Bubbles / voids
Cause
Moisture in resin or trapped air/gas creates internal bubbles. Dry the resin thoroughly and vent trapped gas to prevent them.
Corrective action
Dry resin properly (dew point -40°C), improve venting, reduce melt temperature.
Diagnosis guide: Bubbles or voids in the preform wall
What you will see
Small bubbles, or a single hollow void, inside the preform wall, usually in the thickest section near the gate or the neck transition. They stretch into visible marks in the bottle and are weak points under pressure.
Likely causes
- Hold (pack) pressure or time too low, so the thick section shrinks away inside and leaves a vacuum void
- Wet resin: moisture turns to vapour in the melt and also lowers IV
- Gas trapped by poor venting, or a melt temperature high enough to degrade PET
Do this first
Inspect 30 preforms under bright transmitted light.
How to diagnose it
- Backlight 30 preforms and rotate them slowly; note where each bubble or void sits
- One void in the same thick section on every preform points to hold pressure; scattered small bubbles point to moisture
- Note the cavity numbers of affected preforms
- If moisture is suspected, ask Delta for the lot IV on the Certificate of Analysis — wet resin also lowers IV
How to fix it
- Quarantine the lot
- Contact Delta with photos, positions and cavity numbers