For producing hollow articles, a special tool is required and this process is called extrusion blow moulding.

In the blow moulding process, molten resins are extruded in the form of a vertical tube, which is clamped between two halves of a mould. This tube is inflated so that it takes on the shape of the mould. It is then cooled, and released as the fully formed container. For larger containers, the molten resins have to be extruded very quickly or it will 'sag' due to gravity.