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Manufacturing Processes - Injection Moulding Process

Successful Injection Molding

Injection Moulding Process

The injection moulding process fits under the general category of casting as a manufacturing process. This may seem surprising as the process is dedicated to the production of plastic, rubber and composite components. There are however similarities between pressure die casting and injection moulding in that the molten polymer is forced at high pressure into a metal mould. The mould in an injection moulding process is cooled which promotes a faster solidification time.

The process is suited to complex 3D shapes but has limitations with respect to thick sections which can prove problematic for injection moulding. Also, to prevent distortion, it is usually better to design components to have uniform sections and to make allowances for shrinkage.

The normal mode of operation of an injection moulding process is to open and close the mould using a hydraulic arrangement. The material is fed via a feed hopper, usually in granular form, into a heated screw feed arrangement, raised in temperature to between 220C and 270C. There is a back flow stop valve to prevent back feed of the material once injected into the charge chamber and a hydraulic arrangement is used to push the feed screw forward forcing the material into the mould. This aspect of the system is known as an injection moulding reciprocating feed screw arrangement.

Pros

  1. Suitable for all thermoplastic materials, some thermosetting plastics, some elastomers and rubber
  2. Capable of maintaining close tolerance production down to +/- 2 thousandths of an inch (more typically 8 thousandths)
  3. Suitable for complex 3D shapes
  4. Mechanical properties of the material are largely unaffected

Cons

  1. High operating costs due to required equipment
  2. Only suited to high volume production due to equipment costs
  3. Relatively high waste material, although thermoplastic materials can be recycled

The injection moulding process lends itself very well to the mass production of a multitude of cheap plastic components ranging from items such as bottle tops, children's toys, combs and industrial components that are often disposable items. Many of these can be recycled at the end of their useful life if they are produced in thermoplastic material.

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Injection Moulding Schematic

Schematic provided by Brockey, February 2009 and used here under the Creative Commons license ref: http://creativecommons.org/licenses/by-sa/2.0/deed.en

Schematic provided by Brockey, February 2009 and used here under the Creative Commons license ref: http://creativecommons.org/licenses/by-sa/2.0/deed.en

Injection Moulding Process

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