Theory

Investment casting is one of the oldest manufacturing processes, dating back thousands of years, in which molten metal is poured into an expendable ceramic mould. The mould is formed by using a wax pattern - a disposable piece having the shape of the desired part. The pattern is surrounded, or "invested", into ceramic slurry that hardens during heating into the mould. Investment casting is often referred to as "lost-wax casting" because the wax pattern is melted out of the mould after it has been formed.

Lost-wax processes are one-to-one (one pattern creates one part), which increases production time and costs relative to other casting processes. However, since the mould is destroyed during the process, parts with complex geometries and intricate details can be created, which is unique feature of the process.

Investment casting can make use of most metals, most commonly using aluminium alloys, bronze alloys, magnesium alloys, cast iron, stainless steels, and tool steels. This process is beneficial for casting metals with high melting temperatures that cannot be moulded in plaster or metal. Parts that are typically made by investment casting include those with complex geometry such as turbine blades, firearm components. High temperature materials are also common, which includes parts for the automotive, aircraft, and military industries.

Investment casting requires the use of a metal die, wax, ceramic slurry, furnace, molten metal, and any machines needed for sandblasting, cutting, or grinding. The process steps include the following:

• Pattern - The wax patterns are typically moulded into a metal die and are formed as one piece. Cores may be used to form any internal features on the pattern. Several of these patterns are attached to a central wax gating system (sprue, runners, and risers), to form a tree-like assembly. The gating system forms the channels through which the molten metal will flow to the mould cavity

• Mould creation - This pattern tree or single pattern is dipped into slurry of fine ceramic particles, coated with more coarse particles, and then dried to form a ceramic shell around the patterns and gating system. This process is repeated until the shell is thick enough to withstand the molten metal it will encounter. The shell is then placed into an oven and the wax is melted out leaving a hollow ceramic shell that acts as a one-piece mould, hence the name "lost wax" casting.

• Drying & Pouring - The mould is preheated in a furnace to approximately 250^oC and the molten metal is poured into the gating system of the mould, filling the mould cavity. Pouring is typically achieved manually under the force of gravity, but other methods such as vacuum or pressure are sometimes used.

• Time for Solidification - After the mould has been filled, the molten metal is allowed to cool and solidify into the shape of the final casting. Cooling time depends on the thickness of the part, thickness of the mould, and the material used.

• Casting removal & Finishing - After the molten metal has cooled, the mould can bebroken and the casting removed. Often, finishing operations such as grinding, or filing are used to smooth the part at the gates.

• Heat treatment is also sometimes used to harden the final part.

Advantages

• Investment casting is a manufacturing process that allows the casting of extremely complex parts, with good surface finish.
• Very thin sections can be produced by this process. Metal castings with sections as narrow as 0.5 mm have been manufactured using investment casting.
• Investment casting also allows for high dimensional accuracy. Tolerances as low as 0.08 mm have been claimed in ideal case.
• Practically any metal can be investment cast. Parts manufactured by this process are generally small, but parts weighing up to 25 -30 Kgs have been found suitable for this technique.
• Parts of the investment process may be automated.

Disadvantages

• Investment casting is a complicated process.
• Investment casting is relatively expensive.
• Investment casting is not suitable for heavy objects.