Edge curl is the upward or downward movement of a flange or trimmed perimeter. It is common where one side cools faster, the flange has uneven thickness, or trimming releases a stress gradient through the sheet.
The clamp frame and surrounding web can hold the edge flat until release. If the edge curls only after trimming, the formed part contains an unbalanced stress state rather than a simple cooling-time problem.
Corrective action
Temperature and thickness should be compared across the flange. Cooling air, mold contact, and trim support must be balanced. The part should reach a repeatable stable condition before final dimensional inspection.
A fixture can support the edge during cooling, but the geometry should also be reviewed. A small return, bead, or controlled curvature may provide more stiffness than a broad unsupported flange. Increasing overall cooling time will have limited effect if one side continues to cool much faster than the other.