Injection-moulding

Injection moulding is different from extrusion in that it involves a crosslinking reaction. Therefore, the rate and the mechanisms of crosslinking is an integral part of the process. However, in this chapter, chemistry of vulcanisation is not included. 

Apparently, the material is microscopically inhomogeneous the size of the dispersed carbon black is in the order of 10-100 nm and the size of the rubber domains is approximately 1 pm with irregular topology implying a sub-domain of approximately 0.1 pm. Even though these dimensions are much smaller than the capillary diameter, which is in the order of 1 mm, it may not be appropriate to treat the material as homogeneous. 

Does the material look like a flowable liquid The carbon black forms a network structure, which possesses a yield value. Further, the yield value depends upon the previous mechanical history [25]. 

Reprinted with permission from W. M. Hess in Reinforcement of Elastomers, Ed., G. Kraus, 1965, Interscience Publishers, New York. Copyright 1965, John Wiley and Sons. 

Injection-moulding, widely used in the plastics industry, is ideally suited to long production runs of complex shapes. The process has been adapted for forming ceramics and the last 20 years or so have seen it firmly established as an attractive route for forming structural ceramics. 

Recently injection-moulding has been exploited by L.J. Bowen [10] to fabricate a range of piezoceramic-polymer composites (see Section 6.4.6) for the active elements of sonar devices. 

Injection moulding is one of the most common manufacturing processes in use today. It lends itself to high volume production of both complex and precision parts with a variety of uses. It is a highly versatile process that is as successful and economic for both small and large components from an automotive bumper to intricate wristwatch parts. 

Like an extruder, an injection moulding machine uses a screw to feed and plasticate the melt. There are however, a number of design differences between the two. An injection screw must withstand both higher pressures and intermittent stop-start use. It must be able to inject as well as move back to prepare the next weight of shot. 

A model has been developed to predict degradation during moulding based on a spiral mould and the use of mathematical equations (249). 

The problem of part failure in service has been addressed using diagnostic analytical tools (353). 

The co-injection moulding of PVC-U with other thermoplastics (glass fibre reinforced PVC, polypropylene, ABS and polycarbonate), was investigated using the mono-sandwich process and the properties determined. Polypropylene was the only polymer not to exhibit good adhesion. The mechanical properties of the other samples were intermediate between those of the constituent polymers (104). 

One of the most common processing methods for plastics is injection moulding. Nowadays every home, every vehicle, every office, every factory contains a multitude of different types of articles which have been injection moulded. These include such things as electric drill casings, yoghurt cartons, television 

The first group of studies is concerned with products made by injection moulding. In this chapter we start with an outline description of the process itself, and this is followed by three sections on features of polymers as materials which are particularly important in the design and processing of polymers by this route. 

The basic principle of injection moulding is to inject molten polymer into a closed, cooled mould, where it solidifies to give the product. The moulding is recovered by opening the mould to release it. An injection-moulding machine has two principal parts the injection unit the clamp unit, or press. This is illustrated in Fig. 1.1. 

The mould is mechanically fastened (e.g. bolted) in the clamp unit, but is interchangeable to allow different products to be moulded. The essential features of a mould are listed below. 

After solidification, the moulded article is ejected and the sequence of events is repeated in a cyclic manner. The cooling time constitutes 

In this chapter, we have tried to emphasise the specific behaviour of polymers when used in different materials. There is indeed a very wide range of different 

We have restricted the discussion to polymers used in structural materials. However, a new field of use has recently opened up polymers as functional materials. The list of functions is long electrical conductors, liquid crystals, materials for non-linear optics, biomaterials, and many more. Although their production will always remain very limited when compared with structural polymers, their commercial value is already proven. They carry with them a great potential for innovation. 

1 de Gennes, P.G. (1979) Scaling Goncepts in Polymer Physics, Cornell University Press, New York 

2 Buckley, C.P., Bucknall, C.B., McCrum, N.G. (1988) Principles of Polymer Engineering, Oxford University Press, Oxford 

3 Ward, I.M., Hadley, D.W. (1993) An Introduction to Mechanical Properties of Solid Polymers, Wiley, New York 

---

PoJy (3,3-bis (chloromethyl) oxelane), chlorinated polyether (R CHiCl) is widely used for injection moulding and coaling. 

Polymers owe much of their attractiveness to their ease of processing. In many important teclmiques, such as injection moulding, fibre spinning and film fonnation, polymers are processed in the melt, so that their flow behaviour is of paramount importance. Because of the viscoelastic properties of polymers, their flow behaviour is much more complex than that of Newtonian liquids for which the viscosity is the only essential parameter. In polymer melts, the recoverable shear compliance, which relates to the elastic forces, is used in addition to the viscosity in the description of flow [48]. 

Hiebcr, C, A. and Shen, S.F., 1980. A finite element/finite difference simulation of the injection-moulding filling process. J. Non-Newtonian Fluid Mech. 7, 1-32. 

MaranylNylon Compounds, Injection Moulding, N102,11th ed., Du Pont de Numours Int. SA, Geneva, Switzedand, 1987. 

