Casting With Complexity: How Casting Plus 3D Printing Combine the Strengths of Both

The trade-off that users of additive manufacturing tend to take for granted is geometry versus material familiarity. 3D printing makes possible intricate geometries that could not be achieved without it, and even makes these complex forms easy to realize. However, the material properties are those that come from the AM process — the aluminum alloy produced through laser powder bed fusion will differ in its microstructure from a comparable alloy used in, say, casting. And processes such as casting are what engineers are familiar with and what engineering certifications often expect.

So why not cast the part? That is the question investment casting provider Aristo Cast has been asking with increasing frequency. Casting can be used in conjunction with 3D printing to leverage the strengths of both processes. CEO Jack Ziemba is leading the company in the development and application of a technique he calls PICS, for “printed investment casting shell.” In this method of casting, a temporary ceramic shell 3D printed to give the part its shape is later dissolved out of the metal casting it was used to form. The result is a part with the geometric intricacy of additive manufacturing using the alloys and material properties expected of casting.

The company has had enough success applying PICS that Ziemba now bristles when he hears the assertion that AM is able to achieve geometries casting cannot. Referring to perhaps the best-known example of a geometrically intricate production part produced with AM, Ziemba says, “I believe we could cast the GE Aviation fuel nozzle. I have asked about getting the design file; I hope we can one day prove this.”

Aristo Cast’s process using 3D printing is similar to traditional investment casting, but it avoids an initial step. Investment casting typically begins with a wax pattern, and a ceramic shell is allowed to form around this pattern by dipping it in a slurry. During casting, the shell contains the metal as the wax melts away. The PICS process skips the wax pattern — the ceramic shell is 3D printed directly. Aristo Cast does this using an additive system from Admatec.

The Admatec machine uses digital light processing to 3D print with ceramic material. Ceramic in slurry form is delivered on a continuous conveyor to a UV light source that fuses the material to the part layer by layer. Users of this machine include manufacturers of ceramic implants, but Ziemba says finding this machine and seeing how it could be applied to sacrificial casting tooling was an important discovery that made the AM-enabled investment casting process possible. Aristo Cast uses the machine to print with fused silica, a ceramic material that breaks down in potassium hydroxide and water after casting is done, so the casting remains clean and unharmed while the ceramic entirely dissolves away.

The result is far more than just a different way to do investment casting, Ziemba notes. A wax pattern is limited when it comes to the potential for complexity — too much detail, and the ceramic will not be able to coat it sufficiently during dipping. A ceramic form directly 3D printed does not have this limitation. Ziemba showed the example of a cast metal part with intricate internal surface details in the shape of gyroids. That mathematical form, the gyroid, is a shape found in nature that lends itself well to devices such as heat exchangers. The common assumption is that 3D printing alone is the only way to realize manufactured forms taking advantage of the surface-area benefit of gyroids. Aristo Cast argues that 3D printing is indeed vital, but the company is demonstrating that a much older and more established process — casting — can also be used to produce these complex geometries.