Lost wax casting is now called investment casting. It is a kind of casting technology with little or no cutting. It is an excellent technology in the casting industry, and its application is very extensive. It is not only suitable for casting of various types and alloys, but also produces castings with higher dimensional accuracy and surface quality than other casting methods. Even the complicated, high temperature resistant and difficult to process castings that are difficult to be cast by other casting methods can be cast by investment casting.
Investment casting is developed on the basis of ancient wax casting. As an ancient civilized country, China is one of the countries that used this technology earlier. As far back as hundreds of years BC, the ancient working people of our country created this lost wax casting technology, which was used to cast the bell tripod and utensils with various fine patterns and characters, such as the tomb and zunpan of Zeng Hou Yi in the spring and Autumn period. The base of the zunpan of the tomb of Marquis Yi of Zeng is a series of intertwined dragons. They are connected end to end and interlaced up and down to form a hollow multi-layer cloud pattern in the middle. These patterns are difficult to be made by ordinary casting technology. However, the wax loss casting technology can take advantage of the fact that paraffin is not strong and easy to carve. With ordinary tools, the zunpan I of the tomb of Marquis Yi of Zeng can be carved with the desired one Sample of paraffin based crafts, and then add pouring system, coating, dewaxing, pouring, you can get a beautiful tomb of Marquis Yi.
It was in 1940s that modern investment casting method was applied in industrial production. At that time, the development of Aerojet engine required the manufacture of heat-resistant alloy parts with complex shape, accurate size and smooth surface, such as blades, impellers and nozzles. Because it is difficult to machine heat-resistant alloy materials and the shape of parts is complex, so it is impossible or difficult to use other methods to manufacture them. Therefore, it is necessary to find a new and precise forming process. Therefore, we can learn from the lost wax casting handed down in ancient times, and through the improvement of materials and process, the modern investment casting method obtains important development on the basis of ancient technology. Therefore, the development of aviation industry promotes the application of investment casting, and the continuous improvement of investment casting also creates favorable conditions for aviation industry to further improve performance.
China began to apply investment casting to industrial production in the 1950s and 1960s. After that, the first-in casting technology has been greatly developed, which has been widely used in aviation, automobile, machine tools, ships, internal combustion engines, gas turbines, telecommunication instruments, weapons, medical devices, cutting tools and other manufacturing industries, and also used in the manufacturing of Arts and crafts.
The so-called investment casting process, simply speaking, is to make a fusible model (investment model or model) with fusible materials (such as wax or plastic), apply several layers of special fire-resistant coating on it, after drying and hardening to form an integral shell, then melt the model from the shell with steam or warm water, and then place the shell in a sand box and fill it with dry sand, Finally, the mold is put into the permeating baking furnace for high-temperature baking (for example, when using high-strength mold shell, the mold shell after demoulding can be baked directly without modeling). After baking, the mold or mold shell is poured with molten metal to obtain the casting.
The dimensional accuracy of investment casting is high, generally up to ct4-6 (ct10-13 for sand casting and ct5-7 for die casting). Of course, due to the complex process of investment casting, there are many factors that affect the dimensional accuracy of the casting, such as the shrinkage of the mold material, the deformation of the investment mold, the linear change of the shell during heating and the cooling of the core ash, the shrinkage rate of the alloy and the deformation of the casting during solidification, etc Therefore, although the dimensional accuracy of common investment castings is high, its consistency still needs to be improved (the dimensional consistency of castings using medium and high temperature wax materials needs to be improved a lot).
When pressing the melting die, the pressing die with high surface finish of the cavity is used, so the surface finish of the melting die is also relatively high. In addition, the shell is made by coating the refractory coating made of special high temperature resistant binder and refractory on the investment mold, and the surface finish of the cavity directly contacting with the molten metal is high. Therefore, the surface finish of investment casting is higher than that of general casting, generally ra.1.6 ~ 3.2 μ M.
The biggest advantage of investment casting is that due to its high dimensional accuracy and surface finish, it can reduce the machining work. Only a little machining allowance can be left for the parts with high requirements, and even some castings only have grinding and polishing allowance, which can be used without machining. It can be seen that the investment casting method can save a lot of machine equipment and working hours, and greatly save metal raw materials.
Another advantage of investment casting is that it can cast complex castings of various alloys, especially superalloy castings. For example, the streamlined profile and cooling inner cavity of the blade of jet engine can hardly be formed by machining technology. Using investment casting technology to produce can not only achieve batch production, ensure the consistency of castings, but also avoid the stress concentration of residual knife marks after machining.