Heat Seal parameters are easily measured; Heat Sealability = Temperature + Pressure + Time. Yet maintaining optimum heat sealing conditions in production and recognising the true cause of poor sealing remains a challenge. Measurement of heat sealing parameters is done by creating “controlled seals” on a calibrated Laboratory Heat Sealer according to ASTM F2029, then peeling the seal apart with a Seal Strength Tester at a controlled rate to measure either Ultimate Seal Strength (after cooling, method ASTM F-88) or Hot Tack Strength (peel tested within 150ms of the seal being formed, method ASTM F1921). By varying the sealing temperature and dwell time, the optimum sealing conditions can be found. In studies the third variable, sealing pressure, was found to have little impact after reaching a certain level.
For finished packs, common practice is to perform off-line package integrity quality checks every 15 to 30 minutes. The most common method being a manual squeeze test to feel & see if the pack deflates under the pressure of the operators hand. Other more objective methods include bubble leak testing (submerging packs in a water chamber/vacuum to visually watch for bubbles coming from the inflated pack), inflation testing (packs placed in vacuum chamber to visually check if the inflated pack deflates over a defined time), and blue dye penetration testing (passing a blue dye solution over the seals inside empty packs to observe capillary ingress into seal faults). These methods have the advantage of low capital cost, but are limited to somewhat subjective qualitative pass/fail data.
Another objective and quantitative method for seal strength testing is according to ASTM F2054 (increasing the internal pressure of a pack until seal failure)
