Method of forming tubular plastic material having a flare-top edge using a blow head

Claim 1

In a method of forming extruded tubular thermoplastic film to create a permanent inherent flaring characteristic of the severed edges of the tubular film when flattened and lying in face-to-face two-ply relation, wherein a series of internal annular nozzles is employed to cool the film internal surface with a gaseous medium and form continuous skin layers thereon and wherein the film during formation by streatching is maintained out-of-contact with said internal annular nozzles, the improvement which comprises the steps of directing a plurality of radial streams of cooling gaseous medium from said series of internal annular nozzles forcefully against the interior surface of the extruded tubular film as formed closely adjacent the melt issuing extruder, controlling the removal of the internal cooling gaseous medium from the interior of the extruded tubular film and through the extruder to thereby control the internal pressure and precise dimensions of the extruded tubular film as formed, directing one or more radial streams of heated gaseous medium forcefully against the exterior surface of the extruded tubular film as formed closely adjacent the melt issuing extruder, the cooling and heated gaseous mediums being juxtaposed at least in part and simultaneously applied in facing relation to create a substantial temperature differential between the interior and exterior surfaces of the extruded tubular film as formed to thereby establish different physical characteristics in said surfaces and a permanent flaring tendency at its edges when flattened and subsequently severed.