Injection molding plunger

Injection Molding. Any standard design plunger or reciprocating screw injection machine can be used for PEA 340, although a reciprocating screw machine is preferred (32). Slow injection into mold cavities avoids surface or internal melt fracture, and control of ram speed is important at low... 

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. 

In addition to high-speed photography, which was used to obtain the results discussed above, other methods have been used to visualize flow, for example, the tracer method.279"281 A typical experimental device consists of a capillary viscometer connected to a mold, which is a simple model of plunger-type injection molding unit.279 A special device was used to introduce tracers of different colors in regular succession. After sample solidification, it is possible to examine the position of the various colors on the surface (Fig. 4.47 a) or through the volume by cutting sections (Fig. 4.47 b). It can be seen that the tracers are positioned symmetrically near the surfaces and that they have a V-type shape, with the tip oriented toward the stream. 

Two-Stage" Injection Molding Machine. Instead of using a reciprocating screw to melt the plastic and to push the melt into the mold, the extruder screw may feed the molten plastic into a separate reservoir, and then a separate plunger can feed the melt into the mold. 

Figure 4.12 Schematic of a ram (plunger) injection molding machine...

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. 

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. 

Polystyrene Syringe plungers Injection molding Good To 10,000 kGy Rigid... 

Injection equipment consists essentially of two parts the injection and clamping units. The injection unit melts the polymer and injects it in the mold. The vast majority of modern injection presses use a reciprocating screw. As in an extruder, the screw rotates, melts the polymer and pumps it. Furthermore, in most injection molding units, the whole screw can also axially move forward, acting like a plunger to inject the resin into the mold cavity through runners and gates. 

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. 

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. 

Note that the feed mechanism of the injection molding machine is activated by the plunger stroke. The function of the torpedo in the heating zone is to spread the polymer melt into thin film in close contact with the heated cylinder walls. The fins, which keep the torpedo centered, also conduct heat from the cylinder walls to the torpedo, although in some machines the torpedo is heated separately. 

Injection-molding machines are rated by their capacity to mold polystyrene in a single shot. Thus a 2-oz machine can melt and push 2 oz of general-purpose polystyrene into a mold in one shot. This capacity is determined by a number of factors such as plunger diameter, plunger travel, and heating capacity. 

FIGURE 2.6 Cross-section of a typical plunger injection-molding machine. (After Lukov, L. J. 1963. SPE 13( 10),... 

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