In the extrusion of plastics, raw plastic material in the form of small beads (called resin) is fed from a hopper into the barrel of the extruder. The polyethylene resin is heated to the desired melt temperature of the molten plastic.
A die is an upright cylinder with a circular opening similar to a pipe die. The diameter can be a few inches to more than a foot across. The molten plastic is pulled upwards from the die by a pair of nip rolls high above the die. Around the die sits an air-ring. The air-ring cools the plastic as it travels upwards. In the center of the die is an air outlet from which compressed air can be forced into the center of the extruded circular profile, creating a plastic bubble. The nip rolls flatten the plastic bubble into a double layer of film. During the extrusion process, plastic film is processed into various styles including sheeting, tubing, and centerfold. Polyethylene film can then be spooled or printed on, cut into shapes, and heat sealed into bags.
Poly film can be extruded to meet very specific qualifications for various industries. Among the various certified film that can be produced are FDA certified poly, USPS certified poly, and USP certified poly.
