This solution is aimed at cracking failures in the core and cavity of injection molds, covering assessment, classification repair, stress relief and verification. It is applicable to common mold steels including P20, 718, NAK80, S136 and H13, helping to restore mold performance and avoid repeated cracking.
Common Causes of Mold Core CrackingMold core cracking usually results from a combination of factors. Rapid temperature changes, excessive holding pressure and unbalanced ejection create thermal fatigue and stress concentration. Improper material selection, excessive hardness and residual internal stress from heat treatment increase brittleness. Structural defects such as sharp corners, thin walls and deep ribs further concentrate stress. In addition, insufficient preheating, foreign object crushing, long-term overload and poor maintenance accelerate crack propagation.
Damage Classification and InspectionClass I: Superficial micro-cracks below 0.05mm, only on the surface, repairable by polishing. Class II: Medium cracks 0.5–2mm, linear and non-penetrating, requiring welding and finishing. Class III: Severe cracks deeper than 2mm, penetrating or at stress-concentrated areas, needing insert replacement or full core change. Inspection includes cleaning, visual inspection, dye penetrant testing and ultrasonic depth measurement to confirm crack range and depth.
Class I Superficial Crack RepairFor shallow cracks, polish the surface with 800#–1200# oilstone along the original grain, then finish with 1500#–2000# diamond paste. After polishing, conduct penetrant inspection to confirm no remaining cracks. Apply rust preventive oil and optional low-temperature nitriding to improve surface stability.
Class II Medium Crack RepairUse low-heat-input welding such as laser welding or micro-arc welding with matching material wire. Grind the crack into a V-shaped groove, preheat the core to 80–120°C, and perform segmented welding to reduce thermal stress. After welding, cool slowly and conduct stress-relief annealing at 150–200°C. Then machine the surface to restore dimensions and polish to match the original finish.
Class III Severe Crack RepairFor deep or penetrating cracks, the preferred method is to machine out the damaged area and install a precision insert with interference fit and positioning pins. The insert uses the same material and hardness as the original core. After fitting, grind and polish to eliminate seams. If the core is badly damaged, direct replacement is more reliable and cost-effective.
Acceptance and PreventionAfter repair, the surface must be free of cracks, pores and unevenness, with dimensions within tolerance. During trial production, the mold should run stably without flashing or sticking. To prevent future cracking, preheat the mold before production, control temperature fluctuation, balance ejection and regularly inspect for micro-cracks. Proper heat treatment and rounded corner design also reduce cracking risks.
Safety NotesDuring welding and grinding, use personal protective equipment and ensure ventilation. Control preheating and annealing temperatures to avoid deformation. After repair, run the mold at low speed and low pressure before mass production.
