High-impact polystyrene should form with useful ductility, but grade selection, aging, recycled content, orientation, and temperature strongly affect its fracture behavior. Brittle splits may appear at radii, clamp transitions, trim edges, or highly stretched features.
The fracture should be examined for necking and whitening. A clean sharp split during forming points toward insufficient local ductility, while a thin stretched rupture suggests excessive heat or draw. Material batch, impact-modifier level, regrind history, and storage should be compared when the heat profile is stable.
Sheet temperature should be restored to the grade-specific forming range and sharp transitions reduced. Regrind and aged material should be controlled according to appearance and structural requirements. Additional heat is not a complete solution when the grade or sheet condition lacks the required impact performance.