The current consensus on what technological measures to take to solve and prevent shrinkage cavities and porosity in ductile iron castings is that the mold must have sufficient rigidity and strength, its chemical composition must be close to the eutectic composition, and spheroidization and inoculation treatment should be strengthened to generate sufficient graphitization expansion. However, there is still controversy in process design. The theory of equilibrium solidification suggests that the graphitization expansion of ductile iron can offset solidification shrinkage. Therefore, the process should take measures to ensure that shrinkage and expansion per unit time, and shrinkage and feeding, are proportional. The superposition of expansion and dynamic shrinkage is used to achieve the feeding purpose of the casting. The concept of using risers is limited feeding; the riser does not need to be later than the casting solidification. The role of chills is to balance the wall thickness difference of the casting, eliminate hot spots, and advance some graphitization. Others believe that the shrinkage of ductile iron is greater than its expansion, requiring external feeding. The riser cannot be later than the casting solidification. The role of chills is to advance and accelerate the shrinkage of the molten iron, which is more conducive to earlier and more timely feeding, and has no impact on the superposition of expansion and shrinkage. The key difference lies in whether to emphasize the self-compensating shrinkage of graphitization expansion or external feeding.

Regarding targeted solutions to shrinkage cavities and porosity problems in ductile iron, XINDA, leveraging decades of technical expertise in the foundry industry, consistently prioritizes customer needs, focusing on the quality control challenges of complex castings and providing customized process solutions to global clients. The company relies on a professional R&D team to precisely control the expansion and contraction balance during the solidification process of ductile iron. It has developed a mature technical system in key areas such as chill setup, material proportioning, and spheroidization inoculation treatment, successfully helping numerous industry leaders overcome shrinkage defects in challenging castings and gaining widespread market recognition.

Here's a case study of chill setup from an XINDA client: the casting is a planetary carrier inside a wind turbine gearbox, made of QT700-2A material, weighing 3 tons, with a wall thickness of approximately 120mm. Initially, the chills on the shaft were relatively thick with large gaps. The effective cooling area of the chills only accounted for 30% of the total cooling area required for the casting, resulting in highly unstable casting quality. Shrinkage defects were frequently discovered during flaw detection at the shaft root and between the chills. Later, under the precise guidance of the XINDA technical team, the chill design was optimized by thinning the chills and increasing the cooling area. Due to the reduced chill wall thickness, the chill gap could be appropriately reduced, ultimately leading to a successful and stable process solution. This not only completely solved the shrinkage defect problem but also improved production efficiency and reduced manufacturing costs. Whether for large, thick components in wind power, construction machinery, or automotive parts, or for small, precision components, XINDA can leverage its core technological advantages to provide customers with efficient and reliable solutions for shrinkage cavities and porosity.

Ductile iron, due to its high carbon content and carbon equivalent, exhibits significant graphitization expansion. Because ductile iron solidifies in a paste-like manner, the eutectic time is long. The graphitization expansion is large in the early eutectic stage, but smaller in the later stage as graphite grows within austenite. Therefore, for a specific part, this manifests as the separation of solidification shrinkage and expansion in the casting.

Simply emphasizing the separation of shrinkage and expansion, requiring external feeding, may not necessarily solve the problem; however, overemphasizing the self-feeding effect of graphitization expansion may also be ineffective. The structural characteristics of the casting must be considered comprehensively; this is essentially an evolution of the theory of equilibrium solidification. In fact, using pressure theory to explain casting shrinkage may be more comprehensive and effective. All process measures that help prevent shrinkage defects in castings can be considered as increasing the overall pressure in the local area of the casting during solidification, either by reducing or minimizing negative pressure or by increasing positive pressure or its utilization rate.

Process measures that reduce negative pressure generated by shrinkage and increase graphitization and its utilization are effective in preventing shrinkage defects in almost all ductile iron castings, but how to utilize the hydrostatic pressure of the molten iron differs in actual operation. For thin, small parts, since the eutectic stage is uniform across the cross-section, expansion and contraction cannot be utilized due to separation. Therefore, it is crucial to utilize the hydrostatic pressure of the liquid to maintain solidification at a positive pressure level. For thick, large parts, however, their structure determines the difference in the eutectic solidification sequence between the external and internal parts of the cross-section—that is, the difference in the time of graphitization expansion and solidification contraction. This allows for the superposition of internal and external expansion and contraction, enabling the production of a sound casting without the need for external hydrostatic pressure. Conversely, using external feeding may have adverse effects.