During pouring and molten metal solidification, due to the high temperature, the surface of the casting will be oxidized, and the oxide layer will be uneven. In addition, the metal oxide on the surface of the casting may interact with the oxide in the mold shell to promote the uneven shedding of the surface of the casting. Increase the roughness of the casting surface.
The cooling of the casting under a protective atmosphere is an important part of obtaining a high-quality surface. For example, after the casting is poured, it is cooled under inert gas or a reducing protective atmosphere to combine the prevention of oxidation and the prevention of surface decarburization, until the surface of the casting does not reach the oxidation temperature. The surface of the casting with good surface protection is silver gray, silver white or colored oxide color after being removed from the shell.

Cleaning also has a great influence on the surface roughness of investment castings. The as-cast surface roughness of investment castings is inherently low, so sandblasting or water sand cleaning should be used to clean the surface. The sand blasting method is better than the shot blasting to clean the surface roughness of the castings by more than 2 levels. Shot blasting often damages the as-cast surface.
For stainless steel investment castings with high hardness, shot blasting can also be used to improve efficiency, but the shot blasting diameter should be less than 0.3mm.
In short, the surface quality of castings is an extremely important quality indicator for investment castings, and surface quality includes two related but not identical concepts of surface roughness and surface defects. Surface defect refers to the macroscopic defect with large local size of the casting. Surface roughness refers to the uniform microscopic unevenness of the casting surface. Only by preventing serious defects on the surface of castings can castings with low surface roughness be produced, and low surface roughness of castings can achieve higher dimensional accuracy. The factors that affect the surface roughness of investment castings are well-known, and a complete set of strict process measures from raw materials, molding to cleaning can be implemented to reduce the surface roughness of castings.