Precision Casting

The biggest advantage of precision casting is that because investment castings have high dimensional accuracy and surface finish, it can reduce machining work, just leave a little machining allowance on the parts with higher requirements, even some castings Only the grinding and polishing allowance is left, and it can be used without mechanical processing. It can be seen that the use of investment precision casting methods can save a lot of machine tool equipment and processing man-hours, and greatly save metal raw materials.

Another advantage of the investment casting method is that it can precision cast complex castings of various alloys, especially high-temperature alloy castings. For example, the blade of a jet engine, its streamlined profile and cooling cavity, can hardly be formed by machining technology. Production with investment precision casting technology can not only achieve mass production, ensure the consistency of castings, but also avoid the stress concentration of residual knife marks after machining. Now known as investment casting, it is a precision casting process with little or no cutting. It is an excellent process technology in the precision casting industry and has a wide range of applications. It is not only suitable for precision casting of various types and alloys, but also the dimensional accuracy and surface quality of the produced castings are higher than other precision casting methods, and even other precision casting methods are difficult to cast, high temperature resistance, and difficult to process. All castings can be cast by investment casting.

Investment casting is developed on the basis of ancient wax casting. As an ancient civilization, China is one of the earliest countries to use this technology. As far back as hundreds of BC, the ancient working people of our country created this lost wax precision casting technology for precision casting with various fine patterns. Products such as clocks and dings and utensils with writing, such as the tomb of Zenghou Yi in the Spring and Autumn Period. The base of the tomb of Zenghou Yi is composed of multiple dragons intertwined with each other. They are connected end to end and staggered up and down to form a multi-layer moire pattern hollowed out in the middle. These patterns are difficult to produce by ordinary precision casting techniques, but the lost wax method is used. The precision casting process can take advantage of the fact that paraffin wax has no strength and is easy to engrave. You can use ordinary tools to engrave the same paraffin craftwork as the tomb of Zenghou Yi tomb, and then add a pouring system, coating, dewaxing, Pouring, you can get the exquisite tomb plate of Zenghou Yi.

The practical application of modern investment casting methods in industrial production was in the 1940s. The development of aviation jet engines at that time required the manufacture of heat-resistant alloy parts such as blades, impellers, nozzles, etc., with complex shapes, accurate dimensions, and smooth surfaces. Because heat-resistant alloy materials are difficult to machine and the shapes of parts are so complex that they cannot or are difficult to manufacture by other methods, it is necessary to find a new precision molding process. With the improvement of craftsmanship, modern investment casting methods have gained important development on the basis of ancient craftsmanship. Therefore, the development of the aviation industry has promoted the application of investment casting, and the continuous improvement and perfection of investment casting has also created favorable conditions for the aviation industry to further improve its performance.

However, if the pouring temperature is too high, it will cause defects such as coarse grains, thermal cracking, pores and sticky sand. Therefore, the casting temperature of generally small, thin-walled and messy-shaped castings is about the melting point temperature of steel + 150 °C; the pouring temperature of large and thick-walled castings is about 100 °C higher than its melting point. Considering the poor fluidity of molten steel, in order to prevent cold insulation and pouring defects of stainless steel castings, the thickness of stainless steel castings should not be less than 8mm; the structural requirements of the casting system are simple, and the cross-sectional specifications are larger than those of cast iron.

Dry casting or hot casting is used; the casting temperature is appropriately increased, generally 1520-1600 ° C. Considering the high casting temperature, the high degree of superheat of molten steel, and the long liquid state, the fluidity can be improved. Precision casting refers to the general term for the production process of standard castings. Compared with the traditional sand casting production process, the casting size of precision casting is more accurate and the surface finish is better.

It includes: investment casting, ceramic casting, sand casting, pressure casting, lost wax casting. Precision casting is also known as lost wax casting. Its products are fine, messy, and close to the shape of the parts. It can be used directly without processing or with little processing. It is a near-net-shaped process. However, if the pouring temperature is too high, it will cause defects such as coarse and hard crystals, thermal cracking, pores and sticky sand.

Therefore, the casting temperature of general small, thin-walled and chaotic castings is about the melting point temperature of steel + 150 °C; the casting temperature of large and thick-walled castings is about 100 °C higher than its melting point. Since the shrinkage of cast steel is much greater than that of cast iron, in order to prevent the casting from shrinking holes and shrinkage porosity, methods such as riser and cold iron and subsidies are mostly used in the casting process to achieve sequential solidification.

In addition, in order to prevent the occurrence of shrinkage holes, shrinkage porosity, pores and cracks in steel castings, the wall thickness should be uniform, sharp and right-angle structures should be prevented, sawdust should be added to the sand for casting, coke should be added to the core, and Use hollow cores and oil sand cores to improve the concession and air permeability of sand or cores.