Plunger injection moulding

Polystyrene Syringe plungers Injection moulding Good To 10000 kGy Rigid... 

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. 

Example 5.2 In a plunger-type injection moulding machine the torpedo has a length of 40 mm, a diameter of 23 mm and is supported by three spiders. If, during moulding of polythene at ITO C, the plunger moves forward at a speed of 13 mm/s, estimate the pressure drop along the torpedo and the shear force on the spiders. The barrel diameter is 25 mm. 

Injection moulding A plastic processing technique to produce solid parts with a high degree of precision. The material is injected into a mould by a plunger, and a press keeps the mould closed while the material cools. At the end of the process, the mould is released and the part ejected. 

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.

Most thermoplastic materials are moulded by the process of injection moulding. In this process, the polymer is heated in a cylindrical chamber to a temperature at which it will flow and then it is forced in to a relatively cold, closed mould cavity by means of hydrolytically applied high pressure through a plunger or ram, but recently by means of a reciprocating screw that serves the dual purpose of providing the molten polymer mass and forcing it into the mould. The particulate polymer is picked up by the rotation of the screw, compressed and melted, the melt is mixed, and delivered to the entrance of the mould. The screw then moves forward to force a fixed volume of the molten polymer... 

There are two types of injection moulding equipment plunger-type (also called ram or piston) and screw-type machines. Linear low-density PE (LLDPE) requires more heat to be moulded than LDPE. These materials are more easily moulded in a screw machine in which the mechanical work of the screw adds considerably to the heat delivered by external electric heaters. 

Two principle designs for Injection moulding machines are available and are characterized according to the method of plasticization. A simple plunger machine is shown in Figure 6 and It is held that such designs suffer... 

In the low-pressure systems a shot of material is injected into the mould which, if it did not expand, would give a short shot. However, the expanding gas causes the polymer to fill the mould cavity. One important form of the low-pressure process is the Union Carbide process in which the polymer is fed to and melted in an extruder. It is blended with nitrogen which is fed directly into the extruder. The extruder then feeds the polymer melt into an accumulator which holds it under pressure (14-35 MPa) to prevent premature expansion until a predetermined shot builds up. When this has been obtained a valve opens and the accumulator plunger rams the melt into the mould. At this point the mould is only partially filled but the pressurised gas within the melt allows it to expand. 

However, nowadays this type of machine is seldom used because it is considerably more complicated and more expensive than necessary. One area of application where it is still in use is for large mouldings because a large volume of plastic can be plasticised prior to injection using the primary cylinder plunger. 

The final clamping action forces rubber through the sprue holes leaving a thin (0.13-0.25 mm) transfer pad. The mould is designed with load springs to break open the plates and to help support the top plate and plunger when bottom injection is used. 

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