Method for the manufacture of biaxially orientation-stretched plastic film

Claim 1

A method of manufacturing biaxially orientation-stretched plastic film, comprising the steps of extruding a plastic film through an annular die to form a seamless tube, simultaneously supplying a gas to the inside of the tube during forming to inflate the tube such that the diameter of the tube is at least doubled, simultaneously drawing out the tube at least to the same extent in its longitudinal direction, cooling the tube thus formed, flattening the tube, sealing the tube by conducting the tube between a first pair of driven rolls, expanding the tube formed in the first inflation operation after the flattening between the first pair of rolls by introducing gas, forming a gas reservoir or gas accumulator for continuously retaining a quantity of the introduced gas between the first and second pair of rolls, flattening the tube between the second pair of rolls, controlling the pressure of the gas in the gas reservoir by selectively moving two compression plates toward and away from one another which plates are arranged on either side of the gas reservoir, inflating the flattened tube again with the gas from the reservoir to increase the diameter to orientation stretch the tube, simultaneously further stretching the inflated tube in its longitudinal direction whereby the tube material is biaxially molecular-oriented, said pressure of the gas in the gas reservoir being controlled such that sufficient pressure is provided to ensure a proper subsequent inflation of the tube and to compensate for gas loss.

Claim 2

A method of manufacturing biaxially orientation-stretched plastic film, comprising the steps of extruding a plastic film through an annular die to form a seamless tube, simultaneously supplying a gas to the inside of the tube during forming to inflate the tube such that the diameter of the tube is at least doubled, simultaneously drawing out the tube at least to the same extent in its longitudinal direction, cooling the tube thus formed, flattening the tube, sealing the tube by conducting the tube between a first pair of driven rolls, expanding the tube formed in the first inflation operation after the flattening between the first pair of rolls by introducing gas, forming a gas reservoir or gas accumulator for continuously retaining a quantity of the introduced gas between the first and second pair of rolls, flattening the tube between the second pair of rolls, controlling the pressure of the gas in the gas reservoir by selectively moving two compression plates toward and away from one another which plates are arranged on either side of the gas reservoir, inflating the flattened tube again with the gas from the reservoir to increase the diameter to orientation stretch the tube, simultaneously further stretching the inflated tube in its longitudinal direction whereby the tube material is biaxially molecular-oriented, said pressure of the gas in the gas reservoir being controlled such that sufficient pressure is provided to ensure a proper subsequent inflation of the tube and to compensate for gas loss.