Pressure injection moulding

Compact tension samples (Fig. 7.1) were prepared from cast plates. Pressure injection moulding was used for Phenoxy PKHJ. The toughness tests were performed at ambient temperature according to ASTM-E399 [77]. The results are given in Table 6.1. 

Low-pressure injection moulding, RIM, RRIM for liquid polyamide ... 

TPO skins for low-pressure injection moulding. .. insulation of electric wires, connectors... workbench feet, appliance feet, supports and pads... returnable packaging, totes and bins... handles, soft-touch overmoulding. .. profiles, tubes, pipes, sheathing. .. toys... 

Figure 5.2. Principle of high-pressure injection moulding...

Using the manufacture of PP battery cases as an example, the authors show how to produce thin-walled (2-5mm), foamed articles with good surface properties by means of low pressure injection moulding. Neopor 35 was used as the blowing agent. 5 refs. 

The manufacture of products, e.g. containers and lids, made from filled foamed PE by gas counter pressure injection moulding was studied with special reference to the effect of production conditions on dimensional stability. Results obtained are discussed and evaluated. 7 refs. 

One of the most interesting and fruitful approaches to the theoretical study of the non-isothermal flow of polymer melts has been suggested by Merzhanov and coworkers 82 84), which reduced the set of equations for the flow to the well-known equation of thermal explosion. The authors obtained the profiles of temperature and velocity for the case of stationary flow. However, the thermal explosion was founda5) to become feasible at too high velocities v lOm/s which could hardly be attained at low-pressure injection moulding of polymers. 

Closing the discussion of the area of developed flow, note that a number of ideas concerning the modelling of low-pressure injection moulding may be borrowed from the studies of normal injection moulding which are reviewed in36) and 62). 

Recent work has included tests that examine the effects of shear rate and cure simultaneously (which will also be discussed below). The models derived from these tests are recombined to provide an overall chemoviscosity model to be used in processing applications. The effect of pressure on chemoviscosity has not been studied extensively however, system pressure may be relevant to high-pressure injection-moulding and transfer-moulding processes. 

High pressure injection moulding and crystallization of polycarbonate during such a process has also been described. ... 

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. 

Paste injection moulding processes have also been developed. In one technique used for applying PVC soles to shoe uppers the paste is injected by gas under pressure into a hot mould, the last and shoe upper forming the top half of the mould. The paste gels in the mould, adheres to the upper in the presence of a suitable adhesive and is stripped hot from the mould. 

The polymer melts at 216°C and above this temperature shows better cohesion of the melt than PTFE. It may be processed by conventional thermoplastics processing methods at temperatures in the range 230-290°C. Because of the high melt viscosity high injection moulding pressures are required. 

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. 

Wall sections in foam moulding are thicker than in solid material. Longer cycle times can therefore be expected due to both the wall thickness and the low thermal conductivity of the cellular material. In contrast, however, the injection pressures in foam moulding are low when compared with conventional injection moulding. This means that less clamping force is needed per unit area of moulding and mould costs are less because lower strength mould materials may be used. 

---