Transfer molding screw-plunger

There are essentially two methods of using the plasticating screw. The first is the screw-plunger system also called the two-stage or screw-pot system. The screw rotates in the heated barrel and consequently plasticizes the polymer material. The plasticized material is then transferred into a second heated cylinder from which it is injected into the mold by a plunger. Figure 11.6 shows the basic features of a screw-pot injection molding machine. 

In this proeess, the plastic is heated and melted in a screw and then dropped into a pot of an inverted plunger mold (Fig. 10.15). The molten polymer is transferred to the mold by the same procedure as shown in Fig. 10.12. The screw transfer method and the sequence of the operational steps are depicted in Figs. 10.15(a) and 10.15(b). Screw transfer molding is an excellent process for full automation. 

Melting (plasticating) the plastic is accomplished in a plasticator (screw in barrel as described in Chapter 3). This melt is forced into a clamped mold cavity. The liquid, molten plastic from the injection cylinder of the injection machine is transferred through various flow channels into the cavities of a mold where it is finally shaped into the desired object by the confines of the mold cavity. What makes this apparently simple operation complex is the limitations of the hydraulic or electrical circuitry used in the actuation of the injection plunger and the complicated flow paths involved in the filling of the mold (Chapter 17). Finally opening the mold to eject the plastic after keeping the material confined under pressure as the heat in the melt is removed to solidify the plastic into the shape desired. 

Automation of the transfer/plunger concept is accomplished by the addition of a hopper-fed screw plasticator, in a system called screw transfer, which can replace the preform and preheat operations and automatically load the pot in CM (and is used to load a CM mold cavity). Conventional screw injection molding (IM) is another so-called transfer system (Chapter... 

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