The tubular plastic parison obtained by extrusion or injection molding of thermoplastic resin is heated (or heated to a softened state), placed in a split mold, and compressed air is introduced into the parison immediately after closing the mold to blow the plastic parison It is inflated and adhered to the inner wall of the mold, and is cooled and released to obtain various hollow products. The manufacturing process of the blown film is very similar in principle to the blow molding of hollow products, but it does not use a mold. From the perspective of classification of plastic processing technology, the molding process of blown film is usually included in the extrusion. The blow molding process of the blow molding machine began to produce low density polyethylene vials during the Second World War. In the late 1950s, with the birth of high-density polyethylene and the development of blow molding machines, blow molding technology was widely used. Hollow containers can be up to several thousand liters in volume, and some have been computer controlled. Suitable plastics for blow molding include polyethylene, polyvinyl chloride, polypropylene, polyester, etc., and the resulting hollow containers are widely used as industrial packaging containers.