Things to Know about Injection Moulding Machine

While selecting injection moulding machine the following specifications are required to be evaluated.

INJECTION UNIT To understand

Maximum swept volume cc / max. shot weight gr.

calculation of max. shot weight for a material.

Maximum metering stroke in mm. dependance of quality of melt for consistancy of moulding.

Maximum injection speed g/s or cc/s how it ensurs melt to spread through out when in fluid condition- before it freezes. Its relationship with freezing time.

Maximum injection pressure Kg/cm2 how it overcomes resistance to flow during filling and; pressure phase on account of flowratio and; viscosity.

Maximum Injection Power.Kgcm/sec. how it takes care of difficulty in fiiling for thiner walled and high flow ratio parts.

Plasticizing rate g/s or Kg/hr. how it influences cycle time.

CLAMPING UNIT To understand

Clearance between Tiebars and platen size mm x mm

how it accomodates mould.

Maximum daylight mm and mould open stroke mm

its significance for ejection of deep parts.

Minimum mould height its relationship with mould open stroke and daylight.

Clamping force its dependance on cavity pressure and method to compute cavity pressure.

HYDROMOTOR To understand

Torque Kgm and rpm torque requirement for -viscosity of- melt.

DRIVE POWER -

Power supply frquency 60 or 50 Hz. its influence on speeds.

Pump-motor rating. kW to match the application - usage of machine.

CONTROLS • Conventional / Proportional and Cartridge valve Hydraulic Controls

• Electrical / solid state / microprocessor controls

• Open loop or closed loop controls

NO LOAD CYCLE TIME It indicates the time for non processing part of the cycle time

Things to know about MOULD

MOULD To know about

Assembly of mould – elements of mould,

� Assembly of various plates, function of these plates in the mould.

� Elements of moulds are Sprue bush, Locating ring, Runner, Gate, Fixed halve and moving half, Core and cavity, Ejector pins, Return pins, Sprue puller, Ejector Plate / rod, Cooling channels, Guide pins, Locating plugs, Back plate, Spacers.

Classification and distinction of moulds,

� Based on parting lines, � Ejection system, � Heat exchange system, � Transmission of forces

Feed system in mould - Sprue / runner size balanced type,

� Functions of runner system, � Circular, series, symemmetrical layout, � Factors affecting runner system, Demands

on runner system.

Gate classification, location, size and type,

� Runners – remain with moulding, � Runners – cutoff automatically, � Runners –automatically separated from part

but remains with mould.

Air vent,

Rate of total air expulsion from the mould due to speeding melt front in all direction can not be less than injection rate.

Wall thickness uniform and capable of promoting balancing of melt flow

� Familiarise with plastics PART DESIGN. It has to be over 90% mouldable shape.

� Familiarise with MOLDFLOW analysis.

Balancing of flow in un-symmetric part geometry,

Flow leader or flow deflector techniques are to be used for balancing of melt flow inside the part geometry so that moulded-in stress is avoided.

Mould mechanism For split cavity, unscrewing part, etc. Stripper plate, stripper sleeve etc.

Mould release. � Surface finish, ejection mechanism, � Overpacking in some regions can make it

difficult to eject. Mould steel, Essential characteristics for mould steel,

Heat treatment, Cost of steel is about 20% of cost of mould.

Cooling system and heating system

� Objective of mould cooling, � Design of cooling channel,

Mechanical life of mould – fatigue in mould.

� Understanding of FATIGUE in mould. � Thickness of plates to be generous so that

shut mould height is with in the specification of machine on which the mould is intended to run on production.

POLYMERS

Polymers are produced from intermediates like these parafins:

• Methane, Ethane, Propane, Butane, Pentane. • This parafins must be first converted to its olefin: Methylene, Ethylene, Propylene etc.

These intermediate monomers are polymerised by addition. Thus if 500 or more ethylene monomers polymerise, the result is Polyethylene.

MOLECULAR WEIGHT

Thermoplastics are molecules that are chains of 500 or more carbon atoms, The distance between adjacent carbon atoms being 1.5 x 10 -8 cm. All the molecules may not contain same number of monomers. However, some control over the length of polymer chain is ensured during polymerisation. The polymer is classified or specified by its average molecular weight. This also gives rise to molecular weight distribution of particular grade.

Most monomers are gases. A short polymer chain of low molecular weight will be liquid. Large molecular chain gives solid material. Higher molecular weight results in increased strength and stiffness. Higher molecular weight also increased viscosity of melt. Hence higher molecular weight melt requires higher power ( pressure) to fill the mould. Higher the molecular weight are difficult to process. Ultra high molecular materials are not mouldable.

High molecular weight material is accompanied by a very high melt viscosity. To reduce the viscosity the processing temperature must be increased, but this results in degradation of polymer. Therfore while processing the thermal stability of the polymer dictates the processing temperature and residence time of melt in the plasticising unit of the machine. Limitation of residence time dictates the speed of cooling to avoid the degradation of polymer.

Many thermoplastics are deteriorated by prolonged exposure to oxygen and ultraviolet radiation in sunlight. Resistance to such deterioration is improved by higher molecular weight. It means few molecules with fewer terminal monomers (which are reactive) in the carbon chain. Longer molecules are less mobile. Even if high molecular weight degrades it results in medium molecular weight with acceptable properties instead of a low molecular weight.

CRYSTALLINITY

Inter-molecular order refers to the geometric arrangements of adjacent polymer molecules in the solid mass. There are three types of inter-molecular order.

• Amorphous random coils are isotropic and entangled and their properties depend primarily on molecular flexibility, polystyrene, acrylic, polyphenylene oxide, polysulphane, polycarbonate, are amorphous polymer.

• Crystalline polymers have their molecules arranged in a very regular repeating lattice structure, so precise that every section of the polymer molecule must recur at very specific points in the repeat structure. No polymer is completely crystalline. Highly crystalline polymers are HDPE, PP, Acetal, Thermoplastic Polymer, Nylon.

• Oriented polymers - orientation produces anisotropic properties, i.e., different in different direction.

CLASSIFICATION OF POLYMERS:

Polymers are classified

• Thermosets • Elastomers • Thermoplastics

Thermosets are cross-linking polymers in which the final macro-molecules are formed by chemical reaction under the influence of heat and pressure. Once this reaction is complete, thermosets can not be altered from this state by further application of heat and pressure.

Phonel (PF), Urea (UF), Melamine (MF) formaldehyde resins, Polyester (UP) resines and epoxy (EP) resins are typical Thermosets.

Elastomers are plastics or modified natural substances with a limited degree of cross linking capability. They deform readily under stress but recover their original shape as soon as this stress is removed.

Thermoplastics consist of long chain macromolecules which are not interlinked. Their characteristics property is that they may be moulded when the temperature is increased beyond their softening range, and on cooling revert to the solid state in its new moulded shape. This process may be repeated indefinitely, but it is in fact limited by the ageing stability of the particular material. This means that after undergoing a certain number of processing operations, the original properties of the material are altered as a result of excess thermal stress. HDPE, LDPE, PP, PS, ABS, NYLON, PVC, PMMA, PBT, etc are thermoplastics.

POLYMER BLENDS

By alloying of two polymers it is possible to get in one material advantages of two or more polymers. At present, the following alloys are available.

PVC / Acrylic Tough with good flame and chemical resistance.

PVC / ABS Easily processed with good impact and flame resistance.

PC / ABS Hard with high heat distortion temperature and good notch impact strength.

ABS / Polysulphane Less expensive than unmodified Polysulhone.

PPO / HIPS Improved processability and reduced cost.

SAN / Olefin Good weatherability.

Nylone / Elastomer Improved notch impact strength.

Reinforced Plastics

Reinforcement in plastics enhances the mechanical strength and reduces the shrinkage. There are two types of reinforcement used in plastics.

• Fibrous materials like cotton, nylon, polyester, rayon, glass, carbon/graphite etc and • Non-fibrous mineral fillers like mica, talc, solid glass micro-spheres.

Higher content of fibrous reinforcement would impart higher mechanical properties to the material. It may not necessarily confer high rigidity.

Higher content of mineral reinforcement may give higher rigidity but poor tensile strength. Combination of fiber as well as mineral reinforcement can be possible. To enhance the bonding between various components, processing aid will be required in the compound.

In reinforced polymer, the stresses in the weaker polymer matrix is transferred to stronger fibers. Stress transfer between polymer matrix and fiber can be improved by incorporating binders or coupling agents in the compound. Smaller diameter and longer fiber length provides larger surface area per unit weight of fiber. This would help in enhancing the stress transfer in the compounded material.

Glass fiber is most popular reinforcement in polymers in thermoplastic as well thermoset polymers. It increases the mechanical properties of the compound by 1.5 to 2 times.

Carbon/graphite fiber reinforcement improves the mechanical strength by 3 times. It imparts conductivity to the material.

THINGS TO KNOW ABOUT POLYMER CHARACTERISITICS

• Molecular weight, • MFI, • pvT characteristics, • Response to shear rate (Shear thinning) and limitation, • Heat and Thermal stability, • Shrinkage behaviour, • Maximum flow ratio, • Hygroscopic or not.

PROPERTIES OF PLASTICS

POLYMERS DENSITY MELTING POINT

COEFFICIENT LINEAR THERMAL EXPANSION D696

THERMAL CONDUCTIVITY

SPECIFIC HEAT

Deg. C 10 -6 per C 10-4 cal / cm. sec C

cal / g C

ABS 1.05 Tg 110 -125 65 - 95 4.5 - 8

CA 1.29 Tm 230 80 - 180 4 - 8

POM 1.42 163 9.7 5.5 0.35

PMMA 1.18 100 -= 120 50 - 71 4.7 0.35

PPO 1.06 Tg 100 – 112 38 - 70 3.8

PA6/6 1.14 264 80 5.8 0.5

PC 1.22 Tg 150 68 4.7

PES 1.37 Tg 230 55 3.2 - 4.4

PBT 1.31 Tm 232 - 267 60 - 90 4.2 - 6.9

PPS 1.5 - 2.1 Tm 290 Tg 88 49 6.9 0.25

PETP 1.38 Tm 254 - 259 65 3.3 - 3.6

PS 1.05 Tg 74 - 104 50 - 83 3

HDPE 0.95

LDPE 0.92 109 -125 14 7-10 0.52 - 0.65

PP 0.91 165 -175 70 - 909 2.8 - 4 0.46

RPVC 1.3 - 1.6 75 - 85 190 3 0.26

SPVC 1.1 - 1.14

SAN 1.08 Tg 120 65 - 68 3

TPU 1.20 Tg 120 - 160

The plastics exhibit different characteristics than metal.

