Process Encyclopaedia

Overview

Almost the opposite of blow moulding - with sucking instead of blowing! As a result, the two processes are useful for different types of shape, although both can only produce parts with thin walls. This process is more properly called thermoforming and relies on the sudden drop in strength and stiffness of thermoplastics above a certain temperature.

Process details

Materials and shapes

Only suitable for thermoplastics and some polymer foams. Shapes should have constant section thickness and not 'curve-back' on themselves. Parts cannot have holes or openings. Surface texture good, but fine detail in mould cannot be copied. Suction holes in mould need to be small to avoid leaving a mark on the product. Near-net-shape, but often leaves some waste material that needs trimming (and is difficult to recycle).

Economics

Cycle time is limited by heating and cooling of the sheet. Normally cycle times of 5+ units a minute can be achieved. Production rate can be increased by multi-part moulds, although extra trimming will be required. Manual equipment is cheap enough to use in a school workshop. Fully automated equipment can cost over £250,000. Moulds are usually aluminium (although wood can be used for small-scale production) and so relatively inexpensive. Manual systems viable from 1 - 1000 parts. With fully automated systems, only becomes economically viable for batches over 10,000.

Typical products

Advertising signs, bath panels, washing-up bowls, packaging.

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