Apparatus for cooling tubular plastic films extruded from a film blowing head

Claim 1

Apparatus for cooling tubular plastic films extruded from a film blowing head, comprising: a film blowing head having an annular extrusion die orifice; air cooling means including a plurality of pipes extending from the film blowing head to supply cooling and inflating air for cooling and inflating the film, and a pipe extending from the film blowing head and within the tubular film to withdraw the air from an inflated tubular film; a plurality of vertically spaced apart inner cooling rings disposed substantially concentrically to the axis of and downstream of the annular extrusion die orifice of the blowing head, wherein the inner cooling rings include annular members that define inner nozzle gaps and that have air guiding surfaces which face the inner surface of the film and are convexly curved in a longitudinal section of the inner cooling rings, and which surfaces during the operation of the apparatus are swept by cooling air blown out of said nozzle gaps, said cooling air blown out of said nozzle gaps flows approximately parallel to the direction of travel of the film; said cooling means also including outer cooling means, which concentrically surround the extrusion die and include an outer cooling ring defining an annular outer nozzle gap, which is adjacent to and downstream of the annular extrusion die orifice and serves to direct air along the inner surface of the film and in a direction which is approximately parallel to the direction of travel of the film; an apertured baffle plate spaced downstream from the film blowing head and surrounding the film below a radially outwardly expanded portion adjacent to a downstream end of the cooling path defined by the inner cooling rings, wherein said baffle plate in an axial projection on the axial projection of the expanded portion of the inflated tubular film covers at least the outer portion of the expanded portion of the film, and said baffle plate has a substantially circular inner edge which together with the wall of the film defines an annular gap; wherein the inner cooling rings are concentrically enclosed within the tubular film and surround the pipe serving to withdraw the cooling and inflating air, the outside diameter of the lowermost internal cooling ring is substantially as large as the inner diameter of the annular extrusion die orifice, and the diameters of the inner cooling rings above said lowermost inner cooling ring progressively decrease in a downstream direction relative to the blowing head so that the tubular film will assume an upwardly and inwardly tapering, frustoconical shape having a small included angle before it is inflated to its radially outwardly expanded condition downstream of the baffle plate.