• LOWER DENSITIES. All plastics have low densities generally in the range of 0.85 to 2.5 g / sq.cm. These figures can be

o extended upwards ( up to 0.01 g/sq.cm) by using foaming additives or o downwards by using filled polymers.( up to 3.5 g / sq.cm.).

In comparison, density of aluminium is 2.7 g/sq.cm. and density of stainless steel is 7.9 g/sq.cm.

• TOUGHNESS : Some polymers are extreamly tough and virtually indestructible by mechanical treatment. Others are less tough and others are fragile.

• RESILIENCE : Plastics show some of the behaviour associated with rubbers in accomodating large strains without fracture and in recovering their original shape and dimensions when the stress is removed.

• VIBRATION DAMPING : The quietness of operation of plastics gear trains depends on inherently high degradation of mechanical energy to heat. Metal wire milk bottle crates rattle during transportation and handling whereas plastics crate does not produce irritating noise.

• RESISTANCE TO FATIGUE : Some plastics apper to perform remarkably satisfactorily in situations involving dynamic stresses or strains.

• LOW COEFFICIENT OF FRICTION : Plastics / plastics and plastics / metal combinations have low coefficient of friction and can often perform unlubricated without fear of seizing.

• CORROSION RESISTANCE : Plastics materials are chemical resistant. The degree of resistance is given in the table. Strong acids may cause some attack leading to discolouration and possible embrittlement. Some organic solvents, to which metals are generally inert, may cause swelling, deterioration of properties and eventually dissolution. The degreee of attack is dependent on nature of plastics, and of environment, temperature.

• Some polymer absorb moisture and expand or lose moisture and shrink depending on relative humidity of atmosphere.

• MIGRATION OF COLOUR pigment takes place in plastics.

• INTEGRATED DESIGN : The easy flow characteristics and properties offered allow the design and manufacture of polyfunctional complicated shapes with out the need for assembly.

• STRENGTH AND SURFACE HARDNESS : The general level of conventional tensile strength is not high, by metal standards, being in the range of 5000 - 10000 lb / sq.inch. Most thermoplastics can be scratched by pencils of 9H to HB, and hardest plastics - ACRYLIC - is comparable in hardness with aluminium. Abrasion resistance depends on exact condition of use, and ranges from excellant to poor. Nylon gears are known to outwear meshing metal gears.

• STRESS-STRAIN characteristic of metal is consistent and is not much influenced by temperature variations (except at very high temperature). Hence, modules of elasticity for metal is constant. It is also not influenced by rate of loading. Whereas, stress-strain curve for polymers (plastics) vary substantially with rate of loading and temperature. Hence, the modules of elasticity is not constant in plastics.

• MODULES OF ELASTICITY --KG/CM} •

GLASS 680,000 (varies)

WOOD 100,000 "

CONCRETE 200,000.

ALUMINIUM 680,000.

COPPER 1100,000.

STEEL 2000,000.

POLYETHYLENE 2500

POLYESTERS 31,000

ACRYLIC 27,500

TEFLON 117

NYLONE 17,200

POLYCARBONATE 20,000

POLYPROPYLENE 14,000

POLYSTYRENE 35,000

RPVC 27,500

ABS 30,000

• Modules of elasticity improves with the reinforcement in the polymer. • The STRENGTH in plastics is more in the direction of orientation. Oriented filament

(HDPE, PP, NYLON ) is stronger in tension than a bar or sheet of the same polymer. The blown film is stronger in longitudinal direction than transverse direction.

• THERMAL INSULATION : Plastics are good insulators, their conductivity much lower than that of metals.

• THERMAL EXPANSION : The co-efficient of thermal expansion in plastic is higher (3 to 10 times) than that in metal. It influences converting operation in plastics and also in service condition. Due allowance should be made at the design stage.

• THERMAL CONDUCTIVITY : Plastics are poor conductor of heat. Plastics are good insulators, their conductivity much lower than that of metals.

• TEMPERATURE RESISTANCE : Mechanical properties of many thermoplastics markedly affected by temperature. Continous use at elevated temperatures may also cause deterioration of plastics material with consequent loss of properties. However some polymers are servicable even at 150 deg. C.

• GLASS TRANSITION TEMPERATURE :

Specific volume v/s temperature - characteristics of polymer melt provides

o Tm= melting temperature and o Tg= glass transition temperature.

While cooling the melt, the specific volume of the melt sharply drops at a temperature which is termed as Tm.

While cooling non-crystalline polymer melt there is no sharp drop in specific volume and the melt becomes highly viscous and it appears like solid. Since the glass behaves in this manner the temperature at which the specific volume curve changes its slop is called Tg- glass transition temperature.

Polymer becomes :

o hard, stiff and brittle below Tg o highly viscous but solid at Tg o rubbery, flexible and softer above Tg

• HEAT DEFLECTION TEMPERATURE : The heat deflection temperature of a plastic is useful for assessing load bearing capacity at an elevated temperature. The sample is mounted on supports 4" apart and loaded as a beam. Abending stress of either 66psi or 264 psi is appliedat the center of the span. The test is conducted in a bath of oil, with temperature increased at a constant rate of 2 0 C per minute, The heat deflection temperature is the temperature at which the sample attains a deflection of 0.010in.

• UV RESISTANCE AND OUTDOOR WEATHERING : This can be improved markedly by special additives by suitable formulation.

• FLAMABILITY : Most plastics materials burn to a greater or lesser extent, although some are self -extinguishing and many can be formulated to be more flame resistant

• CREEP is a slow and continuous increase in deformation under a static load and is a permanent deformation. Creep failure in plastic occurs at stress well below the failure stress given by tensile test. Hence, the allowable stress in plastic should be lower than the creep stress. Environment temperature should also be considered while determining the allowable stress.

With the knowledge of characteristics and strength of material the plastic components are successfully designed, manufactured and used in automobile, textile, clock, electrical switch gears, instruments, computers, telecommunication equipments, home appliances, medical instruments and equipment's etc.

• Stress oriented area of moulded part may relieve stress and WARP. This can be controlled by part design and processing conditions.

• ELECTRICAL PROPERTIES : All plastics are electrical insulators, some are outstanding.

MECHANICAL PROPERTIES OF PLASTICS

MECHANICAL PROPERTIES

Tensile strength

Tensile modulus

Compressive strength

Compressive modulus

Flextural strength

Flextural

modulus

Izod impact strength

Hardness

POLYMERS psi 10 3 psi psi 10 3 psi psi 10 3 psi

ABS 4000 -7400 320 - 400 6500 - 7500 130 - 310 9000 - 14000

320 - 400

3 - 12 R100 - 120

ABS / PC 8400 - 9000 370 - 455 11200 - 11300 190 - 440 12000 - 13600

370 - 455

4.1- 10.5 R117 - 119

CA 3000 - 8000 2000 - 16000

CAB 50 - 200 2100 - 7500 50 - 200 1800 - 9300

50 - 200

PMMA 7000 - 11000 325 -470 10500 - 18000 370 - 460 10500 - 19000

325 - 460

0.3 - 0.6 M68 -105

PPO 9600 355 - 380 16400 9500 -14000

330 -400

5 R 118 -120

30% glass filled

16000 - 18500

1000 -1200 17500 20000 -23000

1100 -1150

1.7 - 2.3 R115 -116

PA6/6 8000 230 - 500 15000 17000 420 0.55- 1.0 R120

30-33% glass filled

28000 24000 - 29000 41000 1300 2.0 - 2.2 M100

PC 9000 345 125000 350 13500 340 16 M70

10% glass filled

9500 - 9600 500 - 600 13500 - 14000 520 15000 - 16000

440 1.2 - 2.6 M75 - R 118

30% glass filled

19000 - 20000

1250 - 1400 18000 - 20000 1300 23000 1100 1.7 - 2 M92 R119

PBT 8200 280 8600 - 14500 12000 - 16700

330 -400

0.8 - 1.0 M68 - 78

30% glass filled

14000 - 19000

1300 18000 - 23500 22000 - 29000

850 - 1200

0.9 - 1.6 M90

40 % glass ∓ mineral filled

12000 - 14500

1350 15000 18500 -23000

1250- 1600

0.7 - 1.7 M75 -86

PPS 9500 480 16000 14000 550 <0.5 R123

10% glass filled

19000 - 23000

1100 21000 29000 - 32000

1700 -1800

1.4 - 1.5 R123

Tensile strength

Tensile modulus

Compressive strength

Compressive modulus

Flextural strength

Flextural

modulus

Izod impact strength

Hardness

psi 10 3 psi psi 10 3 psi psi 10 3 psi

PETP 7000 -10500

360-450 11000 - 15000 14000 -18000 350 -450 0.25-0.65 M94-101

30 % glass filled

21000 -23000

1300 - 1440

23000 32000 - 33500

1250-11500

1.6 - 1.9 M100

40-45% glass mica filled

14000 - 17000

1950 21000- 25700 1400 - 1750

1.2 - 1.4 R 118

PS 5200 - 7500 339 475- 12000 - 13000 480 - 490 10000 - 14000

380 - 490 0.35- 0.45

M60 - 75

HDPE 3200 - 4500 155 - 158 2700 - 3600 2700 - 3600 145 - 225 0.4 -4.0

LDPE 1200 - 4550 25 - 41

PP- homo 4500 - 6000 160 - 225 5500 - 8000 150 - 300 6000 - 8000 170 - 250 0.4 - 1.0 R80 -120

PP-co 4000 - 5500 100 - 170 3500 - 8000 5000 - 7000 130 - 200 1.0 - 20 R50 -96

40% talk filled-homo

4300 - 5500 450 - 625 7500 8500 - 9200 450 -625 0.4 - 0.6 R94 -110

40% caco3 filled -homo

3400 - 5000 375 - 500 3000 -7200 5500 - 7000 360 - 450 0.6 - 1.0 R178 - 98

40%glass filled homo

8500 - 15000

1100 - 1500

8900- 9800 10500 - 22000

950 - 1000

1.4 - 2 R102 -111

RPVC 2800 50 - 80 0.3 - 1.0 R98 - 106

SPVC 3500 50 - 80 0.3 - 1.0 R98 - 106

SAN 10000 - 11900

475 - 560 14000 - 15000 530 - 580 11000 - 15000

500 - 580 0.4 - 0.6 M80 - R83

APPLICATIONS OF POLYMERS

Polymers will continue to replace other materials on an increasing scale. The polymers already perform satisfactorily in many applications previously employing metal, wood, paper, glass etc. Usage of polymers are already well established in Automobile, electronics, Telecommunication, Computer, Toys, Medical application, clock, houseware, plumbing, footwear, electrical switch gears, luggage,etc. New polymers with specific properties and applications are being developed. With the result , number of polymers with properties suitable for specific applications are now available.

