Patent GB-1152564-A (GB)
Title: Method of and Apparatus for Stretching Blown Films Longitudinally and Transversely.
IPC: B29C48/90
Filed: 1968-02-01
Abstract: 1,152,564. Tubular films. J. P. BEMBERG A.G. 1 Feb., 1968 [10 Feb., 1967; 9 Aug., 1967; 8 Jan., 1968(4)], No. 5122/68. Heading B5B. In a method of producing a biaxially stretched tubular film a thermoplastic synthetic plastics material is extruded from an annular nozzle in the form of a tube 1 and is expanded in a pres sure chamber 2 to a diameter which is larger than the diameter of the pipe 3 connecting the pressure chamber with a heating chamber 7, and is treated in the pipe 3 in three zones, namely (a) in a first zone 4, in which the tube is cooled whilst substantially reducing wall adhesion, (b) in a second zone 5, in which a braking is effected, and (c) in a third zone 6, in which the tube is preheated before it enters heating chamber 7, and the tube, after leaving heating chamber 7 in which the biaxial stretching takes place, is guided through a temperature controlled calibrating pipe 8, and is wound after withdrawal by means of nip rolls 9.2. Suitably, the tube is so expanded in the pressure chamber that its maximum diameter is 5 to 10% larger than the diameter of the pipe 3, and suitably the length of the tube section in the pressure chamber is not greater than the largest diameter of this tube section. As shown, cooling air flows through the pressure chamber, said cooling air being supplied in the form of at least six streams each of equal pressure and being discharged in the form of a like number of streams of equal pressure. Air automatically regulated, may be supplied to the cooling air streams when the diameter of the tube increases above the desired value, and additional air may be extracted from the discharging air streams when the tube diameter falls below the desired value. Apparatus suitable for such control is shown in Figs. 2 and 3. In Fig. 2, the cooling air passes through pipe 032 into a chamber 04, in which optionally air is additionally mixed with the cooling air stream, chamber 04 having at least six outlet unions 06 and as many connections 07 with the pressure chamber, and as many inlet nozzles 08 into the pressure chamber, said nozzles being uniformly distributed and not spaced more than 30 cms. from one another, just as many uniformly distributed outlet pipes 014 from the pressure chamber as there are provided inlet nozzles and just as many connections 015 to a chamber 016 from which the cooling air stream is discharged and from which, optionally, air can be extracted via conduit 019. The supply and discharge nozzles or pipes of the pressure chamber are each arranged on a distributer ring 09 or 012, air flowing into the pressure chamber from ring 09 and escaping from the latter by means of annular nozzle 011. A switch 020 or automatic pressure regulator is situated inside the pressure chamber 2 in a casing permanently flushed with air to avoid deposition of dirt on the contacts. Only one fan 031 need be provided for supplying added air and extracting additional air by suction, the valves of said fan being automatically actuatable in the pressure pipe and suction pipe. Thus air flows through hoses 015 and nozzles 016 into a chamber 017, from whence it is discharged through 018 and from which optionally air can additionally be extracted through a conduit 019. The switch 020 is actuated by air flowing in through pipe 021 and out through slot 022, through which a lever arm 023 of a sensing member 024 extends into the casing. Sensing member 024 bears on tube 1, so that with tube diameters which differ from the required diameter, a contact arm 025 actuates contacts, whereby electromagnetic valves are opened or closed. Thus if the diameter tube 1 is too large, arm 025 moves in the direction of the arrow (Fig. 3), touching contact 026 and opening valves V1 and V2 and closing valves V3 and V4 (Fig. 2). As a result, air can be drawn in from a conduit 032 by a fan 031 and forced into the chamber 04, so that the pressure in the pressure chamber 2 becomes greater and the tube diameter is reduced until, with the correct diameter, contact arm 025 is not touching any contact in the switch 020. This has the effect that the valves V1 and V4 are opened and the valves V2 and V3 are closed. When the tube diameter becomes too small, contact arm 025 closes the circuit through contact 027, exhausting air from chamber 017 and thus from pressure chamber 2 and thus the tube diameter becomes larger. Desirably, the ratio between the diameter of the annular nozzle and the diameter of the pipe is between 1: 1À2 and 1: 3. As shown, the