Patent GB-629645-A (GB)
Title: Improvements relating to the injection moulding of thermo-plastic materials, particularly in the productionof electrical insulation
IPC: B29C45/72
Filed: 1945-01-10
Abstract: 629,645. Injection-moulding. BRITISH THOMSON-HOUSTON CO., Ltd., and COX, H. E. Jan. 10, 1945, No. 851. [Class 87 (ii)] [Also in Groups XXXV and XXXVI] Thermoplastic material is injected into a mould, and those parts of the moulding that are remote from the point of injection are cooled whilst (a) the mould is kept hot in the region of the point of injection and (b) the injection pressure is maintained on the injected material, and the cooling proceeds progressively from the said remote parts until the entire mass of the thermoplastic material has set. Fig. 1 shows apparatus for moulding a tapering mass of thermoplastic insulation, e.g. "Alkathene " (Registered Trade Mark) around a hollow central conductor 1. The mould 5 is supported on a central pillar 4 carried by wheeled framework 2, and is divided into sections by flanged joints 7. It is made of magnetic material such as iron and is heated by induction, current passing from the secondary winding 13 of a transformer 10, through conductor 1, link 14 and heating coil 9 back to 13. The injection nozzle 8 is heated by a coil 81 fed by conductors 16 from an auxiliary winding on the transformer. With the parts in the full-line position shown, material is injected until the mould is full, air escaping through crevices in the joints 7 or through holes provided in the mould for that purpose. Then the current supply to the transformer is interrupted, link 14 is disconnected and replaced in the broken-line position 15 (thereby cutting out the conductor 1 and leaving in the coil 9 which heats the central part of the mould) and the supply of current resumed. Cooling liquid is then passed through the conductor 1, the mould and its contents being thereby cooled, beginning with the ends thereof. When the mould has cooled to the desired extent, coil 9 is cut out by removing link 15, the injection nozzle only remaining heated and the injection pressure being maintained. When all the material has solidified the heating current to the nozzle is interrupted and the injection pressure released. A drip tray 6 prevents thermoplastic material falling on the lower part of the apparatus. Figs. 2 and 3 show apparatus for moulding insulation around a hollow conductor 1 for forming the primary winding of a transformer (see Group XXXV). The mould comprises an upper part 17 heated by a coil 23, and cooled by water from a trough 29, an intermediate part 18 heated by a coil 24, and a lower part 19 heated by current passed through the conductor 1 and cooled by a collapsable water jacket 25. A coil 42 heats the injection orifice 8. Cooling liquid is passed through the conductor proceeding down through one branch and up through the other. The conductor 1 is positioned within the mould by hollow plungers 38a which extend through sleeves 38d secured in the mould wall and receive thermometers for taking the temperature of the material. Each plunger abuts the conductor and is initially held in place by a threaded collar 38c, this collar being later unscrewed to allow the injection pressure to press the plunger out and fill up the hole left thereby. Vents 40, 40<SP>1</SP> allow escape of air from the mould. In operation all the heating coils are energised, the material is injected, vents 40, 401 are closed, heating of the upper and lower mould sections is discontinued, and heating of the intermediate section by the coil 24 is increased (as a result of bypassing the coil 23 and conductor 1). The mould and conductor are then cooled, the jacket 25 being gradually raised about the container, and plungers 38a are retracted, and finally all heating is discontinued.