There are many reasons for shrinkage and porosity casting defects in castings. Deficiencies in product structure, smelting process, core sand, process design and other aspects may lead to shrinkage and porosity casting defects.
① Product structure: the casting wall is thick and uneven, and the heat feeding is insufficient; The existence of isolated hot knots, local overheating of the product, and the inability to solidify in sequence may lead to the formation of shrinkage cavities and porosity casting defects. Because the product structure is designed by the user according to the product performance, it cannot be easily changed. It is impossible to solve the casting defects by changing the product structure.
② Molten iron smelting and mold and core sand: casting molding sand and core sand have poor heat dissipation effect, which reduces the solidification rate of liquid metal; During the melting or transportation of liquid metal, different factors lead to high gas content, and gas is separated out during the cooling process, which prevents the adjacent metal liquid from feeding there, forming shrinkage casting defects; Poor inoculation of molten iron leads to the precipitation of a large amount of cementite during solidification of molten iron, which increases the amount of solidification shrinkage and also produces shrinkage casting defects. Since our company adopts large-scale production line automatic molding and cupola smelting process, the mold, core sand and molten iron smelting process are relatively mature, corresponding to the production of many different kinds of products with different materials, it is not convenient to adjust the sand and smelting process for this product.
③ Process design: the design of gating system is unreasonable, which violates the solidification principle and is not conducive to sequential solidification; The unreasonable design of inner gate size and riser neck size leads to the reduction of riser feeding effect or the formation of reverse feeding; The riser design is unreasonable, including quantity, position, size, size, etc. Because it is difficult to improve in other aspects, the best way to solve the casting defects of shrinkage cavity and porosity is to start from the process.