting

Increase cooling

Increase draft angle

Decrease

Polish mold

Redesign part

Relocate gate

2.4.1 PARAMETER EFFECT

Table 2.3 Parameter Change versus property Effect (Douglas M. Bryce, (1 997)

Table 2.3 shows some of the property values that can be adjusted by plus or minor

minus change in some of the more common molding parameter. There are some

examples. But the notice how some properties are changed in the same way by

different parameters. For instance "less shrinkage" can be attained by either

increasing injection pressure or increasing mold temperature, and "less degradation"

can be achieved by lowering back pressure as well as lowering melt temperature.

These example demonstrate that the basic molding parameters do work closely

together, and that changing a parameter in one area may affect a value of some

property in another area. By understanding this relationship, it is possible to

minimize the number of adjustments required when it is necessary to make a

correction due to an unexpected change in some variable of the process.

Parameter

Injection pressure (+)

Injection pressure (-)

Back pressure (+)

Back pressure (-)

Melt temperature (+)

Melt temperature (-)

Mold temperature (+)

Mold temperature (-)

2.5 PROPERTIES OF THE MATERIAL

Property Effect

Less shrinkage, higher gloss, less wrap, harder to eject

More shrinkage, less gloss, more wrap, easier to eject

Higher density, more degradation, fewer voids

Lower density, less degradation, more voids

Faster flow, more degradation, more brittle, flashing

Slower flow, less degradation, less brittle, less flashing

Longer cycle, higher gloss, less wrap, less shrinkage

Faster cycle, lower gloss, greater warp, higher

shrinkage

2.5.1 THERMOPLASTIC

Thermoplastic materials that used in high volume, widely recognized applications

and are known as wmmodity thermoplastics. Some resin manufacturer have objected

to the commodity designation because that term can imply that the material are

interchangeable from supplier to supplier without different in properties. Some

differences can be seen, but within a product classification, they are not great. All of

the commodity thermoplastic that will be considered are made by addition

polymerization method. Polymer materials(molecu1ar viewpoint), this method

requires that the monomer have carbon-carbon double bond and all the monomer

meet that requirement. The differences between the monomers used make these

commodity thermoplastics are in the hctional groups attached to the carbons.

Although functional group substitution can be made at four locations on a carbon-

carbon double bond, only one site is used for substitution in all major types of

wmmodity thermoplastics which will be considered.

Where --X can be among other:

----H polyethylene ---CH3 p o l ~ ~ r o ~ ~ l e n e ---CI polyvinylchloride - polystyrene

Figure 2.3 General Representation Of Commodity Thermoplastics (Hans-Geog Elias,

2002).

Therefore, all these commodity thermoplastics monomer and polymer can be

represents by general formula given in figure 2.3 where X represents a functional

group of type. Note that in one hydrogen is wnsidered are also attached to the to the

carbon-carbon double bond. The differences between the wmmodity thermoplastic

arise, therefore from the difference caused by the substitution of one functional group

on the carbon-carbon double bond. One of most important effects is steric that is the

consequences of differences in the size of the functional groups. When is structured

groups are small (such as hydrogen, then little steric hindrance the polymers are

relatively fiee to rotate, bend, and pack together. The carbon double bond, with the

results of restricted polymer motion, less ability to pack densely, and changes in

mechanical, physicals, and chemical properties.

2.5.2 POLYPROPYLENE

Polypropylene is one of those rather versatile polymers out there. It serves double

duty, both as a plastic and as a fiber. As a plastic it is used to make things like

dishwasher-safe food containers. It can do this because it doesn't melt below 160 OC,

or 320 OF. Polyethylene, a more common plastic, will anneal at around 100 OC, which

means that polyethylene dishes will warp in the dishwasher. As a fiber,

polypropylene is used to make indoor-outdoor carpeting, the kind that you always

find around swimming pools and miniature golf courses. It works well for outdoor

carpet because it is easy to make colored polypropylene, and because polypropylene

doesn't absorb water, like nylon does. Structurally, it is a vinyl polymer, and is

similar to polyethylene, only that on every other carbon atom in the backbone chain

has a methyl group attached to it. Polypropylene can be made fiom the monomer

propylene by Ziegler-Natta polymerization and by metallocene catalysis

polymerization.

