What Is Blow Molding?

Blow molding is a molding process used in the manufacturing industry to create hollow objects made of plastic. Like other molding processes, it involves the use of heated, liquid material that’s forced into a mold cavity under pressure. Blow molding is a special type of molding process, however, that leverages the properties of traditional glassblowing. The blow molding process was used to produce low-density polyethylene vials during World War II. In the late 1950s, with the birth of high-density polyethylene and the development of blow molding machines, blow molding technology was widely used.

Blow molding follows the same principle as glassblowing. With glassblowing, a glassblower blows air across heated glass, thereby creating a hollow glass object. With blow molding, a machine blows air across heated plastic that’s placed on top of a mold cavity. The air forces the heated plastic to expand across the interior walls of the mold cavity. Bottling and packaging are the main applications of blow molding which comprise about 49% of the global blow molding market share. This is followed by building and construction, consumer products, and transportation industries. The global market share of blow molding was estimated at around $78 billion in 2019 and is expected to grow annually by 2.8% from 2020 to 2027. Typical raw materials used are polyethylene (PE), polyethylene terephthalate (PET), and polypropylene (PP).

The tubular plastic parison obtained by extrusion or injection molding of the thermoplastic resin is placed in a split mold while it is hot (or heated to a softened state). After the mold is closed, compressed air is injected into the parison to blow the plastic parison. It expands and clings to the inner wall of the mold, and after cooling and demolding, 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 molds. From the perspective of plastic processing technology classification, the molding process of blown film is usually included in extrusion.

There are two main types of blow molding: extrusion and injection blow molding. They differ according to the method of producing the preform or parison. Preform is the more general term used to describe the heated plastic tube while parison is commonly associated with extrusion blow molding. A third type, stretch blow molding, is basically a modification of the two main types that are used for creating biaxially oriented plastics. These processes have their pros and cons that aim to serve a particular application.