Utilização da técnica de conformação por coagulação direta (direct coagulation casting) em massas refratárias de sílica para processos de fundição por cera perdida

Abstract

Refractory masses for lost wax casting process of steel and alloys, especially for dental devices are constituted basically of silica (quartz and / or cristobalite). In these materials, the binding agent is generally based on magnesium phosphate cements, produced by acid-based reaction of NH4H2PO4 with MgO. The main product of the reaction is struvita (MgNH4PO46H2O). The presence of phosphates together with retarders, necessary to control the reaction kinetics, contributes to the sintering and adhesion of the silica investment to the metal material after solidification. Recently it was developed a consolidation process of ceramic powders by direct coagulation (DCC) of a stabilized slurry of high solids content and low viscosity. The direct coagulation is achieved by controlled destabilizing (flocculation/ coagulation) of the suspension either through the gradual change of the pH towards the isoelectronic point or increasing the ionic strength of the medium. In this work the change in pH was accomplished by two different routes. The first one by the use of buffer solutions whose pH controll was close to the one of silica IEP (innovative methodology). The second was made by using citric acid and sodium trypolyphosphate in a competing reaction changing slowly the medium pH towards the silica IEP. Both routes promoted the direct coagulation of silica refractory masses without the presence of phosphates or any other binder. The results were very satisfactory, since in all silica refractory masses (>99% SiO2) used, the coagulation occurred rapidly, at low initial viscosity, and low final water content to be eliminated. The masses kept the required refractory properties for high temperature castings, presenting excellent properties such as high expansion at the metal casting temperatures, excellent surface finishing, low adherence to the cast part and easiness upon demolding. This whole set of properties are ideal for lost wax investment casting processes.