The process involves the production of a tube or parison or soft thermoplastic polymer by extruding it between a circular die and internal mandrel vertically downwards. In its simplest form a two piece mould closes around the parison sealing one end and compressed air is introduced to the inner surface of it which inflates the parison until it is forced against the mould whereupon it takes up the shape of the mould and cools by conduction. Subsequently the mould opens and the formed part is ejected. There are many similarities to the process of glass blown into moulds.
There are many derivations of the basic process. Some designed for very high speed production of large quantities of product will have two or more moulds that alternately close around the parison and then move to another position for the part to be blown thus allowing the other mould to close around the next piece of parison. This allows the extruder to continuously extrude with its output sized to match the forming, cooling and ejection times of the remainder of the machinery.