Patent GB-1092948-A (GB)
Title: Improvements in the stretching of tubular film
IPC: B29C55/28
Filed: 1964-12-21
Abstract: 1,092,948. Stretching tubular film. SEKISUI KAGAKU KOGYO KABUSHIKI KAISHA. Dec. 21, 1964 [Dec. 21, 1963], No. 51880/64. Heading B5B. In stretching tubular film of thermoplastic resin both longitudinally and transversely, undrawn tubular film, the interior of which is maintained at atmospheric or superatmospheric pressure, is passed successively through a prevacuum zone, a heating zone in which it is heated to a temperature between its softening point and its melting point, a stretching zone (distinct from, and of larger diameter than, the heating zone) in which it is allowed to expand in the radial direction and simultaneously is stretched in the longitudinal direction, and a cooling zone in which it is cooled to a temperature below its softening point whilst being maintained in its stretched condition. The pressure is subatmospheric in the pre-vacuum and stretching zones, the pressure in the stretching zone preferably being higher than the pressure in the pre-vacuum zone. The film may be preheated before passing into the heating zone; preferably, a tubular film in flattened form is pre-heated and passed between a pair of pinch rollers disposed in the pre-vacuum zone, the temperature of the film passing between the pinch rollers being insufficient to cause selfsticking of the film. Longitudinal stretching preferably is effected by passing the film through two pairs of pinch rollers, the second pair running at a higher peripheral speed than the first pair. After cooling, the stretched film may be flattened under subatmospherc pressure and cut longitudinally into sheets. The heating or cooling operation may be effected by blowing hot or cold air at subatmospheric pressure against the exterior of the film, the air passing as a thin layer between the surface of the film and the cylindrical internal surface of a passage forming the heating or cooling zone; preferably, the hot air and the film are in countercurrent contact whilst the cold air and the film are in co-current contact. After passing through the stretching zone and before passing into the cooling zone, the film may pass through an annealing zone in which warm air at subatmospheric pressure is passed in co-current with the film between the exterior surface of the film and the cylindrical internal surface of a passage forming the annealing zone and having an internal diameter equal to the diameter of the cooling zone. Alternatively, the heating or cooling operation may be effected by contacting the exterior of the film with heated or cooled walls, the film being successively heated and cooled by passing in sliding contact with the cylindrical interior surfaces of successive passages forming the heating and cooling zones, the outer surfaces of the passages being in contact with hot air and with cold air, respectively. The undrawn tubular film of thermoplastic resin may be stretched in apparatus comprising a first pair of pinch rollers disposed in a prevacuum chamber, a stretching vacuum chamber, a heating chamber (distinct from, and of smaller diameter than, the stretching chamber) disposed between the pre-vacuum chamber and the stretching vacuum chamber, means for heating the film in the heating chamber, means for cooling the film after emergence from the stretching vacuum chamber, and a second pair of pinch rollers capable of being run at a higher peripheral speed than the first pair. As illustrated in Fig. 1, after being preheated by a plurality of heating rollers 2, film 1 enters prevacuum chamber A via inlet 41 having sealing means 4 and is passed by first pair of pinch rollers 3 into the heating chamber which comprises passage 10 connecting the pre-vacuum chamber and stretching vacuum chamber B, the passage having a lower air inlet 11 and an upper air outlet 8 interconnected by pump 9, heater 13 and valve 12. The film then passes through the stretching vacuum chamber and the cooling means which comprises passage 15 having an upper air inlet 14 and a lower air outlet 19 interconnected by pump 18, cooler 17 and valve 20. The film then passes into folding vacuum chamber C to be folded therein by means comprising a plurality of pairs of guide rollers 26 disposed in a downwardly tapered array for gradually bringing opposite side walls of the tubular film towards each other to form a flat double sheet, the final pair of rollers 26<1> being spaced slightly apart to ensure that the interior of the film is at atmospheric pressure. After leaving the folding vacuum chamber via slit 27 having sealing means 27<11>, for example sponge, retained by bracket 27<1> against the wall of the folding vacuum chamber, the flattened film passes to a pair of opening rollers 29 coacting with cutter 28 to longitudinally sever the film into two elongated sheets which pass through the second pair of pinch rollers 30 and further pinch rollers 31 to means 32 for winding up the cut sheets of film. The pressures in the several chambers may be kept below atmospheric by means comprising vacuum pump 25, pressure controller 24 and vacuum tank 23 from which pipe 21 leads to means comprising valves 22, 22<1>, 22<11> connected to exhaust openings 7 in chambers A, B, C, respectively, thereby to control the pressures in these chambers. Alternatively, the heating and cooling passages 10 and 15, respectively, may be provided with means for heating the walls thereof, for example chambers 16<11> and 16, respectively, through which heated and cooled gas or liquid may be passed. In an embodiment of apparatus illustrated in Fig. 4 (not shown), the pre-heating rollers are disposed in the pre-vacuum chamber and a further heating passage forming an annealing zone is located between the stretching vacuum chamber and the passage which forms the cooling means.