What Is the Investment Casting Process?
The investment casting process, also known as lost-wax casting, uses wax patterns and ceramic shells to create near-net shape parts. It's a multi-step procedure that starts with an investment casting die and ends with components that have superior finishes and tight tolerances. The investment casting process steps include:
1. Produce an Investment Casting Die
A metal investment casting die is created based on a master pattern that matches the end product, taking shrinking into account.
2. Create Wax Patterns
Wax is injected into the investment casting die to create replicas of the end product. Cores can be incorporated to manufacture highly complex parts.
3. Assemble Wax Patterns
The wax patterns are attached to a sprue to create a tree. This tree is a gated system that will allow the molten metal to flow freely into every portion of the cast component. If the final product is small, a tree can accommodate numerous wax patterns. For the very largest parts, there may be just one pattern per tree.
4. Create Ceramic Shells
The wax trees are dipped repeatedly into a ceramic slurry to build up a shell. Granular refractory stuccos can also be used. Each layer is allowed to harden before a new layer is created. The size and shape of the end product, along with the temperature of the molten metal, determines the thickness of the ceramic shells. In the end, the dried ceramic shells will be sufficiently hard to contain the molten metal.
5. Remove the Wax
The ceramic shells are treated to melt and remove the wax. The hollowed-out shells contain a cavity that matches the shape of the desired component.
6. Pour the Metal
The ceramic shells are heated to an appropriate temperature, usually around 1,000°F to 2,000°F. At this temperature, the shells are further strengthened, residual wax melts away and any moisture in the shells dissipates. Once the shells reach the target temperature, they're filled with molten metal. The temperature of the metal is usually significantly higher than the mold, ensuring it flows into all areas before solidifying.
7. Cool the Ceramic Shells
Once filled, the ceramic shells are set to cool, allowing the metal to solidify. The cooling rate can be controlled by the thickness of the ceramic shell and the ambient temperature around it. This solidification process can take a few minutes to a few hours.
8. Remove the Ceramic Coating
Once the metal has solidified, the ceramic shells are removed through a knockout operation. The sprues and gates that allowed the metal to flow into the shells are also removed.
9. Finish the Parts
The metal castings go through a finishing process. This can include sandblasting to remove any remaining ceramic material, heat treatment to alter the properties of the metal or minor machining to achieve the final dimensions.
10. Inspect the Parts
The last step is inspecting the final products to ensure they meet all specifications. This may involve visual inspection, dimensional checks or more advanced methods, like non-destructive testing.