Characterization of the refractory coating material used in vacuum assisted evaporative pattern casting process

In the production of the casting using evaporative pattern of expandable polystyrene, the refractory coating applied to the pattern is an extremely critical factor. The coating is not required merely to prevent a direct contact with the sand and metal. But its main task is to support the sand during the brief interval between the gasification and replacement of pattern by the molten metal. In the present study, refractory filler material (flyash) has been used for coating the evaporative polystyrene patterns. Effect of temperature on the powder mixture containing zircon flour, filler material and binder was determined using Differentially Thermal Analysis (DTA) in the range from ambient temperature (20 °C) to 1000 °C. Powder particle morphology was analyzed by the scanning electronic microscope. X-ray diffraction analysis of the coating samples was carried out for 2θ values from 10° to 80°. Prepared slurry was applied on the polystyrene pattern of 20 kg/m3 density. Coating thickness was maintained between 0.15 and 0.20 mm. Al–7% Si alloy was poured at the pouring temperature (700 °C). Microstructure of the casting was investigated. The result reveals that the sample 4 has less material loss. It contains large and small particles and fine pores. Particles of different grain size contribute to better uniform, continuous coating on pattern due to cohesion between the particles. The phases (ZrSiO4, TiO2, Fe2O3, Al2O3 and SiO2) make the stable compound which leads to high temperature refractoriness.