When it comes to metals, investment casting is one of the most widely used rapid prototyping techniques, and it's the one that's moving the fastest toward manufacturing application. Rapid prototyping processes are used to create patterns rather than traditional wax molds. Stereolithography is the process that is most commonly used to create patterns. A honeycombed build pattern is frequently used because it allows the plastic pattern to expand when it is burned out without cracking the mold. Because of the ash that may remain after pattern burnout, selective laser sintering and laminated object manufacturing are used less frequently for this purpose. The Sanders model maker, which uses inkjet technology to produce wax parts with extremely high resolution and accuracy, is also frequently used. Investment Castings are produced using all castable engineering materials, and the process is being used to produce very low volume production components when cost savings on pattern tooling are expected to be significant.

What is Investment Casting and How Does It Work?
For complex-shaped components requiring tighter tolerances, thinner walls, and a better surface finish than can be achieved with sand casting, investment casting is typically used. The method by which the mold is constructed distinguishes investment casting from other types of casting. A wax pattern is created for the part, which is then immersed in a fine ceramic slurry containing colloidal silica and alumina. The mold is dried and heated in an oven to melt away the wax, leaving a ceramic shell mold that can be used for casting. Precision casting of aerospace components such as gas turbine blades is accomplished through the investment casting method, also known as the lost wax process. Compared to sand castings, Investment Casting of aluminum alloys has a lower strength, but it allows for tighter tolerances, a better surface finish, and the ability to produce thin-walled sections.

Both investment casting and die casting are capable of producing parts with similar geometric shapes and sizes. The disposable pattern is created by injecting wax into a mold, as you will learn below. As a result, features that are difficult or expensive to injection mold or die cast (e. g., undercuts) are also difficult or expensive to investment cast. In most cases, investment casting is used when low production volumes (e. g., less than 10,000 pieces) are expected, whereas die casting is typically used when high production volumes are expected.

A wide variety of metal alloys can be used to create investment cast parts. These include aluminum and copper alloys, carbon and low alloy steels, stainless steels, tool steels, as well as nickel and cobalt alloys. You will learn in the following section that die castings must be limited to metals with low melting temperatures, such as zinc and aluminum.

In investment casting, a metal die or mold is created through either machining or casting. The metal dies are more expensive the more complicated the shape (due to undercuts, for example). Following the creation of the mold, wax is injected into the mold to create a pattern. The external shape of the wax pattern is similar to the internal shape of the mold. The wax pattern is removed from the mold and attached to a wax base that contains a gate. The pattern is then cast in resin. The wax patterns are attached to a tree that contains the runners, gates, and other features that will feed and distribute the molten metal if the volume of production is large enough. To complete the process, a metal hollow tube is placed over the wax patterns, into which is then poured a slurry (such as plaster of Paris) to completely cover the wax patterns. The finished mold is placed in the oven, where the wax is removed through a process of melting and evaporation. Following this, the mold is typically placed in a second oven for 12 to 24 hours to allow the mold to cure completely.

To create parts, molten metal is poured into the mold cavity and allowed to solidify. To make filling the mold easier, the melt is poured into the mold while it is still hot. Following cooling of the part, the mold is destroyed and the part removed. Investment casting is capable of producing tolerances and surface finishes that are so good that machining is rarely required.

The investment casting process itself is relatively simple and well established, but producing premium, high integrity, and high-quality components that are suitable for increasingly demanding aerospace applications is not without its challenges. Production of fully dense castings with low porosity, inclusions, oxides, and impurities and a very fine microstructure, as well as a modified eutectic structure is now possible, and thus routinely manufactured.