PET is valued for its glass-like clarity. When a blown bottle turns hazy or shows white, chalky areas, the root cause is almost always micro-structure and process—how the polymer was dried, heated, stretched, and cooled. Below is a practical, shop-floor-friendly guide that explains what’s happening inside the material and how to correct it without guesswork.

What “Haze” and “Whitening” Really Are

• Optical haze comes from light scattering. In PET, scattering typically arises from micro-crystallites, voids, or orientation non-uniformity that form when the preform is reheated or stretched outside the ideal window.

• Stress whitening appears milky in localized zones (shoulder, base petaloids, handle junctions). It’s caused by crazing/micro-voids from excessive local strain, too-cold stretching, or abrupt cooling.

Clarity returns when you control moisture, heat balance, stretch ratios, and cooling so the wall is uniformly oriented and largely amorphous.

Frequent Root Causes and How They Create Haze/Whitening

1) Inadequate Resin Drying

• Mechanism: PET is hygroscopic. Residual moisture hydrolyzes chains during reheat, lowering IV and creating low-molecular fragments that nucleate crystals → haze.

• Tell-tale signs: Generalized dullness across the body; IV drift; higher AA; brittle parts.

• Targets: Dry to ≤0.005% moisture (≤50 ppm) with hopper air at roughly 160–180 °C, −40 °C dew point, 4–6 h residence, and sealed conveying to the machine.

2) Over- or Under-Heating During Reheat

• Under-heat (too cold): Preform surface stretches below the ideal Tg-to-Tm window; molecules cannot orient smoothly → stress whitening in high-strain zones (shoulder, base).

• Over-heat (too hot/hot spots): Local crystallization before blowing and slow quench → haze bands or pearlescent patches.

• Practicals: PET preform skin typically needs a controlled gradient with internal core slightly cooler. Lamp zoning, rotation speed, and reflectors must be tuned to avoid hotspots at gate and shoulder.

3) Aggressive or Uneven Stretching

• Mechanism: Excess local draw ratio or jerk (high strain rate) forms micro-voids/crazes → whitening.

• Watch points: Rod speed profile, pre-blow timing/pressure, and hoop vs axial draw balance. As a rough envelope, axial 3–3.7× and hoop 2–2.8× are common for beverage bottles; skewing too far in one direction invites whitening.

4) Mold and Cooling Problems

• Too-cold mold or poor venting: Trapped air and sudden quench freeze in orientation gradients, causing haze and flow marks.

• Uneven cooling circuits: Temperature deltas across cavities make one panel consistently cloudy.

• Targets: Stable mold water (often ~10–15 °C for CSD/water), verified flow, clean vents, and polished optical surfaces.

5) Preform Quality Issues

• IV outside spec or degraded regrind: Encourages crystallite formation and dullness.

• Gate blush and cold spots: Create bands that turn cloudy after blowing.

• Ovality and wall non-uniformity: Force uneven hoop stretch → whitening on thin zones.

6) Contamination and Additives

• Color masterbatch agglomerates, dust, or metal fines: Act as nucleants for spherulites → scattered haze.

• Excess slip or anti-stat: Can cloud the surface under heat and pressure.

7) Utilities and Air Quality

• Compressed air with oil/water: Deposits on hot walls, dulling gloss and clarity.

• IR oven instability: Voltage swings shift lamp output and heat profile, creating batch-to-batch variability in haze.

Quick Symptom-to-Cause Guide

Process Targets That Protect Clarity

• Drying: Moisture ≤50 ppm; dryer inlet air ≤−40 °C dew point; purge lines closed; sealed transfer.

• Reheat: Uniform external skin 100–120 °C depending on grade and wall; maintain a cooler core for bubble stability.

• Stretch: Smooth rod profile (multi-stage or S-curve), balanced axial/hoop ratios, pre-blow pressure timed after initial rod penetration to avoid shock draw.

• Cooling: Stable mold temperature with verified flow per circuit; clean vents; no oil carryover in plant air.

