Among the special casting methods, metal mold casting and investment casting have the longest history. The real modern sense of special casting appeared at the end of the 19th century and the first half of the 20th century, which was perfected and applied in the second half of the 20th century. Great efforts were made after the 1980s. The wave of global economic integration has a huge impact on the casting industry. There are more and more competitive means in the casting market, which not only requires higher and higher quality of the casting itself, but also requires new changes in the price, delivery period and related service requirements. Therefore, the foundry industry should adapt to the new changes, constantly improve the production process and select the appropriate casting process.
1. Investment casting
Investment casting is developed on the basis of lost wax casting in ancient times. The specific method is: using fusible materials to make the same model as the parts, coating the model with a coating composed of refractories (quartz, corundum, etc.) and high-strength adhesives (ethyl silicate or water glass), or casting with gypsum to form a certain thickness of shell, then adding hot melt loss model, the shell is baked at high temperature and then cast to obtain the casting.
The biggest advantage of investment casting method is that the castings have high dimensional accuracy (cT4 ~ 6) and surface finish (Ra1.6 ~ 3.2 μ m), only a little machining allowance is required for the parts with high requirements, so the machining work can be reduced; even some castings only have grinding and polishing allowance, which can be used without machining. It can be seen that using investment casting can save a lot of machine tools and working hours, and greatly save metal materials.
Another advantage of investment casting is that it can cast complex castings of various alloys, especially superalloy castings, such as blades of jet engines, whose streamline profile and inner cavity for cooling can hardly be formed by machining. The investment casting process can not only achieve mass production, ensure the consistency of castings, but also avoid the stress concentration of residual knife marks after machining.
2. Metal mold casting
Metal mold casting is a method by which liquid metal is poured into a mold made of metal by gravity to obtain castings, which is called metal mold casting. The metal mold can be reused, so it is also called permanent casting.
Compared with sand casting, metal mold casting has the following advantages: realizing “one mold more casting” (dozens to tens of thousands of times), saving a lot of molding materials, working hours and floor space, improving productivity and working conditions; metal mold cooling fast, gradually crystallizing and weaving fine, high mechanical properties and density, For example, the tensile strength of copper and aluminum alloy castings is more than 20% higher than that of sand casting; the casting accuracy can reach it14-it12, the surface roughness Ra 12.5-6.3 μ m, and the machining allowance is 0.8-1.6mm, which can realize less and no machining. The disadvantages are high cost and long cycle of metal mold manufacturing, not suitable for small batch production, not suitable for casting complex shape, large-scale thin-walled parts, and easy to produce white structure. In addition, mechanized and automatic devices must be used for production to improve the working conditions.
3. Lost foam casting
Lost foam casting is filled with refractory coated plastic foam into the sand box. The sand around the mold is filled with dry sand. When the molten metal is cast, the high temperature molten metal dissolves, and occupies the space launched by the foam mold, and finally obtains the casting process.
The EPC process is much simpler than the traditional clay sand casting process. Its characteristics are: high casting precision, lost foam casting is a kind of new technology of near no allowance and precise forming, which does not need to take mold, no parting surface and no sand core, so the casting has no flash, burr and draft angle, and reduces the size error caused by core combination; the design is flexible, which provides sufficient freedom for casting structure design. The structure originally assembled into several parts can be cast by gluing together several foam plastic chips. The holes and holes originally needed to be machined can be directly cast out without sand cores, which greatly saves the cost of machining and core making. There is no chemical binder in the molding sand for cleaning production. The foam plastics at low temperature are completely harmless to the environment. When it is cast, the organic matter discharged is very few, and the time of discharge is short and the location is centralized, so that it is easy for centralized collection and processing. The investment and production cost are reduced, the sand recovery system is greatly simplified, the die life is long, and the loss is less than the traditional die. The production pattern and casting can be automated, which completely changes the image of “bitter”, “dirty” and “tired” of the traditional casting.
4. Pressure casting
Under the action of high pressure, the metal solution is pressed into the cavity with high precision at a high speed, so as to solidify rapidly under pressure, so as to obtain a high efficiency casting method of high quality castings.
In all kinds of investment casting processes of non-ferrous metals, the proportion of pressure casting is the largest. The dimensional accuracy of die casting can reach it13-it11, and the surface roughness Ra is 3.2-0.8 μ m, which can realize less and no machining. Because most of the die-casting parts can be assembled directly without cutting, it can save labor, material and cost. The productivity of die casting is higher than that of other casting methods, and it is easy to realize semi automation and automation. The strength and surface hardness of the casting are high, the structure is fine, and its tensile strength is about 25% ~ 40% higher than that of the sand casting. The disadvantages are high cost of die-casting equipment and die-casting mold, long manufacturing cycle of die-casting mold, only suitable for mass production.
