Minimization of machining by using ceramic cores during the investment casting process of femoral knee components

Humans life standards keep getting to high levels by means of the technology that is improving day by day. In order to decrease the negative effect of medical operations as much as possible on humans life standards, mainly for the ones who had surgeries before, researches and developments are keep getting done. Medical implants that placed in human body are used in many different conditions. Bones are exposed to different stresses, also replaced metallic implants must show same strength like bone. Furthermore, the medical implant that used in human body must have resistance to corrosion and would not be deformed. Most of the medical components that used in arthroplasty knee operations are produced with investment casting method. Investment casting is one of the oldest shaping method. Before B.C. 4000, metals were started to melting inside the moulds and on B.C. 2000, the investment casting started to apply with melting copper and gold. On 10th century, industrial production of investment casting began. In 1897, Dr. D. Phillbrook Council Bluffs made some trials about dentistry. The collimator trial was applied in 1901 by Dr. William H. Taggart. He filled the damaged teeth with wax and took out the wax from teeth. He embedded the wax inside a heat resistant material and dewaxed. Molten gold metal was casted inside the heat resistant material. Taggart applied the basic process of investment casting, furthermore, filling material and bridgework on dentistry. During the Second World War, necessity of special alloys and complex shaped parts gained importance. Investment casting method provided near net shape parts for defense industry and was preferred for the production of gun and warplane spare parts. Complex shaped parts and different alloys can be produced with a high surface quality and near net shape using the investment casting method, however, the process takes long time and high costs, because of the necessity for experienced staff and single use wax models, ceramic shells. A total knee implant has three main parts which are femoral component, tibia component and insert. Femoral component and tibia component parts are generally produced with the investment casting of ASTM F75 Co-Cr-Mo alloy due biocompatibility, fatigue strength, and other mechanical properties. Insert is a polyethylene based material which prevents to wear between two metallic components. After casting, all femoral and tibia components are needed to have machining process. However, ASTM F75 Co-Cr-Mo based materials which has average casting hardness value of 25-35 HRC, are rather difficult to apply machining process. Polishing is applied to external surfaces of femoral and tibia components as well as machining process is applied to some specific places of the parts after the casting process. In order to hold prosthesis on the bone even better, tapping and inserting pegs are applied to the as cast holes. The main aim of this study is to obtain femoral component's holes, mainly coming from casting, to be metric screw shaped in order to remove tapping to femoral component's peg holes which are produced with ASTM F75 Co-Cr-Mo based material. To attain tapped screw holes from casting, ceramic cores must be used on pattern assembly process. For this purpose, first of all, studies which are about designing ceramic core mold were done to produce ceramic screw cores. Shrinkage factor that would occur depending on ceramic raw material that was used and abrasive effect on mold material were observed with trying two different options: steel and aluminum mold materials. Ceramic screw core production begins with preparing the core mixture. This mixture contains 25% fused silicate, 70% zirconia, 5% aluminate and paraffin, polyethylene, polypropylene binders. The particle shape of ceramic core powder material is significant for packing density. Spherical shaped powders have higher packing density than cylindrical, isotropic particles or acicular fibers. Core materials were blended at 140ºC. The prepared mixture was injected to two different molds. During ceramic core production, it was observed that the mold produced with 4140 steel was tougher than the mold produced with 7075 aluminum alloy so that the steel mold had more resistance against abrasions. Injected ceramic cores were debinded and sintered. Visual control was applied for all parts and prepared to use during casting. Ceramic cores were assembled with sticking carefully to peg holes after femoral component wax injection. Femoral components with ceramic cores were assembled like a tree and preliminary prepared to ceramic shell production process. Pattern assembled trees were ceramic coated 7 times to get the shell thickness 9 mm and left to drying about 8 days before dewaxing process. During burnout of ceramic shells ASTM F75 Co-Cr-Mo based ingots were melted in Inductotherm 200 kW induction furnace under argon atmosphere. Burnout process should be applied at least 1 hour at 1050ºC. After casting process, ceramic shells were broken with vibration and remaining ceramic parts and cores were moved away with sand blasting. All peg holes were checked with M8 screw. As a result of this inspection, the produced all femoral components were checked and no any failure was detected with 18 of those femoral components, 4 of them were covered by metal at their one peg holes. Furthermore 1 of 2 remaining parts did not have screw rail and the other one had trouble while M8 screw was applied. Applying the visual control for ceramic cores should be performed more carefully and stick very strong. Chemical analysis was performed according to the ASTM E354 standard after casting process. It was observed that the chemical analysis results conformed to the ASTM E354 standard. Surface roughness was measured with a portable surface roughness tester according to DIN EN ISO 4287 standard. The average surface roughness value must be less than 3.2 µm and the measured value was 2.65 µm. According to ASTM E18 standard, the hardness value should be between 25-35 HRC. The average hardness was measured 33 ± 0.7 HRC on as cast femoral sample. To identify the ultimate tensile strength and yield strength, the tensile tests were performed according to ASTM E8 standard. It was observed that the average ultimate tensile strength and average yield strength of the studied femoral samples were 883.5 ± 16.37 and 691.4 ± 11.77 MPa, respectively, minimum values must be 655 and 450 MPa, respectively. After applying mechanical tests, the non-destructive testing methods i.e., liquid penetrant inspection and radiographic inspection, were applied for all studied femoral casting samples. The liquid penetrant test was performed according to the method of ASTM E1417 Type 2 - Method C, and no linear or circular defects were observed. In addition, the radiographic inspection of the femoral casting samples was made according to the ASTM F629 standard, and any crack, inclusion, casting void or discontinuity type defects were not detected. Overall this study showed that 18 out of 24 femoral components with metric screw holes which were produced by the investment casting method, fulfilled all standard requirements successfully. In addition, producing femoral components with ceramic cores cost 33% less than using machining process. Tapping and drilling to peg holes are not necessary anymore with using ceramic cores.