Design and Development of Acrylic mould for hip bone joint by investment casting

Abstract

The hip joint is an important feature of the human body that controls the movements of daily activities. Total hip replacement has become a common and predictable procedure for hip joint problems since it gives pain relief and improved function. Over the past few years, hip bone joint implant is made using four manufacturing processes such as machining, forging, casting and powder metallurgy of biocompatible material. Investment casting has been chosen as an appropriate manufacturing process to produce hip bone joint implant since this method is able to produce shape with complex and intricate dimensions. Due to the increase number of hip joint problems, there is a need to explore on the fabrication procedures for total hip bone replacement owing to the fact that each patient may have different sizes of bone structures and there are no identical patients that can use similar bone. Thus, an efficient method for mould fabrication and ceramic shell development procedure may improve the practice of investment casting for manufacturing hip bone joint implant. Static load analysis is done on the modified design and the maximum translational displacement for the designed hip bone was investigated in the range of 0.02mm to 0.04mm for load 700N and 1400N respectively. In order to perform this casting, a mould or mould is needed to produce wax pattern to make ceramic investment. This study proposes the fabrication of wax casting mould with acrylic. Two acrylic slabs of the acrylic mould are machined using Vertical Machining Center (VMC). The acrylic mould are made ready for making wax pattern intended to produce the ceramic shell, which is required for investment casting. Throughout ceramic shell development, stucco size system is analyzed between alternating stucco size layers and increasing stucco size layers. The result shows that ceramic shell with alternating grain stucco size (20/40 mesh alternate 80/100 mesh) is harder and gives a homogenous fine pores than the increasing grain size stucco system (20/40 mesh and 80/100 mesh). An ASTM F75 cobalt alloy is selected as the appropriate material for manufacturing hip bone joint by investment casting for its low melting temperature. In this study, the limitation of this alloy in the aspects of brittleness is improved by heat treating the alloy to 1200oC. The microstructures become more uniform and hardness of the alloy is reduced thus may reduce the possibility of stress shielding phenomenon. Essentially, this study may benefit a lot in terms of cost reduction and time saving while promising a better quality of the hip bone joint implant.