Reasons for replacement of trditional materials:

• Availability of stronger, stiffer polymers. • Development of processing techniques to exploit the properties, • Design posibilities of plastics, • Availability of accurate, meaningful data on the mechanical properties

of polymers. • Greater willingness on the part of engineers to consider plastics as raw

materials in their own right, rather than substitute, • Increasing awarness of the cost saving, enegy saving, labour saving

and ease of manufacturing technique.

It is not correct to say that PLASTICS will generally and universally replace all materials. Need for replacement of any part should arise from functional requirement, ease of fabrication, cost with out compromising functional needs, lower weight, lower energy requirement. New polymers with specific properties and applications are being developed.

The part should be designed with plastics material, by considering the

• Functional needs • Service condition • Mechanical loading and duration of loading • Polymer melt behaviour ( flow, shrinkage, response to shearing ) • Strength of material • Properties of plastics and • Processing (conversion- fabrication) technique.

HDPE HIGH DENSITY POLYETHYLENE

LDPE LOW DENSITY POLYETHEYLENE

PP POLYPROPYLENE

PS POLYSTYRENE

CA CELULOSE ACETATE

RPVC RIGID POLY VINYLE CLORIDE

SPVC SOFT POLY VINYL CLORIDE

ABS ACRYLONITRILE BUTADINE STYRENE

POM ACETAL--POLYOXYMETHYLENE POLYFORMALDEHYDE

PMMA ACRYLIC--POLYMETHYL METHACRYLATE

PPO POLYPHENYLENE OXIDE

PA POLYAMIDE --NYLONE

PC POLYCARBONATE

PES POLYETHERSULPHONE

PBT POLYBUTYLENE TETRAPHALATE

PPS POLYPHENYLENE SULPHIDE

PETP POLYETHYLENE TETRAPHTHALATE

PEEK POLYTHERETHERKETONE

PP(copolymer) POLYPROPYLENE

SAN ACRYLONITRILE STYRENE

TPU THERMOPLASTIC POLYURATHANE

APPLICATION OF PLASTICS

POLY-MERS

OUTSTANDING PROPERTIES TYPICAL APPLICATIONS

ABS Toughness, Electroplatable Housing of home appliances,

TV and computor parts, ect.

POM Low friction, low wear. Little change in impact strength

with temp. Resistance to fatigue,

free from biological attack but

susceptible to UV radiation.

Bearing applications, gears,

digit wheels, sprocket, chain, cams, carburetor body, arosol

parts ect.

PMMA Excellent clarity &mp; transparency

Dimensionally rigid, Resistant to

outdoor weathering.

Lamp covers, lenses, reflectors

knobs, transparent panel knobs, covers.

PPO Excellent electrical properties

flame resistant, good toughness,

dimensionally rigid, resistant to

detergent.

Electrical parts, TV back covers , car dashboards ,washing m/c parts.

PA Wear resistant, tough, low friction, low fatigue, withstand temp. Electrical insulation property.

Bearings, gears, electrical socket

plug, cooling fan, powertool

housing, safety belt parts,

bathroom fittings, etc.

PC High impact strength, excellent

clarity, good dimensional stability good weathering resistant, low moisture absorption, high heat deflection temperature.

Replacement for glass, Transparent covers for instrument panel, lighting application. safety helmet, car lamp housing, goggles, lenses, food mixer parts, computer parts, connectors,

PES Lower flamability, execellant electrical. properties, excellent long term load bearing properties at elevated temperature, good toughness, dimensionally rigid.

: Aircraft parts, electrical parts.

automobile parts, micro oven dishes, grills, dishwasher parts, hair drier parts, projector fan.

PBT High rigidity, ultra-low water absorption, excellent electrical properties. withstand high temperature under load.

Electrical components, lamp housing, fuse cases, pump housing, toaster parts, hair drier parts.

PPS Excellent electrical properties, arc resistant, withstand high temperature. good dimensional stability,

Connectors, terminal blocks, socket, coil former, relay parts, lamp holder, switches, carburetor parts, ignition plate.

PETP High stiffness, excellent dimensional stability at elevated temperature, good electrical properties.

Rotary switches, contactors, circuit boards.

PP-co polymer

High impact strength, Improved heat stability, Housing with integral hinges, luggage, house wares, toys, interior parts of car, washing m/c parts, bottle caps, disposable syringe, crates, battery boxes. bobbins, dyeing cones.

PEEK Excellent long term bearing properties at high temperature of 200 degree C., strong, rigid, tough, excellent abrasion resistant.

Wire coating, parts for aerospace application.

SAN Excellent optical properties, tough, no weather resistant, Cup, picnic items, tray, cutlery, cassette storage racks, dials, cosmetics containers,

TPU Flexible, durable, oil resistant,. Seal, washers, rollers, watch straps

shoes soles.

APPLICATION OF COMMODITY PLASTICS

HDPE Bottle crates, containers, housewares.

LDPE Toys, bottle caps, lids, bowls, shopping bags.

PS Toys, containers, tape cassettes, disposable cups, transistor cabinet, appliance housing.

CA Toys, pen, handles for tools.

RPVC Pipe fittings, guttering, plumbing items,

SPVC Washers, soft tubes, soles, heels, footwear's.

PLASTICS PART DESIGN and MOULDABILITY

Injection moulding is popular manufacturing method because of its high-speed production capability . Performance of plastics part is limited by its properties which is not as strong (as good) as metal. There are applications

where the available properties of the plastics can be useful. The strength of plastics can be improved with reinforcement of glass fiber, mica, talk etc.

Plastics generally have following characteristics,

• Light weight - low density, • Low conductivity of heat and electricity - insulating properties, • Low hardness, • Lower strength than metals, • Ductile, • Dimensional stability- not as good as metal,

WALL THICKNESS

Solid shape moulding is not desired in injection moulding due to following reasons.

• Cooling time is proportional to square of wall thickness. Large cooling time for solid will defeat the economy of mass production. (poor conductor of heat)

• Thicker section shrink more than thinner section, thereby introduce differential shrinkage resulting in warpage or sink mark etc. (shrinkage characteristics of plastics and pvT characteristics)

Therefore we have basic rule for plastic part design; as far as possible wall thickness should be uniform or constant through out the part. This wall thickness is called nominal wall thickness.

If there is any solid section in the part, it should be made hollow by introducing core. This should ensure uniform wall thickness around the core.

What are the considerations for deciding wall thick ness?

• It must be thick and stiff enough for the job. Wall thickness could be 0.5 to 5mm.

• It must also be thin enough to cool faster, resulting lower part weight and higher productivity.

Any variation in wall thickness should be kept as minimum as possible.

A plastic part with varying wall thickness will experience differing cooling rates and different shrinkage. In such case achieving close tolerance becomes very difficult and many times impossible. Where wall thickness variation is essential, the transition between the two should be gradual.

CORNERS

When two surfaces meet, it forms a corner. At corner, wall thickness increases to 1.4 times the nominal wall thickness. This results in differential shrinkage and moulded-in stress and longer cooling time. Therefore, risk of failure in service increases at sharp corners.

SINK MARK IS INEVITABLE.

Temperature dependent change in volume - 29% in crystalline and 8% in amorphous-.

Compressibility of melt under pressure is 10-15%.

On falling temperature of melt in the mould, decrease in volume is more than the increase in volume on relaxation of pressure.

Therefore void can not be perfectly filled in. Hence sink mark is inevitable.

CHANGE IN VOLUME and DENSITY OF MATERIAL

Materials Specific volume AT 20

degree C

Specific volume AT

200 degree C

% age change

cubic-cm / g cubic-cm / g

HDPE (crystalline)

1.03 1.33 29 %

PS (amorphous)

0.97 1.05 8%

Density Density

HDPE (crystalline)

0.97 0.75 22.7%

PS (amorphous)

1.03 0.952 7.8%

To solve this problem, the corners should be smoothened with radius. Radius should be provided externally as well as internally. Never have internal sharp corner as it promotes crack. Radius should be such that they confirm to constant wall thickness rule. It is preferable to have radius of 0.6 to 0.75 times wall thickness at the corners. Never have internal sharp corner as it promotes crack.

RIBS for stifness consideration

Ribs in plastic part improve stiffness (relationship between load and part deflection) of the part and increases rigidity. It also enhances mouldability as they hasten melt flow in the direction of the rib.

Ribs are placed along the direction of maximum stress and deflection on non-appearance surfaces of the part. Mould filling, shrinkage and ejection should also influence rib placement decisions.

Ribs that do not join with vertical wall should not end abruptly. Gradual transition to nominal wall should reduce the risk for stress concentration.

Ribs should have following dimensions.

• Rib thickness should be between 0.5 to 0.6 times nominal wall thickness to avoid sink mark.

• Rib height should be 2.5 to 3 times nominal wall thickness.

• Rib should have 0.5 to 1.5-degree draft angle to facilitate ejection.

• Rib base should have radius 0.25 to 0.4 times nominal wall thickness.

• Distance between two ribs should be 2 to 3 times (or more) nominal wall thickness.

MOULDABILITY consideration

While designing plastic part, pitfalls in achieving quality, consistency and productivity must be considered. It is wrong to assume that shapes can be moulded successfully with out any defects. All shapes may not be 100% mouldable. To improve the mouldability injection moulding process has to be understood in depth.

Part design obviously has to be influenced by the intricacies of the process.

Filling phase of the process is influenced by type of gate, location of gate, number of gates, size of gate (also dependent on material viscosity). Gate

should be located at such a position from where flow path to thickness ratio (flow ratio)is constant in all direction. The difference in flow ratio could be as small as possible. In some cases where thickness variation is unavoidable, melt must flow from thin section to thick section for better mouldability. Melt flow from thin to thick results in poor moulding. The size of gate should not result in excessive pressure drop across it. It should be adequate to handle flow rate required.

Resistance to flow and viscosity determines the filling pressure. Filling pressure variation should be gradual and not abrupt. It should be remembered that flow thinner section introduces shearing of melt, resulting in lowering of melt viscosity. This is the shear thinning nature of thermoplastics melt.

