Investment casting

Lost wax casting is now called investment precision casting, is a kind of less cutting or no cutting casting process, is an excellent process technology in the casting industry, its application is very extensive. It is not only suitable for casting of various types and alloys, but also produces castings with high dimensional accuracy and surface quality than other casting methods. Even castings that are difficult to be cast by other casting methods are complex, high temperature resistant and not easy to process can be cast by investment precision 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 far back as hundreds of years BC, the working people of ancient China created this lost wax casting technology, which was used to cast products such as bell tripod and utensils with various fine patterns and characters, such as the tomb of Zenghou Yi in the spring and autumn. The base of Zeng Hou yi's tomb statue plate is a plurality of intertwined dragons, which 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 be manufactured by ordinary casting technology, while the lost wax casting technology can take advantage of the characteristics of paraffin wax that it has no strength and is easy to carve, and ordinary tools can be used to carve handicrafts made of paraffin wax the same as the Hou Zeng yi's tomb plate, and then additional pouring system, coating, dewaxing, pouring, you can get the exquisite Zenghou Yi tomb plate.

Modern investment casting methods were actually used in industrial production in the 1940 s. At that time, the development of aviation jet engines required the manufacture of heat-resistant alloy parts with complex shapes, precise dimensions and smooth surfaces, such as blades, impellers, nozzles, etc. Due to the heat-resistant alloy material is difficult to machine, the shape of the parts is complex, so that can not or difficult to use other methods of manufacturing, therefore, the need to find a new precision molding process, so learn from the ancient lost wax casting, after the improvement of materials and technology, modern investment casting method on the basis of the ancient process to obtain important development. 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.

China began to apply investment casting to industrial production in the 5. and sixties of the last century. Since then, this advanced casting process has been greatly developed, and has been widely used in the manufacturing industries such as aviation, automobiles, machine tools, ships, internal combustion engines, gas turbines, telecommunications instruments, weapons, medical devices and cutting tools, and is also used in the manufacture of arts and crafts.

The so-called investment casting process is simply to use fusible materials (such as wax or plastic) to make fusible models (investment or models for short), coat several layers of special refractory coatings on them, dry and harden to form an integral shell, then melt the model from the shell with steam or hot water, then place the shell in a sand box and fill dry sand around it for modeling, finally, the mold is put into the baking furnace after high-temperature roasting (such as the use of high-strength shell, can not be molded and the mold shell after direct roasting), mold or shell after roasting, in which pouring molten metal castings.

The dimensional accuracy of investment castings is relatively high, generally up to CT4 ~ 6 (CT10~13 for sand casting and CT5 ~ 7 for die casting). Of course, due to the complex technological process of investment casting, there are many factors affecting the dimensional accuracy of castings, such as shrinkage of mold materials, deformation of investment, linear change of mold shell during heating and cooling, shrinkage rate of alloy and deformation of castings during solidification, etc, therefore, although the dimensional accuracy of ordinary investment castings is high, its consistency still needs to be improved (the dimensional consistency of castings using medium and high temperature wax materials should be improved a lot).

When pressing the investment mold, the high surface finish of the cavity is used, so the surface finish of the investment mold is also relatively high. In addition, the shell is made of a refractory coating made of a special binder and refractory material with high temperature resistance, and the inner surface of the cavity in direct contact with the molten metal is high. Therefore, the surface finish of investment castings is higher than that of general castings, generally up to Ra. 1.6~3.2 μm.

The biggest advantage of investment 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, and even some castings only leave grinding and polishing allowance, which can be used without machining. It can be seen that the use of investment casting method can save a lot of machine tool equipment and processing hours, and greatly save metal raw materials.

Another advantage of the investment casting method is that it can cast complex castings of a variety of alloys, particularly superalloy castings. Such as jet engine blades, its streamlined profile and cooling with the inner cavity, with mechanical processing technology can hardly be formed. The investment casting process can not only achieve mass production, ensure the consistency of the casting, but also avoid the stress concentration of the residual knife pattern after machining.