Preplasticizer machines

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. 

Two types of preplasticizer machines are shown in Figs. 4-2 and 4-3. The two-stage plunger system shown in Fig. 4-3 is similar to the single plunger system except the melt is forced into a second chamber instead of the mold. The melt is then forced into the mold by the second plunger. The direction of melt flow is controlled by a 3-way rotary valve. 

Processing and Application. Polystyrene and the other thermoplastics would not have been able to enter on their triumphal march if the manufacturers of processing equipment had not been able to provide efficient fabrication methods. A decisive factor was that between 1950 and 1960 the injection molding machine with a screw preplasticizer was developed and extrusion through slit dies to give sheeting or film followed by thermoforming was evolved. 

FIGURE 2.7 Schematic drawings of (a) a plunger-type preplastidzer and (b) a screw-type preplasticizer atop a plunger-type injection molding machine. (After Lukov, L. J. 1963. SPE/., 13(10), 1057.)... 

In the ram extrusion method the parison is formed in a cyclic manner by forcing a charge out from an accumulated molten mass, as in the preplasticizer injection-molding machine. The transport arm cuts and holds the parison and lowers it into the waiting mold, where shaping under air pressure takes place (Figure 2.33). 

An injection molding machine is used. The clamping unit is used as a press and preplasticizing is done in the injection unit. With this technique, much more precise feeding of the preplasticized mixture keeps the molding tolerances down. 

A Battenfeld (Siemag) Structomat ST 2500/400 S-C-150 machine was used to mould the tank. This has a 150 ton clamping force with a separate screw preplasticizer barrel feeding an accumulator. 

Injection machines are classified generally into three types—reciprocating screw, ram or plunger, and two-stage or preplasticizers— described in the paragraphs that follow. Most injection... 

Some machines have a two-stage injection unit. A rotating-screw preplasticizer feeds molten polymer into an injection cylinder, from which it is injected into the mold by a hydraulically driven plunger. This configuration is claimed to provide better control of... 

---