Nominal sheet thickness does not describe cross-web profile or batch variation. A thicker region heats more slowly and supplies more material; a thinner region heats faster, sags more, and may fail during draw. The same recipe can therefore produce different parts from sheets within a broad purchase tolerance.
Gauge should be measured at defined locations and linked to the formed-part orientation. The profile should be compared with sag and wall-thickness maps. A defect that moves when the sheet is rotated or comes from a new pallet is strong evidence of incoming variation.
Material specifications should include the profile and local tolerance needed by the tool, not only average thickness. Process windows may accommodate normal variation, but heater zoning should not be used to compensate for an uncontrolled or shifting extrusion profile.