Corner Strength of Investment Casting Shells

During the investment casting process, the shell is subjected to high internal pressure and thermal stress, particularly during pattern removal and when pouring steel into the free standing ceramic shell. Most testing methods investigate the properties of the ceramic shell in flat regions while cracks typically form in the sharp corners and edge regions. The corners and edge regions have different structure and thickness when compared to flat regions and experience large mechanical stress during processing. In this study, experimental methods were combined with finite element modeling to predict failure stress in the internal corner regions of the shell. The model takes into consideration the mechanical properties of the ceramic shell to determine the stress developed during loading. The effect of shell porosity on stress concentration in sharp corners was evaluated. A general equation was developed to predict the force necessary for crack formation in the shell based on various geometric variables. The results from the model were experimentally verified and the failure stress in flat and corner regions of the shell were compared in order to develop an improved equation.