Prediction of distortions and pattern allowances during sand casting of a steel bracket

Mechanical interactions between the casting and mould generate unwanted distortions and lead to dimensional inaccuracies. In this study, the effects of mould expansion and mould restraint are investigated through sand casting experiments involving a U-shaped steel bracket. Distortions are quantified by in situ measurements of the evolution of the gap opening between the bracket legs. Mould expansion is observed immediately after filling. Outer mould restraint prevents distortions in the bracket legs until the time of mould fracture, after which the legs are pushed outward. The experiments are simulated using a sequential thermo-mechanical coupling. The steel and bonded sand are modelled using previously calibrated elasto-visco-plastic and Drucker-Prager Cap constitutive laws, respectively. Excellent agreement between measured and predicted pattern allowances (PA) is obtained. Distortions are greatly under-predicted unless mould fracture is considered. Variations in the packing density of the moulds are also shown to have an impact on PA.