By Hank Harvey, Tech Cast Inc., Myerstown, Pennsylvania  

What Is an Investment Casting?

The term “investment” refers to the ceramic materials that are used to build a hollow shell into which molten metal is poured to make the castings. This term is derived from the solid mold process where a plaster-type material is poured, or invested, into a container that holds a clustered “tree” of small plastic patterns that are identical to the casting being produced. After the plaster has set, the disposable patterns are burned out leaving a hollow cavity into which the metal is poured.

The same method is used in the investment casting process (also known as the lost wax process). Wax is injected into an aluminum die to produce a pattern that is an exact replica of the part to be produced. The method is similar to diecasting, but with wax used instead of molten metal.

Because investment casting trees are easy to assemble, many trees for a single part can be built repeatedly for high production runs.

After cooling, the ceramic is vibrated and blasted off the metal parts and discarded. The cast components are broken off the tree and run through cleaning/finishing processes, which are similar to those of other casting processes, such as cut off, grinding, heat treatment, straightening and blasting.

What Alloys Can Be Poured?

Investment casters have the capability to pour a wide variety of alloys.

Additionally, certain applications require the use of specialized “exotic” alloys used primarily in harsh environments. These alloys, such as titanium and vanadium, meet the additional demands that might not be achieved with standard aluminum alloys. For example, titanium alloys often are used to produce turbine blades and vanes for aerospace engines. Cobalt- and nickel-base alloys (with a variety of secondary elements added to achieve specific strength-, corrosion- and temperature-resistant properties), are additional types of exotics. Some examples are hastelloys, which can endure steam and acids found at chemical manufacturing plants, and monels, which have good weldability.

As with any casting process, investment casting alloys can be specifically made for a customer depending on the materials at the metalcasting plant. But unlike other processes, the ceramic molds in investment casting can be brought to more than 1,500F (816C), thus increasing their strength. This allows for high-temperature pours. Ductile iron and steel pour in the 2,600-2,900F (1,427-1,593C) range and monels pour at more than 3,100F (1,704C). These temperatures ensure the most efficient pouring procedures and allow for smoother cooling methods when cast in the ceramic shell.