Investment casting or ‘lost wax’ casting is one of the oldest manufacturing processes used since ancient times. Also referred to as precision castings, it uses an expendable-mold process to lend exceptional accuracy for all sizes and materials to achieve extreme complex forms with exceptional surface qualities. Its basic procedure needs a metal die, wax, ceramic slurry, furnace, molten metal and machines as required for process like sandblasting, grinding etc. It process steps are as follows –
- Pattern creation – First the wax patterns are injection molded into a metal die to form one piece. For internal features, cores may be used. Number of these patterns is attached to a central wax gating system to give way to a tree-like assembly. This gating system constitutes the channel for the molten metal to flow inside the mold cavity.
- Mold creation – The formed ‘pattern-tree’ is now dipped into slurry of fine ceramic particles. Now it is coated with more coarse particle sand and then left to dry. A ceramic shell is formed around the pattern and gating system. The procedure is repeated several time till the shell acquires desired thickness which can easily without the molten metal when it will face it. Now the shell is put inside the oven and wax is melted out to get the ceramic shell or the ‘one-piece mold’.
- Pouring – In the preheated mold of about 1000°C (1832°F), the molten metal is poured using a ladle through the gating system till the mold cavity gets filled. For pouring, normally force of gravity is used but other methods like vacuum or pressure can also be used.
- Cooling – The molten metal is allowed to cool once it has filled the mold completely till it solidifies as the solid casting. Cooling time may vary depending on material used and thickness of part as well as mold.
- Casting removal – Post complete cooling, the mold is broken to remove the casting. Ceramic molds can be broken using water jets but other methods may also be utilized. Now the parts are separated from the gating system by processes like sawing or cold breaking via liquid nitrogen.
To smoothen parts, finishing operation like grinding or sandblasting and even heat treatment is used.