16 - Blow Molding

Abstract

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. Extrusion blow molding is the largest process user of high-density polyethylene. 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. Extrusion blow molding can be classified into two major categories: continuous extrusion and intermittent extrusion. Intermittent extrusion is further subdivided into reciprocating screw, ram, and accumulator parison extrusion. Continuous extrusion has the heated parison being continuously extruded. A stationary extruder plasticizes and pushes the molten plastic through the head to form a continuous parison. A good example of the continuous extrusion process is the wheel machines used by major companies as Graham, Plastipak, Consolidated Container, Ball, Silgan, and the other large volume plastic bottle producers. The intermittent parison extrusion blow molding machine using a reciprocating screw is often referred to as “shot” extrusion. The screw rotates and retracts, plasticizing or melting the resin as it moves back, then charging the shot in front of the screw. The screw then rams forward via hydraulic or electric means, pushing the heated plastic through the head and out of the die head tooling as a parison. One of the most important items in blow mold design is for the designer to understand the plastic resin and its behavior in blow molding the required product.