Thermal Stress Analysis of Gypsum Shell Cracking in Polyjet-Based Rapid Casting of Cellular Metals

Abstract

Cellular (or lattice) metals are increasingly gaining attention for their having combinations of mechanical, thermal, and acoustic properties that provide potential opportunities for diverse multifunctional structural implementations. These include ultra-light structures with high specific strength and high specific strain, excellent impact absorption, acoustic insulation, heat dissipation media and compact heat exchangers. The emerging 3D printing technologies including direct and indirect additive manufacturing processes may accelerate the realization of their structural applications of cellular metals. For indirect additive manufacturing processes, sacrificial patterns are 3D printed with castable polymers, followed by metal filling into a mold cavity to make final cellular metals. With a high stiffness of a castable polymer, e.g., VisiJet® Procast, it is possible to build network lattice cellular structures, replacing wax which has been used for traditional investment casting processes. In general, a high thermal stress is expected during burning-out process of the rapid casing. Due to the castable polymer’s new properties, no literature is available on thermal stress between the castable polymer and ceramic shells for indirect additive manufacturing of cellular structures. The objective of this study is to investigate i) thermal stress by thermal expansion mismatch between a sacrificial pattern made of a castable polymer and a coated gypsum shell and ii) an effect of the thickness of the coated gypsum shell on thermal cracking. Starting with thermal analysis, glass transition temperature, melting temperature and thermal expansion coefficient are obtained from experiments. An analytical model for thermal stress analysis is constructed with thermo-mechanical constitutive equations and compatibility equations, followed by a failure analysis at the coated shells where gypsum is used for coating the sacrificial pattern. The thermo-mechanical analysis is conducted as a function of temperature and coated shell thickness followed by a numerical validation with a finite element (FE) based simulation. The castable polymer has the potential to be used as a base material for manufacturing 3D network cellular sacrificial patterns with thin cell walls over conventional wax materials due to its high modulus and low thermal expansion coefficient during the burning out process of the sacrificial pattern.