Filling phase is influenced by wall thickness variation as it introduces variation in resistance to flow in all directions from the gate. Melt is held in cylindrical shape in plasticating cylinder before injection. When the melt is injected through gate and runner system, melt streams move equally in all directions only when resistance to flow is equal in all direction.

It should be realised that variation in wall thickness, hole / slot, variation of mould surface temperature introduces variation in resistance to flow. Therefore melt moves in number of streams with different velocity in different direction and mould does not fill in balanced manner.

When melt streams reach boundary at the same time it can be called balanced filling. When some stream reaches the boundary early and some other streams reach late - this time lag to complete the filling of part results in induction of moulded-in stresses in the part.

See various results of Moldflow Analysis.

Unbalancing flow can be corrected by using flow-leader / flow deflector and multiple gates so as to form the melt stream shape very close to the projected shape of the part.

Ideally all the melt streams should move with the same velocity till the mould is filled. Variation in cross section area (due to changes in wall thickness or slot) introduces variation in melt stream velocity. Hence the freezing of melt can not be uniform through out the part. It should be realised that while freezing, cross section through which melt can flow reduces thereby introducing increasing resistance to flow. When some stream freeze faster then other, faster freezing streams introduce increasing resistance to flow. Therefore, balance in filling can not occure and moulded-in stresses are induced.

WELD LINE IN MOULDING

Weld line ocures when two melt streams join. Melt stream gets divided at cutout (core) in the part and they join at the other end of the cut out.

Normally weld line region is filled at the end of injection stroke or during pressure phase.

Strength of the weld line is weak when partially frozen melt front meet. The orientation at the joint remains perpendicular to direction of flow -a sign of weakness.

Weld line can form by melt stream flowing in same direction or in opposite direction.

It is not possible to eliminate weld line, but it can be made sufficiently stronger or its position can be altered.

MELT STREAM FROM OPPOSITE DIRECTION

CHANGING WELD LINE POSITION

Over cooled region can also freeze faster than lesser cooled region. When freezing is not uniform, melt moves through narrowing cross section of slow freezing stream and overpacks the slow slow freezing stream region. Hence uniform mould surface temperature distribution is very important. This has to be achieved through proper design of cooling channels for turbulent water flow.

Melt temperature is highest near the gate. Hence freezing likely to be slower near the gate. This happens near the gate during pressure phase of the

process. Here over packing can be controlled through proper profiling of pressure - reducing with time.

COOLING consideration

Volumetric changes associated with changes in temperature and pressure should be understood well. Click here see pvT characteristics of thermoplastics.

Dimensional variation of mould cavity and core during moulding, moulded part before ejection and after ejection and thereafter sever hours later is described in this figure.

Balance in heat exchange during a cycle time ensures the consistency moulding.

EJECTION consideration .

Adequate draft angle, good surface finish, mechanism to handle undercut, stregic location of ejector pins etc should be the consideration of part designer.

SUMMARY

Design Factors To improve mouldability, understand the following;

Gate • Ideally at geometric center of the part. • Melt stream shape is similar to projected shape of

the part by multiple gate or suitable type and size of the gate.

• Locate gate at thickess section so that melt flow from thick to thin section.

Wall Thickness • No variation in wall thickness. Larger the variation means poorer mouldability. Rib thickness 50 -60% of wall thickness.

Pressure drop in runner system

• Runner system should be designed for high pressure drop, thus minimising material in runner, in order to give low runner to part weight ratio.

Flow pattern • Distance (L/T ratio) from gate to boundary in all direction, if not same, provide flow leaders or flow deflectors to balance the flow to improve mouldability.

• Lower the difference in L/T ratios in different

direction, better the mouldability.

Melt temperature variation in side mould

• Variation of melt temperature should be with in 10 degree centigrade. Shearing through narrow wall increases melt temperature.

Filling Pressure • The good mouldability occur when pressure gradient i.e. pressure drop per unit length, is constant along the flow path.

Maximum Shear Stress

• The shear stress during filling should be less than a critical value. This critical value depends on material and application.This data is available with Moldflow software.

Melt stream velocity

• Ideally, all melt streams move at same velocity.This can ensure same cooling time for all melt streams.

• Difference in velocities as less as possible for better mouldability

Avoid hesitation effect

• Melt flow from thick to thin section is better for mouldability.

Weld-lines • Weld-line distance from gate should be as less as possible for better mouldability.

• Weld line can be shifted by using frame of suitable thickness.

Hold-on pressure

(not desin factor but processing factor)

• Multi steps with reducing pressure with time to avoid moulded-in stress near the gate.

Thermal shut off of runners.

• The runners must be sized for thermal shut off when the cavity is just filled and sufficiently packed, to avoid overpack or reverse flow, in and out of cavity, after the mould is filled.

Heat exchange • Consistent mould temperature can only be ensured when there is balance between heat in and heat out during moulding cycle time. Cooling channels must

be designed with the help of MoldFlow software.This should ensure uniform cooling time to enhance mouldability.

Core and Cavity dimensions

• Core and cavity Dimensions computed taking into consideration mould-makers tolerance, mould shrinkage and post moulding shrinkage.

Easy ejection • Proper taper on the part and smooth polished mould surface facilitate easy part ejection.

MECHANICAL consideration.

BOSSES

The boss is required for fixing or mounting some other part with screw. It is cylindrical in shape. The boss may be linked at base with the mother part or it may be linked at side. Linking on side may results in thick section of plastic, which is not desirable as it can cause sink mark and increase cooling time. This problem can be solved by linking boss through a rib to the side wall as shown in the sketch. Boss can be made rigid by providing buttress ribs as shown in the sketch.

Screw is used on the boss to fasten some other part. There are thread forming type of screws and tread cutting type of screws. Thread forming screws are used on thermoplastics and thread cutting screws are used on inelastic thermoset plastic parts.

Thread forming screws produce female threads on internal wall of boss by cold flow - plastic is locally deformed rather than cut.

Screw boss must proper dimensions to withstand screw insertion forces and the load placed on the screw in service.

• The size of the bore relative to the screw is critical for resistance to thread stripping and screw pull out.

• Boss outer diameter should be large enough to withstand hoop stresses due thread forming.

• Bore has slightly larger diameter at entry recess for a short length. This helps in locating screw before driving in. It also reduces stresses at the open end of the boss.

• Polymer manufacturers give guidelines for determining the dimension of boss for their materials. Screw manufacturers also give guidelines for the right bore size for the screw.

• Care should be taken to ensure strong weld joints around the screw bore in boss.

• Care should be taken to avoid moulded-in stress in boss as it can fail under the aggressive environment.

• Bore in boss should be deeper than the thread depth.

Quality of screw connection in plastics

Screw connection would obviously be successful only if driving torque is less than the stripping torque. Torque required to drive in the screw is driving torque. The torque required to tear away the internal thread is called stripping torque. Boss should be designed with factor of safety higher than 2. The ratio of stripping torque to driving torque should be more than 2 and preferably 5.

Stripping torque depends on

• Thread size and • Boss material.

Stripping torque increases as screw penetrates and tends to level off when the screw engagement is about 2.5 times screw pitch.

Driving torque depends on

• Friction and • Ratio of bore size to screw diameter.

When force required to hold something down exceeds the screw pull out force, the screw thread in the plastics boss will shear off .

Pull out force depends on

• Boss material, • Thread dimensions and • Length of screw engagement.

INJECTION PROCESS

The process involves

• Filling phase, • Switchover point, peak pressure occur after slight delay. • Pressure phase, • Cooling phase.

Sequence in the process

• It is desirable to have highest possible filling speed (Injection Rate) during filling phase .

• Filling speed is normally limited by limitation of shearing in engineering polymers. Highly thermally stable material - commodity polymers- can be filled at high filling rate with out any fear of degradation of polymers melt.

• Filling phase covers 85 to 98% of total injection stroke .

• High speed filling is terminated at Switch-over point and low speed Pressure phase commences.

• Due to relaxation of compression (10-15%) of melt during injection stroke, melt expands and fills up remaining empty space in the mould. It overfills to raise the cavity pressure as relaxation completes.

• If we don't leave any space for relaxation of melt then, tremendous amount of pressure generated. This can damage the mould.

• Since mould surface is maintained at lower temperature (than that of melt), melt cools and hence tries to lower its volume as per the pvT characteristics.

• At this point the compensation for shrinkage is supplied at lower speed. Since less than 10% of space is to be filled, it does not require high speed filling. The cavity pressure is just to be maintained in such a manner to compensate for void created by the shrinking melt on account of falling melt temperature. The pressure phase has to be maintained till the melt transfer stops.

• There after the melt has to be cooled further, below heat defection temperature of material. This would prevent deformation of moulding during ejection.

Important note

During filling phase, pressure value should be set 15-25 bar above the actual pressure encountered in the cavity. This is because if the relief valve is actuated during the filling phase, oil would drain and you loss control over the speed. In case of doubt, it is ok to set value at about 80% of maximum pressure for the machine through out the filling phase. The value of maximum hydraulic system pressure is recommended in the manual. To maintain the constant melt front velocity, it is required to adjust the injection speed in stroke dependent multiple steps.

During pressure phase, the filling speed is low and constant through out the pressure phase and pressure can be adjusted in time dependent multiple steps to avoid moulded-in stress.

SETTING POSITION SPEED AND PRESSURE

PRINCIPLE for SETTING OF INJECTION SPEED, PRESSURE AND POSITION

POSITION SPEED PRESSURE

FILLING PHASE

Also known as Speed phase.

Select end of 1st step and; start of 2nd step.

Set SPEED 1

Try to set high.

Also known as Speed phase.

There may be no. of steps available on machine.

Select end of 2nd step and; start of 3rd step

Set SPEED 2

Lower speed for crossing narrow passage / gate

Steps are position controlled.

Select end of 3rd step and; start of SWITCH OVER POINT. This point is at around 80-95% of the injection stroke.

Set SPEED 3

Reduce to lower sink mark / increase to shift weld line.

Only one pressure setting is required during FILLING PHASE.

Pressure Setting should be more than actual filling pressure. as relief valve should not be actuated. If it is actuated, then speed control will be lost.

Filling pressure depends on resistance to move the melt. It depends on flow ratio and viscosity of melt.

PRESSURE PHASE-

Holding phase.

There may be no. of steps available on machine.

Timer controls the Holding pressure steps if available on machine.

Holding pressure time for step 1 is set on a timer,

Holding pressure 1 set just enough to fill cavity without overpacking.

Steps are timers controlled.

