BLOW MOLDING
Introduction
Blow molding is defined as a plastic process whereby a thermoplastic material is heated to its forming temperature, which is below that of the plastic materials being used; at its melting point it is made to form a hollow tube called a parison or preform. This heated homogeneous plastic material is then placed between two female molds that are cooled through some medium. The two female molds close on the heated parison or preform and a gas, usually air, enters via an open end of the parison via a blow pin or needle; the gas is blown into the closed female mold halves, taking the shape of the internal female closed mold, allow to cool, and is then sent out through an exhaust. The two female molds are then separated and the cooled, shaped hollow part is then ejected or allowed to drop out for the cycle to repeat.
In the first attempt over 100 years ago, to blow-mold hollow objects, two sheets of cellulose nitrate were clamped between two female mold halves. Steam injected between the sheets softened the material, sealed the edges, and expanded the heated sheets to form the inside shape of the two female mold halves. The high flammability of cellulose nitrate, however, limited the usefulness of this technique. In the early 1930s, more suitable materials, such as cellulose acetate and polystyrene (PS), were developed; these led to the introduction, by Plax Corp. and Owens-Illinois, of automated equipment based on glass-blowing techniques.
Unfortunately, the high cost and poor performance of these materials discouraged rapid development; they offered no advantage over the glass bottles. Finally, the introduction of low density polyethylene (LDPE) in the mid-1940s provided the advantage of squeezability, which glass could not match.