Patent GB-1490338-A (GB)
Title: Extrusion processes
IPC: B29C48/90
Filed: 1973-10-15
Abstract: 1490338 Sizing extruded tubes BAKELITE XYLONITE Ltd 15 Oct 1974 [15 Oct 1973] 48048/73 Addition to 1422325 Headings B5A and B5B In the parent Specification, a thermoplastics tube is downwardly extruded whilst supported by an internal air pressure, and is internally sized in passage around axially spaced edges of a sizing member supported from the extrusion die, as the tube is internally cooled by water which is continuously delivered to a space below, and between the member and a seal engaging the tube interior, and passes through and to above the member to be continuously withdrawn. The tube may also be externally water-cooled. The modification of the present Specification comprises at least the sizing edges of the member, and/or at least a peripheral portion of the seal which contacts the tube, being of a material which absorbs water so that a lubricating layer of water is formed between the tube, and the sizing edges and/or the seal, thereby facilitating the extrusion of a tube which does not contain a lubricative additive, e.g. of an electrical grade polypropylene. The liquid absorbent material may be a natural or synthetic open pore sponge, e.g. of rubber, polyurethane, or polyvinyl chloride, porous leather, open pored sintered metals, cloth, e.g. as layered or laminated pads, rope, string or cord, open pore minerals, e.g. chalk, paper or cardboard, or felt. The sizing edges and/or the seal may be defined by a peripheral band of the absorbent material around a metal core, or may be defined by a disc of the material which may either be supported, or not supported, on both sides by a rigid material. In the Fig. 1 embodiment, the sizing member comprises sizing discs each defined by a layer 5 of felt supported on each side by aluminium plates 8, to which they are adhered by silicone rubber adhesive, and spaced apart by brass baffle discs 4, the circulation of water upwardly through the discs 5, 8 being permitted by through holes 6 spaced apart adjacent their periphery, and the discs 5 being of smaller diameter and rebated on each side around their periphery. The assembly of discs is clamped together to define the sizing member on a tube 3, between a lower brass nut 2 and an upper threaded baffle disc (not shown) having an upper face with an inwardly inclined periphery which functions as a weir over which the circulating water pours to be removed by suction. The tube 3 is in sealed engagement, at an 0-ring 9a, with a tube 9 by which the water is admitted to below the sizing member. The seal 10 of the Fig. 1 embodiment is clamped on a tube 12 between a threaded nut member 11 and a member 18 secured by grub screws, and comprises two discs 13 of liquid absorbent material separated by a member 14, and two discs 15 of flexible non-absorbent material with separating members 16 and 17. A seal is effected at tube 12 by 0-rings 9b, 9c between which a vacuum is applied at 19 to the discs 13, to withdraw any liquid by-passing the discs 15. An alternative seal, Fig. 2, is an assembly which is glued together with silicone rubber adhesive, and comprises a lower nut-washer 11 threaded on the tube 12, a disc 20 of non- absorbent foam silicone rubber, two discs 13 of felt separated by a disc 21 of harder felt, and two further discs of the silicone rubber and brass spacers 23. A space is provided between the tube 12 and the discs, intermediate 0-rings 9b and 9c, whereby vacuum applied at 19 withdraws liquid passing the discs 22, with air bleed holes 25 being provided in the nut washer 11 and disc 20. In the examples, polypropylene tubes were extruded using either the calibrating disc assembly of Fig. 1, or solid brass calibrating discs, with the seal of Fig. 2, the tube being subsequently converted by conventional bubble blowing into balanced biaxially oriented 0À3 mm. film. The Figs. 1 or 2 seals may also be used with the calibrating disc assembly of Specification 1,422,325, or the seal of Specification 1,422,325 may be employed with the Fig. 1 disc assembly.