Effectiveness of PrimeCast® and Poly (Methyl Methacrylate) additive manufacturing processes to produce patterns for investment casting

Abstract

Investment casting is known as a “near net shape” process as it can produce parts that need very little secondary machining operations. The investment casting process is one of several metal casting processes and it has an advantage in that very complex designs can be produced with good accuracy and surface finish. The investment casting process begins with the fabrication of a sacrificial pattern, typically made of foundry wax, with the same basic geometrical shape as the finished cast part. Normally, the sacrificial wax patterns are produced by injection moulding where hot wax is injected into the desired patterns. Runner and gates systems are assembled and attached onto the completed fabricated wax patterns. Next, the wax patterns with runners and gates are repeatedly dipped into a ceramic slurry, with a drying period in between dipping, to form layers that create a shell. Investment casting sacrifices a pattern and ceramic shell mould for each metal part that is made. The investment casting process suffers long lead times when a new part is designed, due to the fabrication of initial tooling. Cost and lead-time to produce tooling can be prohibitively high and complexity is limited by what is possible with injection moulding in the chosen time. These factors have lead investment casting foundries as well as the companies purchasing the castings to explore alternate methods to create investment cast parts without the cost and time burden associated with permanent tooling. Additive manufacturing patterns provides an alternative method for producing investment casting patterns that can provide dramatic time and cost savings. It also gives the designers freedom to rapidly modify and redesign a product without significant increase in the total development time and cost. Nowadays, the foundries are able to play around with different designs, test them, and reach the optimum design very quickly. It is relatively expensive and timeconsuming to do this using conventional investment casting. Furthermore, by using additive manufacturing, patterns can be made as complex as needed without any impact on the cost. This study determined the difference in dimensional accuracy between PrimeCast® and PMMA patterns produced for investment casting by two different additive manufacturing technologies as well as their corresponding castings. PrimeCast® and PMMA patterns were built at the same time at Central University of Technology, Free State and Vaal University of Technology, respectively. Metrology was performed on all patterns just after manufacturing using a micro-computed X-ray tomography scanner to compare dimensional accuracies of different features of the patterns. Aluminium alloy A356 was cast in the moulds made from both types of patterns. Similar metrology was performed on all the castings to compare dimensional accuracies of different features of the castings from the two types of patterns. The patterns had features such as thin walls, cavities and angles that pose challenges to these additive manufacturing technologies and the investment casting process. From the results of this study, it was found that both technologies provided good dimensional results on simpler shapes. The PrimeCast® pattern had a better dimensional accuracy than the PMMA pattern. However, the casting from the PMMA pattern had relatively better dimensional accuracy than the casting from the PrimeCast® pattern.