INVESTMENT CASTING

INVESTMENT CASTING PROCESS

1. Creating wax pattern:

In today’s manufacturing world, wax patterns are typically made by injecting wax into a metal tool or “die”. With the evolution of Additive Manufacturing, patterns can be printed. 

​In the art community, one of a kind pieces are carved by the artist from wax blocks. For multiple castings, a silicone tool is usually made from the artist’s sculpture and wax is injected or poured into the resulting cavity.

2. Wax Tree (or Sprue) Assembly:

It is typically uneconomical to make small parts one at a time, so wax patterns are typically attached to a wax “sprue”.  The sprue serves two purposes

1. Provides a mounting surface to assemble multiple patterns into a single mold, which will be later filled with alloy
2. Provides a flow path for the molten alloy into the void created by the wax pattern(s)

The wax between the pattern(s) and the sprue are called “Gates”, because they throttle the direction and flow of the alloy into the void made by the pattern.

3. Shell Building:

The next step in the process is to build a ceramic shell around the wax tree. This shell will eventually become the mold that metal is poured into.

To build the shell, the tree is dipped into a ceramic bath or “slurry”. After dipping, fine sand or “stucco” is applied to the wet surface. The mold is allowed to dry, and the process is repeated a number of times until a layered (or laminated) ceramic mold, capable to undergo the stresses of the casting process, has been built.

4. Dewax / Burnout:

Before pouring metal into the mold, the wax is removed. This is typically done using a steam dewax autoclave, which is like a large, industrial pressure cooker. 

Another method is the use of a flash fire oven, which melts and burns off the wax. Many foundries use both methods in concert. Autoclave removes the majority of the wax, which can be reconditioned and reused. Flash fire burns off residual wax and cures the shell, readying it for casting.

5. Metal Pouring:

Before the metal is poured into the ceramic mold or “shell”, the mold is preheated to a specific temperature to prevent the molten alloy from solidifying or “freezing off” before the entire mold is filled.

Alloy is melted in a ceramic cup (called a crucible) using a process known as induction melting. A high frequency electric current creates a magnetic field around the alloy, generating electric fields inside the metal (eddy currents). The eddy currents heat the alloy due to the material’s electrical resistance. When the alloy reaches its specified temperature, it is poured into the mold, and the mold is allowed to cool. 

6. Shell Knock Off:

Once cool, the shell material is removed from the metal. This is typically done via mechanical means:

• Hammer
• High Pressure Water Blast
• Vibratory Table

Shell removal can also be accomplished chemically, using a heated caustic solution of either potassium hydroxide or sodium hydroxide, but this approach is being phased out due to environmental and health concerns.

7. Cut Off:

Once the shell material has been removed, the parts are cut off the sprue and the gates are ground off. Part cut off can be done manually:

• Chop saw
• Torch
• Laser (limited applications)

Parts can also be cut off using automation, that is, the mold can be secured using a fixture on a programmable cut off saw.

8. Finished Castings:

Once the parts are removed from the sprue, and the gates removed, the surface can be finished via a number of means

• Vibratory/Media finishing
• Belting or hand grinding
• Polishing

Finishing can be done by hand, but in many cases it is automated. Parts are then inspected, marked (if required), packaged and shipped. Depending on the application, the parts can be used in their “net shape” or undergo machining for precision mating surfaces.