Blow molding: also known as hollow blow molding, a rapidly developing plastic processing method. The tubular plastic parison obtained by extrusion or injection molding of thermoplastic resin is placed in a split mold while it is hot (or heated to a softened state), and compressed air is introduced into the parison immediately after closing the mold to blow the plastic parison. It expands and sticks closely to the inner wall of the mold, and after cooling and demoulding, various hollow products are obtained. The manufacturing process of blown film is very similar in principle to blow molding of hollow products, but it does not use a mold. From the perspective of plastic processing technology classification, the molding process of blown film is usually included in extrusion.

When selecting resins for blow molding, MI, density, and resin molecular weight distribution should be considered. Generally speaking, when MI is less than 2, the blow molding is better, and the wider the molecular weight distribution, the better the molding performance.

The relationship between stiffness and ESCR that varies with density, the factor that has a great influence on the stiffness of resin and the adjustment of ESCR performance is density. Stiffness and ESCR performance exhibit opposite results as the density changes. Blow-molded products are mainly used for containers, so the outer wall of the molded parts should have rigidity that can withstand a certain pressure and have ESCR performance to ensure that the contents will not leak when used for a long time in hot and cold weather.

The factor that has the greatest influence on the stiffness of the resin and the adjustment of the ESCR performance is the density. Stiffness and ESCR performance exhibit opposite results as the density changes.