This binds the silica together to form the set investment mould. On mixing with water or a colloidal silica solution, a reaction between the phosphate and oxide occurs to form magnesium ammonium phosphate. These materials consist of a powder containing silica, magnesium oxide and ammonium phosphate. Phosphate-bonded Investment Material Composition A very thick, almost dry mix of investment is used and it is vibrated in order to encourage close packing and produce as strong an investment as possible. This process is aided by a gradation of particle sizes such that small grains fill in the spaces between the larger grains. It is necessary to incorporate as much powder as possible into the binder solution to have sufficient strength at the casting temperature. The silicic acid sol forms silica gel on mixing with quartz or cristobalite powder under alkaline conditions achieved by the presence of magnesium oxide in the powder. The solution gels slowly on standing and its viscosity may increase noticeably after three or four weeks, when this happens it is necessary to make up a fresh solution. Stock solutions of the silicic acid binder are normally made and stored in dark bottles. A slow hydrolysis of ethyl silicate occurs producing a sol of silicic acid with the liberation of ethyl alcohol as a byproduct. The binder solution is generally prepared by mixing ethyl silicate or its oligomers with a mixture of dilute hydrochloric acid and industrial spirit (improves the mixing of ethyl silicate and water). On heating, the silica gel turns into silica so that the completed mould is a tightly packed mass of silica particles. These materials consist of powdered quartz or cristobalite which is bonded together with silica gel. Silica-bonded Investment Material Composition The setting expansion of the calcium sulphate dihydrate, when mixed with water partially compensate for the shrinkage of the alloy which occurs on casting.įurther compensation can be achieved by employing the hygroscopic setting expansion. The magnitude of the contraction, which occurs rapidly above 320☌, is significantly reduced in investment materials by the incorporation of sodium chloride and boric acid. Gypsum alone is not satisfactory as an investment for alloy casting since it contracts on heating as water is lost and fractures before reaching the casting temperature. The calcium sulphate hemihydrate reacts with water to form calcium sulphate dihydrate (gypsum) which effectively binds together the refractory silica. These materials are supplied as powders which are mixed with water and are composed of a mixture of silica (SiO2) and calcium sulphate hemihydrate (gypsum product) with minor components including powdered graphite or powdered copper and various modifiers to control setting time. Gypsum-bonded Investment Material Composition The nature of the binder characterizes the material: Binder material: which binds the refractory particles, and may provide additional expansion to compensate for the casting shrinkage of the alloy.
Compensating expansion: the investment mould should compensate for the casting shrinkage, achieved by a combination of setting, hygroscopic and thermal expansion.Thermal stability: the investment mould should be capable of maintaining its shape, integrity and have a sufficiently high value of compressive strength at the casting temperatures.
The accuracy of the casting can be no better than the accuracy of the mould.
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