Brittle tearing is a sharp split produced when the sheet cannot elongate at the required rate. The fracture usually shows little necking, unlike a hot rupture where the polymer has lost melt strength and thins severely before breaking.
Insufficient or uneven sheet temperature is a frequent cause. Cold mold features or a cold plug can freeze a contact zone while adjacent material continues to move. Sharp radii, excessive draw depth, damaged sheet edges, aging, unsuitable grade selection, and aggressive mechanical motion reduce the remaining forming margin.
Diagnosis
A tear that repeats at the same corner points to local geometry or chilling. Random tears near the clamp suggest edge damage, poor grip, or uneven heating. If added heat changes the defect from a clean split to extreme thinning, the original condition was too cold but the revised condition may now be too hot.
The correction should restore ductile forming without sacrificing melt strength. Tool contact temperature, plug motion, vacuum timing, radii, draw ratio, and starting gauge should be reviewed together. Notches and saw damage outside the finished trim line can still propagate into the formed area and should not be ignored.