The gating system, riser and chill must be designed according to the solidification principle of castings in sand casting process. For example, the principle of sequential solidification is mainly applicable to the occasions requiring feeding, and the solidification is suitable for ordinary gray iron and nodular iron castings with small shrinkage. Proportional solidification is a solidification principle that makes use of the dynamic superposition of shrinkage and expansion and takes technological measures to make the shrinkage and feeding per unit time, shrinkage and expansion in proportion. In the whole process of cooling and solidification of gray iron and nodular iron castings, there are not only liquid shrinkage and solidification shrinkage, but also expansion caused by graphite precipitation.
Macroscopically, the volume change in the forming process of sand castings is the net result of expansion and contraction. Balanced solidification technology is based on the self feeding effect of “expansion and shrinkage offset” of graphite iron castings and the law of external feeding of gating and riser, and puts forward the process design principle and production technology reflecting the solidification, shrinkage and feeding characteristics of iron castings.
Equilibrium solidification is similar to sequential solidification, which emphasizes the feeding of sand castings. However, the feeding technology of balanced solidification emphasizes the self feeding of sand castings. The theory points out that the riser only allows the shrinkage of insufficient self compensation, so the riser does not have to solidify later than the sand casting; In addition, the riser should not be placed at the hot spot of sand casting, and the feeding effect of riser is limited. According to the sequential solidification theory, the riser solidifies later than the sand casting, and is placed on the thick part with the highest position of the sand casting. The riser size of balanced solidification is small, and the process yield is relatively improved; On the other hand, because the riser is placed away from the hot joint position, the shrinkage cavity and porosity defects caused by the increase of hot junction volume at the riser root can be avoided and the waste products can be reduced.
Balanced solidification is also similar to simultaneous solidification, which emphasizes that the gating system or riser should be introduced from the thin-wall part of the sand casting to reduce the temperature difference in different parts of the sand casting and avoid local overheating. At the same time, solidification emphasizes reducing residual stress and avoiding cracks and deformation, without considering feeding. Balanced solidification starts from feeding, emphasizing the feeding of thin-walled parts, small parts and parts with uniform wall thickness. Moreover, the pouring riser leaves the hot spot of sand casting, which not only meets the feeding, but also reduces the trend of stress and deformation cracking. Therefore, using the balanced solidification theory to design the thin and small part process can effectively reduce the defects such as shrinkage cavity and porosity, and significantly improve the internal quality of sand castings.