Enhancing the Permeability and Properties of Ceramic Shell in Investment Casting Process Using ABS Powder and Needle Coke

In the investment casting process, initially, a wax pattern is made, around which repeated slurry/stucco coatings are applied. The slurry is made up of very fine ceramic ingredients like zircon flour, etc. Once the ceramic slurry is dried out, a ceramic shell is formed around the wax pattern. After drying out of the ceramic shell, the wax is drained out and the shell is fired in an oven for inducing additional strength. Molten metal is then poured into the ceramic shell. Because of the fine ingredients of the ceramic shell, the investment casting gets a very fine surface roughness, compared to a conventional sand casting. However, the ceramic shell exhibits poor permeability, which often results in porosity defects. Hence, in the present work, an attempt was made to enhance the permeability of ceramic shell by mixing certain fillers like acrylonitrile-butadiene-styrene (ABS) powder and needle coke to the ceramic slurry. Fine ABS powder was added to the inner coat slurry in a very small proportion. The ABS powder, upon heating, got burnt and left micropores inside the inner coating of the shell. In a similar way, needle coke was added to the outer coat slurry in a very small proportion. During the firing of the shell, the needle coke got burnt and, consequently, macropores were created in the outer coating of the shell, which would enable escape of undesirable gases that may generate inside the shell. This ultimately would reduce the porosity defects in the final cast parts. Tests conducted on the ceramic shell modified with ABS powder and needle coke reveal that the modified shell possesses higher permeability compared to the conventional one. The modified shell also possesses adequate mechanical properties like flexural strength, hot tensile strength, etc., and fair surface roughness.