Reciprocating screw injection unit

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 purpose of the injection unit is to heat and melt the polymer, inject the melt into the cavity, and apply pressure during the cooling phase. The most common type of injection molding machine is the reciprocating screw. In this type of machine, the screw rotates to plasticize the polymer, moving backward to deposit a volume of polymer melt ahead of the screw (shot). Once the correct shot size has been built, the screw then moves forward to inject the melt into the mold. Injection molding is a discontinuous process, and the clamping unit allows for the mold to open and close for part removal and to provide pressure as the cavity is filled. This is depicted in Fig. 1.19. 

A combination injection and plasticizing unit in which an extrusion device with a reciprocating screw is used to plasticize the material. Injection of material into a mold can take place by direct extrusion into the mold, by reciprocating the screw as an injection plunger, or by a combination of the two. When the screw serves as an injection plunger, this unit acts as a holding, measuring, and injection chamber. 

Reciprocating-screw molding machines n. A combination plasticating and injection unit in which an extrusion device with a reciprocating screw is used to plasticate the plastic. 

A reciprocating-screw machine is an exunder system that has the advantages of screw plastification and ram injection in the same unit. In this design, as the screw rotates, it is pushed backwards by the molten polymer that collects in front of the screw. At a predetermined point, rotation stops and the nozzle moves in the forward position, acting as a ram to force the melt into the mold. It remains forward until the mold gates freeze, then pumps back to repeat the cycle while the mold opens. 

In the injection molding machine shown in Figure 13, the clamp unit is on the left and the injection unit on the right. A mold is shown in position between the platens of the clamp unit with one half of the mold fastened to the fixed platen and the other to the movable platen. When the mold is opened, the movable platen moves away from the fixed platen and the molded part can be removed. After part ejection, the mold is closed in preparation for the next injection cycle. In contrast to extrusion, the screw in the injection imit rotates only during part of molding cycle. When the screw turns, it pumps melt forward. The rearward movement of the screw is controlled by the placement of a limit switch, which stops the hydraulic motor. The maximum length of reciprocation is approximately three screw diameters. The shot size is directly proportional to the amoimt of screw... 

FIGURE 14.5 Maj or components of an inj ection molding machine showing the extruder (reciprocal screw) unit and injection molds. Top the screw rotates and slides backward to allow polymer melt to accumulate in front of the screw. Bottom the mold clamps up followed by moving the screw forward to inject polymer melt into the hot runner and fill the mold cavities. 

Figure 5.1 Major components of an injection molding machine showing the extruder (reciprocal screw) and clamp units [1]...

These kinds of injection units have mostly been replaced by reciprocating screws, but they are occasionally found in applications such as multicolored parts where a mottled appearance is desired. They excel here because there is very little mixing in the process. 

The hardware associated with the injection unit in the reciprocating screw IMM consists of ... 

Figure 7.4 Typical reciprocating screw injection unit. ...

Reciprocating screw intermittent extrusion blow molding machines use an extruder unit in which the screw is capable of axial as well as rotational movement. The function is virtually identical to that of the injection unit on an injection molding machine, except that the blow molding unit functions at a lower melt... 

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