Patent GB-1111604-A (GB)
Title: Biaxial further-stretching of thermoplastic tubular films
IPC: B29C55/28
Filed: 1966-01-10
Abstract: 1,111,604. Biaxially stretching tubular films. POLYMER PROCESSING RESEARCH INSTITUTE Ltd. 10 Jan., 1966 [11 Jan., 1965], No. 1105/66. Heading B5B. A tubular film 1 which has been partially transversely stretched by inflation and partially longitudinally stretched is passed through the nip of a pair of nip rolls 2 (Fig. 1) and is drawn by means of a pair of nip rolls 11 downwards over an inflated flexible bag mandrel 4 supported on a tube 5 through which a pressurized gas is introduced into the interior of the mandrel and the portion 3 of the tubular film between the nip rolls 2 and the mandrel 4. The pressure of the gas in the inflated portion 3 of the tubular film is automatically maintained at a predetermined value by means of a device positioned at the open upper end of the tube 5 comprising a light ball valve 8 pulled on to the conical seating 9 by a spiral tension spring 10 the tension of which corresponds to the difference between the pressure of the gas in the bag mandrel and that of the gas in the inflated portion 3 of the tubular film. The gas is introduced into the interior of the mandrel 4 through the outlet 7 in the pipe 7 and when the mandrel has been expanded by inflation until the diameter of its cylindrical portion nearly equals the predetermined diameter to which the tubular film is to be expanded, the ball valve 8 opens and allows pressurized gas to enter the portion 3 of the tubular film until the diameter of the latter has been increased to a predetermined value and equilibrium is established between the rate of entry of gas into the film through the valve 8 and leakage of gas through the small clearance between the inner surface of the film and the outer surface of the cylindrical portion of the inflated bag mandrel 4. The flexible bag mandrel may be made from a tubular film of an elastomeric material impervious to gas to form an inner bag which is covered by an outer bag of thin cloth of woven filament yarn which limits the extent to which the inner bag can be inflated. The outer cloth bag is preferably made of polytetrafluoroethylene filaments to reduce friction between the mandrel and the tubular film being drawn over it during the process of the invention. In starting the process the partly stretched tubular film is threaded over the deflated bag mandrel, slit longitudinally by the knife 6 and taken up by the nip rolls 11. The mandrel is then inflated to its predetermined maximum size by introducing pressurized gas into it through the tube 5 and outlet 7. The stretched tubular film 3 is cooled by spraying on to it from an annular pipe 13 a fluid such as water or air. Alternatively the stretching tubular film 3 may be internally cooled by water supplied through a pipe 14 (Fig. 2), a header 15 and a number of parallel narrow branch tubes 16 embedded in the thickened wall of the tube 5. The upper ends of the tubes 16 are bent downwards and terminate in nozzles to which are attached narrow rubber tubes 17 disposed equidistantly over the conical surface of the mandrel and open at their lower ends to discharge cooling water over the cylindrical portion of the mandrel and the inner surface of the portion of the film passing over the mandrel. A thin conical polytetrafluoroethylene film 18 attached to the portion of the tube 5 above the rubber tubes 17 covers the rubber tubes completely and keeps them out of contact with the travelling film 3.