Figure 2.4 Structure Of Polypropylene (Clive Maier and Teresa, 1998)

Polypropylene is an extremely versatile plastic and is available in many grades and

also as copolymer(ethylene/propylene). References from R J Crawford for the book

Plastic Engineering. He said It has the lowest density of all thermoplastic(in the

order of 900kg/m3 and this combined with strength, stifhess and excellent fatigue

and chemical resistance make it attractive in many situations. These include crates,

small machine parts, car components (fan, fascia panels etc), chair shells, cabinets

for TV, tool handle etc. its excellent fatigue resistance is utilized in the molding of

integral hinges ( e.g accelerators pedals and forceps/ tweezers). Polypropylene is also

available in fibre from (for ropes, carpet backing ) and as a film (for packaging).

Table 2.4 Polypropylene At A Glance (Clive Maier and Teresa, 1998),

A. Brent Strong are said In order to obtain regular arrangement of atoms required to

make isotatics PP a catalyst is used to force this arrangement during the

polymerizations of the polymer. Such catalyst are called stereoregular. The Ziegler-

Natta catalysts used to produce HDPE is of this type.m other types of stereoregular

catalyst have been developed that are increasingly used to produce PP, in part

because the ability to control the shape and length of the polymer is even better with

the new catalyst. Therefore commercial grades of PP are made using Ziegler-Natta or

some other stereoregular catalyst.

uses:

Monomer:

Polymerization:

Morphology:

Melting temperature:

Glass transition temperature:

Thermoplastics, fibers, thermoplastic

elastomers

Propylene

Zieglar-Natta polymerization,

metallocene catalysis polymerization

highly crystalline (isotactic), highly

amorphous (atactic)

174 OC (1 00% isotactic)

-17 OC

It is not surprising that PP and PE especially HDPE have similar properties and

compete for many of the same application. However, PP and PE differ in some

important respects and these differences have led to preferences for one or the other

in various application. PP is stiffer than PE so in application requiring flexibility

(such as wire coating) one of the PE material would be used. On the other hand if

greater stifiess is needed PP is preferred resin. This is especially true if the

application also requires abrasion resistance or hardness, such as for gears, toys,

automotive battery cases and seats for stacking chairs. The resistance to environment

factors is similar for PP and PE. PP is somewhat more susceptible to W and

oxidative degradation than is PE but is more resistant to stress cracking than PE.

Hence, cross linking of PP for improve ESCR is not practiced commercially,

partially because the electron beam radiation degrades the PP. PP has a higher glass

transition points and higher melting points than PE. This means that processing

temperature are generally higher, but it also means that service temperature are

higher. Sterilizable medical devices, dishwasher-safe food container and appliance

parts are often of PP for this reason. A very important property differences that has

led to many application for PP is its superior resistance to cracking h m mechanical

stresses. PE material will readily blush and craze when subjected to bending, but PP

will not. Application requiring this polymer include carpets, ropes, strapping tape

and molded item incorporating integral hinges.

The superiors stifiess of PP over PE and low price of PP compared to the

engineering plastics have led to its use in some structural applications. If additional

sti&ess or strength is needed, reinforcement can added to PP. for instance the

addition of 30% shirt fiberglass reinforcement can double the tensile strength and

impact resistance of PP. impact modifier can be added to PP to further improve

impact strength especially for low temperature application where PP is less impact

resistant than HDPE. EPDM the copolymer of PP, PE, and dime monomer, has

improved impact properties and much greater elongation than either PP and PE.

Filler (such calcium carbonate or talc are often added to PP up to about 30%

concentration by weight. The filled plastic has improved stiffness, lower mold

shrinkage and lower cost. Many molded automotive parts have been converted h m

thermoset materials to filled PP because PP can be molded into very complex shapes

using fast molding cycles and still retain the dimensional stability that was

previously provided by the thermoset materials.

