Corner thinning occurs where material must travel the farthest or contacts the tool last. In a deep female cavity, the bottom corners are common risk areas because much of the sheet has already contacted sidewalls before those regions finish stretching.
The defect is controlled by more than draw depth. Corner radius, cavity width, wall angle, first-contact sequence, sheet temperature, plug shape, and local friction determine how much material reaches the corner.
Diagnose the distribution path
Thickness should be measured along a section from flange to base rather than only at the thinnest point. A progressive reduction down the wall suggests insufficient pre-distribution. A sudden thin spot at one radius points toward local geometry, chilling, or plug contact.
Plug assist, bubble pre-stretch, local heater zoning, or a change in mold orientation can move material toward the corner before final forming. Increasing starting gauge may raise minimum thickness but does not correct an unstable distribution pattern. The solution should be validated across the full process window, not only on one acceptable sample.