The majority of polymer films are manufactured by film blower extruders (blown film extrusion). Film blowing process: A single screw extruder is used to melt the polymer and force it into a tubular die. Air is blown into the centre of the extrude tube and causes it to expand in the radial direction. Adjustments to wall thickness and diameter are made in this continuous bubble or tube.
Extension of the melt in both the radial and down-stream direction stops at the freeze line (frost line) due to crystallization of the melt. The nip rolls collect the film, as well as sealing the top of the bubble to maintain the air pressure inside. This is used extensively with polyethylene and polypropylene.
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