Injection Moulding

Injection moulding is one of the most common manufacturing processes in use today. It lends itself to high volume production of both complex and precision parts with a variety of uses. It is a highly versatile process that is as successful and economic for both small and large components from an automotive bumper to intricate wristwatch parts.

Figure 6.7 Injection moulding machine

An injection moulding machine consists of an injection unit and a mould clamping unit and is shown in Figure 6.7. The injection unit must convey, plasticate, mix and homogenise the melt. It must also be able to inject a fixed weight into the mould. This is called the shot weight and will vary depending on the shape and size of the component to be moulded. The clamping unit is responsible for opening and closing the mould as well as part ejection after moulding. The clamping mechanism applies a clamping force on the mould, to hold it together whilst the plastic material is injected.

A simple description of an injection moulding cycle is:

  1. The mould closes,
  2. Material is injected,
  3. The material cools in the mould,
  4. The injection unit meters the next shot,
  5. The mould opens and the part is ejected.

Like an extruder, an injection moulding machine uses a screw to feed and plasticate the melt. There are however, a number of design differences between the two. An injection screw must withstand both higher pressures and intermittent stop-start use. It must be able to inject as well as move back to prepare the next weight of shot.

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