In investment casting process, the fusibility model is made of fusible material (e.g. wax or plastics), which is coated with some special refractory coatings. After drying and hardening, a whole shell is formed, then the model is melted out from the shell by steam or hot water, and then the shell is placed in the sand box. After filling with dry sand around the mold, the mold is finally put into the calciner and roasted at high temperature (for example, when the high strength shell is used, the mold shell after demoulding can be directly roasted instead of moulding). The castings are obtained by pouring molten metal into the mold or shell after roasting.
The dimension accuracy of investment casting is high, generally up to CT4-6 (sand casting is CT10-13, die casting is CT5-7). Of course, due to the complex process of investment casting, there are many factors affecting the dimension accuracy of investment casting, such as shrinkage of die material, deformation of investment mould, linear change of shell during heating and cooling, shrinkage rate of alloy and so on. In the solidification process, the deformation of castings and so on, so the dimensional accuracy of ordinary investment casting is higher, but its consistency still needs to be improved (the size consistency of castings with medium and high temperature wax is much higher).
When pressing the melting die, the high surface finish of the cavity is used, so the surface finish of the melting die is also higher. In addition, the shell is made of a refractory coating made of a special high temperature binder and refractory material, which is coated on the melting die. The inner surface of the cavity directly contacted with the molten metal has a high smoothness. Therefore, the surface finish of investment castings is higher than that of ordinary castings, generally up to Ra.1.6-3.2 um.