A Novel Filling System for Gravity Casting of Cobalt Alloy Prostheses

We discuss the reasons for the growing need in India for the manufacture of prosthetic implants such as implants for total or partial knee replacement. Complex prostheses such as knee replacement implants are highly complex in shape; therefore, they are preferably manufactured using shaped casting. It is now well established that castings made using traditional filling systems suffer from unreliable mechanical properties, particularly the ultimate tensile strength and ductility. Reliability, however, is a critical requirement for prostheses. The present article describes the latest filling system for gravity casting of critical cobalt alloy implants for use in knee prostheses. The novelty of the proposed filling system for casting knee implants lies in the combination of a flush filter and a spin trap, sited appropriately within a bottom-gated naturally pressurized filling system, aimed at reducing entrainment defects, viz. air bubbles and oxide bifilms. An uprunner that serves to both fill and feed the castings and narrow slot gates connecting the uprunner to the casting cavities are the other important elements of the present filling system. Casting simulations of knee implants using the proposed filling system technology indicate reduced surface turbulence of the melt during the early stages of filling, followed by priming of the initial melt to separate entrained defect content, with clean melt eventually entering the casting mold cavities at a sub-critical velocity for surface turbulence (below 0.5 m/s). Thus, for the first time in the production of cast prostheses using gravity casting, reducing entrainment defects to a low and reproducible level to raise reliability appears possible. This indicates a positive forecast for the production of high-quality medical implant castings of extremely low defect content and therefore enjoying high reliability compared to castings made by traditional gravity running and gating methods.