Guide

PET Bottle Defects: Haze, Pearlescence and Rocker Bottom

Investigate haze, pearlescence, rocker bottoms and uneven walls using defect location, material records and controlled process comparisons.

Direct answer

PET Bottle Defects: Haze, Pearlescence and Rocker Bottom: key points

Treat the visible defect as a starting observation, not a diagnosis. Record its location, wall thickness, preform lot and blowing station, then compare heating, timing, pressure and cooling measurements. Whitening can involve local stretching conditions; haze can involve material or thermal history; an unstable base or uneven wall can involve process, preform or tooling conditions. Use controlled trials and measurements to distinguish these possibilities.

Keep accepted and rejected samples with their preform lot, station, time and process settings so comparisons are traceable.
Map the defect through the wall and around the bottle; visual location alone does not distinguish every material, thermal or surface cause.
Check preform appearance and material records as well as reheating conditions when investigating haze.
Persistence across trials is a reason to review the investigation, equipment condition and geometry, not proof that new tooling is required.

Review status

What this page is reviewed against.

ClusterBlow machine selection

Last reviewed2026-07-15

Use this page forShortlisting, RFQ preparation and evidence-aware route selection

Factory and route evidence

Public references linked to this buying decision.

These links point to public route pages, reference media or downloadable worksheets that buyers can review before requesting a project-confirmed quote.

PET stretch ratio calculation Document geometry using consistent dimensions without treating ratios as a diagnosis. PET resin drying parameters Check material preparation evidence when degradation is suspected. Bottle blow mold design checklist Mold inputs to check when the defect is geometric.

Decision table

Use this guide to avoid vague machine or mold quotes.

Each row converts a common buying question into the exact data engineering needs before final route selection.

Pearlescence or whitening	Record affected location and thickness; investigate local temperature, deformation and preform geometry without inferring the cause from colour alone
Thin affected area	Compare the wall map with an accepted sample and review how heating and pre-blow timing distribute material
Thicker affected area	Check the local thermal profile and stretching sequence before choosing a controlled adjustment
Suspected thermal haze	Compare preform and bottle clarity with their thermal histories; no universal temperature threshold is asserted here
Suspected material degradation	Review resin moisture, injection records and relevant material tests; haze alone does not prove hydrolysis
Rocker bottom	Record base shape and temperature at discharge and after cooling; check base cooling, material distribution and mold condition
Uneven wall thickness	Map thickness around the bottle and check preform centring, heating uniformity, timing, pressure and station condition
Blowout or unfilled corners	Distinguish rupture from incomplete forming; record pressure behaviour, preform condition, temperature and defect location
Persistent defect	Review whether trials isolated the suspected variables, then examine calibration, wear, material and geometry before selecting a remedy
RFQ-only confirmation	The actual cause on your line is confirmed by engineering against bottle drawing, preform data and process records

Workflow

Prepare the buyer-side data package.

01 Step 1

Photograph and map the defect, measure the affected wall or base, and retain an accepted comparison sample.

02 Step 2

Record preform lot and blowing station, then compare whether the problem follows the lot, station or operating period without treating correlation as proof.

03 Step 3

Form a testable hypothesis from the material and process measurements; change one approved variable while keeping a baseline record.

04 Step 4

Repeat the check and compare acceptance results. If unresolved, send drawings, samples and trial logs for a combined material, machine and tooling review.

FAQ

Questions this guide answers.

What causes pearlescence in a PET bottle?

Local stretching and temperature conditions can contribute, but a visual description is not enough to select a remedy. Locate the whitening, measure the wall and compare the thermal and stretching sequence with an accepted sample.

How do I tell moisture haze from crystallisation haze?

Use resin moisture and injection records, preform inspection and controlled thermal trials together. A response to heating or a link to one preform lot is evidence to investigate, not a conclusive test separating every cause of haze.

What is a rocker bottom and how is it corrected?

The bottle base does not sit stably on its intended contact surface. Compare base geometry at discharge and after cooling, then check thermal conditions, material distribution and tooling. Choose a correction only after identifying which condition differs from accepted production.

Why is my wall thickness uneven around the bottle?

Possible contributors include preform centring, temperature distribution, blowing and stretch timing, pressure behaviour and equipment condition. Map the wall and compare stations and preform lots; the geometry ratio alone cannot distinguish these causes.

When should I stop adjusting settings and change the preform?

Consider a geometry change when drawing review and controlled trial evidence support it, after checking material and machine conditions. Persistent defects alone do not establish that the pair is outside a universal stretch window or that new tooling will solve the problem.