M. E. Edwards, Chemical Reaction Engineering of Polymer Processing Reaction Injection Moulding Inst. Chem. Eng. Symp. Ser. 8(87), 783—796 (1984). 

When metals are rolled or forged, or drawn to wire, or when polymers are injection-moulded or pressed or drawn, energy is absorbed. The work done on a material to change its shape permanently is called the plastic work- its value, per unit volume, is the area of the cross-hatched region shown in Fig. 8.9 it may easily be found (if the stress-strain curve is known) for any amount of permanent plastic deformation, e. Plastic work is important in metal- and polymer-forming operations because it determines the forces that the rolls, or press, or moulding machine must exert on the material. 

Demonstrations (a) Give four injection-moulded close-packed planes to each student to allow personal building of f.c.c. and c.p.h. (b) Atomix atomic model on overhead projector to show atom packing (Emotion Productions Inc., 4825 Sainte Catherine O, Montreal 215PQ, Canada) or ball bearings on overhead projector. 

When you have to estimate how a change of temperature changes the viscosity of a polymer (in calculating forces for injection moulding, for instance), this is the equation to use. 

Fig. 24.3. (a) Extrusion polymer granules ore heated, mixed and compressed by the screw which forces the now molten polymer out through a die. (b) Injection moulding is extrusion into a mould. If the moulding is cooled with the pressure on, good precision and detail ore obtained. 

In injection moulding, polymer granules are compressed by a ram or screw, heated until molten and squirted into a cold, split-mould under pressure (Fig. 24.3b). The moulded polymer is cooled below T, the mould opens and the product pops out. Excess polymer is injected to compensate for contraction in the mould. The molecules are oriented... 

Both thermoplastics and thermosets can be formed by compression moulding (Fig. 24.5). The polymer, or mixture of resin and hardener, is heated and compressed between dies. The method is well suited to the forming of thermosets (casings for appliances, for instance) and of composites with a thermosetting matrix (car bumpers, for example). Since a thermoset can be removed while it is still hot, the cycle time is as short as 10 seconds for small components, 10 minutes for large tliick-walled mouldings. Pressures are lower than for injection mouldings, so the capital cost of the equipment is much less. 

E. C. Bahardt, Computer-aided Engineering for Injection Moulding, Hanser, 1983. 

A. Whelan, Injection Moulding Materials, Applied Science Publishers, 1982. 

BOBBIN PBT(30% FILL) INJECTION MOULDED WITH ITEM 5... 

Following the tolerance stack through the end assembly, the bobbin dimension of 22 mm from the outside face to the back face of the magnetic pole is analysed next. This characteristic dimension does not include the tolerance on the impact extruded pole. The pole is to be moulded into the bobbin and the pole face is considered to be part of a mould related dimension. The bobbin is injection moulded using 30% filled polybutylene terephthalate (PBT). The tolerance assigned to the bobbin dimension is 0.035 mm. 

Injection moulding - Elastomers Compression moulding - Composites Hand/spray lay-up - Composites... 

Note The injection/compression moulding process capability maps 1, 2 and 3 are used for large parts with a major dimension greater than 50 mm typically and/or for large production volumes. Map 4 is for injection moulded parts that have a major dimension less than 150 mm and which are produced in small volumes. 

Through the development of such techniques as injection moulding it is possible to make highly complex parts in one operation without the need for assembly work or the generation of more than a notional amount of scrap material. 

Deformation of a polymer melt—either thermoplastic or thermosetting. Processes operating in this way include extrusion, injection moulding and calendering, and form, in tonnage terms, the most important processing class. 

The principles of thermoplastic melt processing can perhaps best be illustrated by reference to Figure 8.1 illustrating extrusion, injection moulding, bottle blowing and calendering operations. In order to realise the full potential of the process it is necessary to consider the following factors ... 

The flow process in an injection mould is complicated by the fact that the mould cavity walls are below the freezing point of the polymer melt. In these circumstances the technologist is generally more concerned with the ability to fill the cavity rather than with the magnitude of the melt viscosity. In one analysis made of the injection moulding situation, Barrie showed that it was possible to calculate a mouldability index (p.) for a melt which was a function of the flow parameters K and the thermal diffusivity and the relevant processing temperatures (melt temperature and mould temperature) but which was independent of the geometry of the cavity and the flow pattern within the cavity. 

Some typical data for this mouldability index are given in Figure 8.8. One limitation of these data is that they do not explicitly show whether or not a mould will fill in an injection moulding operation. This will clearly depend on the thickness of the moulding, the flow distances required and operational parameters such as melt and mould temperatures. One very crude estimate that is widely used is the flow path ratio, the ratio of flow distance to section thickness. The assumption is that if this is greater than the ratio (distance from gate to furthest point from gate)/section thickness, then the mould will fill. Whilst... 

Table 8.2 Some collected values for the flow path ratio of injection moulding materials...

An example of the effect on production rates is provided by injection moulding. The longer it takes after injection for solidification of the polymer to occur, the longer will be the overall cycle. (Provided the moulding is not distorted on ejection it will only be necessary to form a rigid skin to the moulding.)... 

---