Patent GB-1098319-A (GB)
Title: Improvements in and relating to processes for the manufacture and/or treatment of tubular film
Inventor: JACK JAMES
IPC: B29C48/10
Filed: 1963-12-03
Abstract: 1,098,319. Stretching tubular films by inflation. BAKELITE XYLONITE Ltd. Feb. 23; 1965 [Dec. 3, 1963], No. 47708/63. Heading B5B. In the production and/or treatment of a continuously advancing tubular film of thermoplastic material, the inner surface of the film forming a boundary of an enclosed stationary zone containing a gas under pressure, the pressure of the gas within the zone is brought to a desired value at least in part by the introduction of gas through a gas transfer tube introduced into the zone and the end of the tube is then caused to be outside the zone. In Fig. 1 a tube 1 of thermoplastic material is extruded from an annular die 2, through the centre of which extends a gas transfer tube 3, and is rapidly cooled by cooling means 4, before passing between a pair of auxiliary nip rollers 5 and a pair of feed nip rollers 6. The pairs of nip rollers 5 and 6 are so arranged that they may be spaced apart to a position where they do not contact the tube, or brought together so that they nip the walls of the tube together and advance the tube. Nip rollers 5 are shaped to provide a passage for gas transfer tube 3 even when brought together, as described, for example, in Specification 787,479. Between pairs of nip rollers 5 or 6 (whichever pair of rollers is closed on to the tube) and take-off rollers 10 the tube is inflated by an enclosed " bubble 7, of gas under pressure, the tube being softened by heating means 8 to permit the pressurized gas to inflate the tube and increase its diameter. The tube is thereby rendered thin-walled and converted into a tubular film. This passes between a converging series of rollers 9 to the pair of takeoff nip rollers 10 which advance the film faster than do pairs of rollers 5 or 6, so that the film is longitudinally as well as transversely stretched in the usual manner. The film is then cut to form a sheet by a knife 11 and wound on to take-up roller 12. In the operation of the apparatus the bubble 7 is normally enclosed between feed rollers 6 and take-off rollers 10, auxiliary nip rollers 5 being open. When it is desired to alter the pressure in bubble 7, auxiliary nip rollers 5 are closed on to the tube to nip its walls together and advance the tube at the same rate as nip rollers 6. Gas under pressure is then fed through the gas transfer tube 3 into the zone enclosed between the pairs of nip rollers 5 and 6 and nip rollers 6 are then opened at least sufficiently to allow the gas to enter zone 7. When the pressure in zone 7 has reached the desired value, the rollers 6 are closed on to the tube, the gas supply through the gas transfer tube 3 is discontinued, the pressure in the zone between pairs of rollers 5 and 6 is adjusted to that in the zone above the rollers 5, the rollers 5 are opened at least sufficiently to prevent the tube 1 touching the gas transfer tube 3 preferably so that the rollers 5 are themselves clear of the tube 1. The friction between the tube 1 and the gas transfer tube 3 when the rollers 5 are closed may be reduced by lubricating the gas transfer tube, e.g. with ethylene glycol, at this position or by providing the outer surface of the tube 3 with a coating of a material such as P.T.F.E. over which the plastic tube 1 slips easily. The P.T.F.E. may be in the form of a sheath or a sintered coating on the gas transfer tube which is of metal.