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Blow Molding 101

Blow molding is a conversion process wherein heat and pressure are applied to pelletized thermoplastic materials that are extruded into a cylindrical tube called a "parison". The molten "parison" is suspended within an aluminum mold closing around it. The mold halves are held together with a large hydraulic press while air is injected into the center of the part, forcing the plastic to push outward and replicate the cavity of the mold. In addition to the actual part cavity, the molds have water lines to assist in cooling and solidifying the part.

It is also possible to incorporate added design features that typically cannot be formed in a traditional blow molding process. These added features can include plastic or metal mounting tabs, injection molded fill necks, hose barbs, threaded pump mount rings, vents, valves and threaded fasteners. This process is called insert molding. Some injection molded plastic or metal features can be inserted into the mold prior to the blow molding process. These “inserts” are captured in the wall of the part as the plastic wraps and forms around them during molding and cooling process.

Once the finished product is removed from the mold, the next step is the de-flashing process where excess material is trimmed and removed from the molded part. Any excess material, or “flash” is ground up using a dedicated grinder positioned at each machine to ensure that each specific material is not mixed with dissimilar plastics from adjacent machines. The “regrind” material is conveyed to a blending unit where it is accurately metered and blended with the prime material and reused into the next part. Because our customers rely on us to provide only the highest quality product, Agri-Industrial Plastics Company uses certified prime virgin materials, unless recycled materials are preferred.

Many products made from metal, wood or other materials can be converted to plastics, in many cases offering greater design freedom and part consolidation. Other advantages of designing for plastics with the blow molding process are weight reduction, improved structural and dimensional stability, chemical resistance and heat resistance.