The blow moulding machine is based on a standard extruder barrel and screw assembly to plasticise the polymer. The molten polymer is led through a right angle and through a die to emerge as a hollow (usually circular) pipe section called a parison.
When the parison has reached a sufficient length a hollow mould is closed around it. The mould mates closely at its bottom edge thus forming a seal. The parison is cut at the top by a knife prior to the mould being moved sideways to a second position where air is blown into the parison to inflate it to the shape of the mould.
After a cooling period the mould is opened and the final article is ejected.To speed production several identical moulds may be fed in cycle by the same extruder unit. The process is not unlike that used for producing glass bottles, in that the molten material is forced into a mould under air pressure.