Patent GB-374472-A (GB)
Title: Process for the production of dentures from thermo-plastic material by injection
IPC: A61C13/20
Filed: 1928-11-02
Abstract: 374,472. Moulding ind:iarubber, artificial resin, and celluloid. JOANNIDES, C., 88, Fenchurch Street, London. Nov. 2, 1928, No. 11391/32. Divided on 374,450. [Class 87 (ii).] In the production of dentures from thermoplastic material by injection, a seamless, drained mould is employed and is maintained in drained condition during injection ; the flask is heated to the required temperature, after removal of the wax, independently of the injecting-device ; steam is prevented from entering the mould ; scouring is effected by water uncontaminated by contact with the flask; and removal of the wax and draining or drying of the mould are effected in the same apparatus, the drained or dried condition being maintained during injection. The invention is described in connection with the apparatus described in Specification 374,450. A wax model is prepared and the teeth set therein as usual and a gum facing of pink rubber is applied. To prevent displacement during injection, an accelerat r in powder form with or without an activator such as zinc oxide is rubbed on to the wax or pink rubber, or both, prior to the application of the teeth. The model is screwed centrally in the flask and is connected to the flask openings by wax wires or metal spines, one preferably at the extreme edge of the rear face and the other on the further side of the centre. The flask is then filled with plaster, closed by a cover and allowed to set. Meanwhile the boiler 10, Fig. 1, is filled with water and heated by the gas burner 46 or electric heater 56. When the plaster is set, the cover 20 is removed and the flask 14 is inserted into the recess 12 which is isolated from the water in the boiler. To facilitate heating of the flask, steam may be admitted to the recess through the steam valve, the scouring attachment 60 having been previously clamped in position. The packings 59 prevent entry of steam into the mould. The melted wax flows out of the tube 65 by gravity. The base 62 is then connected to the tube 61, and after opening the pressure-reducing valve of the boiler, the valve 63 is opened to allow hot water to flow from the boiler through the pipe 61 and nozzle into the mould and thence through the tube 65. The water is followed by steam to drive out water from the mould, a filter of rolled gauge being provided in the valve 63 to prevent impurities from the boiler entering the mould. The valve 63 is then closed, the hose 62 is disconnected and the boiler valve is closed, heating being continued so as to thoroughly heat the mould and partially vulcanize the pink rubber. Preferably the mould is heated to a temperature below the melting point of sulphur, the temperature being determined by a test injection. Air, preferably hot, may be forced through the mould to ensure thorough draining. The injecting-device 84, Fig. 3, as described in Specification 374,470 is then fitted in place of the scouring attachment, the injector being previously heated, by placing it on top of the boiler, to such a degree that the screw 85 rotates easily. A reel of strip base rubber, as described in Specification 374,471, is fitted on to the holder 90c and the end is led into the feed aperture 84<1>, the intervening strip 190 of non- adhesive material being stripped off. The injecting-device is not subjected to the direct heat of the boiler, and as it is not in operative position during heating of the flask, the mould may be subjected to a higher degree of heat. To reduce the time of vulcanization, an accelerator may be applied to the base rubber, e.g. by means of a brush. When the rubber begins to exude through the lower flask opening 33, the injector is removed and closing plugs are inserted into the openings 33a in the cover 20 after allowing escape of excess rubber. Before the plugs are screwed completely home, steam is admitted to the recess to drive out air ; the plugs are then tightened down, pressure in the boiler is increased and vulcanization is effected as usual. After vulcanization, the steam valve is closed, the cover 20 is removed and the flask 14 is withdrawn. When producing dental plates from artificial resin, the mould is drained to a bone dry condition and the injection is effected through the lower mould opening. Hardening is also effected by dry heat. Thus, the mould openings are closed by a strip 96, Fig. 3, which slides in grooves in the flask and is pressed down by a screw 98 in the cover 20. To prevent discolouration of the artificial resin by the plaster the model is covered with tinfoil prior to waxing-up. The invention may be applied to a two-part mould which is partly packed by hand and completed by injection. For large numbers of dentures, each flask may be heated and scoured by water from a separate supply while the preceding flask is undergoing injection; and, after filling, the flask is removed to another vulcanizer. To avoid repeated filling of the boiler scouring may also be effected, while the flask is in the apparatus, by hot water from an external source. The Provisional Specification refers also to celluloid.