Development of high-temperature permeability and shock-tearing measurements for shell molds

Shell Moulding in Foundry processes became popular because a skin or shell of sand forms the mould and the finished mould is obtained without ramming. Shell permeability is the capability of a cured shell specimen to allow air to pass through it. Not much work is reported on high temperature permeability, a knowledge of which is highly essential to prevent mould dilation and gas porosity, in order to obtain close tolerance castings. Hence an experimental set up is developed to measure high temperature permeability for testing shell raoulds. Test results reveal that an increase in mould temperature and sand fineness ninaber, decreases the permeability drastically. Cracking or tearing of shell mould caused under the influence of thermal shock of molten metal while pouring, results in metal penetration and runout. This tendency is due to the quality or quantity of resin, catalyst and other process variables in mould making. Experimental set up is developed to observe and record cracking tendency of shell mould by simulating pouring condition. Test results indicate that insufficient air flow rate and air temperature during resin coating, increase crack failures. An increase in resin content decreases shock tearing tendency. Catalyst hexamine addition below 16 percent and beyond 20 percent is prone to shock tearing.