The Seven Basic Defect Categories in Sand Castings

Sand-mold castings can develop numerous defects due to mold spalling. Molds typically fail for one of two reasons: either the wrong material was used, or compaction was improper.
The first type is mold erosion, which occurs when the liquid metal flows into the mold and gradually wears it down. This defect typically happens only in sand-mold castings, as molds used in most other casting processes are significantly more robust. As a result, the surface of the cast part often features rough spots and excess material. Additionally, sand particles may become embedded within the casting metal, compromising its ductility, fatigue strength, and fracture toughness. This issue can arise either due to insufficient sand strength or excessively high pouring speeds. To address this, redesigning the gating system—such as incorporating larger runners or multiple sprues—can help slow down the pouring process. Another common related defect is "dripping," where small amounts of sand fall from the mold cavity into the molten metal while the casting is still in its liquid state. This problem often occurs when the mold hasn’t been properly compacted during preparation.
The second defect is metal penetration, where molten metal seeps into the molding sand. This results in a rough surface, typically caused by either excessively coarse sand grains or overly high pouring temperatures. If the pouring temperature is too high—or if low-melting-point sand is used—the sand may actually fuse with the casting itself. When this happens, the resulting cast surface develops a brittle, glassy appearance.