The apparent shrinkage time of iron castings only accounts for a part of the total solidification time, and the external feeding only plays a role in the apparent time. Therefore, the solidification of iron casting riser does not have to be later than the casting, and its shrinkage supplement amount and shrinkage supplement action time are limited, which is called limited shrinkage supplement. According to this principle, the riser design of iron castings need not be larger than that of castings in modulus or size. Before reaching the equilibrium point P, the solidification process of iron castings must rely on the feeding of external molten iron, and the difference of self feeding deficiency of iron castings must be supplemented by risers or gating systems. When the cooling solidification process reaches the equilibrium point P, the liquid iron in the mold cavity will no longer produce volume loss. No matter whether the gating system or the riser is unblocked with the cast iron, the cast iron will no longer suck the molten iron in the gating system or 1:3, and the feeding effect of the gating riser system will stop. After the equilibrium point, the cast iron can carry out self feeding. At this time, as long as the shrinkage per unit time is no longer greater than expansion, the cast iron will not have shrinkage defects in this stage.
As a casting shrinkage characteristic, the shrinkage rate of iron castings is rarely paid attention to. In fact, it is more important to study its influence on the solidification shrinkage of various iron castings than to pay attention to its quantity. Contraction rate refers to the distribution of contraction per unit time. For a large thin-walled piece, the shrinkage is the product of the shrinkage and the volume of the cast iron. Obviously, the larger the volume is. In fact, due to the fast cooling speed of thin wall cast iron castings, the phenomenon of “concentrated shrinkage and sudden expansion” will appear, and the shrinkage time will be shortened. In the process of mold filling, the feeding effect of the later poured molten iron on the first poured molten iron (i.e. post feeding) and the short-time unblocking of the inner sprue after filling make the gating system play the role of riser, which can meet all feeding of iron castings. The design of riser should adopt large riser body and small riser neck to adapt to large shrinkage and short shrinkage time.
The cooling of thick iron castings is slow, the apparent shrinkage time is long, and the shrinkage distribution is relatively dispersed. The edges and thin parts of iron castings cool quickly, and the graphite precipitates earlier. The heat dissipation of thick and large parts is slow, and the shrinkage per unit time is easily offset by the expansion of graphite precipitated m in advance at the same time. Therefore, the shrinkage of iron castings as a whole is small or even zero. The design size of riser can be very small, or the casting process without riser can be selected. Due to the fast cooling speed, thin-walled nodular iron castings may have the phenomenon of concentrated shrinkage and sudden expansion, that is, the superposition mode of expansion and contraction. Due to the concentration of initial shrinkage, the graphitization expansion of this solidification mode is delayed. Although nodular cast iron with high carbon equivalent should have large graphitization expansion value, due to the existence of “expansion shrinkage separation”, the graphitization expansion can not be used for feeding in the early and middle stages of solidification, so relatively large external feeding is required.
When the design size of the riser is large enough, a large amount of molten iron is sucked from the riser due to the apparent shrinkage of thin-walled nodular iron castings. When a large amount of graphitized eutectic expansion occurs, the increased volume of iron castings offsets part of the simultaneous shrinkage, and the excess part will squeeze the mold wall and expand the cavity. When the design size of the riser is too small and the feeding capacity of the riser is insufficient at the initial stage of solidification, the iron castings will have shrinkage defects such as shrinkage cavity and porosity, but at the same time, there will be large cavity expansion. This is the reason why nodular iron castings sometimes shrink and the mold wall migrates greatly.
To sum up, it is more practical to strengthen the feeding technology of medium and small parts of nodular cast iron, which is more effective than pursuing to improve the rigidity of the mold to improve the compactness of the casting. It is not necessary to use the expansion and shrinkage of graphite in the later stage of solidification of iron castings, but it can also play the role of supplement in the later stage of solidification. Especially for iron mold sand coated casting, the utilization degree of graphitization expansion is relatively improved, the apparent shrinkage time of iron castings is shortened, and the demand for external feeding is reduced.