wall of the first zone 4 of the pipe 3 may project into the pressure chamber. Reduction of wall adhesion in the first zone 4 may be effected by using a wall roughened to a depth of at least 30 millimicrons, or the wall may be provided with intersecting helical grooves and also with an annular groove 4.6 disposed at the end of the zone 4, and a regulating valve 4.7, so as to cause a controlled quantity of air to flow from the pressure chamber 2 between the zone wall and tube. For cooling the tube in this zone, the coolant enters the jacket 4.1 and 4.2 and leaves it at 4.3. Braking in the second zone is obtained by lining the wall 5 with a textile material, or by using an air-permeable wall, e.g. one made of sintered metal, to the outside of which a vacuum is applied. If the wall is air-permeable, the zone is provided with a jacket 5.1 with vacuum connection 5.2. The third zone 6, has a roughened wall, a roughening depth of less than 30 millimicrons being sufficient, a jacket 6.1 with an inlet 6.2 and an outlet 6.3 for heating medium. The heating chamber 7 is so heated by known means, e.g. by infra-redradiators7.1, thatahigher temperature may be set in the upper part than in the lower part. Following heating chamber 7 is calibrating pipe 8 which can be either heated or cooled and consequently has a jacket 8.1 with an inlet 8.2 and outlet 8.3 for the temperature control medium. When manufacturing tubes which are predominantly orientated transversely, zone 2 is either entirely omitted or the jacket 5.1 is placed under a light over-pressure by means of connection 5À2. Hence braking of the tube after leaving the pressure chamber is substantially avoided in the pipe 3. When manufacturing tubes which are predominantly orientated in the longitudinal direction, heating chamber 7 is omitted, so that pipe 8 is directly connected to zone 6 of the pipe 3, pipes 8 and 3 having the same diameter. As shown, the provision of flanges at the junctions between the members enables the zones of the pipe duct and the calibrating pipe to be interchangeable for zones and pipe ducts of different dimensions, or for zones of the pipe duct to be omitted. The biaxially stretched tube is finally flattened in known manner, a system of rollers 9 driven by endless belt 9.1 being satisfactory. Flattening is complete by driven pair of nip rolls 9.2, which maintains the air pressure in the tube and exerts the tension necessary for the longitudinal stretching. The tube is thereafter reeled by means of an arrangement 10. To avoid the so-called "piston rings" in the finished roll, winding arrangement 10, roller system 9, nip rolls 9.2 and preferably also the calibrated pipe 8 are rotated backwards and forwards slowly about the longitudinal axis of the tube in known manner. The same effect can, however, also be obtained by the blowing head with the annular nozzle carrying out the same movement or revolving, the pressure chamber being constructed in the upper part as a "stuffing box" in order to accommodate a short pipe section which is correspondingly rotated with the blowing head. In this case the entire following apparatus need not be moved. If it is desired to give a controlled post-shrinkage to the tube withdrawn by means of cylinder 9.2, driven heated cylinders are interposed between 9.2 and reeling arrangement 10. In this case, the roller system 9 and nip rolls 9.2 cannot be moved in rotation. Such heated cylinders may run at peripheral speeds reduced in stages up to altogether 5%, whereafter the tube is then carried forward to the reeling arrangement by means of cooled cylinders having a speed which is reduced by up to 5% compared with the nip rolls 9.2. Hence a controlled post-shrinkage of up to 5% may be effected. The ratio between the diameter of the annular nozzle and the diameter of pipe 3 is not critical, but it is advantageous to select a smaller ratio for substances with a high stretching factor than for substances with a smaller stretching factor. By stretching factor, there is understood the number which indicates by how many times a shaped film of a thermoplastic material can be stretched. 1: 1À5 was found to be a particularly favourable ratio between annular nozzle diameter and pipe 3 diameter for polypropylene, which has a stretching factor of about 6, and 1: 2 was found desirable for polyvinyl chloride which has a stretching factor of about 2. The calibrating pipe has the diameter which is finally to be given to the tube which is to be manufactured. Examples are given relating to the production of tubular films of polypropylene, polyvinyl chloride, and copolymers of vinyl chloride with vinyl acetate.