Sag is the downward movement of a heated sheet under its own weight. Some sag is normal in many cut-sheet processes. It becomes a defect when the sheet approaches heaters or machine components, develops an unstable shape, shifts the pre-stretch bubble, or changes material distribution before forming starts.
Read the sag pattern
Uniformly deep sag usually indicates excessive overall heat or insufficient melt strength for the unsupported span. A tilted, twisted, or locally deep profile points to uneven temperature, gauge variation, residual orientation, airflow, or clamping. Cycle-to-cycle changes can also come from inconsistent regrind or a dirty or misaligned sag sensor.
Stabilize the process
Sheet temperature should be returned to the grade-specific forming window. Heating time, zone balance, and heater distance must be reviewed as one system. Controlled air support can keep the sheet clear of lower heaters, but it should not conceal an incorrect thermal profile.
For deep draws, pre-stretch must begin from a repeatable sheet condition. Bubble pressure and timing cannot compensate reliably for a different sag depth on every cycle. Production records should therefore include an acceptable sag position or geometry, not only oven time and heater percentages.