What is ceramic shell investment casting?

Ceramic Shell Investment Casting (CSIC) is one of the near net shape casting technologies. The process is based on expendable wax patterns for producing joint-less moulds that are required for near net shape castings. 

Before we start the discussion on the ceramic shell investment casting process,it is desirable to understand in brief, the process sequence and details of investment casting process,already discussed in the earlier lecture. 

The main difference between investment casting and ceramic shell investment casting is that, in the former process, before dewaxing the wax pattern, it is immersed in a refractory aggregate. Whereas in the ceramic shell investment casting, a ceramic shell gets built around the tree assembly through repeated dipping of the pattern into slurry (refractory material such as zircon with binder). After getting the required thickness of cross section, the tree assembly is de-waxed. The shell obtained is further immersed in a refractory coating and the metal is poured into it. 

In this process, a wax pattern/assembly is first dipped into a ceramic slurry bath for its primary coating. Thereafter, the pattern is withdrawn from the slurry and is manipulated to drain of the excess slurry to produce a uniform coating layer. The wet layer further stuccoes through sprinkling the relatively coarse ceramic particles on it or by immersing it into such fluidized bed of particles.The ceramic coating is built by successive dipping and stuccoing process. This procedure is further repeated till the shell thickness as desired is obtained.Upon completion,the entire assembly is placed into an autoclave or flash fire furnace at a high temperature. In-order to burnout out any residual wax, the shell is heated to about 982℃ which helps to develop a bonding of high-temperature in shell. Such molds are stored for future use wherein they are preheated for removing the moisture content from it and then, molten metal can be poured into it. 

Sequence of Producing Ceramic Shell Investment Casting

The process sequence of Ceramic shell investment casting is given below and some of steps are shown  

Steps: 

1. Manufacturing of the master pattern of wax through the master dies. 
2. Preparation of wax blend and injecting it into the die. 
3. Manufacture of wax pattern and assembly of wax pattern 
4. Investment of wax with slurry (coating the slurry) 
5. Drying of shell thickness (stuccoing) 
6. De-waxing of raw moulds followed by heating and baking of the shells 
7. Pouring of moulds with molten metal 
8. Once the metal is solidifed, the shells are removed. 
9. Cutting off the gates / risers (fettling) followed by finishing operations. 

Advantages: 
 
*Complex shapes that are difficult to produce by other casting methods are very easily possible to be produced by this method. 
*Thin cross sections and intricacies can be made by this process. 
*Finish machining is considerably reduced or eliminated on the castings made by this process, making it economical in cost. 
*The process has no metallurgical limitations. 
*This process produces castings with excellent surface finish. 
 
 Disadvantages: 
 
*Expensive process due to the cost of ceramics and pattern (wax cost). 
*As the shells are delicate, the process is limited by the size and mass obtained. 
*Making intricate and high quality pattern increases the process costs. 
 
Applications: 
 
*Aircraft: Turbine blades; carburetor and fuel-pump parts; cams; jet nozzles; special alloy valves.
*Chemical Industries: Impellors; pipe fittings; evaporators; mixers 
*Tool and Die: Milling cutters; lathe bits; forming dies; stamping dies; permanent molds etc. 
*General and Industrial applications: cloth cutters, sewing machine parts; welding torches; cutter, spray nozzles; metal pumps; etc.