Blow Molding

Blow molding is a technique for forming nearly hollow articles and is very commonly practiced in the formation of PET soft-drink bottles. It is also used to make air ducts, surfboards, suitcase halves, and automobile gasoline tanks. Blow molding involves taking a parison (a tubular profile) and expanding it against the walls of a mold by inserting pressurized air into it. The mold is machined to have the negative contour of the final desired finished part. The mold, typically a mold split into two halves, then opens after the part has cooled to the extent that the dimensions are stable, and the bottle is ejected. Molds are commonly made out of aluminum, as molding pressures are relatively low, and aluminum has high thermal conductivity to promote rapid cooling of the part. The parison can either be made continuously with an extruder, or it can be injection molded; the method of parison production governs whether the process is called extrusion blow molding or injection blow molding. Figure 1.40

shows both the extrusion and injection blow molding processes. Extrusion blow molding is often done with a rotary table so that the parison is extruded into a two-plate open mold, and the mold closes as the table rotates another mold under the extruder’s die. The closing of the mold cuts off the parison and leaves the characteristic weld line on the bottom of many bottles as evidence of the pinch-off. Air is then blown into the parison to expand it to fit the mold configuration, and the part is then cooled and ejected before the position rotates back under the die to begin the process again. The blowing operation imparts radial and longitudinal orientation to the plastic melt, strengthening it through biaxial orientation. A container featuring this biaxial orientation is more optically clear, has increased mechanical properties, and reduced permeability, which is important in maintaining carbonation in soft drinks. Injection blow molding has very similar treatment of the parison, but the parison itself is injection molded rather than extruded continuously. There is evidence of the gate on the bottom of the bottles rather than having a weld line where the parison was cut off. The parison can either be blown directly after molding while it is still hot, or it can be stored and reheated for the secondary blowing operation. An advantage of injection blow molding is that the parison can be molded to have finished threads. Cooling time is the largest part of this cycle and is the rate-limiting step. HDPE, LDPE, PP, PVC, and PET are commonly used in blow molding operations.

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