212 Philips Tech. Rev. 44, No. 7, 212-217, March 1989

Analysis of the injection-moulding process

J. F. Dijksman

To mark Dr E. A. Muijderman 's retirement a colloquium was held at Philips Research Labor-atories, Eindhoven, on 27th May 1988. All the subjects discussed were closely associated withDr Muijderman's work in science and engineering - work that has included research onplastic products and has provided valuable spin-off to other research. This was evident fromone of the talks at the colloquium, in which Dr Dijksman examined the injection-mouldingprocess used for the manufacture of plastic products. The text given below is closely based onthe talk.

In very many cases we can say that the interfacebetween the user of a Philips device and its function isa plastic product. The casing of the Philishave, forexample, is made of plastic, sometimes finished with ametal-coloured coating to make it look like metal.The outer casings of coffee makers, kitchen machines,irons, vacuum cleaners, and sometimes parts inside,are formed from plastic components. Very complexshapes can be made (jig. 1), and this means that anumber of functions can be integrated, such as• load distribution,• shielding rotating parts and electrical connections,• sound insulation,• replacement of fasteners such as screws by snapconnectors,• design.One component that has to take a very heavy load isthe plastic tub of a washing machine; see jig. 2. An-other highly stressed component is found in the Philipstop-loader washing machines. This is the internal topframe of fibreglass-reinforced plastic from which thedrum is suspended. The control knobs of amplifiers,tuners, Compact Disc players, record players and soon are made of plastic. In a remote-control unit for a

television set the integration of functions has pro-gressed so far (push buttons, guide and return-springmechanism for the pushbuttons, snap connectors be-tween top and base) that it only consists of a fewplastic components, LEDs (light-emitting diodes) anda number of electronic components. This is a varia-tion on the integrated-circuit theme, for here we havemechanical components in which various functionshave been integrated.

To generalize again, whenever you encounterplastic components in our products, you will usuallyfind that these components have been made by injec-

Fig.1. The turntable of a record player is an example of a plasticDr Ir J. F. Dijksman is with Philips Research Laboratories, Eind- product with a complicated shape. The photograph shows a half ofhoven. a turntable, seen from below.

Philips Tech. Rev. 44, No. 7 INJECTION MOULDING 213

Fig. 3. Injection-moulding process, schematic. Plastic granules fed via the hopper H to the cylin-der E are transported towards the runner R by the plunger screw P as it moves to the right. Thegranules are heated during this transport by internal friction and the electrical cuff heaters T.When sufficient melt has collected to its left, the screw is used as a plunger to inject the plastic intothe cavity C of the mould M in a single stroke.

Fig. 2. The tub of a washing machine has to take a very heavy load.It is suspended in a frame and holds the water used in the wash. Thebearings for the drum go through one side of the tub. It also hasblocks of iron or concrete to increase the mass so that the machinedoes not move during the spin.

tion moulding. This process is widely used for themass-production of plastic devices and components.(The same process is used for metal, too, but then it iscalled pressure die-casting.) The shaping of the com-ponent takes place during this process almost alwaysin the following way. The original material, granulesof the plastic to be used, is melted in the injectionunit, the 'extruder', which is a cylinder containing aplunger screw; seefig. 3. The heat required is suppliedpartly from the friction between the granules as theymove as a result of the rotation of the screw and part-

ly from electrical cuff heaters wrapped round the cyl-inder. While it rotates the screw moves slowly back-wards (to the right), so that the molten material col-lects in the space formed in front of the screw. Assoon as sufficient molten material has collected, thescrew stops rotating. The screw, which now takes onthe role of a plunger, is next pushed forward, forcingthe molten material into the mould. Flowback is pre-vented by a non-return valve or the resistance alongthe screw. When the mould is full, the material has tocool until the product has become strong enough to beremoved. Meanwhile new material for the next prod-uct is being melted and transported.

I should now like to explain the injection-mouldingprocess from a number of different viewpoints - as itis seen by• the materials scientist,• the designer of the component or product,• the mould designer,• the rheologist.So far I have been talking about plastics in general.

The materials scientist knows however that many dif-ferent types of materials lurk behind that word'plastics', and I shall name some of them.

