Blister Seal Strength Testing: Peel, Burst and Which Limit to Write Down

Why the number alone is not enough

Two suppliers can both quote "seal strength above 1.5 N/15 mm" and mean different things. One measured peel on a strip cut from the flange, the other burst-tested a whole cavity. Both are valid tests; they answer different questions, and the acceptance limit belongs to the method.

That is why the specification has to carry three items together: the method, the sample position and the acceptance rule. Miss one and the number stops being a specification and becomes a marketing line.

Peel and burst answer different questions

A peel result is a material and parameter signature. A burst result is a pack-level statement. A specification that needs both should say so, rather than quoting whichever figure looks stronger.

Where the sample comes from

Seal strength varies across the cavity and around the flange. A sample cut where the seal is flat and wide will read higher than one cut at a corner where the foil bridges a radius. State the sampling position: which side of the cavity, how far from the corner, and how many cavities per run.

The same logic applies to incoming inspection of the lidding or cavity film. Material tests on a flat laboratory sealer do not represent the geometry of a production tool, which is why line trials are still part of qualification.

Which limit to write down

1. One method per clause. If peel is the release criterion, say peel, and state the strip width and test speed. 2. A position rule. Define where the strip is taken, so results are comparable between batches and suppliers. 3. A statistical basis. A mean alone hides weak cavities; consider a minimum value per sample set as well. 4. A pack-level check. Add burst or leak testing for products where a single failed cavity is a recall risk. 5. A handling clause. Seal performance depends on plate flatness, pressure and dwell; record the machine parameters that produced a passing result.

How seal failures usually start

Most seal complaints trace back to four causes: product or dust in the seal area, worn or uneven sealing plates, parameters drifting outside the validated window, and a lidding lacquer that does not match the cavity material. The first two show up as channels in visual inspection; the last two show up as a batch-wide shift in peel values.

Recording parameters with every batch turns a complaint into a comparison. Without them, the investigation restarts from zero each time.

What to send with a seal complaint

• The batch number of the lidding and the cavity film
• Seal parameters from the run, including plate condition
• Peel or burst results from the batch and from the previous passing batch
• A photograph of the failed seal, with the position marked
• The product and the cavity geometry

Our engineers review seal data against the nominated [standard blister PVC](/products/standard-blister-pvc), [PTP aluminium foil](/products/ptp-aluminum-foil) or [high-barrier PVC](/products/high-barrier-pvc) pairing and return the parameters that produced the validated result. If the issue sits with the structure rather than the machine, the comparison notes in our technical articles usually point to the right test. Send the record through the contact form and we will work from the data rather than from the sample alone.