Characterization and modeling of anisotropic SL pattern during investment casting process

Photopolymer-based stereo-lithography (SL) rapid prototyping patterns with honeycomb internal structure reduce lead time and material cost for pattern manufacturing and also minimize ceramic mold residue after pattern removal in investment casting process. However, relatively larger thermal expansion of SL pattern tends to cause ceramic shell cracking during the pattern thermal removal. The anisotropy of SL pattern properties incurs different stress states of ceramic shell depending on the orientation of the honeycomb structure. In this work, a comprehensive thermo-mechanical coupled finite element model was developed to characterize the anisotropy of SL pattern and its effect on stress in investment casting shell molds. An experimental-based thermal degradation material model was developed considering anisotropy of the thermo-mechanical properties of SL pattern. The static analysis of slurry dipping and transient thermo-mechanical analysis of the pattern thermal removal process were performed. Both the pattern and ceramic shell behavior were studied and validated with experimental findings. Facesheet thickness/facesheet-to-core ratio and orientation of honeycomb structure were found to be the key factors to shell stress during pattern firing. Decreased facesheet thickness reduces stress in shell but also increases shell stress sensitivity to anisotropy of SL pattern. The developed tool provides an effective and efficient way to evaluate and optimize the structure of SL pattern for investment casting process.