ASTM F88 Specimen Preparation for Width, Conditioning and Test Speed

ASTM F88/F88M measures the force needed to separate a test strip containing the seal, and it identifies how the specimen failed. The seal can join two flexible materials, or a flexible material to a rigid or semi-rigid one.

ASTM describes seal strength as a quantitative measure for process validation, capability and control. That is why preparation matters. If two labs cut, condition or pull specimens differently, each lab ends up measuring its own method rather than the sealing process.

For the scope, test conditions and report contents of the 2023 edition, see our ASTM F88/F88M-23 standard overview.

Cut every specimen in a series to one of the three widths F88/F88M-23 gives for the specimen cutter, which are 25 mm, 15 mm and 25.4 mm (0.984 in, 0.591 in and 1.00 in). The recommended tolerance for the cutting tool is ±0.5%, and any deviation from the tolerance or width needs a documented rationale or supporting data.

Pick one unit system and stay with it. F88/F88M says SI and inch-pound values are not exact equivalents, and combining the two systems may result in non-conformance. A 25 mm strip and a 1.00 in strip are different specimens, so record which one you cut.

F88 requires the specimen cutter to meet section 6.5 of Test Method D882. Check that requirement before you buy or make a cutting die. Our laboratory heat sealers prepare the seals you then test to F88.

Questions about a model or a test? Email Amy Zhao, Export Manager, at info@heatsealtester.com.

F88/F88M-23 refers to ASTM E171 when conditioning before testing is desired and appropriate. E171 defines the standard temperature and humidity for conditioning and testing flexible barrier packaging at nominally ambient conditions.

E171 also explains when conditioning is worth the time. Many packaging materials show no meaningful change across normal lab temperature and humidity, so routine testing often skips it. ASTM says conditioning should be considered in three situations.

• You are comparing results between labs, for example a supplier and a customer.
• Temperature or humidity is expected to affect the outcome.
• Other sources of variation in the results must be kept to a minimum.

Hygroscopic materials are the main concern, because their properties shift with temperature and relative humidity. F88 does not fix a minimum conditioning time. It says heat seal conditioning periods may be set by experiment, long enough for seal strength to stabilize, and that conditioning can be adjusted for specific test objectives.

For freezer, refrigerated or abusive storage conditions, set your own conditions, because E171 does not cover them.

Run the tensile tester at one constant rate of grip separation between 200 and 300 mm/min (8 to 12 in/min), the range F88 sets for the test. Keep the same rate for the whole series. F88 has an appendix on how variation in grip separation rate affects results.

Part of the force you measure can come from bending rather than from the seal alone. F88 therefore describes three ways to hold the specimen tails.

• Technique A, unsupported. Each tail sits in an opposing grip and the seal is left unsupported.
• Technique B, supported 90° by hand. The seal is held loosely at a right angle to the tails, without gripping it.
• Technique C, supported 180°. For flexible-to-flexible seals, the less flexible tail lies flat on a rigid alignment plate.

In Technique C the more flexible tail is folded back over the seal. With rigid or semi-rigid packages, the package structure can be kept on the less flexible side and gripped or held in a fixture.

ASTM recommends using one technique throughout a test series, because each one affects results differently. Some materials move or flip at the tail during the test, and ASTM says this should be reported with the results. If you validate the method, account for any fixture or alignment plate and for which material goes into which grip.

If you make the seals in the lab, our STH-3A touchscreen heat seal tester prepares the seal samples, and seal strength is then measured separately on the tensile tester per ASTM F88.

The table lists key fields to capture for every series. It is not the full report list, which F88 sets out in section 10.1.

Section 10.1 also asks for the sealing and test equipment, ambient temperature and humidity, seal width, which material sits in each grip, and the number of specimens and sampling method. Report average peel force only when the strip peels apart in the seal area, not when separation is mostly breaking, tearing or elongation.

We sell direct from our factory, so you work directly with our factory team. Questions about a model or a test? Email Amy Zhao, Export Manager, at info@heatsealtester.com.