The primary task in casting process design is to select the appropriate casting method. Each casting method has its own advantages, disadvantages, and applicable scope. The selection of a casting method should be based on factors such as the applicable alloy types, the structure and size of the castings, technical requirements for dimensional accuracy, surface roughness, and internal quality, production batch size, and the existing production conditions of the enterprise. A comprehensive consideration and comparative analysis should be conducted from technical, economic, production condition, resource utilization, and environmental protection perspectives to choose the most economical casting method that can produce castings meeting quality requirements with existing or potentially available process equipment.
In terms of applicable alloy types, the choice of casting method depends primarily on the heat resistance of the mold material. The silica sand used in sand casting has a refractoriness of up to 1700°C, which is about 100°C higher than the pouring temperature of carbon steel, making it suitable for castings made from various metal materials such as cast steel, cast iron, and non-ferrous metals. The shell molds used in investment casting are made from more refractory materials like alumina powder, pure quartz powder, and silica sand, enabling the production of alloy steel castings with even higher melting points. Permanent mold casting, die casting, and low-pressure casting typically use metal molds and metal cores. Even with refractory coatings applied to the surface, their refractoriness is not high, so they are generally only used for non-ferrous metal castings with low melting points.
Regarding applicable casting sizes, the choice of casting method is mainly related to conditions such as mold dimensions, metal melting furnace capacity, and lifting equipment capacity. Sand casting and lost foam casting have fewer limitations and can produce small, medium, and large castings. Investment casting is generally suitable only for producing small and medium-sized castings due to the susceptibility of wax patterns and shells to deformation during the process. For permanent mold casting, die casting, and low-pressure casting, the difficulty and high cost of manufacturing large metal molds and cores, coupled with equipment tonnage limitations, typically restrict their use to producing small and medium-sized castings.
Concerning the applicability to casting structure, shape, and complexity, methods like sand casting, investment casting, and lost foam casting impose no limitations on the shape or complexity of the castings. Centrifugal casting is more suitable for specific shapes like pipes and sleeves. Permanent mold casting is suitable for producing castings with relatively simple shapes and is not ideal for complex thin-walled castings. Die casting can produce castings with complex shapes, but the high cost of die-casting molds means it is only economical for mass production. However, die casting remains economically viable overall for producing parts due to its high productivity and significant savings in machining time.
Regarding casting dimensional accuracy and surface roughness, the choice of casting method is primarily related to the precision and surface finish of the mold. Sand castings generally exhibit poorer dimensional accuracy and higher surface roughness values. In investment casting, the pattern dies can be machined very precisely and smoothly, resulting in accurate wax patterns. The shell molds have no parting lines, leading to excellent dimensional accuracy and very low surface roughness values for investment castings. Due to forming under high pressure and high speed in die casting, and the die-casting molds being precisely machined, die-cast parts also exhibit very good dimensional accuracy and very low surface roughness values. The metal molds (cores) used in permanent mold casting and low-pressure casting are not as precise or smooth as die-casting molds. Forming under gravity or low pressure results in castings with better dimensional accuracy and surface roughness than sand castings, though not as good as die castings.
In terms of production batch size, the casting method should also correspond to the casting production volume. Sand casting, investment casting, and lost foam casting can be used for single-piece, small-batch, and mass production. However, casting methods like low-pressure casting, die casting, and centrifugal casting, due to the high cost of equipment and molds, are only suitable for batch production. For mass or batch production of hollow rotational castings like sleeves, pipes, and cylinders, centrifugal casting can greatly simplify the production process, eliminate the need for gating and riser systems, and achieve high yield rates. Furthermore, lost foam casting, utilizing a vacuum negative pressure dry sand molding process, can significantly reduce waste emissions, minimize dust, smoke, and noise pollution, facilitate shakeout, and utilize residual heat for water toughening treatment of high manganese steel castings and solution treatment of heat-resistant steel castings. Therefore, lost foam casting should be prioritized for high manganese steel casting production to achieve comprehensive resource utilization and environmental protection goals.
