The Principles of Blow Moulding A simple explanation of the principle of blow moulding is a balloon. If you blow air into a plastic tube that is closed on all sides except the point at which the air enters, the tube will expand and take the shape of the mould that is around the tube.

The process begins with a plastic resin hot tube called a parison or pre-form. The parison is placed within a split mould with a hollow cavity.

The mould sides are then clamped together, pinching and sealing the parison tube.

Air is blown into the tube, which expands the hot resin wall into the shape of the cavity; the mould is cooled with water solidifying the resin into the shape of the part.

Once cooled, the part is ejected from the mould and trimmed.