Patent GB-805282-A (GB)
Title: Moulds for precision casting of metal
Inventor: EMBLEM HAROLD GARTON
IPC: B22C1/16
Filed: 1955-04-07
Abstract: A method of making a shell mould comprises forming a multi-layer coating around an expendable pattern by dipping the pattern alternately in a solution of acid hydrolysed organic silicate having ground refractory material suspended therein and a solution of sodium aluminate having ground refractory material suspended therein; each dipping being followed by the application of a stucco material to the inner surface of the coated pattern. The expendable pattern may be of wax or of a low melting point metal, e.g. a tin-bismuth alloy, or it may be of thermoplastic synthetic resin, e.g. polystyrene or cellulose acetate.  Its eventual removal may be achieved by melting or, in the case of wax, by dissolving in trichlorethylene.  The pattern may include a p  refractory insert which becomes part of the mould.  An initial or primary refractory coating may be applied by spraying the pattern with, or dipping it in, a suitable solution.  An example is given of the preparation of such a solution in which a silica aquasol was diluted and its pH value adjusted by the addition of ammonium phosphate solution, a wetting agent was added, prepared by reacting ethylene oxide with octylcresol, and sieved zircon powder was stirred into the mixture.  Another wetting agent, i.e. a solution of sodium dioctyl sulphosuccinate in aqueous ethyl alcohol, was added and lastly a mixture of higher aliphatic alcohols as a foam inhibitor.  In some cases the primary coating may be dusted, while still wet, with coarse grains of refractory material.  The shell is next formed on the pattern by dipping it alternately in the organic silicate slurry and in the sodium aluminate slurry, with an application of refractory stucco material after each dip by holding the pattern, still wet, under a shower of dry particles, turning it meanwhile.  After two or three dippings to produce a shell about 1/4 inch thick, the last dipping being in the organic silicate slurry, the material which forms the shell is allowed to dry on the pattern, which is then melted or dissolved.  Before metal is cast in the shell mould it is supported in e.g. a heat resistant steel flask, surrounded by fireclay grog, and fired, preferably being raised to a temperature from 900 DEG C. to 1100 DEG C. in 8-10 hours and held at that temperature for 2 hours.  As an alternative to the fireclay grog, sillimanite may be used or fritting agents as described in Specification 731,445.  The organic silicate is an ester of orthosilicic or polysilicic acid which can be hydrolysed under acid hydrolysis conditions to give a solution which will set to a gel by reaction with a sodium aluminate solution.  Any acid hydrolysed organic silicate solution may be used, preferably with a minimum gel time of 24 hours.  Ethyl silicate is preferred, through a methyl silicate, e.g. tetra methoxy silane, or a butyl silicate, e.g. tetra n-butoxy silane, may be used.  Suitable acid hydrolysed ethyl silicates may be prepared from tetra ethoxy silane or, preferably, by reacting silicon tetrachloride with ethyl alcohol as described in Specification 290,717 [Class I(1)].  Hydrochloric acid may be used to promote hydrolysis, which is best carried out in a liquid which is a mutual solvent for water and ethyl silicate.  Numerical examples are given of the preparation of a suitable acid hydrolysed ethyl silicate solution wherein a Proprietary ethyl silicate is mixed with methylated spirits and hydrochloric acid, forming a solution which should preferably be used after not more than two weeks.  Sulphuric acid may be used as an alternative to hydrochloric acid.  The term sodium aluminate includes sodium meta aluminate and other sodium aluminates containing excess caustic soda or polyhydroxy compounds such as tartrates or small quantities of silica giving a stable aqueous solution.  The most suitable sodium aluminate solutions are those which are miscible with the corresponding organic silicate solutions or readily dispersed through them.  The sodium aluminate solution must be stabilized to prevent atmospheric carbon dioxide from reacting with it, which may be done by excess sodium hydroxide, by polyhydroxy compounds such as tartrates, or by the use of small quantities of silica.  The refractory material incorporated in both the organic silicate solution and in the sodium aluminate solution may be one or more of silica, zircon, zirconia or sillimanite.  An ethyl silicate slurry may be prepared by adding silica flour to acid hydrolysed ethyl silicate solution. A sodium aluminate slurry may be prepared by adding ethyl alcohol to an aqueous solution of sodium aluminate, followed by silica flour and sillimanite.  The stucco material may be one or more of sillimanite, zircon, fireclay grog, alumina or calcined kaolin. Specifications 674,137, 678,035 and 722,816 [Group XXII] also are referred to.