GB722816A - Improvements relating to precision casting by the lost-wax process - Google Patents

Abstract

722,816. Moulds; moulding processes. METROPOLITAN - VICKERS ELECTRICAL CO., Ltd. Nov. 6, 1952 [Nov. 15, 1951], No. 26795/51. Class 83 (1). A process of precision casting comprises providing a wax pattern with a multiplicity of superimposed dip coatings so that the pattern is invested with a shell of refractory material of substantial thickness, e.g. “ to ¢ inch, removing the wax pattern from the shell by means of the hot vapour of a solvent and surrounding the shell with a mass of dry refractory material, e.g. dry sand and fire-brick grog with or without a fritting agent. The wax is removed by a solvent, e.g. trichlorethylene vapour or steam, and the shell may be provided with a sprayed-on initial coating of, e.g. zirconium silicate powder used with a bonding agent consisting of, e.g. ethyl silicate, methylated spirits, hydrochloric acid and water, or a dipped coating of, e.g. silica flour used with the same bonding agent. Other usable refractory materials are sillimanite, alumina and molochite, and bonding agents sodium silicate, cement or plaster of paris. The composition of the initial coat is chosen in accordance with the surface finish required of the casting and the metal to be cast. When the initial coat is dry the shell is produced by repeated dipping in the refractory slurry, a stucco of coarse grained refractory, e.g. crushed fire-brick, being applied after each dip, until the desired thickness is reached. The drying of each coat may be accelerated by the use of an accelerator, e.g. ammonia vapour or magnesium oxide, or by placing the pattern after each coat in an air stream, e.g. in a wind tunnel. The refractory slurry may comprise a dry mixture of silica sand and flour mixed with the above bonding agent. When the shell is completed and the wax removed, it is placed in a mould box of heat-resisting steel and packed with the dry refractory material, the grain size of which is chosen according to the desired rate of cooling of the casting, and may also be varied through the mould to obtain progressive solidification of different parts of the casting. After casting, the shell may be removed by hammering or vibrating the runner. Specification 722,884 is referred to.