Reciprocating Screw Machine

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

1. In stage one, as shown in Figure 1.6, material is injected into the tool.

2. In stage 2 (Figure 1.7), the screw begins to turn and retract, metering a specified weight of molten material for the next shot. The previous shot is now cooling in the closed tool.

3. In stage 3, the injection unit moves back from the clamping unit as shown in Figure 1.8.

4. Stage 4 is shown in Figure 1.9. In this stage the tool opens to reveal a cooled injection moulded

component.

5. Stage 5 is ejection of the part as shown in Figure 1.10. The injection unit will then move forward to the clamp unit to start a fresh cycle as shown in stage 1.

Thermoplastics as well as thermosets and classic elastomers can be processed with screw injection

units. These material types will be discussed in more depth in Chapters 6 and 7.

Thermoplastics are injected into a cold mould. The temperature of the mould must be sufficiently below the melting temperature of the material for it to solidify. This is because solidification is a physical process.

Thermosets and classical elastomers are injected into a hot mould to make the crosslinking of the

material possible. Crosslinking is a chemical process.

In the screw piston injection unit, the material is dosed and plasticised simultaneously. The material is kneaded thoroughly by a rotating, axially movable screw. It is heated up to the processing temperature by the heat transfer of the hot cylinder wall and by friction. The material is transported by the screw to the screw tip. As the nozzle opening (cylinder opening) is still closed, the screw moves backwards. As

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