What is investment casting?

2021-06-04 by No Comments

Lost wax casting is now called investment casting. It is a casting process with little or no cutting. It is an excellent process technology in the foundry industry and has a wide range of applications. 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, and even complex, high-temperature resistant, and difficult-to-process castings that are difficult to 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 earliest countries to use this technology. As far back as hundreds of BC, the ancient working people of our country created this lost wax casting technology, which was used to  cast various fine patterns and characters. Zhong Ding, utensils and other products, such as the tomb of Zeng Hou Yi in the Spring and Autumn Period. The base of the tomb of Zenghou Yi’s tomb is composed of a number of intertwined dragons, which are connected end to end and staggered up and down to form a hollowed-out multilayer moiré pattern. These patterns are difficult to produce by ordinary casting techniques, but are cast by the lost wax method. The craftsmanship can take advantage of paraffin wax’s lack of strength and ease of engraving. Ordinary tools can be used to engrave the same paraffin craftwork as the tomb of Zenghou Yi’s tomb, and then add a pouring system, coating, dewaxing, and pouring. You can get the exquisite tomb plate of Zeng Hou Yi.

The practical application of modern investment die 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 parts are complex in shape, it is impossible or difficult to manufacture by other methods. Therefore, it is necessary to find a new precision molding process. The improvement of modern investment casting methods has achieved 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.

my country began to apply investment casting to industrial production in the 1950s and 1960s. Since then, 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 equipment, weapons, medical equipment, and knives, and other manufacturing industries. Manufacturing of fine art.

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 or model), apply several layers of special refractory coatings on it, and then dry and harden it. After forming an integral shell, use steam or hot water to melt the model from the shell, then put the shell in the sand box, fill the mold with dry sand around it, and finally put the mold in the roasting furnace through high temperature Roasting (if a high-strength mold is used, the mold shell after demolding can be directly fired without modeling). After the mold or mold is fired, molten metal is poured into it to obtain a  casting.

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 complicated process of investment casting, there are many factors that affect the dimensional accuracy of castings, such as molds. The shrinkage of the material, the deformation of the investment mold, the linear change of the shell in the heating and cooling process, the shrinkage of the alloy and the deformation of the casting during the solidification process, etc., so the dimensional accuracy of ordinary investment castings is high, but its Consistency still needs to be improved (the size consistency of castings with medium and high temperature waxes should be improved a lot).

When pressing the investment mold, the pressure type with high surface smoothness of the cavity is used, so the surface smoothness of the investment mold is also relatively high. In addition, the shell is made of a refractory coating made of a special high-temperature resistant adhesive and refractory material, which is coated on the investment mold, and the inner surface of the cavity that is in direct contact with the molten metal has a high smoothness. Therefore, the surface finish of investment castings is higher than that of general die castings, generally reaching Ra.1.6~3.2μm.

The biggest advantage of investment casting is that because investment castings have high dimensional accuracy and surface finish, machining work can be reduced, but only a small amount of machining allowance can be left on the parts with higher requirements, and even some castings only have Leave a grinding and polishing allowance, and it can be used without mechanical processing. It can be seen that the use of investment casting method 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 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.