The shrinkage rate of cast iron generally refers to the dimensional manifestation of volumetric contraction occurring during the solidification and cooling processes. Due to the relatively high carbon and silicon content in cast iron-and the fact that graphite precipitation provides a degree of "expansion compensation"-its overall shrinkage rate is lower than that of cast steel.
Typically, the linear shrinkage rate for gray cast iron castings is around **0.7% to 1.0%**. Ductile iron, having a microstructure closer to that of steel, exhibits a slightly higher shrinkage rate, generally in the range of **1.0% to 1.3%**. These values vary depending on factors such as casting geometry, wall thickness, and cooling conditions.
In production, the shrinkage rate is not a fixed value but a design parameter that must be adjusted based on specific casting processes. For instance, thick-walled castings cool more slowly, resulting in relatively stable shrinkage; conversely, thin-walled castings cool rapidly and are prone to localized shrinkage concentration, necessitating the inclusion of a shrinkage allowance in the mold design to compensate.
Factors such as pouring temperature, inoculation treatment, and cooling rate all influence shrinkage behavior. Improper control can lead to defects such as shrinkage cavities or porosity. Consequently, during the casting design phase, the shrinkage rate serves as a critical basis for determining pattern dimensions and designing the feeding system.
