Damage outside the finished trim line can still initiate a tear during heating and stretching. Nicks from saws, shears, pallet straps, forklifts, or manual handling concentrate stress at the clamp boundary and may open only after the sheet softens.
Sheet edges should be inspected under lighting that reveals small cracks and delamination. The tear origin should be traced backward; a smooth fracture beginning at a notch is different from a ragged hot rupture in the formed wall. Damage that repeats at the same loading position may come from the machine rather than the supplier.
Cutting and handling methods should produce supported, clean edges. Sharp machine contacts and loading guides must be repaired. Increasing heat or clamp force to overcome a damaged edge usually moves the failure without removing the initiation point.