Blow moulding is a manufacturing process by which hollow plastic parts are formed. In general, there are three main types of blow moulding: extrusion blow moulding, injection blow moulding, and injection stretch blow moulding.
The blow moulding process begins with melting down the plastic and forming it into a parison or, in the case of injection and injection stretch blow moulding (ISB), a preform. The parison is a tube-like piece of plastic with a hole in one end through which compressed air can pass.
The parison is then clamped into a mould and air is blown into it. The air pressure then pushes the plastic out to match the mould. Once the plastic has cooled and hardened the mould opens up and the part is ejected.