Effect of mould expansion on pattern allowances in sand casting of steel

For steel castings produced in sand moulds, the expansion of the sand and have a significant impact on the final size and shape of the casting. Experiments are conducted using a cylindrical casting to study this effect for different sands (silica and zircon) and different sand binder systems (phenolic urethane and sodium silicate). The type of sand has a significant effect on the final casting dimensions, in particular because the expansion of silica sand can be irreversible. The sand expansion effect is enhanced by the presence of sodium silicate binder. In addition, the size of the core, which in the present experiments controls the amount of steel in the mould and thus the heat input to the mould, strongly affects the internal and external dimensions of the resulting casting. A combined casting and stress simulation code is used to predict the dimensional changes of the castings. In several cases, the pattern allowances are predicted successfully both for free and hindered shrinkage cases. Disagreements between the simulation results and the measurements can be attributed to the fact that the stress model does not account for the irreversible nature of the silica sand expansion, which Is important when silica sand is heated to temperatures above ∼1200°C; and the outer mould sand surrounding the casting, which can cause inaccuracies when there is significant early mould expansion, hindrance, or movement.