Common Casting Defects: Causes and Prevention Methods – Part II

Six Common Casting Defects: Causes and Prevention Methods (Part 2)

In this continuation, we cover three additional common casting defects and their causes, along with prevention methods to help reduce defects in your foundry operations.

4. Crack (Hot Crack, Cold Crack)

Features: Cracks in castings can be straight or irregular curves. Hot cracks typically have a dark gray or black oxidized surface with no metallic luster, while cold cracks have a cleaner appearance with a metallic sheen. Outer cracks are often visible to the naked eye, while inner cracks require more advanced detection methods. Cracks often appear at inner corners, thickness transitions, or where the pouring riser connects with casting hot sections. Cracks are frequently associated with other defects such as porosity and slag inclusions.

Causes:

•   • Metal mold casting tends to develop cracks because the mold lacks flexibility, leading to rapid cooling and increased stress in the casting.
•   • Opening the mold too early or too late, or improper pouring angles, can create stress.
•   • Thin paint layers or cracks in the mold cavity can also contribute to cracks.

Prevention Methods:

•   • Ensure uniform transitions in casting wall thickness to reduce stress concentrations.
•   • Adjust coating thickness for uniform cooling rates, minimizing stress.
•   • Control metal mold temperatures, adjust mold rake, and manage core cracking times for optimal cooling.
•   • Use proper mold design to avoid internal cracks.

5. Cold Shut (Bad Fusion)

Features: Cold shuts appear as seams or surface cracks with round edges, indicating a lack of proper fusion. They often occur at the top wall of casting, on thin horizontal or vertical surfaces, at the junction of thick and thin walls, or on thin panels. Serious cold shuts can result in incomplete casting, leading to structural weaknesses.

Causes:

•   • Poorly designed exhaust systems in metal molds.
•   • Operating temperatures are too low.
•   • Inadequate or poor-quality coating, whether due to human error or inferior materials.
•   • Incorrectly positioned runners.
•   • Slow pouring speeds.

Prevention Methods:

•   • Design a proper runner and exhaust system to ensure adequate ventilation.
•   • Use appropriate coatings with sufficient thickness to maintain consistent cooling.
•   • Increase mold operating temperatures if necessary.
•   • Use inclined pouring methods for better flow.
•   • Consider mechanical vibration during metal casting to reduce defects.

6. Blister (Sand Hole)

Features: Blisters are relatively regular holes found on the casting surface or inside, resembling sand grains. These can be visible on the surface, where you can often remove sand particles. Multiple sand holes can give the surface an orange peel-like texture, indicating underlying issues with sand cores or mold preparation.

Causes:

•   • Sand core surface may shed grains, which get encased in metal and create holes.
•   • Inadequate sand core strength, scorching, or incomplete curing can lead to blisters.
•   • Mismatched sand core and outer mold sizes can cause sand core crushing.
•   • Mold dipping in sand graphite water leads to surface issues.
•   • Friction between sand cores and ladles or runners can cause sand contamination in the casting cavity.

Prevention Methods:

•   • Manufacture sand cores according to strict processes and check quality regularly.
•   • Ensure sand core and outer mold sizes match to avoid crushing.
•   • Clean graphite water promptly to prevent contamination.
•   • Reduce friction between ladles and sand cores to avoid sand contamination.
•   • Clean mold cavities thoroughly before placing sand cores to ensure no loose sand particles are left behind.

For more information on casting defects and other foundry solutions, please contact us at info@dinsenmetal.com. We’re here to help you with your casting needs and provide guidance on reducing defects in your production processes.