Common Causes of Casting Defects and Methods for Prevention

　　I’m sure you’ve encountered these common casting defects—so, why do they occur, and how can they be prevented? Well, let’s dive in together to learn about the causes of typical casting defects and discover effective prevention methods!

　　01 Porosity (bubbles, pinholes, gas pockets)

　　Characteristics: Subsurface defects appear as typical pear-shaped cavities. The blowholes have irregular shapes and rough surfaces, while the gas porosity exhibits concave indentations on the casting surface, which is otherwise relatively smooth. Gas porosity refers to voids—whether on the surface or within the casting—that typically appear in round, oval, or irregular forms. Sometimes, multiple gas pores cluster together to form a gas bubble. These defects can be identified through visual inspection of open pores, whereas subsurface gas porosity can only be detected after mechanical machining.

　　Cause of casting formation:

　　1. The mold preheating temperature is too low, causing the liquid metal to cool down too rapidly as it flows through the gating system.

　　2. Poor mold venting design prevents gases from escaping smoothly.

　　3. The coating is poor, with inadequate self-venting and the release or decomposition of gases from within itself.

　　4. The mold cavity surface has holes and pits, creating gas traps. After the molten metal is injected, the gases trapped in these holes and pits rapidly expand, compressing the liquid metal.

　　5. The mold cavity surface has rusted because it wasn't cleaned thoroughly.

　　6. Raw materials (sand cores) were improperly stored and were not preheated before use.

　　7. The desiccant is ineffective, used in insufficient quantity, or handled improperly, among other issues.

　　Methods to prevent casting defects:

　　1. The mold is thoroughly preheated, and the coating (graphite) has a slightly coarser particle size with excellent permeability.

　　2. Pour using the inclined pouring method.

　　3. Raw materials should be stored in a ventilated, dry place and preheated before use.

　　5. The pouring temperature must not be too high.

　　02. Shrinkage Holes (Shrinking)

　　Cast features: Shrinkage cavities are rough, surface-defect-like holes that appear on or within the casting. They commonly occur near the gating system, at the base of risers, in thick-walled areas, at transitions between thick and thin sections, and in thick-walled regions with large flat surfaces. Mild cases typically manifest as numerous small, dispersed shrinkage cavities—individual shrinkage voids—or as coarse-grained structures around shrinkage cavities and porosity zones.

　　Cause of casting formation:

　　1. The mold's working temperature did not meet the requirements for directional solidification.

　　2. Improper coating selection and inadequate control of coating thickness in different areas.

　　3. Improper design of the casting position within the mold.

　　4. The design of the riser did not effectively compensate for sufficient shrinkage.

　　5. Pouring temperature is too low or too high.

　　Cast component prevention and treatment methods:

　　1. Increase the temperature of the grinding wheel.

　　2. Adjust the thickness of the coating layer to ensure even spraying, preventing uneven application that could lead to localized buildup when reapplying the coating.

　　3. Partially heat the mold or use insulating materials for localized thermal preservation.

　　4. Place copper blocks over the hot spots to induce rapid local cooling.

　　5. Design cooling fins on the mold to accelerate local cooling rates using water or other methods, or spray water or mist directly onto the exterior of the mold.

　　6. During continuous production, to prevent insufficient cooling of the quenching blocks themselves, use detachable quenching blocks and alternate their placement within the cavity.

　　7. Install a pressurization device on the riser mold.

　　8. The gating system design must be precise, and the appropriate pouring temperature should be selected.

　　The above introduces the common causes of casting defects and their prevention methods. If you'd like to learn more, feel free to contact us anytime!