Holding pressure time for step 2 is set on a timer,

Holding pressure 2 set just enough to fill cavity without overpacking.

Holding pressure time for step 3 is set on a timer,

Set SPEED low value say up to 35% not more.

This can be one step of speed for different Holding steps.

Speed set is low, as there is less or no space to move the melt.

Holding pressure 3 set just enough to fill cavity without overpacking.

PLASTICATING

Plasticating refers to conversion of plastic granuals to flow-able melt. It happens inside the screw barrel assembly of the injection unit in the Injection Moulding Machine.

• The plastic granules move inside the screw channel when screw is rotated.

• The screw has three sections. FEED ZONE, COMPRESSION ZONE and METERING ZONE.

• In the compression zone the material is gradually compressed. It therefore rubs against the barrel wall. This sets up shearing forces on the material.

• Plastic material under shear changes its viscosity. This is SHEAR THINING characteristic of plastics.

• Melt is then homogenised in metering zone.

At the tip of the screw, non-return valve is fitted. It allows the melt to flow ahead through this valve while screw is rotated.

Non return valve does not allow the melt to slip back through it to the screw channel.

BARRIER SCREW DESIGN

MOULDING PROBLEMS- Mould, Machine and Material related

MOULDING PROBLEMS

MOULD related MACHINE related

IN-CONSISTENT CRITICAL DIMENSIONS

• Unequal shrinkage due to variation in mould surface temperature.

Cooling circuit design related.

• Core and cavity dimension related. Correct shrinkage is

Hold-on pressure and hold-on time influences the shrinkage. High pressure and time reduces the shrinkage. Lower pressure and time increases the shrinkage of the part. shrinkage in the direction of orientation is more.

not considered for core / cavity dimension.

dimensions could be controlled by controlling cooling rate in different areas of the mould when wall thickness is not constant. Cooling rate could be increased by using Ba-Cu insert in the mould.

SINK MARKS Part design related- non uniformity of wall thickness.

• Injection speed profile and hold on pressure profile can minimise to some extent.

• Gas injection Moulding technique can be considered.

WARP • Use technique to improve part stiffness with part design.

• Random distribution of mould surface temperature. Cooling circuit design related.

...

WEAK WELD LINES

Part design related. Injection speed can influence.

DEGRADATION Check dimensions of runner system for excessive shearing.

Excessive Residence time due to use of oversized injection unit.

Lowering barrel temperature and injection speed can have some influence

EJECTION DIFFICULTY

Part design related -Unbalanced melt flow. Use flow leader to reduce unbalance in flow.

...

POOR IMPACT strength

Radius at projections and sharp corners as it act like notch from where crack propagates.

...

IN-CONSISTENT WEIGHT.

... % age utilisation of shot capacity is closer to 90% resulting in inconsistent melt quality- if metering stroke is

more than 3 times screw diameter. Borderline case for shot weight.

IN-CONSISTENT FILLING

Increase gate size to reduce pressure drop across the gate.

Available maximum injection rate is not adequate for flow ratio of part being moulded. Borderline case for injection rate.

STRESS CRACKING

Melt flow is unbalanced. Part design related.

Over packed part. Set correct pressure profile during pressure phase.

... MATERIAL RELATED PROBLEMS

SINK MARKS Filled polymer can reduce sink mark.

POOR MECHANICAL STRENGTH

Consider high molecular weight polymers.

Consider filled polymers. Pre heat polymer in dehumidifier if required for polymer as recommended for polymer.

DIMENSIONAL VARIATION

Check shrinkage characteristics of material.

DEGRADATION OF MELT

Check thermal stability of polymer as well as pigments and other additives if any used with material.

DIFFICULTY IN FILLING

Consider polymer of higher MFI

FACTORS INFLUENCING QUALITY OF MOULDING

MATERIAL MOULD MACHINE

Characteristics of Polymer

Design factors in Part and; Mould

Specification to meet the requirement of Part and; Mould

• Molecular weight,

• MFI, • pvT

characteristics,

• Response to shear rate (Shear thinning) and limitation,

• Heat and Thermal stability,

• Shrinkage behaviour,

• Maximum flow ratio,

• Hygroscopic or not.

• Gate location, size and type,

• Sprue / runner size balanced type,

• Air vent, • Wall thickness

uniform and capable of promoting balancing of melt flow,

• Balancing of flow in un-symmetric part geometry,

• Mould release.

• Maximum shot capacity of plasticating unit,

• Residence time, • Maximum injection

rate, • Number of stroke

controlled steps for filling phase,

• Maximum injection pressure,

• Number of time controlled steps for pressure phase,

• Plasticating rate, • Clamp force.

PROCESSING LIMITATIONS FOR VARIOUS POLYMERS- GUIDEL INES

Poly-mer

Density

Room

Temp.

Process temp.

degree c

Density process temp

%age injection utilisa-tion capacity

Permissible residence time - min.

Max-flow ratio for 1mm wall thickness

Screw cushion

mm

Shrinkage %

Output ratio related to

Ps

Screw rpm related to

Ps

ABS 1.05 220-260

0.96 40 - 80 100-140 4-6 0.4 - 0.7 0.83 0.83

CA 1.29 170-250

1.10 50 - 80 5- 8 300 3 0.4 - 0.7 NA NA

POM 1.42 192-215

1.16 30 - 80 15 100-230 2 - 6 2 NA NA

PMMA 1.18 220-260

1.09 35 - 80 4- 8 100-130 2 - 6 0.4 - 0.8 0.94 0.74

PPO 1.06 250-290

NA 30 - 80 8 100-140 3 - 4 0.4 - 0.7 NA NA

PA6 /6 1.14 220-280

0.95 40 - 80 5 100-230 2 - 4 NA 0.58 0.50

PC 1.22 280-310

1.08 50 - 80 2- 3 60-100 4 0.7 - 0.8 0.67 0.57

PES 1.37 330-400

NA NA NA 60- 120 4- 5 0.6 0.95 0.83

PBT 1.31 220-270

NA 40 - 80 3- 4 125-185 4 NA NA NA

PPS 1.5 - 2.1

300-360

NA 50-80 2- 4 120-150 4 - 6 0.2 NA NA

PETP 1.38 260-300

1.2 50 - 80 2- 4 50-90 4 1.3 - 1.5 0.8 0.74

PS 1.05 220-270

0.95 25 - 90 2-4 150-200 4 0.45 1.00 1.00

HDPE 0.95 220-280

0.74 10 -85 170 2 - 6 1.5 - 2 0.73 0.94

LDPE 0.92 180-280

0.74 10 - 95 200 2 - 6 NA 0.82 1.05

PP 0.91 250-275

0.73 10 - 85 170 4 1.2 - 2.2 0.63 0.86

RPVC 1.3 - 1.6

170-190

NA 25 - 80 20 60 0.5 - 0.2 NA NA

SPVC 1.1 - 1.14

180-200

NA 20 - 80 15 180 4 1 .2 NA NA

SAN 1.08 240-270

0.99 40 - 80 5- 6 140-180 4 0.5 0.96 1.12

TPU 1.20 180-225

NA 40 - 80 30 50 3 1 - 1.15 NA NA

A TYPICAL BARREL TEMPERATURE SETTINGS -

(appro.) Ascending profile

Polymer Melt

Temp

Feed

Throat

Rear Middle Front Nozzle Polymer Melt

Temp

Feed

Throat

Rear Middle Front Nozzle

. C C C C C C

. C C C C C C

ABS 220

260

35

40

150

180

180

230

210

280

222

280

PETP 260

280

60

80

240

250

245

255

250

260

250

260

CA 170

250

40 135

165

140

185

165

200

185

200

PS 220

270

20

30

150

200

180

230

210

260

220

280

POM 195

245

30

40

150

180

180

200

190

215

195

215

HDPE 220

280

20

30

160

230

200

260

220

280

210

270

PMMA 220

260

50

60

135

180

185

200

200

250

200

250

LDPE 180

280

20

30

120

200

180

260

200

280

210

270

PPO 250

290

40 190

240

230

270

250

290

240

275

PP 250

270

20

30

150

210

210

250

240

290

240

300

PA6/6 220

250

60

80

265 260 280 280

RPVC 170

190

30

40

135

160

165

180

180

205

180

210

PC 260

310

70

80

235

270

285

310

305

350

310

350

SPVC 180

200

40 125

150

150

175

160

200

150

200

PES 330

400

NA 320

390

350

410

360

430

360

420

SAN 240

270

NA 150

200

200

250

210

260

210

250

PBT 240

275

NA 320

260

250

270

260

275

255

270

TPU 180

225

NA 150

190

170

200

180

210

190

220

PPS 300

360

NA 290

310

300

320

310

360

305

320

THERMAL STABILTY CURVE

You can observe two regions - operating region and degradation region in the graph. If you maintain the material in operating region moulding can be with out silver streaks / gas bubbles. If the condition of material shifts to degradation region, then you must expect silver streaks / gas bubbles in the moulding.

During the production run, varying residence time can vary the colour shade, if the pigment used is sensitive to residence time.

Residence Time :

Percentage Utilisation of Machine shot capacity

(K x Max. Shot Volume or weight of Machine)

Residence Time = -------------------------------------------- x Cycle Time Actual Shot Volume or weight of Mould

K x Cycle Time Residence Time = -------------------------------------------------------------- (Actual Shot weight of Mould / Max. Shot weight of machine) K x Cycle Time x 100 Residence Time = ---------------------------------------------------------- Percentage utilisation of max- shot capacity of machine K x Cycle Time x 100 Min.%age UTILISATION of shot capacity of m/c = ---------------------------- Permissible residence Time

Example

Consider Max Shot Weight of m/c=200grm.; Thermal stability is 200 sec for melt.

Actual shot weight of mould 50 grm with Cycle time of say 30sec.

It was found practically that K=2 for 50 dia screw of L/D=18.

Residence time=(400/50) 30 = 240 sec. If thermal stability is for 200 sec at moulding temperature then melt would degrade. However if cycle time is 20 sec then residence time would be 160 sec. This cycle time would be workable with out degradation.

If actual shot weight of mould is say 70grm, then residence time=(400/7)30=

say 6 x 30=180sec then it is possible to run with out degradation. It would degrade only if there is interruption during production run resulting in increased residence time.

