Flow marks are streaks, waves, or bands produced as adjacent areas of the sheet move differently. They often follow the direction of stretch but can also trace a plug edge or a contaminated tool region.
Uneven temperature changes local viscosity. Plug friction, surface pickup, and early contact can make one region stop while another continues to move. Some apparent flow marks are actually extrusion die lines enlarged by forming.
Diagnosis
The sheet should be rotated relative to the tool. A mark that rotates with the sheet originates in the stock; one that stays with the mold, heater, or plug position is process-related. Thickness measurements can show whether the visual line represents a real strain difference.
The correction should target the source: balance heat, clean and refinish contact surfaces, adjust plug motion, or reject inconsistent sheet. Increasing pressure may sharpen detail but will not remove a flow path created before final contact.