Seal strength is generally measured using ASTM F88, where a strip cut across a heat-sealed sample is pulled apart in a tensile tester at a controlled rate, and the resulting force, usually reported in pounds per inch of seal width or newtons per 15 millimeters, defines the seal strength. The test also notes the failure mode, meaning whether the seal peeled apart cleanly at the bond line, or whether the film itself tore or stretched before the seal gave way, which tells an engineer something different about what's actually limiting performance.
Seal strength depends heavily on the sealant resin used (commonly LDPE, certain LLDPE grades, or specialty sealant resins formulated specifically for sealing performance), seal bar temperature, dwell time, and pressure during the sealing process itself. Packagers typically want a seal strong enough to survive distribution and handling without failing, but in some applications, like an easy-open package, a deliberately weaker, more peelable seal is the actual goal, so seal strength isn't always a simple maximize-it metric.
Hot tack, a related but distinct measurement, looks at seal strength immediately after sealing, while the film is still warm and before it has fully cooled and crystallized, which matters enormously on high-speed form-fill-seal equipment where a package may be moved or stressed before the seal has had time to fully set.
Glossary
Seal Strength
The force required to peel or break apart a heat seal joining two layers of film, used to confirm a package will hold together without failing in use.
Example
A snack food bag running on a high-speed vertical form-fill-seal machine needs strong hot tack as much as final seal strength, since the package is often still warm and moving through the machine before the seal has fully cooled and set.
Nuance
Seal strength and hot tack are sometimes used interchangeably, but they measure different things: seal strength is typically tested on a fully cooled, cured sample, while hot tack specifically measures strength in the brief window right after sealing while the seal is still warm. A film can have excellent final seal strength but poor hot tack, which would still cause problems on fast packaging equipment.