GB854368A - Production of seamless tubing - Google Patents

Abstract

<PICT:0854368/IV(a)/1> <PICT:0854368/IV(a)/2> Seamless tubular films are made by continuously extruding hot thermoplastic material through an annular die, inflating the extruded tubing to a predetermined diameter and blowing a cooling gas against the outer surface of the tubing, the cooling gas being directed against said surface from an annular source which is coaxial with the extrusion orifice and is rotated to and fro about its axis, e.g. through an angle within the range 30 to 120 degrees and with an oscillation frequency of 1/2 to 1 1/2 cycles per minute. As shown in Fig. 2, heated thermoplastic material, such as polyethylene, is supplied under pressure from an extruder 11 to a conventional heated extrusion die 12 having an annular opening 14 through which the material is extruded in the form of tubing 16 and having also a central passageway 17 through which air under pressure from a source 13 is introduced into the interior of the tubing to inflate it to a predetermined diameter. Simultaneously an annular stream of cooling air is directed on to the outer surface of the tubing by means of an annular supply chamber 18 comprising upper and lower annular members or rings 19, 21 secured together by peripheral bolts 22 and rotatably mounted on a stationary annular support 23 by means of rollers 24 (Figs. 1 and 2) and ball bearings 26. Air is supplied from the stationary blower 27 to an inlet 28 in the annular support 23 and passes through the space between the upper and lower rings 19 and 21 to an annular passage 29 therein from which it is discharged on to the outer surface of the tubing 16. In order to render the air flow through the space between the members 19 and 21 more uniform, there are provided concentric baffles, 31, 32, 33. Leakage of air between the stationary support 23 and the rotating chamber 18 is prevented by means of a pair of annular rubber seals 34, 36 which are cemented on to the annular shoulders 37, 38 of the support 23 and engage the rounded portions 39, 41 of the rings 19, 21. The apparatus also includes means (described in Specification 854,369) for adjusting the width of the air discharge passage 29. The cooling air supply chamber 18 is rotated to and fro by means of a motor 56 (Fig. 1) which drives the chamber through a variable speed gear 56a, pulley 57 and belt 58 disposed in a circumferential groove 59 in the upper ring 19 (Fig. 2). The motor 56 is controlled by a reversing switch 61 (Fig. 1) which, like the motor, is mounted on a bracket 62 secured to the support 23. The switch has two operating levers 63, 64 positioned at different distances from the chamber 18, each lever being adapted to be engaged by a corresponding tripper 66, 67 mounted on, and extending from, the upper ring 19. The apparatus enables films of uniform thickness to be produced even when the wall thickness exceeds about 0,003 inch. Specifications 654,550, 716,160 and U.S.A. Specification 2,461,976 also are referred to.