Thermal defects
There are five types of casting defects caused by heat: thermal cracking, cold shut and thermal fatigue.
1. Tears
Heat tears are some of the important casting anomalies that occur due to heat. This defect occurs during the solidification process of metal castings. During this stage, heat-induced tensile strains and stresses are generated in the metal. Heat tears occur when the metal contracts while solidifying. You usually see them as irregular or linear patterns on the die cast part.
Causes
• · Thermal shrinkage of the metal surface.
• Inadequate metal pouring temperature.
• Improper solidification process and gate placement.
Remedies
• · Use good fillets at the joints.
• Accurately place the gate near the chip.
• Use standard solidification methods.
• Ensure uniform thickness throughout the material.
2. Cold Close
A cold shut defect is a crack with rounded edges. It occurs when metal flows into the mold from multiple points. However, the metal is too cold to fuse properly to form a perfect piece. Therefore, a cold shut occurs when a rounded edge crack in the middle of the piece creates a weak spot on the casting.
Causes
• Improper gating system design.
• Low temperature of liquid metal.
• Reduced fluidity of molten metal.
• Thin sections of castings.
Remedies
• Optimize gating system to prevent narrow cross paths and ensure proper metal flow.
• Improve mold permeability.
• Increase metal pouring temperature to prevent early solidification.
3. Heat fatigue
This defect is common in aluminum die casting because the mold is subjected to high thermal stresses. Thermal stresses are generated by thermal gradients that occur during surface heating and cooling during injection, jetting and lubricant spraying. Since die casting molds are subject to high thermal and mechanical loads, subjecting them to repeated cycles of very low and very high temperatures can lead to structural weaknesses and cracks inside the part.
Causes
• Continuously subjecting the mold to high temperatures.
• Corners with too small a radius.
• Temperature gradients too close to the gate.
Remedies
• Give corners and edges an allowable radius.
• Reduce temperature gradients close to the gate.
• Allow the die to cool for some time.
