The seal window is the whole job

A seal window is the range of temperature, pressure and dwell time in which a material seals reliably. Laminates built over decades have wide ones. Newer mono-material structures often have narrow ones, and a narrow window is not a defect — it is a constraint that has to be designed around.

The failure we see most often is not that the window is too narrow. It is that nobody measured it on the line that has to hold it.

Why lab windows mislead

A lab sealer holds temperature to within about a degree and applies even pressure across a flat platen. A production jaw on a vertical form-fill-seal machine does neither. It has a heater on one side, a temperature gradient across its width, and tooling that expands as the shift goes on.

We have measured a nine-degree spread across a single jaw at hour three. A material with a twelve-degree window survives that. A material with a six-degree window does not, and the failures start after lunch.

What we measure now

Three things, on the client's own machine:

Window width at temperature, measured across the full jaw rather than at the centre. Drift across a shift, logged from cold start to hour eight. Seal strength after conditioning, because a seal that passes at the machine and fails at four weeks is the expensive kind.

The number that matters

We look for a window at least twice the measured drift. Below that, the substitution can still work, but it needs a tooling change or active temperature control, and that cost belongs in the business case rather than in a surprise six months later.