Injection moulding illustration

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 earliest injection moulding machines were of the plunger type as illustrated in Fig. 4.30 and there are still many of these machines in use today. A predetermined quantity of moulding material drops from the feed hopper into the barrel. The plunger then conveys the material along the barrel where it is heated by conduction from the external heaters. The material is thus plasticised under pressure so that it may be forced through the nozzle into the mould cavity. In order to split up the mass of material in the barrel and improve the heat transfer, a torpedo is fitted in the barrel as shown. 

This is an injection moulding method which permits material costs to be reduced in large mouldings. In most mouldings it is the outer surface of an article which is important in terms of performance in service. If an article has to be thick in order that it will have adequate flexural stiffness then the material within the core of the article is wasted because its only function is to keep the outer surfaces apart. The philosophy of sandwich moulding is that two different materials (or two forms of the same material) should be used for the core and skin. That is, an expensive high performance material is used for the skin and a low-cost commodity or recycled plastic is used for the core. The way that this can be achieved is illustrated in Fig. 4.44. 

One of the major innovations in recent years is the use of pulsed pressure through the gates to introduce and control the orientation of the structure (or fillers) in injection moulded products. A special manifold is attached to the machine nozzle as illustrated in Fig. 4.46. This diagram relates to the double live feed of melt although up to four pistons, capable of applying oscillating pressure may be used. 

The steps in the process are illustrated in Fig. 4.48. Initially a preform is injection moulded. This is subsequently inflated in a blow mould in order to produce the bottle shape. In most cases the second stage inflation step occurs immediately after the injection moulding step but in some cases the preforms are removed from the injection moulding machine and subsequently re-heated for inflation. 

Equations (5.21), (5.22) and (5.23) are useful for the high strain rates experienced in injection moulding or extrusion but unfortunately they do not predict the low strain rate situation very well where plastic melts tend towards Newtonian behaviour (ie n -) 1). This is illustrated in Fig. 5.7. 

The effects of processing will be illustrated by considering injection moulding of a semicrystalline polymer. The molten plastic is injected into the mould under high pressure and temperature. The edges of the mould retard flow and cool more rapidly, leading to a boundary layer of high shear, which in semicrystalline polymers leads to orientation of the polymer chains and of short fibre reinforcements parallel to the direction of flow. At the centre the structure is less oriented. Where two separate flow streams meet, there is a... 

The most successful approach and the one suited to economic mass production is by injection-moulding (see Section 3.6.8), as illustrated in Fig. 6.19(b), and composites having rod diameters down to 100 /mr are produced by this route. 

Injection moulding allows the automated mass production of articles that may have complex shapes and whose properties can be varied by choosing different materials and the appropriate processing conditions. The basic principle of the method is shown in fig. 1.8(a). Polymer contained in a hopper is fed into a cylinder where it is pushed by a ram or plunger through a heated region, in which it melts, into a mould. The filled mould is allowed to cool and the solidified product is ejected. The ram is then drawn back and the process is repeated. This simple process suffers from a number of disadvantages and modern injection moulders are always of the screw-injection type illustrated in fig. 1.8(b). 

Figure 21.16 Illustration showing the production of polymer objects. On the left side compression moulding (a) starts with a preheated polymer pellet (1), which is subsequently moulded (2). After cooling (3j the object is released from the mould. On the right side, injection moulding (b) involves the injection of melted polymer in a mould by means of a plunger-type screw extruder.

The operation of an injection moulding machine is illustrated in Figure... 

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. 

Because of the interest shown by the medical device industry and demands for different compounds a wide range of extraction resistant flexible PVC compounds has now been formulated by BIPVL and this is illustrated in Table 5. While basically formulated for extrusion applications, some of these compounds have also been shown to be perfectly satisfactory for injection moulding of small components for medical devices, and also for film production by calendering and blown lay-flat film processes. 

Comonomer ratio certainly has a profound effect on mechanical properties of the biopolymers. This is illustrated in Table 6 for a range of injection-moulded random HB-HV copolymers. These samples contained a small amount of talc as a nucleating agent and were annealed at 60 C for 24 h before mechanical testing at room tempera-... 

The improvement in the initial modulus of as-spun PP fibres which is provided by the LCPs is illustrated in Table I. This observation accords with those of Baird and coworkers (12,13), who noted improvements in the moduli of injection-moulded samples of PP where LCP was incorporated. The tensile strengths are, however, still low, so that fibre drawing remains essential, to develop an oriented molecular structure for the PP matrix. 

We look at the costing of an injection moulding tool for the manufacture of turbo fan grill as illustrated in Fig. 7.7. The front grill of the turbofan is a plastic moulded part and the tooling for it is illustrated in Fig. 7.8. The... 

Figures 18.29 and 18.42 show the effect of temperature on the falling weight impact strength for compression moulded and injection moulded specimens, respectively, of a range of materials. The reduction in impact strength with decrease in temperature is particularly marked with the injection moulded samples of the two otherwise higher impact resistant types, PP copolymers and ABS terpolymers. The superior impact strength at low temperatures of injection moulded HOPE is clearly illustrated.

The operation of an injection moulding machine is illustrated in Fig. 7.3 this is a screw preplasticator injection unit. The plasticating screw is essentially an extruder which feeds melt into the second barrel. As it does this it pushes back... 

A single-screw injection moulding machine is illustrated in Fig. 7.2S. The machine is shown with an opened and empty mould. Liquid polymer has been generated at Y by screw rotation, as in an extruder. The nozzle, of small diameter, is sealed by cold, solidified polymer. In the moulding cycle, the next step is the closing of the mould. The screw is then rammed forward in order to inject the polymer liquid. As the screw travels forward, a non-return valve at the tip of the screw prevents liquid from travelling backwards between the... 

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