Wrinkles are compression folds within the part geometry. They are not the same as webbing between separate mold features. A wrinkle forms when material is forced into a shorter path faster than it can redistribute or thicken.
The location reveals the mechanism. Lower-corner wrinkles on positive tools often result from material being compressed as the sheet is pulled down the sidewalls. Wrinkles in grooves or recesses occur when some regions contact early and force later-contacting material into compression. Flat-area wrinkles may simply indicate that the heated sheet has more surface area than the tool can absorb, often after excessive sag.
Correction
The forming sequence should be adjusted so material is stretched and positioned before compression develops. Pre-stretch, plug assist, downholders, or controlled initial vacuum can change first-contact points. Sheet temperature must support viscoelastic redistribution; a sheet that is too cold cannot compress cleanly, while an overly soft sheet may fold before strain spreads.
Persistent wrinkles tied to the same geometry require a tool or process-aid change. Increasing vacuum force after the fold has formed rarely removes it and can sharpen the crease.