Forming method and material selection of composite mold for sand casting

Driven by the technology of mold less sand casting, according to the principle of mold forming, the molding of mold less can be divided into two categories: additive manufacturing and additive manufacturing. Among them, the representative CAD Moldless casting processes include selective laser sintering (SLS) and ink jet bonding 3D printing, both of which are based on discrete stacking. Sand mold NC milling is a mold forming technology based on the principle of removal machining. Multi axis NC milling machine can be used to mill the mold cavity. Both of them are carried out in a relatively closed space to reduce the emission of casting dust in the workshop and manufacture molds that meet the requirements of production performance, which reflects the advantages of patternless modeling in technology and environment.

Although Moldless sand casting has certain advantages in mold forming, sometimes a single technology can not fully meet the casting production needs of enterprises. For example, at present, sand casting adopts disposable mold, that is, the formed mold can only complete the production task of one casting. Based on the development of wood mold, the mold forming efficiency of sand mold 3D printing and sand mold NC milling is far lower than that of traditional pattern modeling, so it is difficult to meet the demand of large product batch; At the same time, the equipment and consumables of sand mold 3D printing are expensive, which is difficult to balance the input and output value; The NC milling of sand mold can not adapt to the processing of all structures in the mold cavity. The milling depth and the processing of fillet and chamfer are limited by the size and model of milling tools; The forming quality of mold is greatly affected by milling process parameters. Improper process parameters are easy to cause irreparable mold defects such as collapse angle and crack.

White sand Yellow sand

In addition, mold less sand casting has a variety of material systems, such as selective laser sintering process, which is suitable for modeling materials such as ceramic powder, nylon and polycarbonate; Inkjet bonded sand mold 3D printing is suitable for ceramic powder, furan resin sand and other modeling materials; Sand mold NC milling is suitable for molding materials such as sodium silicate sand, coated sand and furan resin sand. Facing a variety of mold forming methods and comprehensive molding material system, it is necessary to reasonably select mold forming methods and molding materials according to the structural characteristics of castings and the production conditions of foundry.

As mentioned above, for the parts with high surface accuracy requirements in castings, the mold forming method with higher accuracy grade should be adopted, and the molding sand material with smaller particle size should be selected to meet the requirements of high-precision mold surface. As shown in the figure, there are two molding sand materials commonly used in sand mold 3D printing center. The particle size of white sand is 70-140 mesh. Compared with yellow sand, it can cover more and more uniform adhesive per unit area, so as to achieve higher mold tensile strength. The molding sand casting process also needs the molding sand material to have good fluidity, filling, static stability and air permeability. For the mold and sand core with high complexity, such as the mold structure with staggered stiffeners, multiple bosses, deep grooves and large differences in overall dimensions, as well as the sand core structure with complex shape, thin section and thin flanges, edges and corners or ribs, it is applied to the forming of castings such as cylinder heads of automotive internal combustion engines and oil passages of various valves, It is necessary to adopt mold forming methods such as sand mold 3D printing and selective laser sintering to form an important non machined inner cavity through high-precision mold and sand core. For the part surrounded by liquid metal after pouring, there are certain requirements on the dry strength and toughness of the mold and sand core, and the molding sand material with good fluidity and collapsibility and low gas generation should be used. In addition, for the large-size mold with relatively regular and simple forming surface, shallow groove characteristics and moderate wall thickness, the mold forming method of sand mold NC milling can be adopted.