Deeply Understand the Investment Casting II


As a matter of fact, nearly any metal that can be poured can be investment cast. However, because it is so material efficient, economics are particularly attractive for high performance alloys. High on the list is titanium with its superior strength-to-weight properties at high temperature and in corrosive environments. Zirconium is similar, and is often the choice for parts such as valve components in extreme corrosive applications. Other metals include cobalt-chrome alloys for biomedical implants, a number of stainless steel alloys, and nickel alloys where very high temperature creep resistance is needed such as in aerospace applications.

Shape complexity will steer you toward investment casting, the more impossible a part is to machine, the more likely that it will be investment cast. For instance, manifolds with multiple internal passages that curve in different directions, valve bodies with intersecting holes and non-uniform passages, compressor airfoils, biomedical implants and custom surgical instruments, and complex firearms receivers would be impractical to form by machining.

Unlike other fabrication processes, shape complexity has virtually nothing to do with the processing costs of investment cast parts. The primary expense lies in creating the tool. For instance, once the tool is built, a turbocharger impeller with its contoured blades is no more difficult to cast than a simple straight rod.

To get the best parts from investment casting, remember that the flow process is essential. Keep junctions smooth, and avoid sharp corners with radius less than 0.030 in. Avoid sharp edges and allow at least 0.060 in. outside radius. Also, avoid major transitions in section thickness as these tend to cause flow problems as the metal freezes. Space out the intersections where sections join, and for large flat areas, use ribs and stiffeners to avoid warping.

Investment casting lends itself to all state-of-the-art modern modeling and rapid-prototyping techniques. For a potential product, the designer provides 3-D model and a 2-D print that details specifications and tolerances. If the product has a complex shape with multiple changes in wall thickness, the caster will evaluate it using a numerical solidification model to analyze metal flow and potential shrinkage problems. The next step is to make a wax prototype of the part by stereolithography or 5-axis wax machining. Gate positions are set and a prototype shell is made from which the prototype is cast.

The prototype is captured with white-light scanning, digitized to create an electronic model, and compared to the original CAD design. The digital part model is modified if necessary to accommodate non-uniform shrinkage. At that point, investment casting parts are ready to be made.