Introduction to molds（2）

The control unit and control cabinet allow the machine operator to control and monitor the process.

Control systems will be discussed in Chapter 5.

The injection unit is there to melt and meter the material into the tool. Machinery will be discussed in Chapter 3.

The clamp unit holds the injection moulding tool and gives the required clamp force to hold the two sides of the mould tool together. Tooling will be discussed in Chapter 4.

The machine base with hydraulics houses the systems that drive the movement of the machine. Each machine has a certain ‘footprint’. This is the floor space it requires for operation.

2Practical Guide to Injection Moulding

1.3 The Injection Moulding Process

In injection moulding the mould and the plasticising area are separated from each other. The plasticising area, i.e., the plasticising cylinder temperature, is kept at the level of the processing temperature. The mould on the other hand, is kept cold enough for demoulding of the injection moulded part (thermoplastics) or warm enough for crosslinking (thermosets). The plasticised material is injected into

the clamped mould. In an injection moulding machine, the clamping unit which contains the mould and the injection unit are integrated. Completely automated production is possible if the mould is installed with a vertical parting line. This enables the parts to fall down and out of the mould after demoulding.

Injection moulding machines are typically used for the processing of thermoplastics. There are two types of injection unit available: a piston injection unit and a screw piston injection unit (reciprocating).

The reciprocating screw method is the most common. For the processing of thermosets only screw piston machines can be used. This is because without the screw, the dwell time would be too long and the risk of early crosslinking would be too high. The injection sequence for both types of machine now follows.

1.3.1 Piston Injection Unit

Injection sequence: 1. Injection starts (clamped mould, start of piston movement) (Figure 1.3)

2. Injection and dosage

3. Holding pressure (to balance the solidification shrinkage) (Figure 1.4)

4. Ejection (Figure 1.5)

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