Plunger-type molding machines

Injection molding is not new. A patent was issued in 1872 for an injection-molding machine for camphor-plasticized CN, celluloid. Almost all of the machines used today are reciprocating or two-stage screw types. Both types employ a reciprocating Archimedean-like screw similar to that of a screw extruder. A few are of the plunger type. 

Fluoropolymers can be injection-molded in plunger or ram-type equipment but a screw machine works best with this family of plastics. The screw-type injection molding machines have a number of advantages including those listed in Table 6.23. Various components of screw-type machines for processing fluoropolymers are described in this section. 

The majority of commercial blend grades has been developed specifically for injection molding and thus can be processed in conventional machines as long as the processing conditions recommended by the blend manufacturer are followed. Reciprocating screw machines are preferred over plunger-type ones. 

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.)... 

An extruder. Fig. 17, has essentially the same structural components as the plasti-fication unit in a screw-type injection molding machine. Fig. 11. The screw in this case is not an axial plunger in the extruder it only rotates. The significance of extrusion per se is the continuity of the process. The method is used to produce plates, tubing, profiles, films, etc. 

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. 

There are two types of machines that can be used for injection molding—one fitted with a plunger and the other a screw type injection unit, the latter being preferred for carbon fiber applications. Section 17.9.2 explains the functions of the individual parts of an injection molding machine and describes the terminology used for this application. 

Use of an injection molding machine is the most common method for thermoplastic material. PE or other polymer samples are preheated in a cylindrical chamber to a temperature at which it will flow and then is forced into a relatively cold, closed mold cavity by means of quite high pressure applied hydraulically through a ram or screw type plunger. The screw is moved forward to force a fixed volume of the molten polymer into the closed mold, where the polyethylene is soHdified. Finally, the screw rotates and moves backward to charge the polymer for the next cycle. 

As shown in schematic form in Figure 15.14, an injection-molding machine is essentially a screw extruder attached to a mold. The action of the extruder results in a pool of molten polymer directly in front of the screw tip, and this causes a buildup in pressure that forces the screw to retract backward. Once a predetermined amount of polymer (the shot size) has been collected, screw rotation stops and the entire screw moves forward like a plunger pushing material into the mold. This type of machine is therefore known as a reciprocating-screw injection-molding machine. Once the polymer has sohdiiied, the mold is opened. 

Z molding compound (ZMC) was first prepared in France in 1979. This compound needs a special type of injection machine—a combination of plunger and screw—and was indigenously developed by Billion in France. The machine uses a screw to homogenize and measure the shot. The injection is made like a plunger by the displacement of the screw and inner barrel inside the main barrel. Compared to SMC, ZMC parts have lower mechanical properties, but higher performance when compared to conventional injection molded BMC. 

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. 

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