Thermo-Mechanical Properties of SLA Pattern Materials and Their Effect on Stress in Investment Shell Molds

The process of photopolymer stereo-lithography (SLA) provides a unique opportunity for the rapid prototyping of investment castings having both high surface quality and complex geometry from both ferrous and non-ferrous metals. A specific internal honeycomb structure gives a combination of high pattern stiffness needed for precise replication of geometry by investment shell molds with an extremely lightweight of pattern, which minimizes the residue after pattern removal. In this paper, the thermomechanical properties of SLA patterns (coefficient of thermal expansion, modulus, softening and decomposition temperatures) were experimentally evaluated. The anisotropy of the thermo-mechanical properties of SLA pattern were shown to depend on the orientation of the honeycomb structure. A novel approach for finite element method (FEM) modeling of the anisotropic SLA patterns was suggested. A three-dimensional thermomechanical coupled pattern/ceramic shell modeling was used to predict both stress in the ceramic shell during pattern removal and the tendency for crack formation. Experiments were performed to verify these modeling predictions. Recommendations for the optimal structure of SLA pattern for investment castings were formulated.