GB851093A - Ring guide for freshly extruded film tubes - Google Patents

Abstract

<PICT:0851093/IV(a)/1> <PICT:0851093/IV(a)/2> Apparatus for making tubular film by the trapped bubble process comprises, in association with a tube-forming plastics extruder and a pair of pinch rolls disposed with their nip centres transversely on the extrusion centre-line, a metal ring of internal diameter greater than that of the orifice of the tube-forming extruder but smaller than that of the inflated film tube to be produced in the apparatus, said ring having a smooth bead circumferentially disposed along one edge of the inner face of the ring to provide a single, circumferential line of contact between the ring and the inflated tube, the surface of said bead consisting of a layer of a solid inert polymer of the recurring structure -(CF2-CFX)n-in which X is chlorine or fluorine, the ring being mounted coaxially with the tube-forming extrusion die and spaced therefrom a short distance to serve as a guide for the inflated film during operation of the apparatus. The film-contacting surface of the guide ring may consist of a thin layer or coating of polytetrafluoroethylene or polymonochlorotrifluoroethylene. The guide may be hollow and cooled by the circulation therethrough of a heat-exchange medium. As shown in Fig. 1, plastic film tube 7 is extruded through a die 4 having an annular orifice 6 coaxial with an extrusion centre-line 8 passing longitudinally through the centre of the die. Air or other gas is introduced into the tube through a passageway 5 in the die 4, and the tube 7, is withdrawn from the die by the pinch rolls 11 through a guide ring 9 which centres the expanding tube about the extrusion centre-line. After being oriented by the distending gas bubble 32, the tubular film is drawn by the pinch rolls 11 through the collapsing guides 10. As shown in Fig. 3, the ring 9 comprises a steel housing 12 of generally rectangular cross-section within which is provided an inner circumferential film-contacting guide bead 20 coated with a layer 21 of polytetrafluoroethylene about 0,001 inch thick. The coating facilitates the passage of the film tube through the guide. Specification 823,011 and U.S.A. Specification 2,461,975 are referred to.