Patent GB-1315661-A (GB)
Title: Plastics extrusion process and apparatus
IPC: B29C48/38
Filed: 1969-06-18
Abstract: 1315661 Extruding foamed plastics BRITISH INSULATED CALLENDER'S CABLES Ltd and GENERAL ENG CO (RADCLIFFE) Ltd 17 June 1970 [18 June 1969] 30876/69 Heading B5A In an extrusion process wherein thermoplastics material, e.g. polyethylene, elastomers or thermosetting plastics, are fed in a molten state and under pressure into the inlet end of a heated screw extruder 12 wherein gas, e.g. nitrogen, under pressure is dissolved therein in a screw zone 30 which is partially filled with the material and the material then passes, via a screw zone 32 of increasing compression, to completely fill an extrusion zone 34 prior to discharge from a cross-head die as a coating on one or more wires W (which may be pre-covered with polymeric material) which expands to a cellular structure, the location of the transition between the partially and completely filled screw zones is maintained constant, e.g. at the screw zone 32 of increasing compression, by manually or automatically varying the speed of the screw 14 or of a screw 13 feeding the molten material thereto in dependency upon a condition measured within the extruder barrel. The automatic control may be effected by the apparatus of Specification 1,315,662. When the speed of screw 14 is varied, the speed of the wire W is adjusted accordingly. The condition may be measured by an ultrasonic, radio isotope, or X-ray device, or by measurement of thermal conductivity, electrical pressure, or capacitance, but in the described embodiment is measured by means of one or more pressure transducers. A pair of transducers may be utilized one 41 whereof is located in the zone 30, at which gas is introduced at 26, and located midway between the inlet 24 for the melted material and the compression zone 32, and the other 42 whereof is located <SP>1</SP>/ 3  of the way downstream of the zone 32. Additional pressure transducers 43, 44, 45 may be utilized. Alternatively, a single transducer 43 may be utilized located adjacent or downstream of the screw compression section 32. It is stated that the respective pressures at 32, and at 34 may be in the ratios of 1 : 6 to 1 : 8. In the described embodiment, the wire W is drawn through a preheating device into the cross-head, and thence through a cooling trough and past or through quality control instruments comprising a capacitance monitor, diameter monitor, eccentricity gauge, and high voltage fault detector, prior to winding on a take-up drum or coiling in a container. The extruder 12 is fed with the molten material from a further independently driven screw extruder 11 containing a breaker-plate and screens 25 and which connects to the inlet 24, via a bent pipe 22 which is flexible under thermal expansion effects thereof or of the two extruders, and the outlet end of the extruder 11 is preferably anchored with the inlet of the extruder 12 and with the inlet end of the extruder 11 mounted on slidable bushes or a carriage to permit thermal expansion movement. The extruders 11 and 12 in this arrangement may be mounted with their axes parallel, or at right angles as shown. Alternatively the barrel outlet end of extruder 11 may be directly anchored to the barrel inlet end of extruder 12 with the other ends of the barrels on slidable bushes or carriages. The inlet end of the screw 14 passes through an oil lubricated seal, and comprises a part 36 adjacent the seal and having a narrow helical groove 38, of reversed hand intermediate its ends to prevent flow of both the lubricating oil from, and of the molten material towards, the seal.