Blow molding was designed to create hollow products without having two separate pieces that have been fused together. This process revolutionized the bottling industry. It allowed manufacturers to mass produce light weight, cheap packaging. Before the blow molding process was created the only option was blown glass containers. Blown glass was expensive and had to be handled with care. The introduction of plastic containers created a much safer packaging option. Thermoplastic resins are the primary plastic material used in blow molding. There are six steps to the process. First, the resin is heated into a liquid state. Second, the heated material is pressed through a die head to form a hollow parison. Third, the parison is enclosed by two mold halves. Fourth, heated air is blown into the parison, expanding it to completely line the mold walls. Fifth, the mold is cooled to solidify the molten plastic. Finally, the mold is opened and the finished, hollow product is ejected. There are four variations of blow molding: extrusion, injection, stretch and reheat.