Blow moulding has several variations, but the basic process involves either a tube of molten extruded material (the parison) or a heated preform being inflated within a mould. This forces the material against the inner form of the mould, where it cools and solidifies. The mould then opens so that the part can be removed.
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Extrusion blow moulding is the most common and simplest production method. Hot extruded plastic forms a tube (parison) that has one end open. The mould closes over the parison, and compressed air then forces the material outward to conform to the inside surface of the mould. Once the product has sufficiently cooled, the mould is opened and the hollow component removed.
Examples of blow moulded products are bottles, fuel tanks, watering cans, and containers.
This process can manufacture a wide range of products for an economical setup cost and in high volume. It is, however, difficult to control wall thickness, scrap rates are relatively high, and products tend to have high levels of flashing around the split line that has to be removed.
Stretch blow moulding is often used to make PET bottles and adds a material stretching process. It is a two-stage process that involves pre-forms to be injection moulded in advance. The pre-forms are then transferred to the stretch blow station where they are reheated before being inflated within a mould.
The production rate can be considerably higher than with the other two forms of blow moulding, but setup costs are also higher.