Understanding Dimensional consistency:

Shrinkage and Warpage

Dimensional in-consistency and warpage are on account of shrinkage behaviour of the plastic melt in mould. Plastic melt shrinks volumetrically- in all three dimensions. If shrinkages are equal in all the three directions, then part size reduce with out any distortion. In reality, it does not happen that way. Shrinkages along the direction of flow is more that that in transverse direction. This is called area shrinkage and it is with constrains. Wall thickness reduces due to shrinkage with out any constrain. This unequal shrinkage causes distortion of the part that is called warpage. Linear shrinkage can be influenced by restrains in the mould, Crystallinity and orientation.

Shrinkage and Crystallinity

Crystalline content increases with lowering of cooling rate and decreases with faster cooling rate. Higher crystallinity means higher shrinkage. It means that part dimensions could be controlled by controlling cooling rate in different areas of the mould when wall thickness is not constant. Cooling rate could be increased by using Ba-Cu insert in the mould.

Shrinkage and Process

Melt follows its PVT characteristics described by Wander Walls equaltion ie. Relationship of p,v and T. Here specific volume reduces with reduction of temperature at constant pressure. At filling phase, melt compresses 10-15% and hence it relaxes at switch-over point. This causes the smooth change over from filling to (Hold-on/ follow-up) pressure phase.

Hold-on pressure and hold-on time influences the shrinkage. High pressure and time reduces the shrinkage. Lower pressure and time increases the shrinkage of the part.

Melt must spread with uniform velocity in all direction from the gate. Velocity and freezing rate should be such that it should be possible to maintain flow up to the boundary of the part. Gate should freeze last as hot melt flowing during filling phase keeps the gate region hotter that the end of flow region. Faster freezing of part may not allow full part to be filled, even if just filled it may shrink more. Slower freezing of part would extend the cooling time. Gate must freeze after adequate melt moved from nozzle to the gate to compensate for shrinking volume of the part.

The problem occurs when part has varying wall thickness. Thinner section freeze faster than thicker section. Hence cooling design should take care of such problems by placing cooling channel should be closer towards thicker section and cooling channel at farther distance from the thinner section. Thicker section of the part can have Ba-Cu insert. Ba-Cu insert would cool faster because of it's high thermal conductivity.

Shrinkage can vary from gate to the end of flow path (boundary) if melt does not freeze uniformly. It is less at the gate and more at the end of flow. This can cause distortion.

Shrinkage and Orientation

Alignment of polymer chains are stretched in parallel -oriented- with shearing. With high cooling rate this orientation can be trapped in the moulded part. This can result in stressed area in the moulded part. With low cooling rate the

stretched chain gets chance to relax and hence shrink to natural chain. Hence shrinkage in the direction of orientation is more.

In a wall section, it is observed that, the frozen layer in contact with core and cavity is formed with least shear-least orientation. Next layer of melt is subjected to more shear stress. It freezes the moment flow stops, thereby trapping the orientation. The next -central layer is subjected to less shear and cooling rate is also less here. This gives more time for oriented chains to relax in the melt. Hence orientation is less in the central layer. It means that wall thickness consist of layers of different tensions (stress). If the core and cavity surface temperatures differ, then, it is bound to distort.

CYCLE TIME

Any manufacturing activity would like to have optimised productivity and quality. In injection moulding of plastics, if quality is taken care of by part design, mould design and mould precision, then productivity is also ensured on account of zero defect moulding with out rejection and optimised cycle time.

Cycle-time optimisation starts at design stage. Cooling time takes up over 50% of cycle time. Therefore understanding of cooling in the mould becomes very important.

Injection moulding is a cyclic operation. The cycle consist of

• Mould close and clamp, (few seconds -depends on machine speeds)

• Injection - Fill (speed) phase, (few seconds) Switchover and Pack (pressure) phase,(few seconds)

• Cooling time, (40 to 60% of cycle time)

You will observe that total injection stroke is divided in to two phase by a switch over point on the scale of injection stroke. First phase has 80-95% of injection stroke and remaining part of stroke is for pressure phase.

The melt gets compressed in plasticising cylinder prior to entry of melt in to the mould. At the end of filling phase inside the mould, melt is relaxed - i.e. it expands resulting in filling up of remaining space in the mould. This causes pressure peak inside the cavity.

Initially there is no resistance to flow of melt. Resistance increases as the cavity is being filled up -as more and more resistance felt-. This is seen as pressure rising before switch-over. Fill pressure is the measure of resistance to flow of melt. Resistance is directly proportional to melt viscosity and maximum length of flow and inversely proportional to thickness of flow. It means Viscosity &mp;mp; Flow ratio.

During pressure phase, the melt flows into the mould in order to compensate for the shrinkage due to falling melt temperature.

Fill pressure drops to minimum with increasing fill time in the beginning. This is due to isothermal behaviour of melt before switchover. There is enough space for melt expansion resulting in fill pressure drop during shorter fill time.

Further increase in fill time rises the fill pressure. This is due to heat exchange in mould. Falling melt temperature means increasing melt viscosity. This in-turn responsible for increasing pressure.

At lower melt temperature, fill pressure is higher on account of higher melt viscosity. With glass reinforcement, melt visosity increases, hence fill pressure increases.

This equation gives minimum cooling time.

Alfa is the thermal diffusivity of the material, h, is the wall thickness, Tw is the mold wall temperature, Tm is the melt temperature, capital Te is the ejection temperature.

An example calculation has been shown here with typical values for the different variables. In the example, the minimum cooling time for the part centerline to reach the ejection temperature is calculated to be 23 seconds.

It is observed that cooling time is proportional to square of wall thickness.

Cooling time increases in a non-linear fashion with increasing part wall thickness.

The cooling time for a semi-crystalline material like Polybutylene Terephthalate is always higher than that for an amorphous material like a blend of Polycarbonate and ABS.

Cooling Channel Design for Mould- Design tips

Moulds are usually built with cooling channels. These channels are usually connected in series with one inlet and one outlet for water flow. The water flow rate may not be enough for turbulent flow because the water pump capacity itself may not be adequate. This obviously leads to random temperature variation on the mould surface. With the result, uncontrolled temperature drift, varying part dimensions and irregular warped surface appears on mouldings.

The mould designer should take care of following points:

• Thermal conductivity of mould steel influences the rate of heat transfer though mould steel to cooling channel.

• Pure Ethylene glycol can be used as Primary fluid transfer medium in closed loop cooling system. Ethylene glycol does not produce rust and mineral deposits in cooling channels. Mixture of water and Ethylene glycol can also be used for circulation through the cooling channel.

• Cooling channel diameter should be more for thicker wall thickness:

• For wall thickness upto 2mm, channel diameter should be 8 - 10 mm., • For wall thickness upto 4 mm, channel diameter should be 10 - 12 mm., • For wall thickness upto 6 mm, channel diameter should be 10 - 16 mm.

• Cooling channels should be as close as possible to the mould cavity / core surfaces. The distance of cooling channel from mould surface should be permissible by the strength of mould steel against possible failure under clamp and injection forces. It could be 2 to 2.5 times diameter of cooling channel.

• The difference between the inlet and outlet water temperature should be less than 2 to 5 degrees C. However, for precision moulding, it should be 1 degree C or even 0.5 degree C.

• Cooling circuits should be positioned symmetrically around the cavity. There can be sufficient number of independent circuits to ensure uniform temperature along the mould surface.

• The coolant flow rate should be sufficient to provide turbulent flow in the channel.

• There should be no dead ends in the cooling channels. It could provide opportunity for air trap.

• Many a times it is difficult to accommodate cooling channels in the smaller cores or cores with difficult geometry. In such case the core should be made of Beryllium copper which has high thermal conductivity. These core inserts should be located near the cooling channel.

• The seals of coolant system should not leak inspite of application of frequent clamping force and mould expansion / contraction due to thermal cycle during moulding. The O-ring should be positioned so that there is no chance of them being damaged or improperly seated during mould assembly. Seal and O-ring grove should be machined to closely match the contour of the seal. It should ensure that seal is slightly compressed when the mould is assembled.

• Mould temperature above 90 degree C normally requires oil as the heating medium. Heat transfer coefficient of oil is lower than that of water.

• There is enough scope for confusion while giving water connection to mould when there are more number of cooling circuits particularly on bigger moulds. A sketch indicating cooling circuits should be available during mould set up.

• Hot runner mould should be provided with compression resistant insulating plate between back plate and machine platen. This is to prevent the heat flow from mould to machine platen, which can create an unbalanced heat flow in the mould. With out insulating plate machine platen will act like a big heat sink, there by destabilising the possible balance between heat given to the mould by the hot melt, and heat taken away by circulating water through mould.

• The cooling channel layout is suitable when the isothermal i.e. the equi-potential lines, are at a constant distance from surface of the mouldings. This ensures that heat flow density is same everywhere.

• Provision for thermocouple fixing should be available at specific one or two places in core as well as cavity to monitor the temperature of mould.

• Use efficient sealing methods and materials to eliminate cooling leaks.

• Poor mould surface temperature control can cause following quality problems: Axial eccentricity, Radial eccentricity, Angular deviation, Warpage, Surface defects, Flow lines,

The mould has to be heated or cooled depending on the temperature outside mould surface and that of environment. If heat loss through the mould faces is more than the heat to be removed from moulding, then mould has to be heated to compensate the excess loss of heat. This heating is only a protection for shielding the cooling area against the outside influence. The heat exchange takes place during cooling time. The design of cooling system has to depend on that section of part, which requires longest cooling time to reach demoulding temperature.

Cooling Channel layout depends on :

• part geometry, • number of cavities, • ejector and cam systems, • part quality, • dimensional precision, • part surface appearance, • polymer etc.

The sizing of cooling channels is dependent on the rate of cooling and temperature control needed for controlling part quality. CAE software like MOLDFLOW or C-Mold can be used to determine the optimised dimension of cooling channel and distance from mould surface, distance between cooling channel, flow rate.

Understanding & Design of Cooling Channels for Moulds and

Cooling Lines with Cooling Tower for Injection moul ding shop.

Introduction

Injection moulding process is cyclic in characteristic. Cooling time is about 50 to 75% of the total cycle time. Therefore, optimising cooling time for best performance is very important from quality and productivity point of view.

Cooling time is proportional to square of wall thickness. Therefore part design should ensure more or less uniform wall thickness through out the part.

Part design should also ensure that the melt flow is uniform in all direction from the gate and melt should reach the boundary of the part more or less at the same time.

Cooling channel design - location and size and type - should ensure that melt freezes uniformly inside the mould. Cooling channel design can be perfected with the help of MOLDFLOW analysis.

It is necessary to understand Heat Exchange and Cooling Channel design in the mould.

