The seal specimen is strong. Why does the production pack leak?

A flat specimen and a production bag see different conditions

The laboratory may use clean, flat film. Production also involves roll condition, tension, tracking, print registration and product entry. Wrinkles or product in the sealing area change the joining surfaces. First retain evidence of the actual defect location.

For the example, photograph the bag with its orientation and seals identifiable. Record collection time, film lot, speed, settings and whether the defect followed a stop or restart. Retaining good and defective packs from the same conditions is more informative than an isolated settings list.

Ilhan and colleagues review material, process, contamination and later influences on seal integrity.[1] That map helps avoid attributing every leak to temperature; it does not diagnose this particular line.

Separate the hot seal from the cooled seal

For heat-sealed films, resistance immediately after joining differs from strength measured after cooling. The former is commonly called hot tack. Labthink’s technical explanation distinguishes hot-tack performance from ultimate seal strength.[2]

If film tension or product load acts immediately after sealing, a cooled specimen result does not describe that moment. Conversely, good early resistance does not establish the package condition after storage and handling. Select a test according to the stage and question it must address.

Results need the method details that make them comparable: specimen width, direction, test speed, sealing conditions and the relevant cooling or waiting period. Different test methods should not silently share one ranking column. Agree applicable methods and acceptance values for the intended package.

Strength and leakage describe different properties

A seal-strength test measures resistance to separating a specimen. A leak evaluation asks whether the completed package contains a path through which its contents or gas can pass. A strong section does not rule out a channel beside it.

For this comparison, record how each specimen failed as well as the force—for example, whether the seal opened or the film tore. That keeps the number connected to what actually happened in the test.

The enlarged diagram illustrates one possible channel at a wrinkled seal. It is not a claim that all leaks have this geometry. Choose an evaluation suited to the package and contents, with the type and size of defects it can detect understood.

Link strength and leak results through sample IDs and collection conditions. Where separate tests destroy the pack, use separate samples taken under matched conditions. “Strong but leaking” need not be contradictory: the tests describe different properties. No defect detected also does not establish the absence of defects beyond the method’s detection capability.

Preserve an explanation for an improvement

Changing temperature, speed, pressure and contact time together may reduce leaks without revealing why. Establish a reference with the current film and change planned factors for the candidate. Do not equate the machine’s displayed temperature with the heat actually experienced at the interface.

Begin with a limited comparison; where factor combinations matter, extend the design with the equipment and material teams. Changing one factor at a time does not reveal every interaction. Set authorised ranges, stopping criteria and sampling methods before trial work.

Include real operating states such as startup and restart, not only stable running. Connect feeding, wrinkles, registration, early sealing, cooled results and handling. Sample quantities and tests should follow the risk and question rather than the convenience of the available bags.

One good bag does not establish an operating range

Record the conditions authorised for use, the evidence within that range, unacceptable states and pending tests. Also identify what must be reassessed when material specifications or equipment change.

For this film, the next investigation is the relationship between wrinkling and leaks after conveying. Following where the bag changes condition creates options beyond repeatedly raising temperature.