Heat Seal & Peel Strength Testing of Flexible Packaging: F88, F2029 & the Seal Curve

Every pouch, sachet and bag lives or dies at its seal. Too cold a seal leaks; too hot a seal burns through and embrittles. Two instruments define seal quality: a laboratory Heat Sealer (per ASTM F 2029) that makes seals under precisely controlled temperature, pressure and dwell time — and a Peel Tester that measures their strength per ASTM F 88.

Together they generate the seal curve — the single most useful chart a flexible-packaging lab produces.

Making Seals: The Laboratory Heat Sealer

Production sealing jaws vary; a laboratory heat sealer removes that variation. Jaw temperature is controlled to ±1 °C, pressure is set by regulator, and dwell time by timer — so a seal made today is identical to one made next month. By sealing specimens across a range of temperatures at fixed pressure and dwell, you map exactly where your film starts sealing and where it starts degrading.

Measuring Seals: ASTM F 88 Peel Testing

Sealed strips 15 or 25 mm wide are pulled apart in a tensile frame at constant speed, with the tail either supported, unsupported or held at 90° — the standard's three techniques, which give different numbers and must be reported. Results are recorded as peak and average seal strength in N/15 mm or N/25 mm, along with the failure mode: peel, delamination, break at seal, or material break.

• Seal initiation temperature — where measurable strength first appears.
• Plateau strength — the stable maximum across the operating window.
• Operating window width — wide windows forgive machine variation; narrow windows cause production leakers.
• Hot tack — strength while still warm, critical for vertical form-fill-seal lines dropping product onto fresh seals.

Why the Seal Curve Pays for Itself

A documented seal curve lets converters set production jaw temperatures scientifically, qualify alternative resins or recycled-content films in days, and defend against customer leak complaints with data. Pair these instruments with tensile and dart impact testing to complete the flexible-packaging bench — see our packaging industry testing equipment overview.

Frequently Asked Questions