Clamp burn marks appear near the sheet perimeter as discoloration, gloss change, scorching, or a sharp contact pattern. They may come from excessive radiant heat or direct contact with an overheated frame component.
Perimeter zones are often set higher to compensate for heat loss into clamps and transport systems. If that compensation is excessive or the frame temperature changes during production, the edge can overheat while the center remains correct.
Correction
Sheet-edge temperature should be measured at several positions. Heater output, shielding, frame temperature, element distance, and dwell should be checked. A failed switching device can leave one perimeter zone on continuously even when the displayed setting appears normal.
The edge needs enough heat to form without becoming the hottest and weakest region. Local shielding or recipe correction is preferable to reducing the entire oven and underheating the useful forming area.