Method for the preparation of plastic film

Claim 1

In a method for the preparation of extruded thermoplastic film wherein a tube of a synthetic thermoplastic resinous material is extruded from an annular die and inflated by means of a gas contained therein to provide a tube having a diameter generally commensurate with the diameter of the die from which it is extruded, the tube being subsequently enlarged without restriction to a desired diameter substantially greater than initial tube diameter by means of a trapped volume of gas therein, the enlarged tube being subsequently collapsed to a flattened form to provide a plastic film, the film subsequently wound onto a roll, the improvement which comprises rotationally oscillating the tube relative to both the extruder and a collapsing means by frictionally engaging at least a portion of the periphery of the tube with a rotating means at a location between the die and prior to enlargement of the tube, the rotating means being positioned external to the tube, the rotating means consisting of a plurality of externally peripherally disposed rolls about the axis of the inflated tube, the rolls having their axes of rotation disposed in planes generally normal to the axis of the tube, with the further limitation that the rotating means has a variable speed drive and the rotating means is rotated in accordance with the following schedule: maximum speed decreasing to zero, direction of rotation reversed and speed increased to maximum and then decreased to zero and the direction of rotation again reversed; the tube being oscillated through an angle of about 180° or an integral multiple thereof at a generally constant frequency.