Plug chilling occurs when contact removes heat from the sheet faster than the surrounding material can continue to deform. The frozen region may show a dull mark, stress whitening, a thickness step, or a tear near the plug edge.
Plug material, temperature, thermal mass, contact time, speed, and surface pressure all affect heat transfer. A plug temperature that works for one polymer or gauge may be unsuitable for another.
Correction
Plug surface temperature should be measured after thermal equilibrium, not only before startup. The contact sequence should be reviewed to determine whether the plug remains against the sheet longer than necessary before final forming pressure develops.
Temperature control, insulating plug materials, suitable coverings, smoother motion, or revised timing may reduce chilling. The plug must still remain dimensionally stable and resist sticking. No universal offset from sheet temperature should be applied without trials on the actual material and part.