Blow molding makes hollow parts, such as containers or bellows. We create a part by extruding a tube of plastic, called a parison, and then closing a mold on it. We pinch the tube at the bottom, welding it together, and then insert a blow pin in the top, which seals the top of the mold. We then inject air through the blow pin, which inflates the parison to conform to the shape of the mold. The mold opens, and the new part drops out .