Mechanism and multi-factor prediction of skid line defects in metal sheet stamping

Skid lines are a critical surface defect that severely degrades stamped panel quality, originating from hardening traces caused by relative sliding between the punch fillet and sheet metal during stamping. This study employs a multi-factor testing device to investigate skid line formation under varying parameters. By combining contact stress and bending stress analysis, the defect’s formation mechanism is elucidated. Key factors examined include blank holder force (BHF) difference, punch fillet radius, friction coefficient, draft angle, and sheet thickness. Grey relational analysis (GRA) identifies the BHF difference as the most influential factor, followed by draft angle, fillet radius, sheet thickness, and friction coefficient. Response surface methodology (RSM) establishes prediction models for skid line sliding distance and severity, achieving a prediction accuracy of 97.6%. Validation through an automotive hood outer panel case study confirms the method’s effectiveness: the predicted sliding distance of 9.15 mm closely matches the simulated result of 9.39 mm. Through process optimisation, the sliding distance is reduced from the initial 10 to 3.5 mm, successfully confining skid lines to the fillet region after three trial adjustments. This research provides a theoretical basis, technical guidance, and practical solution for eliminating skid line defects in stamping, significantly advancing surface quality improvement for automotive outer panels.