Packaging buyers often ask for "a strong seal" when what they actually need is seal integrity. The two get used interchangeably, but they measure different things, and a film can score well on one while failing the other.
What seal strength actually measures
Seal strength is a force measurement: how much force it takes to pull a sealed sample apart, usually expressed in grams or pounds per inch of seal width. It is a useful, repeatable lab number. It tells you nothing directly about whether the seal has a pinhole, a channel, or a contamination gap somewhere along its length.
What seal integrity actually measures
Seal integrity asks a different question: is the seal a continuous, unbroken barrier along its entire length? A seal can pull apart at a high force in a lab test and still have a microscopic channel that lets air, moisture, or bacteria through over time. Integrity is usually verified with dye penetration tests, bubble leak tests, or vacuum decay testing, not a pull tester.
Why both matter, and why neither is sufficient alone
A seal with excellent strength but poor integrity might survive shipping and handling, then fail to protect shelf life. A seal with excellent integrity but weak strength might leak-test perfectly in the lab, then fail the first time the package gets dropped or compressed in a case. Specifying one without the other leaves a real gap in your packaging's actual performance.
What to ask for
If shelf life and product protection matter more than physical abuse resistance, prioritize integrity testing in your spec. If the package needs to survive distribution more than it needs a perfect hermetic seal, strength testing matters more. Most production specs should include both, with thresholds appropriate to the product.