A chill mark forms when hot sheet touches a relatively cold tool or plug and the contact zone stiffens before surrounding material finishes moving. The mark may appear as a line, dull patch, local thickness step, or frozen stretch pattern.
The temperature difference is important, but contact timing and heat transfer also matter. A conductive metal edge produces a stronger effect than an insulating plug surface at the same measured temperature. High contact pressure and a rough surface accelerate local freezing.
Correction
The first-contact pattern should be identified and compared with the mark. Tool or plug temperature should be stabilized within the validated process window, and the contact surface should be smooth enough for the intended material. Plug covers can reduce marking in some applications, but they must remain clean, consistent, and suitable for the required finish.
Do not raise the entire sheet temperature solely to erase a local chill mark. That approach can create sag, thinning, or degradation elsewhere. The local thermal and contact condition should be corrected first.