Investment Mold Precision Casting

Lost wax casting is now known as investment precision casting, is a less cutting or no cutting casting process, is an excellent process technology in the casting industry, its application is very wide. It is not only suitable for the casting of various types and various alloys, but also the dimensional accuracy and surface quality of the castings produced are higher than other casting methods, and even the complex, high temperature resistant, and not easy to process castings that are difficult to cast by other casting methods can be cast by investment precision casting.

Investment mold precision casting is developed on the basis of ancient wax mold casting. As an ancient civilization, China is one of the earliest countries to use this technology, as far back as hundreds of years BC, China's ancient working people created this lost wax casting technology, used to cast with a variety of fine patterns and characters of the bell triding and vessels and other products, such as the Spring and Autumn period of Zeng Hou Yi tomb. The base of the tomb of Zeng Hou Yi is a number of intertwined dragons, which are connected end to end and interleaved up and down, forming a multi-layer moire pattern hollow in the middle. These patterns are difficult to produce with ordinary casting process, but the lost wax casting process can take advantage of the characteristics of paraffin wax, which has no strength and is easy to carve. You can use ordinary tools to carve out the same paraffin material as the tomb plate of Zeng Hou Yi, and then attach the pouring system, coating, dewaxing, pouring, you can get the exquisite tomb plate of Zeng Hou Yi.

The modern investment casting method was actually applied in industrial production in the 1940s. 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 and nozzles. Because the heat-resistant alloy material is difficult to mechanical processing, the shape of the part is complex, so that it can not or is difficult to use other methods to manufacture, therefore, the need to find a new precision molding process, so learn from the ancient lost wax casting, through the improvement of materials and processes, modern investment casting method in the ancient process on the basis of 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 1950s and 1960s. Subsequently, this advanced casting process has been greatly developed, and has been widely used in aviation, automobiles, machine tools, ships, internal combustion engines, gas turbines, telecommunications instruments, weapons, medical equipment and tools and other manufacturing industries, but 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 a fusible model (referred to as investment mold or model), coated with several layers of special refractory paint, after drying and hardening to form a whole shell, and then use steam or hot water to melt the model from the shell, and then the shell is placed in a sand box, filled with dry sand around it. Finally, the cast is put into the baking furnace through high temperature roasting (such as the use of high-strength shell, you can do not need to shape and the mold after the release of the shell directly roasting), the cast or the shell after roasting, in which the molten metal is poured to obtain the casting.

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

When pressing the investment mold, a 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 high temperature resistant special binder and refractory material made of refractory coating on the investment mold, and the surface of the cavity in direct contact with the molten metal is high finish. Therefore, the surface finish of investment castings is higher than that of general castings, generally up to Ra.1.6~3.2μ m.

The biggest advantage of investment casting is that due to the high dimensional accuracy and surface finish of investment casting, machining work can be reduced, only a little processing allowance can be left on the parts with higher requirements, and even some castings only leave grinding and polishing allowance, which can be used without mechanical processing. It can be seen that the use of investment casting method can greatly save 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 various alloys, especially superalloy castings. For example, the blade of a jet engine, its streamlined profile and cooling cavity can hardly be formed by mechanical processing. The production by investment casting process can not only achieve mass production, ensure the consistency of castings, but also avoid the stress concentration of residual blade after machining.