Plastic is first melted and extruded as a hollow tube. A two-part mould closes around the molten tube and the mould is then separated from the extrusion, with a piece of the tube caught inside. Air is blown through the opening in the top (the neck) of the mould, inflating the tube to the shape of the mould. After the plastic has cooled sufficiently, the mould is opened and the part is ejected. Blow moulding is used to process many different plastics, including high-density polyethylene, polypropylene, PVC, and polycarbonate.
The process is used to produce milk bottles, shampoo bottles, watering cans, automotive ducting, and hollow industrial parts.
It is a low-pressure process with simple moulds, and the capital investment is therefore relatively small. This makes the process feasible for both large and small production runs.