AM to Enhance the Investment Casting Process
February 17th, 2012
By: Joe S. – Project Manager
As additive technologies become more well-known to the engineering world, they continue to find new applications for streamlining processes and providing the potential to manufacture products in completely new ways.
Investment casting is a casting process that has been used for centuries to make parts with complex geometry and minimal material waste. The process most traditionally uses a wax part as the “investment” in the process. Through history this wax investment is typically sculpted or carved into shape. This wax part is then covered with a ceramic coating. Once the high temp ceramic coating has hardened, the soft wax is melted from the cavity and is ready to have any molten material (i.e. aluminum, steel, titanium, etc.) poured into it. Once the part material has hardened the shell can then be broken away to reveal the part.
Additive technology puts a revolutionary spin on this process by providing a method for producing highly accurate investments with almost any geometry conceivable. Some 3D printers are capable of producing investments in wax directly. Plastic parts produced on 3D printers can also be used, however there are a few technical details regarding plastic investments that should be considered. When the investment is heated to eventually be melted from the shell it increases in size due to its positive thermal expansion coefficient. In order to keep the hard shell from cracking due to this expansion, it is often necessary to hollow and join the plastic investment part to allow it to melt in upon itself. An example of a hollowed plastic investment is shown below in the CAD screenshots. The image on the left shows the complete part and the two consecutive images to the right shows the part as it was printed, prior to being joined and coated.
The beauty of using additive technologies to produce investments is that designs can change during the development process without the need for a change in tooling. One can simply alter the 3D model data and produce a revised investment.
In my past experience I have used this process to produce proprietary suspension parts in Ti-6Al-4V (titanium). The geometry of these parts was too complex for machining and lent itself nicely to this additive manufacturing method. For me this process was very eye opening and even made my job as a designer more fun by removing the geometric constraints tied to conventional machining, casting, and material consolidation processes such as welding and fabrication. Additive technology is something that designers and engineers should make sure they have in mind as another manufacturing tool to ensure they are designing the best parts in the most practical manner. Additive technology isn’t just plastic mock ups anymore!