Production of tubular film

In a process for the production of an oriented tubular film of a thermoplastic polymeric material by extruding a tube of the material from an extrusion orifice, cooling the tube to a solid state by withdrawing the extruded tube over a cooling and sizing mandrel located within the tube, reheating the cooled tube to its stretching temperature, introducing gas under pressure, from a distributor beyond the end of the mandrel remote from the extrusion orifice, to expand the heated tube, and withdrawing the expanded tube, the improvement comprising sealing the tube by means of a cup seal, between the mandrel and distributor, said cup seal being in peripheral engagement with the internal surface of the tube, and having a peripheral region extending, in an upstream direction towards the mandrel, beyond at least a part of the upstream surface of the seal, thereby presenting a substantially concave surface to the mandrel.