Aqueous electrophoretic deposition as a method for producing an investment casting shell mould ceramic face-coat. Part 1: formation of a carbon-filled investment casting wax electrode material

Aqueous electrophoretic deposition (EPD), using a zircon, i.e., zirconium silicate (ZrSiO4) suspension, has been investigated as an alternative to the conventional slurry dip-coating process for producing the face-coat of investment casting ceramic shell moulds. This is because EPD has the potential to (a) increase dimensional tolerances in the resultant casting, and (b) form a uniform face-coat on the entire mould-surface of complex shell moulds, including the small and/or complex cavities that are a problem for conventional dip-coating. Part 1 of this work addresses the formation of carbon-filled investment casting wax composite electrode materials. A carbon black powder and a micronised graphite powder were used as the alternative fillers in a water-emulsified pattern wax, an unfilled pattern wax and an unfilled runner wax. The runner wax composites exhibited consistently higher resistivities for both filler types and across the range of filler concentrations. Electrical resistivities of 1200 and 240 Ω cm were attained for the 16 vol% micronised graphite-filled and carbon black-filled straight pattern wax composites, respectively. The higher conductivity values associated with the use of carbon black filler are attributed to its high-surface area and hierarchical agglomerated structure. Rheologically, the micronised graphite-filled runner wax and water-emulsified pattern wax composites followed the Krieger–Dougherty model, which is helpful given that investment casting waxes are invariably injection moulded. In summary, these results confirm, in conjunction with Part 2, the feasibility of developing EPD as a promising technique for forming the investment casting shell mould ceramic face-coat.