Wax loss casting, now known as investment casting, is a casting process with little or no cutting. It is an excellent process technology in the casting industry, and it is widely used. It is not only applicable to the casting of various types and alloys, but also produces castings with higher dimensional accuracy and surface quality than other casting methods. Even the complex, 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 civilization, China is one of the countries that used this technology earlier. As early as hundreds of BC, the working people in ancient China created this wax loss casting technology to cast bell tripods and utensils with various fine patterns and characters, such as the zunpan of the tomb of Marquis Yi in the spring and Autumn period. The base of the zunpan of the tomb of marquis Zeng Yi is composed of several intertwined dragons, which are connected end-to-end and staggered up and down to form hollow multi-layer moire patterns in the middle. These patterns are difficult to manufacture by ordinary casting process, while the wax loss casting process can make use of the characteristics that paraffin has no strength and is easy to carve. With ordinary tools, you can carve paraffin crafts that are the same as the zunpan of marquis Zeng Yi’s tomb, Then add the pouring system, coating, dewaxing and pouring, and you can get the exquisite zunpan of Marquis Yi’s tomb.
The practical application of modern investment casting method in industrial production was in the 1940s. 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 the heat-resistant alloy material is difficult to machine and the shape of the parts is complex, it is impossible or difficult to manufacture by other methods. Therefore, it is necessary to find a new precision molding process. Therefore, drawing lessons from the lost wax casting handed down in ancient times, through the improvement of materials and technology, the modern investment casting method has obtained important development on the basis of ancient technology. Therefore, the development of aviation industry promotes the application of investment casting, and the continuous improvement and perfection of investment casting also creates favorable conditions for the aviation industry to further improve its performance.
China began to apply investment casting to industrial production in the 1950s and 1960s. Since then, this advanced casting technology has been greatly developed, and has been widely used in aviation, automobile, machine tool, ship, internal combustion engine, gas turbine, telecommunication instrument, weapon, medical instrument, cutting tool and other manufacturing industries. At the same time, it is also used in the manufacture of Arts and crafts.
The so-called investment casting process is simply to make a fusible model (referred to as investment model or model) from fusible materials (such as wax or plastic), coat it with several layers of special fire-resistant coating, dry and harden it to form an integral mold shell, then melt the model from the mold shell with steam or hot water, and then place the mold shell in a sand box and fill dry sand around it for modeling, Finally, put the mold into the roasting furnace for high-temperature roasting (for example, when using high-strength mold shell, the mold shell after demoulding can be directly roasted without modeling). After roasting, the mold or mold shell is poured with molten metal to obtain the casting.
The dimensional accuracy of investment castings 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 affecting the dimensional accuracy of castings, such as shrinkage of mold, deformation of investment mold, linear change of mold shell during heating and cooling, shrinkage rate of alloy and deformation of castings during solidification, although the dimensional accuracy of ordinary investment castings is high, However, its consistency still needs to be improved (the dimensional consistency of castings with medium and high temperature wax is much improved).
When pressing the investment mold, the pressure mold with high cavity surface finish is adopted, so the surface finish of the investment mold is also relatively high. In addition, the mold shell is made by coating the refractory coating prepared by high temperature resistant special binder and refractory materials on the investment mold, and the surface finish of the mold cavity in direct contact with the molten metal is high. Therefore, the surface finish of investment castings is higher than that of ordinary castings, generally up to RA 1.6~3.2 μ m.
The advantage is that there is no need to leave a little machining allowance on the casting surface, and even the maximum machining allowance can be reserved on the casting surface. It can be seen that the investment casting method can greatly save machine tool equipment and processing hours, and greatly save metal raw materials.
Another advantage of investment casting method is that it can cast complex castings of various alloys, especially superalloy castings. For example, the streamlined outline and cooling inner cavity of the blade of jet engine can hardly be formed by machining process. The investment casting process can not only achieve mass production and ensure the consistency of castings, but also avoid the stress concentration of residual knife marks after machining.