GB870836A - Precision casting - Google Patents

Abstract

A coating composition for shell moulds is made from a slurry of fine refractory powder such as sillimanite, silica or zircon in an aqueous binding liquid comprising an electrolyte free silica aquasol, an anionic or non ionic emulsion of unplasticized polyvinyl acetate, an aqueous solution of sodium carboxy methyl cellulose and an anionic wetting agent such as sodium di(2 ethylhexyl sulphosuccinate). The weight ratio of aquasol to emulsion is from 4:1 to 5:1 and a de-foaming agent such as C7-C9 aliphatic alcohols, e.g. 2 ethyl hexanol may be included. Minus 150 mesh refractory powder is used and an expendable, e.g. wax, pattern is coated with the slurry by dipping. After drying further coatings are applied until a shell of the required thickness is obtained. Each coating is dusted with coarse fireclay grog or sillimanite before drying, minus 30 plus 80 mesh powder being used for the initial layers and minus 30 plus 50 mesh for the outer layers. After drying firing is effected at 1000 DEG C. minimum, dewaxing being effected during firing or prior to firing by solution in trichlorethylene. To improve adhesion the initial coating may include trisodium phosphate.ALSO:A coating composition for shell moulds is made from a slurry of fine refractory powder such as sillimanite, silica or zircon in an aqueous binding liquid comprising an electrolyte free silica aquasol, an anionic or non ionic emulsion of unplasticised polyvinyl acetate, an aqueous solution of sodium carboxy methyl cellulose and an anionic wetting agent such as sodium di(2 ethyl hexyl sulphosuccinate). The weight ratio of aquasol to emulsion is from 4 : 1 to 5 : 1 and a de-foaming agent such as C7-C9 aliphatic alcohols e.g. 2 ethyl hexanol may be included. Minus 150 mesh refractory powder is used and an expendable e.g. wax pattern is coated with the slurry by dipping. After drying further coatings are applied until a shell of the required thickness is obtained. Each coating is dusted with coarse fireclay grog or sillimanite before drying, minus 30 plus 80 mesh powder being used for the initial layers and minus 30 plus 50 mesh for the outer layers. After drying firing is effected at 1000 DEG C. minimum, dewaxing being effected during firing or prior to firing by solution in trichlorethylene. To improve adhesion the initial coating may include trisodium phosphate.