Casting process refers to the process of pouring high temperature liquid metal into the mold to obtain a certain shape of casting after cooling and solidification. The spiral bevel gear studied in this paper belongs to small thin-walled precision steel casting. The casting material 20CrMoH belongs to low alloy steel, which has high liquidus temperature, poor liquid metal fluidity, obvious oxidation tendency, large shrinkage of casting body, and is prone to casting defects such as shrinkage porosity, insufficient pouring and sand sticking during solidification.
At present, the casting process in the market can be divided into sand casting, pressure casting, EPC, coated sand casting and other types.
Sand casting is widely used because of its low cost and easy to obtain mold materials and strong casting adaptability. The number of castings produced by this process accounts for more than 80% of the total amount every year. However, the surface precision of the castings produced by the traditional sand casting process is not high, and the casting defects such as sand inclusion, porosity, shrinkage porosity and so on are easy to appear.
Pressure casting is a method of rapidly forming castings by pressing molten metal liquid into mold cavity at high speed. Compared with other casting processes, this method has the highest production efficiency and is suitable for mass production of parts. However, this method is easy to involve gas in the process of high pressure filling metal and is only suitable for the casting of low melting point metal.
Lost foam casting is based on the size of parts, made of plastic foam, and then coated with fire-resistant material on the surface. After pouring, the high temperature molten metal will melt the plastic foam mold and eventually fill the cavity and solidify. However, if the low alloy steel is used in EPC process, it is easy to have the effect of carburization, which will lead to the increase of carbon content in the casting and affect the forming effect of the casting.
The above casting processes are not suitable for the production of precision steel castings for driven spiral bevel gears.
Resin coated sand casting is a new type of precision casting process. In this method, the resin coated sand is first used to cover the metal mold which is preheated to a certain temperature and then heated to make a thin shell with certain strength and rigidity; then the thin shell is put into the sand box as a casting mold and compacted; finally, the metal is poured and the thin shell is broken after solidification to obtain the precision casting with smooth surface. This method has a series of advantages, such as high production efficiency, less molding sand, less production dust, low surface roughness of castings, ideal size accuracy, etc. it is very suitable for large-scale production of spiral bevel gear precision castings. Therefore, this paper decided to use coated sand casting as the casting process of driven spiral bevel gear.