Amorphous plastics. Amorphous materials have thesame structure in the liquid phase and the solid phase(there is no crystalline structure) and are often trans-parent. Well-known transparent plastics in this groupare polycarbonate (PC), which is used as a substratefor the Compact Disc, and polymethyl methacry-late (PMMA), which is used as the substrate in theLaserVision disc; see jig. 4.Partially crystalline plastics. These materials have afairly complete crystal structure in the solid phase. Asin metals the crystal structure is highly dependent on

214 J. F. DlJKSMAN Philips Tech. Rev. 44, No. 7

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"Fig.4. Schematic configuration of a double LaserVision disc.S transparent substrate of polymethyl methacrylate (PMMA)that carries the picture and sound information in the form of pits.M metal reflecting coating, 0.6 urn thick. A adhesive layer,100 urn thick.

the previous history of temperature changes. Polypro-pylene, one of the few coffee-resistant plastics, be-longs to this group, as do polyethylene and polyvinylchloride, the plastics that are widely used for the in-sulation of electrical wire.Monomers. The plastics named above are polymermaterials. In a number of cases, however, we startfrom monomers and carry out the polymerization inthe mould under the influence of temperature or light.The vulcanization of rubber is an example of such aprocess. Materials such as phenolformaldehyde (Phi-lite), some kinds of nylon, polyurethanes and mostpolyesters are also examples of materials that polym-erize during the shaping process.Mixtures (Blends). To improve the properties ofplastics or adapt them to specific requirements manyplastics are supplied as mixtures of different materi-als. There are many examples of this. Rubber par-ticles are dispersed in polystyrene to increase its im-pact strength. The addition of large quantities of sandreduces the coefficient of expansion. It is used in en-capsulations for ICs and in precision products like the'arms' in Compact Disc players and for bearings. An-other example is the addition of fibreglass to increasestiffness and strength. Finally, I should mention theaddition of talc: this is done to lower the price.The designer of the product or component has laid

down a number of specifications for it. These mightinclude• mechanical strength and stiffness (how much loadcan the component take and how much is it allowed to'give' under this load),• creep (how much may the material flow under con-tinuous load),• temperature range,• sensitivity to moisture, electrical insulation, dielec-tric properties, magnetic properties,

• colour,• resistance to chemicals and radiation,• shape stability (how large are the tolerances),• toxicity, flammability,• re-use (can it be recycled).Working from these specifications, the designer en-deavours to choose shape and material so that theproduct has the required properties. In practice thismeans that he consults the materials scientist.

In choosing the original material account must betaken of the fact that the material does not only haveintrinsic properties. A number of properties canchange because of the shaping process. This alsomeans, of course, that these properties can beaffected. They include• optical properties (in Compact Discs, for example,the birefringence is affected by molecular orientationand inhomogeneous cooling),• mechanical properties (as a result of the molecularorientation properties such as stiffness, strength andimpact resistance may be anisotropic; inhomogeneouscooling can lead to internal stresses, resulting in re-duced load-bearing strength),• thermal properties (the thermal conductivity andthe coefficient of expansion may become anisotropic),• density (this may vary inside the product or compo-nent),• the degree of crystallization,• microstructure.This list of properties that can be affected to a greateror lesser degree clearly shows the need for research in-to the relationship between shaping and productproperties.

This research on plastics, which was mostly doneon the actual components or products, provided valu-able spin-off to other research at Philips ResearchLaboratories, e.g. on plasticity of materials [11, flowmechanics in molten polymers [21 and the mechanicalaspects of polymer materials, particularly in relationto precision injection moulding [31.

We now consider the matter from another view-point, as the mould designer sees it. The mould is theunit containing the cavity, and is mounted on the cyl-inder that contains the reciprocating screw. Thechoice of product geometry and materiallands on thedesk of the mould designer, who finds he has to an-swer the following questions:• How should I choose the dimensions of the cavityso that I can compensate for the thermal shrinkage ofthe component? How can I prevent the component orproduct from warping and how can I avoid unwantedanisotropy?• What will be the consequences of forced cooling? Ingeneral the product will not just be left to cool down,

