What is Investment Casting?
Investment casting is a metalworking process in which an intricate metal part is produced to match an original wax pattern. "Investment" describes how the wax pattern is invested (surrounded) by ceramic material to create a mold, or shell. Molten metal is then cast into the shell cavity, which is an exact duplicate of the desired part.
Materials that can be investment cast include Stainless Steels, Carbon Steels, Aluminum, Brass, and Nickel- and Cobalt-based alloys. Due to the shell's hardness, investment casting can yield parts with exceptional surface finish — thus reducing the need for secondary machining. Having been in use for over 5,000 years, investment casting is valued for its accuracy, repeatability, and integrity in a variety of metals and high-performance alloys.
How Does Investment Casting Work?
Investment casting occurs in the following steps:
1. Produce a master pattern
A mold-maker creates an original pattern from wax, clay, wood, plastic, or metal. Modern investment casting commonly uses 3D-printed wax or SLA resin patterns.
2. Create a mold
A mold, or master die, is made to fit the master pattern. The die will commonly resemble an injection mold that is used to replicate the original pattern.
3. Produce wax patterns
Wax patterns can be produced by (A) pouring wax into the mold and swishing around until an even coating covers the inner mold surface; or (B) filling the entire mold with molten wax and letting it cool as a solid object. Cores can be created using either soluble wax or ceramic.
4. Assemble wax patterns
Multiple wax patterns can be created and assembled into one large pattern to be cast in one batch pour. In this situation, patterns are attached to a wax sprue to create a pattern cluster, or tree. As many as several hundred patterns can be assembled into a tree.
5. Apply investment materials
The ceramic shell, known as the investment, is produced by repeating a series of steps — coating, stuccoing, and hardening — until a desired thickness is achieved.
Once ceramic shells have fully cured, they are turned upside-down and placed in a furnace or autoclave to melt out or vaporize the wax. Any wax that runs out of the shell is usually recovered and reused.
7. Burnout preheating
The shell is then subjected to a burnout, which heats the mold to between 870° C and 1095° C to remove moisture and residual wax, and to sinter the shell. This heating can also be used to preheat the shell before casting, which helps the metal to fill all mold details and increase dimensional accuracy.
The investment shell is then placed open-side up into a tub filled with sand. The metal may be gravity poured or forced by applying pressure, vacuum, or centrifugal force.
The shell is hammered, media blasted, vibrated, waterjeted, or chemically dissolved to release the casting. The sprue is cut off and recycled. The casting is then cleaned up to remove signs of the casting process, usually by grinding.
After grinding, the completed casting is then subject to finishing via hand tooling, welding, and occasionally straightening on a hydraulic press.
Vacuum Investment Casting
A specialized version of investment casting uses gas pressure and a vacuum to improve the quality of the casting and minimize porosity. The most common materials for vacuum investment casting are the high Nickel-based alloy and super-alloys.
Advantages of Investment Casting
Investment casting offers many advantages, some of which include:
Excellent surface finish
High dimensional accuracy
Extremely intricate geometries as cast
Unlimited material options
No flash or parting lines
Effective utilization of metal
Fewer environmental hazards from the foundry process
Investment Casting Materials
Investment casting can be used with almost any castable metal. Most commonly:
Low Alloy Steels
Nickel Alloys such as Inconel 718 / 713 / 625, Rene 41, Mar-M-247, Haynes 230, and Nickel X
Cobalt Alloys 31 / 25 / 21
Copper Alloys such as Silicon Bronze and Yellow Brass
Titanium Alloys such as Ti6Al-4V
Investment Casting Applications
Investment casting is used in a wide range of industries and products. Some examples include:
Aerospace: Turbine nozzles and blades, afterburner consumables, high-temperature valves, fluid management components
Defense: Firearm receivers, triggers, hammers, ammunition feeding systems, missile structurals, rocket nozzle ejection systems, land combat vehicles, portable communication towers
Industrial: Food processing systems, heavy machinery, safety shutoff valves, network router chassis hardware
Medical: Patient-specific implants, prosthetics, surgical tools, medical equipment components
Transportation: Race car turbo chargers, boat hoists, exhaust manifolds, air valve bodies