Investment Castings

Investment casting is one of the oldest and perhaps the most versatile of the casting processes. Most any castable metal is available, and very intricate shapes can be produced. Common applications include jewelry, turbine blades and veins, valve bodies, and myriad other industrial components.

The Process

A wax positive of the part to be cast is molded at low pressure in a tool. Investment cast tooling is generally made of aluminum. Advances in SLA technology has enabled the investment caster to produce the models without tooling for prototyping purposes. The wax part is then attached to a wax “tree” which enables multiples to be cast at the same time. The tree then has a shell built around it which is composed of ceramic and silica. After the shell has been built in multiple layers, it is then “dewaxed” typically by placing it in a steam autoclave. The wax positive has now formed a cavity in the shell.

The shells are then fired in a furnace in order to burn out any remaining wax residue as well as to prevent the shell from cracking when exposed to molten metal. The shells are then poured with the specified alloy and allowed to cool. The shell is then mechanically removed from the tree and castings. Castings are then cut off of the tree and the tree material is recycled for the next pour. Grinding and machining operations are generally used to clean up the areas of the part where the metal entered, commonly known as the gate witness area.

Design Considerations

General tolerance – +/- .010” for the first inch, +/- .005 per inch thereafter

Wall thickness – Dependent on part configuration. .125” min is a good guideline for ferrous materials.

Radii – 1/64” to 1/32” for all internal and external corners is preferred.

Finish 125 micro inch typical.

Avoid deep blind holes. These often require the use of cores which add cost. Often it is less expensive to drill blind holes as a secondary machining operation.

Economic/Purchasing Considerations

Tooling – Investment casting tooling is generally built for one of two types of wax presses, manual or automatic. A manual tool is generally taken apart to retrieve the wax every time the machine cycles. An automatic wax press operates in a similar manner to an injection molding machine. Manual tools are generally built for lower volume applications. The wax injection stage of the investment casting process is low temperature, low pressure process, therefore tool life is virtually limitless. Tools are generally transferable between foundries.

The investment casting industry is a niche business. The type of equipment available in a foundry determines the size, complexity and volumes of parts which can be most efficiently produced. One foundry may want to produce parts the size of a brick where another producer may specialize in parts weighing only a couple of ounces. Finding the foundry with the right fit for a particular part is key to obtaining a competitive price.

How we can help

How do I know that the metal specified is what is being poured? Does it make sense to employ a ceramic core to form a hole, or drill the hole as a secondary operation? What rapid prototyping options are available through the investment casting process? Send us your drawing and we can answer these questions and many more.