What is investment casting

Update:11 May 2020
Summary:

Investment casting is a manufacturing process in which […]

Investment casting is a manufacturing process in which wax molds are coated with refractory ceramic materials. After the ceramic material is hardened, its internal geometry is the shape of the casting. The wax melts and pours the molten metal into the cavity where the wax mold is located. The metal solidifies in a ceramic mold, and then the metal casting is broken. This manufacturing technique is also called lost wax process. Investment casting was developed 5,500 years ago and can be traced back to ancient Egypt and China. Parts manufactured industrially through this process include dental fixtures, gears, cams, ratchets, jewelry, turbine blades, mechanical parts and other parts with complex geometries.

The first step in investment casting is to make wax molds for this process. The pattern of this process can also be made of plastic; however, since wax is easy to melt and can be reused, it is usually made of wax. Since the pattern is destroyed during processing, one is required for each casting. When producing any number of parts, a mold for making patterns will be required. Similar to molds that can be used in expanded polystyrene casting processes to produce foamed polystyrene patterns, molds that produce wax patterns can be cast or machined. The size of the master mold must be carefully calculated. The shrinkage of the wax, the shrinkage of the ceramic material put on the wax mold and the shrinkage of the metal casting must be considered. To obtain the correct size may require trial and error, so these molds may be expensive.

Since there is no need to open the mold, castings with very complex geometries can be manufactured. Multiple wax molds can be combined together for one-time casting. Or, in general, many wax molds can be joined together and poured together to produce many castings in a single process. This is done by attaching a wax mold to the wax rod, which serves as the center gate. The ceramic pouring cup is connected to the end of the rod. This arrangement is called a tree, which means that the casting pattern on the central runner beam is similar to the branches on the tree.

The metal mold is then immersed in a refractory slurry, the composition of which includes extremely fine silica, water and binder. A ceramic layer is obtained on the surface of the pattern. The pattern is then repeatedly immersed in the slurry to increase the thickness of the ceramic coating. In some cases, the pattern can be placed in the flask, and then the ceramic slurry is poured on the flask.


Refractory mud invested in wax

Once the refractory coating on the pattern is thick enough, it can be dried in air to harden.

Dry refractory coating in air

The next step in this manufacturing process is the key to precision casting. Turn the hardened ceramic mold upside down and heat to a temperature of approximately 200F-375F (90C-175C). This causes the wax to flow out of the mold, leaving a cavity for metal castings.

Wax melted by investment

Then heat the ceramic mold to about 1000F-2000F (550C-1100C). This will further strengthen the mold, eliminate residual wax or contaminants, and drain water from the mold material. Then pour the metal casting while the mold is still hot. Casting when the mold is hot allows liquid metal to easily flow through the mold cavity and fill the thin and thin parts. Pouring metal castings into a hot mold can also provide better dimensional accuracy because the mold and the casting shrink together when cooled.

Investment casting mold heated before casting

After pouring the molten metal into the mold, as the solidification process progresses, the casting is solidified.

Solidification of investment casting

The final step of the manufacturing process involves breaking the ceramic mold from investment casting and cutting the parts from the tree.

Decomposition investment casting