Analysis on process characteristics of sand mold casting without mold

Traditional sand casting mostly constructs the mold cavity and core seat through wood mold or metal pattern. It has an important impact on the geometry, dimensional accuracy and surface roughness of the casting, and is directly linked to the production efficiency and cost of the casting. The pattern and core box are usually made into upper and lower parts in the form of split casting, so as to facilitate the molding process. Sand casting without mold does not need the use of patterns in the molding process. It is a rapid manufacturing method of complex metal parts based on technological innovation such as digital design, automatic control and new materials. On the basis of realizing integrated modeling, the flexibility and forming accuracy of casting production are greatly improved, which can promote the acquisition of high-quality castings. Similarly, its process is different from the traditional pattern modeling method.

Take sand mold 3D printing and sand mold NC milling as examples. Sand mold 3D printing is a manufacturing method based on the discrete stacking principle. The mold is formed by selectively spraying liquid binder through the printing nozzle and stacking molding sand layer by layer. It abandons the processes that consume a lot of time and manpower, such as sand filling, vibration and mold lifting, and concentrates the molding process in the printing and molding warehouse, which effectively avoids the mold defects caused by human factors. The sand mixing is completed in the sand treatment center of the machine, and the molding sand is transported to the forming bin in stages according to the printing process and the remaining sand amount of the sand dropping funnel. Thanks to the forming principle of layer by layer stacking, 3D printing has no special restrictions on the structure of mold and cavity, which can realize the printing of complex structure and avoid the generation of flash and size deviation. Moreover, the mold strength is much higher than the general sand mold (core) strength requirements, which can fully meet the performance requirements of gravity casting for sand mold (core). After molding, whether to carry out flow coating on the mold surface can be selected according to the mold surface strength and fire resistance.

In the traditional sense, material reduction machining is to obtain relevant workpieces or products by removing materials. In the foundry industry, it is mostly used in the post-treatment stage of castings, such as the removal of sprue and riser, and the grinding wheel is used to polish the fracture structure and flash on the surface of castings. The sand mold NC milling is also based on the principle of material reduction manufacturing. The machining system automatically generates the tool path according to the imported CAD model, roughens and finishes the sand mold surface in combination with the accurate tool path of the milling cutter, and finally forms the mold cavity. At present, it has been applied in practical production, such as the machining of engine cylinder block, impeller blade and so on. It is worth noting that the processing object of milling is prefabricated sand blocks. Sand mold milling requires sand mixing, sand filling, vibration and other operations in the early stage of the process. The only difference is the solidification of sand blocks before milling. Because the cutting force of the milling cutter acts directly on the sand block, the curing treatment can make the prefabricated sand block meet certain strength requirements. The mass ratio of raw sand, resin and curing agent can be set to 1000:25:5, and 24h curing can be completed by applying extrusion force of different strength. In terms of molding materials, molding sand with round grain shape is preferred to reduce the damage to the machined surface and tool wear caused by milling. Among them, the grading of molding sand with particle size of 50-100 and 70-140 is applicable to sand milling.

According to the production requirements and process characteristics of the casting process, the traditional casting workshop usually has five production departments, namely molding department, core making department, sand treatment department, melting department and cleaning department. Among them, the molding department is the core Department of foundry production. The production pressure of the molding department can be further reduced by using single technology or dieless composite forming. In the preparation stage, it is necessary to design the 3D model of castings and molds, divide the molds based on composite forming, and put them into production after pouring verification and determining the feasibility of the process scheme. Each casting process involves material, energy flow and pollution flow. Specifically, resource consumption and energy consumption depend on the design of mold forming method and casting process scheme.

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