This is the primary processing technique used to fabricate hollow plastic objects, particularly bottles, which do not need a very uniform distribution of wall thickness. It is a secondary shaping technique that inflates the preprocessed plastic (usually extruded) against the inside walls of the mold with a blow pin. In addition to extrusion blow molding, injection blow molding and stretch blow molding are commonly employed. With most polymers, especially when the product size is
Plastic tube extruded
Cold air injected
Figure 2.15. Blow molding of plastic bottle.
Cold air injected
large, extrusion blow molding is used; while injection blow molding is typically used with smaller products with no handleware. Semicrystalline materials that are difficult to blow are molded by stretch blow molding. Common resins such as PVC, PS, PP, LDPE, HDPE, and PET are blow molded routinely. Figure 2.15 illustrates the steps involved in extrusion blow molding.
In extrusion blow molding, the most common blow-molding process, an extruder is used to produce a thick-walled plastic tube called the parison. The parison is extruded directly into a water-cooled cavity mold, which is then closed, and air injected through the top or the neck of the container. The softened polymer in the parison inflates against the wall of the mold, which cools the melt and solidifies it into the mold shape. The mold opens and the part is removed and deflashed to remove any excess plastic. While the wall thickness of the parison itself is uniform, that of the product formed (a bottle) will not be uniform because of its different geometry. This variation in wall thickness needs to be taken into account when designing products intended for blow molding. In this processing cycle most of the time is spent on cooling the mold. Therefore, it is usual to have several molds set up on a rotating table that takes up sections of parison from a single continuous extruder to optimize the process.
In injection blow molding, an injection-molding machine replaces the extruder. In the first stage a parison with the threads of the finished bottle molded in is injection molded onto a core element. The injected parison core is then carried to the next station on the machine, where it is blown up into the finished container as in the extrusion blow-molding process above. In some instances the parison might be stretched inside the mold to obtain a biaxially oriented plastic product. As the parison is injection molded, there is good control of the weight of the final product in this type of blow molding.
In stretch blow molding (for resins such as PET used in soda bottles) an injection-molded preform (usually obtained from a separate specialized vendor) is used. The preform is loaded into a simple machine that heats it to soften the plastic and stretches it inside the mold to shape the plastic into a bottle.
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