Philips Tech. Rev. 44, No. 7 INJECTION MOULDING 215

since the economics requires the shortest possiblecycle time; therefore the product must be cooled asquickly as possible. Certainly in the vicinity of thewalls there is more likely to be 'quenching' ratherthan unaided cooling, with all the associated conse-quences for the structure of the injected material:extra shrinkage, warping and anisotropy can arise .• How should I choose the stiffness and strength ofthe mould so that it does not distort too much underthe influence of the forces resulting from the highpressure in the cavity. Sometimes the distortion canbe put to use to compensate for shrinkage .• Where should I put the gates? A gate is the connec-tion between the runner channels in the mould and thecavity. When you buy a model aeroplane, you get the'runners' as well. The gates are where you break offthe parts of the aeroplane. Together with the injectiontemperature - the temperature of the melt as pre-pared by the extruder - and the injection rate, the lo-cations of the gates determine the flow pattern in thecavity, and therefore the fill time and the pressure re-quired. Provided the material in the gate has notsolidified ('frozen'), material can be added during thecooling to compensate for shrinkage (packing).Otherwise, the positions of the gates are partly deter-mined by aesthetic considerations (e.g. the position ofa weld line, i.e. the line where the flows of materialfrom different gates first meet). The flow patterndetermines the anisotropy in the outer layer of theproduct.• How should I design the layout of the runnersystem (jig.5)? The idea is to have as little waste aspossible, with balanced injection and packing.• How should the mould be divided so that it can beopened and the product ejected (jig. 6)?

Fig. 5. Eight spring clips with their 'stalks' or runner cores, whichreproduce the geometry of the runner system. The runner systemconsists of the long vertical feed and the horizontal channel witheight branches. (The eight vertical branches are used in ejecting theproduct from the mould.)

a

b

Fig. 6. a) Splitting the mould to free the product - part of the radiocabinet in the foreground. b) In practice each of the two parts ofthe mould is built up from a large number of individual compo-nents. In the photograph the lower part of the mould has been dis-mantled.

[I] H. van Wijngaarden, Constitutive equations for metals withan application to the extrusion of lead, Thesis, Eindhoven1988.

[2] A. A. M. Flaman and B. Veltman, Injection moulding experi-ments, a challenge to numerical simulation programs, suppl.to Rheol. Acta 26 (Proc. 2nd Conf. of Eur. Rheologists, Pro-gress and Trends in Rheology 11, Prague 1986),129-131,1988.

[3] F. P. T. Baaijens, Compressible solidifying flow of a moltenpolymer, in: A. W. Bush, B. A. Lewis and M. D. Warren(eds), Flow modelling in industrial processes, Ellis Horwood,Chichester, to appear shortly.

216 J. F. DIJKSMAN Philips Tech. Rev. 44, No. 7

Finally, let us have a look from the viewpoint of therheologist. Rheology is the science of the deformationand flow of matter due to external forces. So what doesthe rheologist see?To start with, he sees a material witha complicated behaviour. Complicated, because• the coefficient of expansion is high,• the mechanical properties depend closelyon the tem-perature and sometimes on the time in service,• the viscosity in the molten state is extremely high;this must be properly taken into account during theshaping (water at 20°C has a viscosity of 10-3 Pa s,for example, whilepolystyrene at 250°C has a viscos-ity of about 104 Pa s),• the material is viscoelastic in both the solid phaseand the liquid phase,• most engineering plastics are extremely poor con-ductors of heat (diamond has a thermal conductivityof 2000Wm-1K-1, copper 300-400Wm-1K-1, steel40Wm-1K-1 and polystyrene 0.13Wm-1K-1).This last property must also be properly taken into ac-count in the design of plastic products. In practice,because of the cooling time, only thin-walled products(fig. 7) or very small products can be manufacturedeconomically from plastics.We sawearlier that the product designer has to

cooperate closelywith the materials scientist, and here- because of the rheological behaviour of the materi-al in the mould - it turns out that the rheologist andthe mould designer will have a great deal to discuss. Infact, of course, a good product will only result from acooperative effort by all concerned: the materialsscientist, the product designer, the mould designerand the rheologist.The title of my talk is 'Analysis of the injection-

moulding process'. So far I have drawn your atten-tion to various aspects of the material, the product de-sign, the mould design and the rheology. A good pro-duct or component not only meets its specification,but can also be manufactured at an acceptable price.This means that the design of that component or pro-duct and the choice of the manufacturing processmust go hand in hand. Computer aids can be very use-ful here.The use of a flow-modelling program [41 such as

