Laizhou Dingtai Machinery Co., Ltd.

　　Surface depressions in ductile iron castings are a common casting defect. Their formation is complex and is mainly related to casting processes, material properties, and the cooling process. The following are the main causes of surface depressions in ductile iron castings:

　　1. Uneven Cooling

　　When the wall thickness of a casting is uneven, the thin-walled sections will cool before the thick-walled sections, leading to inconsistent cooling times. This uneven cooling process prevents impurities from being discharged in time, thus forming depressions on the surface.

　　In addition, excessive local heat in the mold will also cause slower solidification in the hotter parts, further exacerbating the formation of surface depressions.

　　2. Influence of Solidification Method

　　Ductile iron solidifies in a mushy manner. Graphite spheres grow within the austenite shell, and the movement speed of carbon atoms is limited, resulting in the inability of the casting surface to quickly form a solid outer shell. When the casting solidifies, if the shell strength is insufficient, it may collapse under atmospheric pressure, forming surface depressions.

　　3. Poor Gas Discharge

　　During the casting process, if the gas inside the mold cavity cannot be discharged in time, it will be squeezed to the interface between the casting surface and the molten steel, resulting in surface defects, including depressions.

　　4. Influence of Alloying Elements

　　Alloying elements (such as silicon) that increase the eutectic solidification range in the molten iron increase the risk of shrinkage and depressions in the casting. The wider the solidification range, the more obvious the early precipitation expansion effect of graphite, which may lead to surface depressions.

　　5. Improper Process Parameter Control

　　The choice of filling speed and specific pressure has a significant impact on the quality of the casting. For thin-walled or complex castings, selecting a higher filling speed and relatively lower specific pressure helps reduce surface depressions.

　　6. Insufficient Mold Rigidity

　　Insufficient mold rigidity may cause the casting to deform during cooling, resulting in surface depressions. Improving mold rigidity (such as using resin sand) is an effective measure to reduce such defects.