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Guide

PET Stretch Ratio: Axial, Hoop and Planar Calculation

Calculate axial, hoop and planar stretch ratios from documented drawing dimensions, and understand why geometry checks do not establish a universal processing window.

Direct answer

PET Stretch Ratio: Axial, Hoop and Planar Calculation: key points

Calculate ASR = Lb / Lp, HSR = Db / Dp and planar ratio = ASR * HSR. For this mid-wall convention, Dp = (preform OD + preform ID) / 2 at the same body section; it is not the internal diameter. Use a documented average bottle body diameter Db between shoulder and base, and matching below-neck length datums for Lb and Lp. These drawing ratios describe geometry, not a guaranteed processing window or finished-bottle performance.

  • Review axial and hoop ratios separately: equal planar products can hide different axial and circumferential deformation.
  • Weight alone does not determine either ratio. Obtain preform length, OD and ID rather than inferring dimensions from grams.
  • A change in preform weight may change geometry or wall distribution; recalculate from the revised drawing rather than assuming both ratios rise or fall.
  • Use resin-specific development evidence and bottle trials to evaluate a candidate pair; a published range using different dimensions is not a pass/fail rule.
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 preform mold route Review preform dimensions if trials indicate a geometry change is needed. Bottle blow mold design checklist Bottle drawing inputs needed before the ratios can be calculated. Preform mold vs bottle blow mold Which tooling scope changes when the pair does not fit.
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.

Axial stretch ratio (ASR) Lb / Lp: corresponding stretchable bottle and preform lengths below the neck; record the support-ring and base reference points
Hoop stretch ratio (HSR) Db / Dp, where Dp = (OD + ID) / 2 for the same round preform body section
Planar stretch ratio ASR * HSR; retain both component ratios when reporting the product
Bottle diameter convention Use the average body diameter between shoulder and base, identify its drawing surface convention and section locations, and use the same method for each candidate
Preform diameter convention Measure OD and ID at the same section; mean wall diameter must not be labelled internal diameter
Non-round or tapered geometry Record the selected sections and local dimensions; one average diameter does not describe local shoulder, corner or base deformation
Weight and geometry Preform mass is not an input to these geometric equations; a new weight requires checking whether the drawing dimensions changed
Observed bottle defects Wall thinning, haze or whitening can have several causes; compare geometry with heating, timing, pressure and material evidence
Process development Use resin and equipment guidance to define trial conditions; no universal reheat temperature or stretch-ratio band is specified here
Acceptance Confirm wall distribution, dimensions and application-specific performance on trial bottles, not from the ratio alone
Workflow

Prepare the buyer-side data package.

01 Step 1

Mark corresponding below-neck and base length datums on both drawings. Record the bottle body sections and diameter surface convention used for the average Db.

02 Step 2

Measure the preform OD and ID at the same body section and calculate Dp = (OD + ID) / 2. Record Lp without including the formed neck finish.

03 Step 3

Calculate ASR and HSR with matching units, then multiply them. Compare only with references using the same dimension definitions and relevant resin and bottle conditions.

04 Step 4

Evaluate the candidate using controlled trials and measured wall distribution. Review preform geometry and process settings together before deciding whether tooling needs to change.

FAQ

Questions this guide answers.

What is a good stretch ratio for a PET bottle?

There is no universal acceptance number in this guide. A useful comparison requires the same dimension convention, resin and bottle requirements. Review axial and hoop values separately and confirm the resulting bottle through trials.

Why do published stretch ratio figures disagree?

References can use different preform diameters, bottle sections and length datums, as well as different resin and container designs. A ratio calculated with internal preform diameter is not directly comparable to one calculated with mean wall diameter.

Do I measure the preform including the neck?

No. The neck finish is injection-formed and does not stretch, so axial length is measured from below the support ledge to the preform base. Including the neck inflates the preform length and understates the axial ratio.

Can machine settings compensate for a bad stretch ratio?

Settings do not change the drawing-based ratio, but heating, stretch timing and blowing conditions can affect how material is distributed. An unfamiliar ratio alone does not prove that settings cannot help or that a new preform is required; use trial evidence to decide.

What does stress whitening mean on a finished bottle?

Whitening can be associated with local stretching and temperature conditions, but appearance alone does not identify a cause. Record its location, local wall thickness, preform lot and process settings before testing a geometry or heating hypothesis.