A plug mark is a visible imprint, line, texture change, or gloss difference created where the mechanical assist contacts the sheet. It may be acceptable on a hidden surface but is a defect on appearance-critical parts.
The mark is influenced by sheet softness, plug temperature, contact pressure, surface finish, contamination, and dwell. A rough or cold plug can leave a strong witness even when its geometry distributes material correctly.
Corrective action
The contact area should be compared with the finished thickness map. If the mark is accompanied by a thickness step, plug contour or motion is involved. If thickness is smooth but gloss changes, surface finish and heat transfer are stronger suspects.
The plug should be clean, stable in temperature, and finished for the material. Fabric covers can reduce marking in some processes, but wrinkles, seams, wear, and contamination in the cover can create new defects. Reducing overall sheet temperature to hide a plug mark can damage forming detail and should be avoided.