Preform Engineering Report · Stretch-Ratio Analysis
Preform engineering calculator: understand the stretch.
Enter your bottle target and a candidate preform to compute stretch ratios (ASR/HSR/TSR), predicted bottle wall, section material distribution and CO₂ retention — the engineering numbers behind a sound preform spec.
How biaxial stretching works
Why TSR matters
When PET is stretched in both directions at ~120 °C, its polymer chains align — like wood grain. This biaxial orientation triples tensile strength, halves CO₂ permeability, and improves clarity. Below TSR 5 the bottle is amorphous (weak, hazy). TSR 5–8 is the practical zone for most CSD 500 ml designs — workable but needs careful base design. TSR 8–12 is the optimum for water and juice bottles. Above 13: over-stretching causes splits.
CO₂ retention for CSD
A CSD bottle loses ~15% of its CO₂ over shelf life; beyond that, it tastes flat. Loss rate is proportional to surface area / wall thickness. Halving the wall doubles the CO₂ loss rate. A lightweight preform that's perfect for water can fail CSD shelf life because its wall section is too thin to retain pressure.
Lightweight design with ribs
Below typical preform weight for the volume, the bottle's structural rigidity drops. Compensate with horizontal ribs (3–5 in the body), petaloid base for pressure, and vacuum panels for aseptic. Ribs add 15–25% top-load and buckling resistance without adding material.
Filling pressure & preform length
CSD filling at 4–5 bar requires a preform long enough to achieve ASR ≥ 1.8 — shorter preforms give insufficient axial stretch and the bottle balloons under pressure. For tall bottles (1.5 L, 2 L), use a longer preform body even if weight stays the same. The advisor flags this when ASR < 1.8.
Nitrogen dosing for lightweight water
Liquid Nitrogen Dosing (LIN) injects a precise droplet of LN₂ (~0.2 ml) into the bottle headspace just before capping. As it vaporizes, it expands ×682 — pressurizing the bottle to ~1-2 bar internal. This "rigidification" lets thin-walled lightweight bottles survive stacking, conveying, and shipping without panel collapse. Industry-proven by Linde, Messer, Gulf Cryo and Sidel (StarLite Nitro base). Enables 30-40% lighter still water bottles vs. non-pressurized designs. Requires a LIN doser on your filling line — usually installed between the filler and the capper. Ozone, by contrast, is a sanitation step and does not strengthen the bottle structurally; the two are often used together (ozone for sterility, N₂ for rigidity).
Bottle shape effects on stretch
Stretch ratios are calculated from cylindrical geometry, but real bottles are rarely perfect cylinders. Oval bottles: the major axis sees ~15% higher hoop stretch than the nominal diameter suggests — design with a shorter preform to compensate. Square & rectangular bottles: corners receive ~2× the hoop stress of a round equivalent because PET resists stretching tangentially around sharp edges. Specify a corner radius of at least 12 mm and use a heavier preform (+10-15%) to avoid corner thinning and cracking. Rectangular long-axis: another ~20% HSR amplification on top of corner stress. Waisted / hourglass: the narrowest section may not stretch fully (preform doesn't reach the wall), risking thick localized walls — needs a specialty mold and validated FEA. The advisor applies these multipliers automatically when you pick a shape.
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Certifications: FSSC 22000 V6 · ISO 9001 / 14001 / 22000 / 45001 · EcoVadis · SEDEX. Stretch ratios and predicted bottle wall are engineering estimates from your inputs — final preform specification confirmed by Delta El Nile engineering before order. · © 2026