INJECT-3 [5], developed at the Philips Centre for Manu-facturing Technology, will give the product designer,the mould designer and the rheologist a better under-standing of the flow pattern in the cavity while it isbeing filled. This in turn will provide a clearer pictureof the expected orientation of the molecules and anyanisotropy in the product (prediction of the fill pat-tern). A knowledge of various quantities is requiredhere, such as viscosity/viscoelasticity, thermal con-ductivity, specific heat capacity, compressibility,

coefficient of expansion. As yet the program can onlybe used for amorphous thermoplastics. In partiallycrystalline materials matters such as latent heat,crystallization kinetics and the effect of the orienta-tion due to the flow also come into play [6] •

When the mould is being filled the thermal and me-chanical properties of the mould play no part. Themould fillsquickly, and the heat from the plastic onlypenetrates a few millimetres into the wall. The min-uscule extra movement of the flowing filling materialdue to thermal expansion and mechanical forces onthe mould is negligible in comparison with the mainflow during the fill.

Fig. 7. One exceptionally thin-walled product is the top cover of avacuum cleaner. (The handle is out of sight, on the left.) The pro-jection on the right of the rim, used for attaching the lower cover, ismade hollow to maintain the thin wall. If this was not done therecould be undesirable shrinkage on cooling.

The mould designer should however show a keeninterest in the thermal and mechanical properties ofthe mould if he wants to follow the cooling of theproduct and obtain a better understanding of the finaldimensioning of the product. As seen from the solid-ifying liquid, the pressure dependence of the glass-transition temperature (for amorphous thermoplas-tics), the viscosity, the viscoelasticity and the coeffi-cient of expansion are important. As seen from themould, the stiffness, the strength and the thermo-mechanical aspects of the mould and the runner lay-out are important.

Software relating to the solidifying liquid and devel-oped at Philips Research Laboratories is becomingavailable (prediction of packing and cooling). For themould we can use standard finite-element packagessuch as MARC, ANSYS and ASKA.

Philips Tech. Rev. 44, No. 7 INJECTION MOULDING 217

When the product is ejected from the mould, itsnaps into a shape determined by the equilibrium ofthe internal forces. This results in shrinkage and pos-sibly warping. Changes in shape due to further cool-ing can be calculated with new software now beingdeveloped at Philips Research Laboratories (predic-tion of errors in shape and dimensioning [3] [71).

Physical ageing, the gradual recovery of thermody-namic equilibrium in the quenched product, is de-

[4] c. w. M. Sitters, Numerical simulation of injection moulding,Thesis, Eindhoven 1988.

[5] A. H. M. Boshouwers and J. J. van der Werf, INJECT-3, a sim-ulation code for the filling stage of the injection moulding pro-cess of thermoplastics, Thesis, Eindhoven 1988.

[6] G. Eder and H. Janeschitz-Kriegl, Theory of shear-inducedcrystallization of polymer melts, Colloid & Polyrn. Sci. 266,1087-1094, 1988.

[7] A. A. M. Flaman, Het voorspellen van de eigenschappen vangespuitgiete kunststofprodukten, Materialen, No. 2 (February),40-47, 1988.

[8] L. C. E. Struik, Physical aging in amorphous polymers andother materials, Elsevier, Amsterdam 1978.

[9] F. J. Lockett, Nonlinear viscoelastic solids, Academic Press,Londen 1972, pp. 32-33.

scribed by a theory that has been developed else-where [81. For determining the viscoelastic propertiesof the product use can be made of the correspon-dence [9] between linear elasticity and linear viscoelas-ticity. Standard packages can therefore be used hereas well.

Summary. This article is based on a talk given at a colloquium heldin May 1988 at Philips Research Laboratories. A brief review of anumber of plastic products is followed by a short account of theinjection-moulding process. This process is analysed as it would beseen by several different specialists: the materials scientist (amorph-ous plastics, monomers, mixtures, etc.), the product designer (me-chanical strength, temperature range, sensitivity to moisture, etc.),the mould designer (dimensions of the cavity in connection withshrinkage, runner systems, separating the mould, etc.) and therheologist (coefficient of expansion, viscosity, thermal conduction,etc.). The relationship between shaping and the properties of theproduct is also examined. The article ends with a few words on thehelp that software can offer in predicting the fill pattern, packingand cooling as well as errors in shape and dimensioning.