Sheet pull-in occurs when material slips or is drawn inward from the clamped perimeter during forming. The finished part may be off-center, thin near one side, incomplete at the opposite side, or surrounded by a distorted trim flange.
Insufficient clamping force is a common cause, but edge temperature is equally important. An overheated clamp zone loses strength and can neck under load; an underheated edge can transmit unusually high forces to a small gripping area. Smooth clamps, uneven pressure, contaminated surfaces, and damaged sheet edges reduce holding ability.
Corrective action
Witness marks around the entire perimeter should be compared for depth and position. Clamp pressure and parallelism must be checked, along with mechanical grip features and frame condition. Heater zoning near the frame should then be balanced so the edge remains secure without becoming brittle.
Increasing clamp force without checking sheet condition can cut or crush the edge. The clamping system must hold the required forming load while preserving a stable boundary for material distribution.