An air ring for supplying cooling air to a hot extruded plastic film after the film has been extruded from an annular extrusion die at an elevated temperature has an annular body which surrounds the tubular film after the film has left the extrusion die. The body has a circumferentially extending air passage through which air can be supplied from an external source and which directs cooling air from the passage between the body and the film with a venturi-like effect. A circumferentially extending series of individually operable actuators are each operable to vary the venturi-like effect at its circumferential location and cause the film to become nearer to or further from the body at said location to cause the thickness of the film at said location to decrease or increase.
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