Pearlescence (stress whitening)
Cause
Over-stretching cold preform or excessive orientation causes pearly white appearance.
Corrective action
Raise preform temperature, reduce stretch ratio, balance pre-blow; verify TSR within 8–12 range.
Diagnosis guide: Pearlescence / white haze in body
What you will see
Body wall shows pearly white shimmer or milky haze, particularly visible against dark backgrounds. The bottle is structurally intact but loses optical clarity. Typically appears uniformly across body or in stretched zones.
Likely causes
- Preform body undercooled — insufficient orientation during stretch
- Stretch rod velocity too high — material did not orient before main blow
- Pre-blow pressure too low — bottle did not pre-stretch enough
- Lamp output decayed (typical at >2000 hours)
Do this first
Increase preform body temperature by 2-3°C and rerun a 30-bottle sample.
How to diagnose it
- Inspect last 30 bottles: is pearlescence uniform, or localized to specific zones?
- Check lamp hours log — lamps >2000 hr typically lose 15-25% output
- Measure preform surface temperature at oven exit with IR pyrometer
- If localized to one cavity, the issue is cavity-specific (lamp or valve)
How to fix it
- Increase lamp power in middle zones by 5-10%, hold one cycle to stabilize
- If still pearlescent, reduce stretch rod velocity by 0.1 m/s
- Check oven exit temperature with IR pyrometer; should be in target range for application
- Verify neck cooling water flow — overcooling neck can radiate down into body
- Inspect lamps for end-of-life output drop (typical at >2000 hours)