Effects of material variables and process parameters on properties of investment casting shells

Manufacture of investment casting shells is a complex process. The choice of raw materials - refractory powders or grains, binders and additives - affects the properties of investment casting shells. In this study, different systems of shells were prepared, according to a design of experiments, with commercially available raw materials that differ in chemistry, particle size or particle size distribution. Shell strength was measured in green, fired and cooled, and hot conditions and the results were analyzed for strength – material property relation. Various microstructures of polished cross sections of these shells were characterized using scanning electron microscope. It was determined that the amount of matrix holding the stucco grains was dominant factor affecting green strength. Fired and hot strengths were observed to vary depending on interactions between different phases of matrix and stucco.;In addition to the material properties, control of shell building parameters is critical to achieve quality shells. Process parameters affect strength of the shell by providing a means to change the relative amounts of stucco, slurry and porosity. To study the microstructural variations, shells were prepared by varying process parameters like slurry viscosity and stucco size. Data from image analysis of different microstructures were correlated to their respective fired strengths. It was determined that the shells prepared from high viscosity slurry and fine stucco had the highest strength.