Heat Exchange in mould

During every injection moulding cycle following heat transfers take place:

• from the hot melt to mould steel (heat input to the mould) and

• from mould steel to coolant flowing through cooling channel of the mould. (heat removal from the mould)

If heat input is more than heat removal, then the mould temperature would keep on increasing from cycle to cycle. Therefore moulding quality would not be constant from cycle to cycle. The moulding quality would be erratic- i.e. varying from cycle to cycle. Therefore, there is a need to balance between the heat input and heat removal in the mould after the desired mould surface temperature is reached. In other words, removal of heat by circulating coolant through the mould cooling channel would arrest the rise of mould temperature above the desired value. In practice, it may not be possible maintain constant mould temperature with respect to time. However, the mould temperature would fluctuate between two values around the desired value.

During injection moulding cycle heat flow takes place from polymer melt to mould steel by

• effective thermal difusivity of polymer melt and

• conduction.

This heat is to be removed by circulating cooling fluid through the cooling channels in core as well as cavity during cooling period in order to maintain the desired temperature. Uneven temperature of the mould surface results (uneven shrinkage) in parts with moulded-in stresses, warped sections, sink marks, poor surface appearance and varying part dimensions from cycle to cycle and even cavity to cavity.

Cooling Channel Design for Mould- Design tips

Moulds are usually built with cooling channels. These channels are usually connected in series with one inlet and one outlet for water flow. The water flow rate may not be enough for turbulent flow because the water pump capacity itself may not be adequate. This obviously leads to random temperature variation on the mould surface. With the result, uncontrolled temperature drift, varying part dimensions and irregular warped surface appears on mouldings.

The mould designer should take care of following points:

• Thermal conductivity of mould steel influences the rate of heat transfer though mould steel to cooling channel.

• Pure Ethylene glycol can be used as Primary fluid transfer medium in closed loop cooling system. Ethylene glycol does not produce rust and mineral deposits in cooling channels. Mixture of water and Ethylene glycol can also be used for circulation through the cooling channel.

• Cooling channel diameter should be more for thicker wall thickness:

• For wall thickness upto 2mm, channel diameter should be 8 - 10 mm., • For wall thickness upto 4 mm, channel diameter should be 10 - 12

mm., • For wall thickness upto 6 mm, channel diameter should be 10 - 16 mm.

• Cooling channels should be as close as possible to the mould cavity / core surfaces. The distance of cooling channel from mould surface should be permissible by the strength of mould steel against possible failure under clamp and injection forces. It could be 1.2 to 2 times diameter of cooling channel.

• Cooling system of the mould should have adequate number of cooling channels of suitable size at equal distance from each other and from cavity walls. The center distance between adjacent channel can be 1.7 to 2 times diameter of the channel. This is also governed by the strength of mould steel.

• The difference between the inlet and outlet water temperature should be less than 2 to 5 degrees C. However, for precision moulding, it should be 1 degree C or even 0.5 degree C.

• Cooling circuits should be positioned symmetrically around the cavity. There can be sufficient number of independent circuits to ensure uniform temperature along the mould surface.

• The coolant flow rate should be sufficient to provide turbulent flow in the channel.

• There should be no dead ends in the cooling channels. It could provide opportunity for air trap.

• Many a times it is difficult to accommodate cooling channels in the smaller cores or cores with difficult geometry. In such case the core should be made of Beryllium copper which has high thermal conductivity. These core inserts should be located near the cooling channel.

• The seals of coolant system should not leak inspite of application of frequent clamping force and mould expansion / contraction due to thermal cycle during moulding. The O-ring should be positioned so that there is no chance of them being damaged or improperly seated during mould assembly. Seal and O-ring grove should be machined to closely match the contour of the seal. It should ensure that seal is slightly compressed when the mould is assembled.

• Mould temperature above 90 degree C normally requires oil as the heating medium. Heat transfer coefficient of oil is lower than that of water.

• There is enough scope for confusion while giving water connection to mould when there are more number of cooling circuits particularly on bigger moulds. A sketch indicating cooling circuits should be available during mould set up.

• Hot runner mould should be provided with compression resistant insulating plate between back plate and machine platen. This is to prevent the heat flow from mould to machine platen, which can create an unbalanced heat flow in the mould. With out insulating plate machine platen will act like a big heat sink, there by destabilising the possible balance between heat given to the mould by the hot melt, and heat taken away by circulating water through mould.

• The cooling channel layout is suitable when the isothermal i.e. the equi-potential lines, are at a constant distance from surface of the mouldings. This ensures that heat flow density is same everywhere.

• Provision for thermocouple fixing should be available at specific one or two places in core as well as cavity to monitor the temperature of mould.

• Use efficient sealing methods and materials to eliminate cooling leaks.

• Poor mould surface temperature control can cause following quality problems: Axial eccentricity, Radial eccentricity, Angular deviation, Warpage, Surface defects, Flow lines,

The mould has to be heated or cooled depending on the temperature outside mould surface and that of environment. If heat loss through the mould faces is more than the heat to be removed from moulding, then mould has to be heated to compensate the excess loss of heat. This heating is only a protection for shielding the cooling area against the outside influence. The heat exchange takes place during cooling time. The design of cooling system has to depend on that section of part, which requires longest cooling time to reach demoulding temperature.

Cooling Channel layout depends on :

• part geometry, • number of cavities, • ejector and cam systems, • part quality, • dimensional precision, • part surface appearance, • polymer etc.

The sizing of cooling channels is dependent on the rate of cooling and temperature control needed for controlling part quality. CAE software like MOLDFLOW or C-Mold can be used to determine the optimised dimension of cooling channel and distance from mould surface, distance between cooling channel, flow rate.

CIRCUIT FOR WATER PUMP, COOLING LINES AND COOLING T OWER

Typical Cooling lines with Cooling Tower

for Injection Moulding Shop.

The figure shows number of pumps (each with bypass lines) connected in parallel supplying water through supply line. Two pumps are for main operation. Where as middle pump can be stand by pump. When ever there is problem on any one operational pump, it can be taken up for repair after the standby pump is put on operation. This ensures uninterrupted water supply for moulding shop.

Water reservoir is partitioned to separate cold and warm water. Water from cold reservoir is pumped to process and returns warm to warm part of reservoir. Warm water is again pumped by a separate pump- of same flow

rate but lower head- to cooling tower and returns to cold part of reservoir. The partition will have interconnecting hole at suitable height to avoid overflow on account of any unbalance in water transfer. This is shown in separate figure to avoid over crowding of lines.

Each pump should be connected the pump manifold or main line or supply line through flexible connection. This can save time when pump requires to be removes off line for repairs or maintenance.

Pressure at pump side should be between 5 and 6 bar. Pressure loss across mould is about 2 to 3 bar. This pressure loss represents the productive use of power in cooling the mould and heat exchanger of machine.

The supply line as well as return line have additional pressure equalizing line. Supply line with pressure equalizing line forms main supply ring and similarly return line with equalizer line forms main return line ring. Pressure equalizer lines ensure uniform pressure at each supply terminal (inlet valve) on machine as well as mould. In the absence of equalizer lines on supply as well as return line, the inlet pressure would be different at different machine and mould. Highest at the first machine from pump and lowest at the last machine

Any other pressure loss in the system is waste. Therefore, adequate size of pipe should be used for supply and return lines. Pressure equalizer lines should also have same size as that of supply line. Return line and its equalizer line pipes can be of larger size than supply lines as there should not be any pressure loss on return line and equalizer line.

End of supply line and end of return line is connected through pressure differential valve. This valve automatically ensures pressure loss across mould is 2 to 3 bar. In case this valve is not available then, a gate valve should be used. But this requires adjustment of flow when ever there is a mould change.

Connections to mould as well as machine terminals should be through separate gate valve. Connections to Heat exchanger should be through flexible hose pipe. This saves time during regular preventive clean up of heat exchanger. Select correct pipe for heat exchanger as specified by the machine manufacturer.

A manifold with adequate number of connections for in and out of mould should be connected to the terminal inlet valve for mould. It should be noted that there should be no reduction of water passage area from manifold to cooling channel of mould. Normally hose fittings have smaller cross section area inside thereby throttling the flow. This can prevent turbulent flow. In other words, the water passage for 10 mm channel should have hose fitting with minimum internal diameter of 10 mm. Any thing less will not give turbulence. Turbulence is required for efficient heat exchange resulting in power saving. Therefore ensure that mould should have larger pipe fitting to accommodate this point of view.

Please note that at 3 bar pressure loss across the mould;

• 6 mm channel requires 6.5 lpm to generate turbulent flow, • 10-12 mm channel requires 12-20 lpm.

Smaller the diameter of channels higher the pressure drop across. Higher the channel diameter lower the pressure drop across. Therefore, it is better to have all the channels of same diameter through out the mould. If there are different diameters for channels, then the smaller diameter will have larger pressure drop and hence it will have turbulent flow of water, but larger diameter channels will not have turbulent flow. To achieve turbulence in larger diameter channels the flow rate is required to be increased.

Tips for Design of cooling lines and cooling tower.

1. Compute total requirement of flow rate and pressure for pump selection.

2. Decide the pump / pumps from manufacturers catalogues. 3. Decide reservoir size which should be more than 30 min flow. 4. Decide pipe diameter recommended for the pump and select supply

line and return line diameters. 5. Measure the lengths of each line and prepare Bill of Materials. 6. Include valves and pressure gauges, hose pipes etc.

GLOSSARY OF TECHNICAL TERMS - Polymer Process

Amorphous Polymer: Amorphous means irregular, having no discernible order or shape. In the context of solids, the molecules are randomly arranged, as in glass, rather than periodically arranged as in a crystalline material. Amorphous polymer has a glass like structure with tangled chain and no long range order.

Cavity Pressure: Pressure on the melt in side the space between core and cavity as the melt moves to fill the mould.

Change over- Fill/Pack: Switch over point fill to p ack: The point on injection stroke at which filling phase with speed profile ends and pack or pressure phase with pressure profile starts.

Clamp Force: It is force applied by the machines clamping unit to the mould during filling, packing and holding phases of moulding cycle. Unit is Tons or kN.

Cooling Channels : It is the channels through which coolant is flown to remove heat from the mould. The channels are to be located thoughtfully in the core and cavity so that the temperature distribution over the mould surface is constant with little acceptable variation.

Co-polymer: Polymers that are derived from more than one species of monomers.

Cross linking: A process in which bonds are formed joining adjacent molecules. At low density, these bonds add to the elasticity of the polymer and at high density eventualy produce rigidity in the polymer.

