Prediction of shrinkage allowance for tool design of aluminium alloy (A356) investment casting

Abstract

Investment casting process has been adopted by the industries to produce castings having good surface finish, complex shape and better dimensional accuracy. Gradually, dimensional tolerance is decreasing day by day to reduce the overall weight of the casting. Manufacturing of dimensionally stringent investment casting products require number of shop floor trials and pattern die/tool rework to produce castings with acceptable dimensional tolerance. This is because of uneven shrinkage of a casting during the solidification process in complex shape castings. In this paper shrinkage allowances were determined for H-shape investment casting made up A356 alloy. Experiments were conducted for three types of H-shape castings (small, medium and big). There are two types of shrinkage (free and restricted) has been identified and determined for H-shape casting. The dimensions of die tooling, wax pattern and casting were measured, in order to determine the actual shrinkage allowance for the die. The results revealed that the shrinkage percentage of free and restricted solidification areas is different. However, both shrinkage allowance factor is linear in nature. Further, the shrinkage pattern at free and restricted area has been correlated with microstructure to support the new findings. This study has enlightened the importance of shrinkage allowances during tool making of investment casting products to achieve the desire dimension of the casting. Therefore, it is necessary to identify the free and restricted shrinkage area of a casting before we assign the shrinkage allowance to the die/tool for a particular investment casting product. As a result, number of shop floor trials can be avoided, time required to develop of a casting may be reduced resulting reduction of energy utilization and cost of production.