Blow Moulding

Blow moulding is the third most commercially important process for plastics production after extrusion and injection moulding. It is used to produce a range of hollow articles for example bottles, fuel tanks and other large containers. There are two main variations, injection blow moulding and extrusion blow moulding. The process sequence for both is the same.

•The material is either extruded or injected to produce a tube like preform.

•The preform is blown out to the shape of the mould and then cooled.

Injection blow moulding is most commonly employed for the production of transparent soft drinks containers. However, extrusion blow moulding is the one most commonly employed for mouldings such as shampoos and detergent containers, plastic drums and milk bottles and is described below.

The material is fed through a transfer screw (which is very similar to an extruder), into a die head where the material is melted and passes out through a die as a tube like extrudate termed a parison as shown in Figure 1.15. This process can be either continuous or with larger articles intermittent. The parison extrudes down vertically and relies on the hot strength of the plastic to hold the parison weight in shape.

For these reason blow moulding uses far more viscous materials than would normally be employed for the injection moulding process. A low viscosity material would simply pour out of the die onto the floor or split off before the parison had formed.

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