Blow molding covers three main thermoplastic processes: extrusion blow molding, stretch blow molding, and injection blow molding. Extrusion blow molding is the largest of the three, followed by stretch blow molding and injection blow molding. The total blow molding industry is growing approximately 3–5% annually and will continue to grow at this rate.
Extrusion blow molding is the largest process user of HDPE. “Plastic News” reported through Sep. 2008 that United States production of HDPE was 12.8 billion pounds. The extrusion blow molding markets for HDPE that are growing at a greater pace than the bottle market, in general, are for large holding tanks, 55 gal drums, and automotive fuel tanks. Approximately 40% of the HDPE produced goes into the blow molding market, making it the workhorse process of the HDPE industry.
Blow molding is the forming of a hollow object by inflating or blowing a thermoplastic molten tube called a “parison” in the shape of a mold cavity. The process consists of extruding or “dropping” a parison on which female mold halves are closed. The female mold halves contain the shape of the product to be produced. The bottom opening of the parison is pinched shut by the closing female mold halves. A pressurized gas, normally air, is introduced into the parison blowing the heated parison out against the cavity walls to form the product. Fig. 13.1 is a general schematic of the process.
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