The investment casting process begins with the production of wax replicas of the desired castings. These replicas, called patterns, are injection molded in metal dies. A pattern must be manufactured for each casting to be produced. A number of patterns (depending on size and complexity) are attached to a central wax stick, or sprue, to form a casting cluster, or assembly. After some initial pre-dips, which thoroughly clean the wax, the assemblies are immersed, or “invested,” into a liquid ceramic slurry, and then into a bed of extremely fine sand to form a shell. The first critical layers are often applied by hand. Between each layer the ceramic is allowed to dry. The later, heavier layers are often applied by automated equipment or special shell building robots. Enough layers must be applied to build a shell strong enough to withstand subsequent operations. After the shell is completely dry, the wax is melted out in a high pressure steam autoclave, leaving a hollow void within the mold, which exactly matches the shape of the assembly.
Prior to casting, the shells are fired in an oven where intense heat burns out any remaining wax residue and prepares the mold for the molten metal. In the conventional gravity pouring method, metal is poured into the shell through a funnel shaped pour cup and flows by gravity down the sprue channel, through the gates and into the part cavities. As the metal cools, the parts, gates, sprue and pouring cup become one solid casting.
After the casting has cooled, the ceramic shell is broken off and the parts are cut from the sprue using a high speed friction saw. After minor finishing operations, the castings,which are identical in configuration to the wax patterns which shaped them, are ready for certification and shipment to the customer.