Apparatus for forming hollow plastic articles

Abstract

Abstract 1,131,322. Table tennis balls. DUNLOP CO. Ltd. 6 Oct., 1965 [14 Oct., 1964], No. 41827/64. Heading A6D. [Also in Division B5] Apparatus 39 for welding two shaped sheets 10, 38 of thermoplastic material together to form a table-tennis ball comprises a pair of electrodes each comprising an open-end cylinder 42 having an internal diameter at the open end substantially equal to the external diameter of a table-tennis ball and the cylinders 42 being arranged so that the open ends thereof are directly opposed, means to connect the electrodes to a source 46 of high frequency alternating electric current, and means to reduce the separation of the electrodes 42 during passage of the current therebetween. The sheets 10, 38 may be previously shaped by the method described in Specification 1,131,361. The apparatus 39 comprises a number of rod-like cylinders 42 on parts 40, 41 and which are shaped to conform to hemispherical projections 43 on the sheets 10, 38. The cylinders 42 are mounted to form electrodes on base plates 44, 45 which are connectable to the source 46. The periphery of each cylinder 42 is chamfered so as to produce a line of high concentration of the alternating current and the end surfaces 48, 49, Figs. 7 and 8 (both not shown), of an opposed pair of cylinders 42 are shaped to form a V-section gap 50 the angle of which is from ¢‹-7‹, preferably 3‹. The upper part 40 is mounted on a bar 51 slidable in a cylinder 52 and when the source 46 is connected, the bar 51 is moved, as the sheets 10, 38 are softened, to bring the part 40 closer to the part 41 so that only a very fine gap remains between opposed cylinders 42. This movement forces some of the thermoplastic material into the interior of the ball to form a reinforcing bead 5. Fig. 8 (not shown). After welding, the balls are removed from the surrounding excess sheet material. In modification (not shown), the electrodes may be hollow cylindrical projections with or without a shaped core of insulating material. The apparatus may be maintained at a controlled elevated temperature to raise the temperature of the thermoplastic material prior to passage of the high frequency current but not sufficiently to cause the material to be deformable. Specified thermoplastic materials are polyvinyl chloride, terpolymers of acrylonitrile with butadiene and styrene, copolymers of diphenylolpropane and epichlorhydrin, polyamides, e.g. nylon, and polycarbonates.

Inventors

• 👤 Jones-Hinton James
• 👤 Thomas E H Gray

Bibliographic Details