Advances in jewellery microcasting

Traditional moulds for jewellery casting are made of SiO2 refractory particles agglomerated by a bonding phase. Typical precious alloys are moulded around 1100 °C, a temperature that might lead to partial thermal decomposition of the bonding phase, typically CaSO4. The degradation process usually causes release of gas, e.g. SOx, which is responsible for high porosity and roughness in the casting. These gas imperfections are responsible for about 10% of the overall casting failures. The study of novel bonding phases, developed to eliminate gas defects and improve mechanical strength, is reported in this paper. In traditional moulds, bonding is generated during the investment stage, the new phases are obtained during ceramic burnout. New moulds are made from a mixture of quartz and CaO powders, and in the investment stage, quartz particles are embedded in a network of Ca(OH)2. In the next firing, Ca(OH)2 first dehydrates and then reacts with the surface of SiO2 particles to form CaxSiOx+2 phases. These interfacial compounds provide a refractory scaffold for SiO2. The process has been studied by TG–DTA. Different firing temperatures have been tested and the silicate moulds have been studied by XRD. Mechanical and casting performances have been evaluated by compression tests and microstructural analysis. Cast comparison with respect to the traditional refractory is also illustrated.