Precision casting products are precise, complex, and close to the final shape of the parts. They can be directly used without or with minimal processing, making precision casting an advanced process of near net shape formation, also known as investment casting. Generally, suitable investment casting materials (such as wax) are used to make the molds. The process involves repeatedly dipping the mold in refractory coating and sprinkling with refractory sand, then hardening the shell, drying it, and melting the internal mold to obtain the cavity. The shell is then fired for sufficient strength and to burn away residual investment casting materials, after which the required metal material is poured, solidified and cooled, and the shell is removed to obtain the high-precision precision castings.
The characteristics of precision casting are: it is widely adaptable and not limited by the size, thickness, or complexity of the casting. The production process is flexible, and a wide range of materials can be used, including waste metal and recycled resources. Compared with forging, precision casting consumes less energy. The castings have a certain dimensional accuracy and small machining allowances, which saves processing time and metal materials. The comprehensive economic performance is good and the energy, material consumption, and cost are low.
Precision castings can be applied to all industrial sectors, especially those that require finer details such as electronics, petroleum, chemical industry, energy, transportation, light industry, textile, pharmaceuticals, medical equipment, pumps, and valves etc.
Due to the significantly larger shrinkage of stainless steel in precision casting than cast iron, measures such as sprue, chill, and supplement are mostly used in the casting process to achieve sequential solidification to avoid shrinkage and looseness.
To avoid shrinkage, looseness, porosity, and cracking of stainless steel castings, it is necessary to ensure a uniform wall thickness, avoid sharp corners and right-angle structures, add sawdust in the molding sand, add coke in the core, and use hollow cores and oil sand cores to improve the collapsibility and permeability of the sand mold or core.
Due to the poor fluidity of the molten steel, to prevent cold shut and incomplete casting of the steel casting, the wall thickness of steel casting should not be less than 8mm, and dry casting or hot casting should be used. The pouring temperature should be appropriately increased, generally ranging from 1520 to 1600℃. A higher pouring temperature can improve the fluidity of the molten steel because the overheating and liquid-holding time of the molten steel are long. However, a high pouring temperature can cause drawbacks such as coarse grains, thermal cracks, porosity, and sand sticking.
Therefore, for small, thin-walled, and irregularly shaped precision castings, the pouring temperature is about the melting point temperature of steel + 150℃. The structure of the pouring system should be kept simple, and the sectional size should be larger than that of cast iron. For large, thick-walled castings, the pouring temperature should be about 100℃ higher than the melting point temperature.