Plunger machine

Fig. 13.1 (a) Reciprocating-screw machine, injection end. (b) Two-stage screw-plunger machine. 

Because of the much better temperature homogeneity of the injected material, the required injection pressure is two to three times lower than in a plunger machine, under comparable conditions regions where the polymer is hardly molten cause a considerable increase in viscosity. Plunger machines are, therefore, only used for mass production of very small articles. A variant is the combination of screw and plunger, each in their own cylinder, both of which are discharged into a chamber just in front of the injection point the screw forms and transports the melt, which is the injected into the mould by the plunger. 

The early injection molding machines were plimger machines. The plastic was melted in a heated cylinder and injected into the mold by the plunger. This type of machine is no longer found, except in research laboratories, for making very small quantities of experimental articles. Plunger machines are not discussed in this book. 

An improvement in the two-stage plunger machine is the replacement of the preplasticating ram with a rotating screw. The screw extrudes the plastic, which is melted by the combination of mechanical work by the screw and the heat for the heater bands. The use of the screw results in more thorough plastication combined with better control of the melt temperature through the mixing action of the screw. 

Figure 6.48 (a) Schematic of a plunger injection molding machine identifying the principal machine variables, (b) Relation of fundamental injection molding variables to plunger machine variables. (From Ref. 65.)... 

Preplasticizer machines have been developed to overcome melt degradation and uneven melt heat problems associated with the larger capacity plunger machines. In preplasticating units, plastication is carried out in a separate cylinder which feeds molten material into an injection cylinder. 

Color concentrates are also used for some materials. Pellets of plastic containing high concentration of color are mixed with the clear pellets in a ratio of about 1 10 for plunger machines and 1 20 for screw machines. The correct shade and intensity of color is ascertained by experimenting with the amounts of colorant used and the length of tumbling time. ... 

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... 

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