Table 2.5 Properties of the Polypropylene (R J Crawford, 1998),

2.6 MEASURING MECHANICAL PROPERTIES OF MATERIAL

Name of plastic

Polypropylene

2.6.1 TENSILE TEST

Method for determining behavior of materials under axial stretch loading. Data h m

Polymer repeat unit --(--C---C--)n--

! C

test are used to determine elastic limit, elongation, modulus of elasticity, proportional

limit, reduction in area, tensile strength, yield point, yield strength and other tensile

properties. Tensile tests at elevated temperatures provide creep data Procedures for

tensile tests of metals are given in ASTM E-8. Methods for tensile tests of plastics

are outlined in ASTM D-638,

Properties comparisons

Resists stress cracking Strongerand stiffer than HDPE Resistant to water and solvent Lowcost

A tensile test, also known as tension test, is probably the most fundamental type of

mechanical test can perform on material. Tensile tests are simple, relatively

inexpensive, and l l l y standardized. By pulling on something, very quickly

determine how the material will react to forces being applied in tension. As the

material is being pulled, and then will find its strength along with how much it will

elongate.

Typical uses

Containers with integral hinges Microwave containers Utility fibers (woven bags, ropes, w t s )

Brand Names

Marlex (Philips) Polyfort (schulman) Pro-fax (montell) Vistalon (exxon) Vrestolen Oluls)

"Moh'd Sarni" Ashhab said tensile test is a type of mechanical test that is performed

on a material. The tensile test is the most widely used mechanical property test. The

intent is to measure inherent material behavior. Tensile tests are simple, relatively

inexpensive, and l l l y standardized. By applying a variable tension load (usually

increasing) to a metallic specimen can learn a lot of properties about the material

such as the maximum load it can with stand and elongation-load relationship.

In the tensile test, an axial load is applied to a cylindrical tension specimen which has

an initial length Lo and initial diameter do. The test specimen is really longer but the

length under test in which change in length is measured is the central long section

(refer to Figure 2.6 below). The tensile test is performed with a universal testing

machine (UTM).

Figure 2.4: Test specimen under axial tension load (James S. Preraro ,2000)

2.6.1 UNIVERSAL TESTING MACHINE (UTM)

A universal testing machine (UTM) is used to determine modulus of elasticity

(Young's modulus), tensile strength at yield or break, and stress-strain curves. The

test specimen is loaded between the grips of the UTM. A mechanical (contact)

extensometer is used to measure strain up to a speed of 20000 mmlmin. The

specimen is stretched at a constant speed while load and strain are recorded. The

speed of testing is specified by the relevant standard, or client request. This test

provides high strain rate stress-strain data for the simulation of impact situations. It is

usually combined with stress-strain data spanning several decades of strain rates to

create a rate dependant model of the material.

A UTM should comply with the major and applicable standards, but additional,

special features will increase the purchase price for things that might be unnecessary

for a particular application. Some of the most relevant features to consider when

selecting a UTM test system are discussed below.

Load-Frame Capacity And Dimensions

The selection of load b e capacity is based on the maximum force required to

cause the material being tested to fracture. Specifications for UTM load frame

capacity and dimensions are vital to the equipment-selection process.

Dimensional specifications must take into consideration clearances between columns

and vertical clearance to adequately handle the products being tested. Some

materials, such as elastomers and soft polymers will elongate substantially, and

sufficient vertical travel must be available to allow the material to stretch as far as

necessary without running out of travel space. Also, consideration should be given to

any special grips, fixtures and environmental chambers that could require additional

space in both directions.

Frame Stiffiess

In some instances, h e stiffiess is a feature that can be overrated. The stiffiess of

the test frame could be an important factor where only crosshead motion is being

used instead of a separate extensometer or deflection-measuring device. Most

applications that comply with international tensile-testing standards call for the use

of an extensometer or deflection-measuring devices.

There are many machine components that can affect the frame stiffiess including

screw diameter, ball-nut fit, crosshead stiffiess, screw-bearing fit and frame stiffiess

In addition, compliance of the specimen itself, pull rods and the specimen-gripping