Pre-stretch is intended to distribute material before the sheet contacts the tool. It becomes excessive when the bubble consumes too much available elongation or places material in the wrong region before final forming begins.
Signs include thinning at the bubble apex, rupture before tool contact, weak corners after final draw, and large sensitivity to small changes in air pressure or timing. A bubble can look smooth and repeatable while still being too large for the part geometry.
Correction
Bubble height and shape should be measured rather than judged only by operator appearance. Airflow, pressure, duration, sheet temperature, clamp condition, and unsupported span all affect the result. The target is not the largest stable bubble; it is the bubble that produces the required final thickness map.
Pre-stretch should be reduced or reshaped while the finished part is measured. A lower bubble may require a different plug stroke or vacuum sequence. One variable should be changed at a time because sheet temperature and air timing interact strongly.