Analysis and design of internal web structure of laser stereolithography patterns for investment casting

Shell cracking in investment casting with webbed stereolithography patterns may be problematic during the pattern burnout process. It is due to the maximum thermal stress generated in the ceramic shell exceeding the modulus of rupture of the shell material at a temperature (i.e. the shell rupture temperature) which is lower than both the glass transition temperature of the pattern material and the web link buckling temperature. This paper presents a finite element analysis (FEA)-based numerical simulation of the thermal stresses generated during the pattern burnout process. For efficient numerical simulation, an equivalent force technique is developed for the FEA. The predicted thermal stresses agree fairly well with the measured stresses by strain gauges mounted on the test samples of webbed epoxy patterns. Using the finite element analysis, three internal web structures of stereolithography patterns are compared. The FEA results show that the maximum thermal stress with the hexagonal web structure is 22% less than that with the square web structure and 32% less than that with the triangular web structure. The FEA results are useful to the development of computer-aided engineering tools for the design of the pattern’s internal web structure.