Step-By-Step Troubleshooting Playbook

1. Verify resin and dryness. Measure moisture; if high, increase residence time or temperature and confirm dryer performance with a dew-point meter.

2. Map the preform temperature. Use contact thermocouples or IR camera to capture shoulder, body, and gate temperatures; adjust lamp zones and preform rotation to remove bands.

3. Re-profile stretching. Reduce first-stage rod speed, add a plateau, and delay pre-blow slightly to cut strain rate peaks.

4. Balance mold cooling. Flush scale, confirm flow with a flowmeter, and equalize inlet temps.

5. Audit contamination. Inspect loaders, lines, and hoppers; check masterbatch dispersion; refresh filters.

6. Stabilize utilities. Verify compressed air dryness/filtration and oven voltage stability; replace aging IR lamps.

Preventive QA and SPC

• Moisture log: Record every hopper discharge—keep moisture, dryer outlet temp, and dew point in control charts.

• Oven recipe control: Lock validated lamp profiles by preform weight/IV; recalibrate after lamp replacements.

• Stretch/blow recipe: Document rod speed curve, pre-blow and blow pressures/timings, and mold temp.

• Optical QC: Use haze meters and transmittance tests on blown panels; correlate with cavity and timestamp.

• Material management: Limit regrind %, standardize masterbatch let-down, and segregate lots to avoid mixing IVs.

Engineering Tips for Specific Zones

• Shoulder whitening: Raise shoulder zone temperature slightly, smooth the initial rod ramp, and increase pre-blow delay by a few milliseconds.

• Base haze in petaloids: Balance base cooling circuits, reduce hot-spotting from central lamps, and ensure preform base isn’t overheated relative to body.

• Panel banding: Re-aim reflectors and shields; verify preform rotation speed so the same sector isn’t repeatedly over-exposed.

Example Setup Window (Illustrative)

Values are typical starting points. Always validate against your resin grade, preform design, and bottle geometry.

Design Choices That Reduce Risk of Haze

• Preform design: Adequate wall in high-stretch zones, controlled gate geometry to prevent hot bands, and low ovality for even hoop draw.

• Material selection: Consistent IV grade for the target size; keep regrind within validated limits.

• Color system: High-dispersion masterbatch with verified filtration; avoid additives that migrate under heat.

Choosing Equipment That Makes Clarity Easier

Machines with precise, repeatable control reduce haze-related downtime:

• Closed-loop IR ovens with lamp-by-lamp power control and real-time preform temperature feedback.

• Multi-segment stretch-rod profiles and servo control for smooth axial draw.

• Zoned blow control for staged pre-blow and main blow pressure ramps.

• Balanced mold cooling manifolds with inline temperature and flow monitoring.

• Clean, dry air management and integrated dew-point monitoring for dryers and compressed air.

If you are evaluating new equipment or retrofits, consider platforms that package these controls and provide recipe management and traceability.

A Practical Partner for Clear PET Bottles

If you’re looking to stabilize clarity across different bottle sizes and resins, BOHANG offers stretch-blow systems with fine-grained oven zoning, servo stretch control, and robust cooling circuit design. Their process-recipe management and diagnostic tooling make it straightforward to lock in the window that prevents haze and stress whitening while maintaining cycle time.

Takeaway Actions You Can Apply Today

• Measure and document moisture before production; don’t rely on time-in-hopper alone.

• Map preform temperatures and remove hot/cold bands with lamp zoning and rotation tuning.

• Re-profile rod speed and pre-blow timing to eliminate strain spikes.

• Balance mold cooling and confirm flows with instruments, not assumptions.

• Standardize material inputs (IV, regrind %, masterbatch) and keep loaders, lines, and air systems clean and dry.

With disciplined drying, a stable reheat profile, balanced stretching, and controlled cooling, PET bottles maintain the high transparency that end-users expect—and your line spends less time troubleshooting and more time producing.