Reciprocating screw machine

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... 

Horizontal reciprocating screw machines (Fig. 20). Vertical screw/ram machines (Fig. 21). 

In recent years, the trend has been toward an increasing use of the reciprocating screw machines. One of the most common reasons given for the change is that the first in-last out principle applies for screw/ram... 

To ensure consistency, the two critical areas for control specific to injection molding are shot size control and temperature control on the injection system, i.e., screw portion for the reciprocating screw machines, and both the screw and ram sections for the screw/ram machines. As mentioned previously, for temperature and cure control, the use of microprocessor controls has significantly improved the performance of injection molding machines, including control of the shot size. 

Fig. 13.1 (a) Reciprocating-screw machine, injection end. (b) Two-stage screw-plunger machine. 

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. 

Cyro produces a family of acrylic-based resins Acrylite , Cyrolite and XT). All types of injection molding machines can be used to mold these blends. If reciprocating screw machines are used, the screw compression ratio should not exceed 3.5 (to avoid material breakdown). Injection temperature should be the lowest at which the part will fill. Since the material has sparkling clarity, great care should be taken to avoid contamination. 

Easily processed on conventional reciprocating screw machines... 

Ease of molding Reciprocating screw machines preferred... 

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. 

The reciprocating screw machine consists of a barrel or cylinder and a screw which rotates to melt and mix the material and then moves forward to force the melt into the mold (see Fig. 4-1). 

As previously stated, GRTP can be processed on either screw or plunger (ram) type machines. However, because reciprocating screw machines are preferred and generally used today, this paragraph and processing data and discussion presented in Table 4-1 are applicable to reciprocating screw machines. 

For a reciprocating screw machine the process cycle can be split into five stages ... 

A reciprocating screw machine is shown in Figure 7 in which materia flows in the channel of an helical screw the entire length prior to injection. The rotational flow and close contact with the heated barrel bring the material to a uniform temperature and metering, homogeneity, injection pressure control, reproducibility and cycle time are thereby improved ... 

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