PCO 28mm is one of the most common neck finishes for beverage preforms. When injection problems appear, they usually fall into a few familiar patterns. Here are four common ones — with causes and fixes.
1. Whitening at the Preform Bottom
A white, hazy patch at the base of the preform. The PET at the gate area is over-stressed or too cold. Low hot runner nozzle temperature and fast first-stage injection are common culprits.
Raise the nozzle and melt temperatures in small steps. Check that the gate area is properly polished. Slow the first-stage injection until the patch clears.
2. Opaque or Cloudy Preforms
PET must stay under 50 ppm moisture before molding. Above that, the whole preform turns milky instead of crystal clear. Moisture is the number one suspect.
Confirm drying: typically 160–175°C for 4–6 hours. Check the dryer dew point and the material feed. Under-dried resin or a failing dryer both show up here.
3. Haze on One Side
One-sided haze points to unbalanced fill. The gate sits off-center, or holding pressure is uneven across cavities. The cloudiness shows only on the thin side.
Reduce injection and holding pressure to balanced levels. Verify the gate position is centered. Check individual nozzle temperatures across the hot runner.
4. Yellowing or Silver Streaks
Turning up the heat feels like a fix. It cooks the PET instead. Yellowing or silver streaks mean the resin is degrading from excessive heat or shear.
Lower the melt temperature. Reduce back pressure if excessive. Check residence time in the barrel. Purge with fresh resin until the color clears.
If the screw or barrel is worn, schedule maintenance.