Shape imposition apparatus for the extrusion of tubular thermoplastic film

Claim 1

In an apparatus for forming tubular thermoplastic film comprising extruder means; annular extrusion die means, adopted in combination to extrude a single tube of thermoplastic film; pinch rollers spaced downstream from said annular extrusion die adopted to flatten said tube; and means for introducing a fluid under pressure into said tube whereby to biaxially expand said tube and reduce the wall thickness thereof; the improvement, whereby reducing gauge non-uniformity in said film, which comprises a configuration imposition assembly comprising a diverging housing member, disposed concentric to said extruded tube downstream of, and adjacent to, said annular die, said tubular thermoplastic film passing through said diverging housing member upon exiting from said die, said assembly being further characterized by having multiple rows of channels therethrough, each of said channels terminating in an aperture on the inner surface of said housing, said apertures being arranged in a series of streamwisedly spaced pairs of diverging aperture rows adjacent to and directed towards said extruded tubing in an overall diverging shape, said streamwisedly spaced pairs of aperture rolls having individual apertures in one row of the pair which diverge from the adjacent apertures in the second row of the pair, means for forcing a fluid through said apertures towards said extruded tubing and then away from such tubing, whereby creating a suction and thus drawing said extruded tubing toward said assembly.