Clamp slip allows the sheet boundary to move during forming. The part may become off-center, thin on one side, incomplete on the other, or surrounded by an irregular trim flange.
Insufficient force is one cause, but clamp geometry, surface condition, edge temperature, contamination, and sheet thickness are equally important. A cold stiff sheet can transmit high load to the frame, while an overheated edge can neck and pull through even under high clamp pressure.
Diagnosis and correction
Witness marks should be inspected around the full perimeter. Unequal marks indicate pressure or parallelism problems. The frame, seals, pins, and gripping surfaces should be checked for wear and contamination.
Clamp force should be increased only within machine and sheet limits. Mechanical grip features or a larger clamping area may be required. Heater zoning near the frame should keep the edge strong enough to hold while allowing the forming area to stretch normally.