Investment casting

Investment casting is used to make precision parts with a good surface finish. It is used to make turbine blades from a single crystal Ni-based superalloys. As the moulds are made from ceramic, metals with high melting points can be cast in this way.

• A pattern of the required component is formed out of wax usually through injection moulding into a metal mould. A runner and gate are included within the wax pattern. Pre formed ceramic cores can also be added, so that hollow castings can be made.
• Many wax patterns are then connected together to form a tree, and this is then dipped into a ceramic slurry, which sets through drying. Different layers of ceramic are created around the wax patterns, increasing in coarseness.
• The wax patterns are then melted out, ready for the molten metal to be poured in to create the castings.
• The ceramic mould is broken to reveal the castings, and any ceramic cores added at the start can be chemically etched out to leave a hollow structure.

Investment wax	Investment mould	Investment blades

Video of investment casting

Advantages

• Metals with a high melting temperature can be cast due to ceramic mould
• Complex shapes can be formed by using ceramic liners in the original wax patterns
• Good surface finish can be obtained using fine ceramic material

Disadvantages

• Expensive as mould cannot be reused
• Time consuming (drying times for ceramic range roughly 24 hrs)