Draw lines are visible streaks or bands that follow the direction of material movement. They may be cosmetic only, but they often reveal a nonuniform strain path or an incoming-sheet variation.
Extrusion gauge bands and die lines can become more obvious after stretching. Thermal gradients create adjacent regions that move at different rates. A plug edge, surface contamination, or local sticking can also leave a line that resembles a flow mark.
Trace the line to its origin
The unformed sheet should be examined under directional light and measured across its width. If the line stays aligned with the extrusion direction when the sheet is rotated, the source is the sheet. If it stays fixed to the mold or plug position, tooling or heating is responsible.
Corrective action depends on that distinction. Heater balancing will not remove a die line, and changing sheet stock will not correct a rough plug edge. Wall-thickness mapping can confirm whether the line is associated with a real change in strain or only a surface appearance difference.