Crystalline: It is the structure of polymer where the molecules are arranged in a very regular repeating lattice structure.

Cure: The process of changing property of polymer into a more stable and usable condition. This is accompolished by the use of heat, radiation or reaction with chemical addition.

Cushion: It is the a small amount of melt that is left in the barrel at the end of injection, follow up pressure. The cushion prevents the screw tip from making contact to the head of

barrel.

Cycle Time; A sequence of operation that is repeated regularly, the time it takes for one such operation. It is the time required to complete one moulding cycle.

Dosing Stroke: Metering stroke: It is stroke of screw that determines the quantity of melt to be injected to the mould.

EUROMAP: Euromap is the non profit organisation of the national association of machinery manufacturers for plastics and rubber industries in Austria, France, Germany, Italy, Luxembourg, Netherlands, Spain, Switzerland, and U.K. It represents 600 companies. It's Technical Commission deals with mechanical and electrical sandardisation, the communication protocol, and interface for various types of processing machinery and safety standards. It's technical work started in early seventies describe recommendations for functional and specification, testing of machines etc.

FAQ: Frequently Asked Questions.

Feed System: It is the main connecting channel between the machine nozzle and the part cavities of the mould.

Feedback: Feedback in a closed loop system represents the return signal or response of the system to input instruction.

Fill Pattern: It is a visual history of how a mould fills under a specific set of moulding conditions.

Fill Time: Fill time is the time in seconds that it takes to just fill the mould with melt during the filling phase (up to switchover point) of the moulding cycle.

Flow Balancing: Flow balancing is the process of choosing a mould design strategy, which promotes even filling of mould. First, a gating strategy is selected which will promote ease of filling. Second, a runner layout is developed to feed material to the gates. Finally runner dimensions are identified which will cause all of the flow paths to fill at the same time. Even wall thickness can be manipulated by flow leader or flow deflectors.

Flow path: A flow path describes the route that is traveled by a melt front as melt fills a section of a mould.

Flow Rate: Flow rate is a way of describing how much material goes through or past a specific point in a fixed period of time. If nozzle tip is used as reference, flow rate can be described how much melt is flowing out of the machine per second during injection.

Flow Length: Flow length is the total distance that melt must travel from the machine nozzle along a particular flow path in order to fill a section of the mould.

Follow up Pressure: It is pressure on melt after the switch over point in the moulding process.

Fountain Effect: The fountain effect describes how the melt front behaves during the filling of the mould. The leading melt front swells into the shape of a bubble; it behaves in much the same way as water flowing from a fountain.

Frozen Layer / skin: The frozen layer is a skin of solid melt that forms next to the mould surface during the filling phase of the moulding cycle.

Gating Strategy: It is the approach you use to choose the number, dimension and location of gates in a mould.

Glass Transition Temperature: It is the point at which the polymer hardens into an amorphous solid.

Hold on Pressure: Follow up pressure: It is pressure on melt after the switch over point in the moulding process.

Homo polymer: A polymer that is constructed of identical monomers.

Injection Rate: It is the flow rate of melt (cc/sec) coming out of nozzle. Melt comes out in the form of jet. Then it spreads inside the mould in shape of the space between the core and cavity.

Injection Speed / velocity profile: Before the switch over point the set pressure remains constant but set injection speed is varied with stroke position. After switch over point injection speed should remain at lower value and pressure is changed with time till the mould is just filled with out over packing. The variations in the set speed through out the injection stroke is called Injection speed profile.

Injection Pressure Profile: Before the switch over point the set pressure remains constant but set injection speed is varied with stroke position. After switch over point injection speed should remain at lower value and pressure is changed with time till the mould is just filled with out over packing. This set pressure for filling stage and follow up / hold on pressure change with respect to time is called Injection pressure profile.

Maximum Shot Capacity of Machine: In machine specification it is given in terms of weight for PS. For other polymers the density of polymers at moulding temperature should be multiplied to the maximum swept volume of the machine barrel to get the weight for the given polymer.

Metering stroke: It is stroke of screw that determines the quantity of melt to be injected to the mould.

Melt expansion: During the injection stroke melt is compressed by about 5 to 15%. After melt enters the mould it gets the chance to relax and expand.

Melt Front: As the mould is filling, the melt at the leading edge of flow is called the melt front or stream.

Melt Front Velocity: It is not same as injection piston (screw velocity) velocity. As melt enters mould it spreads in all direction depending on the resistance to flow in each direction. Melt front velocity is determined by the area of flow, which does not remain constant with time.

Melt Temperature: Melt temperature is the actual temperature of the melt during processing. The melt temperature is constantly changing. It varies with time and will not be the same at different locations in the mould.

MFI: It stands for Melt Flow Index. It is the weight of polymer melt in grams extruded in 10 minutes through a standard nozzle or die under standard load condition at a certain temperature.

Mould Cooling: Mould cooling describes the process by which the melt temperature is reduced to the point where part can be removed from the mould.

Mould Packing / Follow up pressure / Hold on phase: Mould packing is the process of delivering an additional amount of melt to the mould, to compensate for the shrinkage after filling.

Mould Temperature: It refers to temperature of mould surface in contact with melt. It varies from point to point on surface if cooling design does not provide uniform heat extraction. It also fluctuates if heat is not balanced.

Moulding Cycle: The moulding cycle is the series of steps that result in the machine producing a part. The cycle is usually described by breaking down into four separate phases. The amount of time that it takes to complete one cycle is called cycle time.

No Flow Temperature: No flow temperature is the temperature at which the viscosity of the melt is so high that it effectively can not be made to flow.

Orientation: Orientation is the change in shape that polymer molecules can undergo when they are made to flow.

Parting line: Contour line on the part separating core and cavity.

Polymer: Long chains of covalently bonded atoms.

Pressure Drop: Pressure drop is the loss of pressure that occures when the melt is pushed into a section of the mould during the filling phase.

Pressure Profile: After switch over point injection speed should remain at lower value and pressure is changed with time till the mould is just filled with out over packing. This follow up / hold on pressure change with respect to time is called Injection pressure profile.

pvT diagram: Polymer Melt follows law similar to famous gas law (Boyel's law). It is the relation ship between pressure, volume and temperature for plastic melt.

RAPRA: RAPRA technology Ltd., (UK,) is a leading rubber and plastics consultancy firm with over 75 years of experience. They offer conduct training courses in Rubber and Plastics.

Repeatability: Repeatability is closeness of agreement of a tool movement position from one part to another when cutting several copies of the same part.

Residence Time: Residence time is the length of time that the material is held at melt range temperatures in the barrel.

Residual Stress / Moulded-in Stress: Residual stress is a term that describes the level and pattern of stress, which is left in the part after it is removed from the machine. It can be due to unbalanced flow, non uniform freezing of melt and over packing.

Runner Layout: It is the channels used to get the melt from the machine nozzle to the gates.

Shear Heating: Shear heating is due to friction caused by flow of melt through narrow passages in the mould during filling phase.

Shear Rate: Shear rate is a way to describe how quickly the velocity of the melt changes from the mould surface to the center of flow for a given cross section. The size of the shear rate gives an indication of the shape of the velocity profile for a given situation.

Shear stress: Shear stress is the result of the force that is generated in a melt to overcome its resistance to a particular flow situation. Shear stress is the product of a material and shear rate.

Shear Thinning: Shear thinning is a the description for the physical effects of orientation and affect the flow behavior of the polymer. Shear thinning causes the melts viscosity to drop when it is made to flow within a certain shear rate range.

Shot Composition: The shot size setting and the switch over point give the machine information about how the melt has to be injected into the mould. The stroke positions correspond to volumes of melt that play a role in different portions of the moulding cycle. While the settings relate to the machine barrel, they correspond with the actual events that happen during the cycle.

Shot size Setting: Shot size setting is control setting which limit how far back the screw will travel as it rotates in the cooling phase of the cycle. The shot size setting is measured as a distance from the front of the barrel.

Shrinkage: It is the deviation of dimensions of the moulded part from the dimensions of cavity when measured on moulded part after certain hours. Compressibility, thermal expansion and pvT characteristics influence the dimensions of moulded part.

Sink Mark: A depression on the surface of moulded part caused by differential shrinkage.

Specific Heat: Temperature is measure of heat energy level whereas heat is a measure of total internal energy contained in a body. When the same quantity of heat is given to equal masses of different substances, they do not result in the same rise in temperature. The specific heat is defined as the quantity of heat energy which will rise the temperature of unit mass (1kg) of a substance by 10C. Heat = mass x specific heat x Temperature rise.

Specific Volume : It is inverse of density. It is volume per unit weight. Unit is cc/gr.

Speed profile: Injection speed set up with respect to stroke positions for the filling phase so that melt front speed remains near constant during filling phase.

Split Points: Split points are locations in the a mould cavity where a melt front will split up and advance in more than one direction at the same time. It is also refered as flow junctions, branch points, or nodes.

Stress Cracking: A crack, either external / internal, in a plastic part caused by tensile stress less than its short time mechanical strength.

The moulded-in stress left in the moulded part - due to unbalanced filling, over packing or non uniform freezing - can fail in service condition. The service

environment can have objectionable temperature or contact with aggressive chemicals causing the plastic part to fail. Avoiding or minimising moulded-in stress in plastic part can increase the performance of part even under adverse condition.

Switch over point fill to pack: The point on injection stroke at which filling phase with speed profile ends and pack or pressure phase with pressure profile starts.

Thermal Stability: It is polymers thermal characteristic. It is the time interval for which polymer remains stable at certain temperature. Beyond that time if the polymer is exposed to longer duration for a given temperature, it degrades.

Thermoplastic: Linear plastics of finite molecular weight that can be fabricated in a complex shape by melting and injection moulding.

Thermoset: A type of plastics that must be cured, forming network like structures that do not soften at high temperature.

Valcanisation: A process by which a network of cross linking is introduced in to an elastomer to strengthen it.

Viscosity response: The viscosity response is a way of describing how the viscosity of a particular polymer responds to the changes in temperature and shear rate.

Warpage: Uneven bending, Twisting etc on account of differential cooling, differential shrinkage, or non uniform freezing of melt in the mould can cause these conditions in the moulded part.

Weld Lines: Weld lines are locations in the moulded part where two met fronts meet.

Wireframe: It is a geometric model that describes 3D geometry by outlining its edges.

Work Done: Work is said to be done if an unbalanced force moves its point of application through a distance measured in the direction of force. Work = Force x Distance