The clamp frame must hold the sheet with reasonably uniform force around the forming area. Unequal cylinder pressure, linkage wear, frame misalignment, damaged pads, or uneven jaw contact can allow one edge to move while another remains fixed.
A witness grid or marked sheet can show where movement begins. Clamp force, cylinder pressure, jaw gap, and mechanical position should be compared around the frame under the actual heated-sheet load. Equal regulator settings do not guarantee equal force when cylinder areas, friction, or linkage geometry differ.
The mechanical cause should be corrected before clamp pressure is raised globally. Excessive force can crush sheet edges, damage masking, distort the frame, or overload the machine. The validated setup should specify the clamping hardware and contact condition, not only pneumatic pressure.