GB797514A - Process for manufacture of casting moulds for the casting of metals - Google Patents

Abstract

A shell mould for precision casting of metals is made by applying to an expendable pattern, e.g. wax or tin-bismuth alloy, successive coatings of a liquid composition comprising three ingredients, viz. a thermosetting resin hardenable below 50 DEG C., comminuted refractory and an alkali phosphate, each coating having applied thereto, whilst it is still wet, powdered refractory, i.e., the pattern is coated alternately with liquid composition, and refractory. The resin is caused to harden by a method that does not melt the pattern (the pattern does not melt below 50 DEG C.), the pattern is removed by liquefaction, and the shell is fired to decompose the resin and to cause the alkali phosphate to bind the shell material. The shell may be supported or invested in a refractory material before material is cast thereinto. The alkali phosphate may be sodium or potassium or ammonium phosphate, or sodium ammonium phosphate. The resin may be phenol or urea or melamine formaldehyde, which may be hardened by heating to 35-40 DEG C. Phosphoric acid, which binds the shell material on firing, may be used as a catalyst or accelerator for hardening a phenol-formaldehyde resin. An unsaturated polyester resin, with or without a monomer such as styrene, may also be used and may be hardened at normal temperature by using a catalyst such as benzoyl peroxide. The refractory material used in the liquid composition may be zircon, zirconia, silica, sillimanite or fireclay grog.ALSO:A shell mould for precision casting of metals is made by applying to an expendable pattern, e.g., wax or tin-bismuth alloy, successive coatings of a liquid composition comprising three ingredients, viz. a thermosetting resin hardenable below 50 DEG C., comminuted refractory and an alkali phosphate, each coating having applied thereto, whilst it is still wet, powdered refractory. That is, the pattern is coated with liquid composition, refractory, liquid composition, refractory, and so on. The resin is caused to harden by a method that does not melt the pattern (the pattern does not melt below 50 DEG C.), the pattern is removed by liquefaction, and the shell is fired to decompose the resin and to cause the alkali phosphate to bind the shell material. The shell may be supported or invested in a refractory material before material is cast thereinto. The alkali phosphate may be sodium or potassium or ammonium phosphate, or sodium ammonium phosphate. The resin may be phenol or urea or melamine formaldehyde, which may be hardened by heating to 35 DEG -45 DEG C. Phosphoric acid, which binds the shell material on firing, may be used as a catalyst or accelerator for hardening a phenol-formaldehyde resin. An unsaturated polyester resin, with or without a monomer such as styrene, may be hardened at normal temperature by using catalyst such as benzoyl peroxide. The refractory material used in the liquid composition may be zircon, zirconia, silica, sillimanite or fireclay grog. The liquid composition may include a volatile solvent for the resin, and may be applied to the pattern by spraying, dipping or painting. The first coating may be sprayed or painted and the remainder dipped. After two coatings have been applied the resin in them may be cured before further coatings are applied. The powdered refractory that is applied to the liquid coating on the pattern may be zircon, sillimanite or fireclay grog, and may be dusted on. The first dusting may be of very fine material, the second dusting applied to the second layer of liquid composition) may be not so fine, the third still coarser, and so on. The hardened shell may be strengthened, before or after removal of the pattern, by immersing it in acid hydrolysed ethyl silicate solution and heating to gell the solution; before the solution is gelled it may be dusted with refractory: this immersing-dusting step may be repeated before gelling. Alternatively, the shell may be strengthened, after removal of the pattern, by immersion in sodium silicate or orthophosphoric acid prior to firing; before firing, refractory may be applied to the sodium silicate &c coating.