A hazy, milky or pearlescent patch in a clear PET bottle is one of the most common cosmetic rejects — and it is almost always a process or design signal, not a resin fault.
What stress whitening is
PET clears because biaxial stretching aligns the polymer into a fine, transparent structure. Push the stretch too far — or stretch cold material — and you get micro-voids and crystallites that scatter light: the wall looks white, hazy or pearlescent. It usually shows in the most-stretched zones — the base, the shoulder, or thin sidewalls.
The stretch-ratio link
Every preform–bottle pair has a usable window of hoop, axial and overall (areal) stretch ratios. Stay inside it and the wall is clear and strong; exceed it — typically by blowing a large bottle from too small or too light a preform — and you over-stretch into whitening. Under-stretch (too heavy a preform) wastes material and can leave the wall under-oriented. The fix is to size the preform to the bottle, not to chase it on the blow-moulder.
Process causes to check
- Preform too cold — low reheat or oven imbalance; cold PET stretches unevenly and whitens.
- Stretch-rod / pre-blow timing — a late rod or weak pre-blow lets the material thin and tear-stretch.
- Preform too light for the bottle — pushes areal stretch past the window.
- rPET / IV drift — lower or inconsistent IV narrows the process window.
Design it out before tooling
Validate the stretch ratios for your bottle–preform pair before you commit to tooling: compute HSR, ASR and BSR in the Preform Engineering calculator and confirm the right starting weight in the preform weight guide. If whitening is already in the field, isolate it against other faults in the Defect Library. Delta El Nile for Industry will recommend the preform weight and neck that keep the wall clear on your line.