The fundamental process of metalcasting consists of five basic elements:

• Molding—The mold cavity must be formed from a material that will withstand the operating temperatures and conditions of the chosen casting process and metal.
• Pouring—The molten metal is poured into the mold and travels through its passages to fill the mold cavity.
• Solidification—During the solidification process, the metal cools and becomes a solid shape.
• Mold Removal—The cooled casting is removed from the mold.
• Secondary Operations—The casting is trimmed, cleaned, heat-treated, machined, inspected, painted, etc.

In many cases, casting metal in the natural environment produces quality results, but in some instances metalcasters have figured out how to act against nature in order to achieve optimal properties from their cast components. By using a vacuum to evacuate air in an enclosed area, either to produce a mold, pour castings or melt metal, metalcasters can successfully produce complex, difficult castings.

Because a vacuum is used in various ways during different steps of the metalcasting process, the term “vacuum casting” can be unclear. An aerospace engineer may associate vacuum casting with the method of melting high-alloy or superalloy material under a vacuum to avoid oxidation of the highly reactive elements.

But an engineer working with pumps and impellers may think of the mold-filling method using differential pressure caused by a vacuum to draw metal into the mold cavity.

This article will take you through a few of the vacuum casting methods used in a range of molding processes.  From thin walls to dimensional accuracy, vacuums can use the unnatural to make certain metal castings possible.

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.

This article provides an explanation of the three main types of plastic patterns that can be used to produce a cast component.

This family of casting processes is unique in that ceramic and plaster are used as molding media. These processes offer a high degree of precision in regard to dimensions, as well as excellent surface finishes.