GB787052A - Moulds for casting metals and alloys - Google Patents

Abstract

A shell mould for precision casting is made by applying to an expendable pattern a coating of finely-divided refractory material suspended in a liquid containing ammonia and sodium silicate (or potassium silicate or a mixture of the two silicates) and hardening the coating by applying carbon dioxide. The carbon dioxide reacts with the silicate to give silica and sodium (or potassium) carbonate, and may be applied by blowing CO2 gas onto the coating, or by suspending the coated pattern near evaporating solid CO2, or by a combination of both of these methods. The liquid contains sufficient water to prevent precipitation of the silicate by dehydration. The function of the ammonia is to form a uniform strong gel. The silicate may be in solution in water, and the solution may contain (a) an organic solvent miscible with water, e.g. ethyl alcohol or glycerol, as a diluent, and (b) wetting and/or de-frothing agents. The coating process may be performed more than once, each coating being hardened by carbon dioxide before the next coating is applied. Each coating may be dusted with refractory material prior to hardening. Before the initial coating is applied the pattern may be coated with refractory material. For castings having intricate surface details a preliminary coating of silica aquasol-refractory slurry may be applied to the pattern. The pattern may be of wax, tin-bismuth, alloy, polystyrene or cellulose acetate. The coating may be applied by spraying, dipping or painting. The pattern may include a preformed refractory core to make a hollow in the metal casting, or the core may be built up as the pattern is coated. The shell mould may be hardened, before or after removal of the pattern by melting or dissolving out, by immersion in hydrolysed ethyl silicate solution. The refractory material may be zircon, zirconia, sillimanite, mullite, alumina, or fireclay grog. The shell mould is supported in a refractory material, e.g. sillimanite powder or fireclay grog and fired; or the mould may be invested in an aqueous slurry containing fireclay grog having an alumina content of 44 per cent acid sodium silicate solution and hexamine (gellation accelerator) dried and fired. Specifications 650,532 and 678,035 are referred to.