A sheet that reaches different temperatures across its surface will not stretch uniformly. Hot regions deform first and become thin; cooler regions resist motion and may lose detail or tear. The resulting defects include asymmetric draw, differential sag, gloss variation, webbing, and warpage.
Likely causes
Typical sources are failed or aging heater elements, contaminated reflectors, incorrect zone settings, uneven heater spacing, drafts, clamp-frame heat loss, and inaccurate temperature measurement. Sheet gauge, color, surface finish, and pigmentation can also change radiant absorption even when the oven field is stable.
Diagnostic sequence
The defect should be mapped against the oven layout. A failure that stays in the same machine position suggests a heater, reflector, control, or airflow problem. A failure that moves with the sheet or batch suggests material variation. Surface temperature must be checked at several points immediately before forming; one center reading is not enough for a large sheet or multi-cavity array.
Faulty hardware should be repaired before zone settings are used to compensate. Local screens may be appropriate for controlled pattern heating, but their position and specification should be documented as part of the recipe.