The first step is to make a tooling or a die. This is made in aluminum or steel employing various techniques. The cavity is an exactly opposite replica of the part to be made with allowance added for shrinkage. You can call it a “negative”.
Next step involves injecting wax into the die. This is done in controlled atmosphere by way of specially designed wax injection press which holds wax in semi liquid condition and injects wax under high pressure. The resultant part is called pattern .for each casting ,you need one wax pattern. You can call it a “positive”.
Making the “trees”
Unlike sand castings, which are large in size, an investment casting is typically smaller. Hence it is neither possible nor economical to pour individual casting. Hence a number of wax patterns are assembled around a common gating system, also made of wax. The assembled shape of a tree and hence named as such!
Now we start the process of making a shell or mould. This is done by dipping the tree in slurry consisting of special ceramics and binders and then spraying it with refectory materials. A number of coats are given with a period of 6 to 8 hour between to let the coat dry. The whole process can take up to five days though can be speeded up if required by using special chemicals. Once the shell is made, it is usually 8 to 10 mm thick. This whole process is also done in a controlled atmosphere.
De-waxing The Shell
Now that the shell is made, we no longer require the wax. Hence the wax is then taken out by using either an autoclave or by “hot wax de-waxing” where the shell is dipped in a bath containing molten wax at high temperatures which melts the wax inside the shells and takes it out. You now get a shell with cavities which are again “negative”.
The shells are fragile as far as their capacity to with stand metal temperature and weight is concerned. Hence they are cured in a shell baking furnace, while simultaneously liquid metal is prepared in a melting furnace. Once the metal is ready, shells are poured while they are red hot.
Once the shells are cooled, the ceramic material is knocked out and castings are separated from the gating system. Finally, you get a “positive”.
Fettling & Finishing
The castings are then fitted, heat treated, blasted, or treated with required surface finish and are offered for final quality checks.
Final Quality Checks
The casting are then subjected to various quality checks like chemical testing, physical properties testing, dimensional checking, NDT and then shipped.