Injection molding screw-plunger

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. 

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. 

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. 

A new option for micromolders is an injection machine that replaces the traditional screw with a motor-driven plunger. Named for the part sizes it produces, the Sesame is a small electropneumatic micromolding machine made by Hull Corp. (Warminster, PA). The machine can mold 20 thermoplastic sesame seeds from a single pellet. Instead of a screwing action, the Sesame uses a tiny plunger to push material into the mold. This plunger, that can be as small as 1.5 mm in diameter, is... 

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. 

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. 

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. 

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... 

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. 

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