GB1159861A - Method of Casting Metal Products - Google Patents

Abstract

1,159,861. Coating processes. PRECISION METALSMITHS, Inc. 22 March, 1967, No. 13378/67. Heading B3F. [Also in Divisions B5 and C3] . In a method of investment casting a shell mould 11, comprising a central sprue passage 13, individual mould cavities 14 gated at 15 into the said passage 13, and a core 12, positioned within the passage 13 to define a tubular sprue passageway 16, is placed within a vacuum chamber 25 on a platform 28 that has a hole 31 leading into the core 12 which may be hollow, and a vacuum is drawn so that the core 12 is held in position by differential gas pressure during pouring of the molten metal. Control of the degree of vacuum in the core 12 relative to that around the shell 11 is achieved by selecting an appropriate size of hole 31 by the use of plugs 33. This is necessary due to a difference in permeability of different materials used for making the core 12 and/or shell 11 They may both be made of a low permeable ceramic shell material but if the core 12 only is made of a highly permeable resin-bonded sand material the hole 31 size is increased. The core 12 may be pre-heated separately to above the temperature of the shell 11 to promote directional solidification. Alternatively the core 12 may be at a lower temperature than the shell 11 before pouring and cheaper resin-bonded sand cores used. The shell 11 may be made by coating a destructible pattern with several coats of a slurry with a drying stage between each coat. The slurry may comprise a suspension of a refractory powder such as zircon, zircon silicate, and fused silica in a binder solution comprising colloidal silica sol and small amounts of an organic film former, a wetting agent, and a defoaming agent. The wet slurry of each coating is stuccoed with coarse refractory particles of, for example, zircon, fused silica, silica, and various aluminium silicate grogs including mullite, sillimanite, fused alumina, and tabular alumina. The core 12 may be made by coating a wax form with a similar slurry as the shell 11 or by casting it in a suitable mould. A ceramic hollow core 12 is made by coating a hollow aluminium mandrel with a layer of low melting point wax, building up a refractory coating by dipping it into a refractory slurry, stuccoing with coarse refractory particles and drying, pouring hot-water in the mandrel to melt the wax and release the core and then firing the core. A suitable mould wash may be applied to a resin-bonded sand core to prevent its disintegration during casting. The vacuum chamber 25 includes gaskets 30, 40 that may be made from asbestos pads or blankets or ceramic fibre felt made from refractory fibres such as aluminasilica. The vacuum in the chamber 25 may act through the permeable shell 11 to draw the molten metal into the mould cavities 14.