Characterization and Properties of Ceramic Shells in Investment Casting Process

Investment casting is an effective way to produce very thin and most complex components with very high-dimensional accuracy and surface finish. A wax pattern is repeatedly dipped into ceramic slurry, and coarse stucco is sprinkled over it to develop a ceramic shell. Very fine ingredients are used to achieve smooth internal surface in the ceramic shell. These fine ingredients of the inner coat reduce the shell permeability, which lead to porosity defects on the cast part. In the present work, an attempt is made to enhance the permeability of ceramic shell by mixing fine needle coke and coarse needle coke powders to inner and outer coat slurries, respectively. After firing, fine needle coke and coarse needle coke got burnt and left some microspores and macrospores in the inner and outer layers of the shell, respectively. These pores improved the shell permeability to a considerable extent. Modified ceramic slurry with needle coke offered higher slurry retention rate as compared to the conventional slurry. Modified shell also possesses sufficient flexural strength and fair inner surface roughness. Field-emission scanning electron microscope (FE-SEM) is used to examine the microstructure of conventional and modified shells, and it is found that some pores are developed at the inner and outer layers of the modified shell, which lead to increment in permeability.