When casting with a wax, the investment casting is also called "lost wax casting". Investment casting usually refers to forming a pattern in a fusible material, coating a plurality of layers of refractory material on the surface of the pattern to form a shell, and then melting the pattern to discharge the shell, thereby obtaining a mold having no parting surface, and after being baked at a high temperature A casting scheme that can be filled with sand. Investment molding is often referred to as "lost wax casting" because the pattern is widely manufactured using waxy materials.
The alloys that can be produced by investment casting are carbon steel, alloy steel, heat resistant alloy, stainless steel, precision alloy, permanent magnet alloy, bearing alloy, copper alloy, aluminum alloy, titanium alloy and ductile iron etc.
The shape of the investment casting is generally complicated, the minimum diameter of the castable hole on the casting can be up to 0.5 mm, and the minimum wall thickness of the casting is 0.3 mm. In the production, some parts that were originally composed of several parts can be directly cast by investment casting by changing the structure of the parts and designed as integral parts, so as to save processing time and metal material consumption, and make the part structure more reasonable.
The weight of investment castings is mostly tens of gram (from a few grams to a dozen kilograms, generally not more than 25 kilograms), and castings that are too heavy are more troublesome to produce by investment casting. The investment casting process is complicated, difficult to control, and expensive to use and consume. Therefore, it is suitable for producing small parts with complicated shapes, high precision requirements, or difficult to perform other processing, such as blades of turbine engines.
Investment castings have high dimensional accuracy, generally up to CT4-7 (water glass lost wax casting is CT10-10 sand casting for CT10~13). Of course, due to the complicated process of investment casting, there are many factors affecting the dimensional accuracy of castings. For example, shrinkage of the molding material, deformation of the investment mold, variation in the amount of the shell during heating and cooling, shrinkage of the alloy, and deformation of the casting during solidification, etc., so the dimensional accuracy of the ordinary investment casting is high. However, the consistency still needs to be improved (the dimensional consistency of castings using medium and high temperature waxes is much improved).
When the investment mold is pressed, the molding having a high surface finish of the cavity is used, and therefore, the surface finish of the investment mold is also relatively high. In addition, the shell is made of a refractory paint made of a special binder with high temperature resistance and a refractory material, and is coated on the investment mold, and the inner surface of the cavity which is in direct contact with the molten metal has a high surface finish. Therefore, the surface finish of the investment casting is higher than that of the general casting, and generally reaches Ra.3.2~12.5μm.
The biggest advantage of investment casting is that because of the high dimensional accuracy and surface finish of the investment casting, it can reduce the machining work, but only a small amount of machining allowance is required on the parts, even some castings only Leave the grinding and polishing allowance and use it without machining. It can be seen that the 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 process is that it can cast complex castings of various alloys, particularly casting high temperature alloy castings. For example, the blade of a jet engine, its streamlined profile and cooling cavity, can hardly be formed by machining. Production by investment casting process can not only achieve mass production, but also ensure the consistency of castings.