ABSTRACT
Blow molding is a low pressure process (25-350 psi) or (0.17-2.41 MPa) for forming hollow thermoplastic parts. An extruded parison or preform is surrounded by a mold and pressurized to take on the contours of the mold cavity. This process is similar to injection molding in that plastic resin is put into a hopper where it is fed through a heated barrel by a continuous screw. As the resin moves forward through the barrel, it melts and is then extruded through a vertical head die as a hollow tube called a parison. As the parison is extruded, the mold halves close around the parison and air is forced into the inside of the parison by means of a blow pin, which causes it to expand and take on the contours of the mold. The part is allowed to cool and is then ejected from the mold once it opens. Excess material is typically trimmed from the part by pinch-off surfaces in each mold half. In some cases, excess material is trimmed as a secondary operation. The cycle is then repeated. Figure 3.1A depicts the blow molding process. Photographs of blow machines are shown in Figures 3.1B and 3.1C.
