Blow molding ( BrE moulding ) is a manufacturing process by which hollow plastic parts are formed. The parison is formed by melted plastic material being pushed through an extruder. The parison is a tube-like piece of plastic with a hole in one end through which compressed air can pass. When the parison reaches a certain length, the two halves of the mould close around the parison sealing it at the bottom. Compressed air is then used to inflate the parison to form the shape of the cavity inside the mould.