4.1 Seal strength is a quantitative measure for use in process validation, capability, and control. Seal strength is not only relevant to opening force and package integrity, but to measuring the packaging processes' ability to produce consistent seals. Seal strength at some minimum level is a necessary package requirement, and at times it is also desirable to have an upper limit to the strength of the seal to facilitate opening.

Note 1: Seal strength values are a measurement of the output of the seal separation and may also involve mechanical properties of the materials that form the seal, given the potential for deformation or elongation over the course of the test. This separation is indicative of the area of the package being sampled and does not take into account simulation of a user interfacing with an entire package during the opening process.

Note 2: Lower seal strength specifications are typically utilized to provide assurance of package closure, which can contribute to seal integrity.

Note 3: Upper seal strength specifications are typically utilized to limit the amount of force required to open a package, ensuring that a user is able to open the design. Upper seal strength specifications are typically limited to seals that are intended to be peeled by the end user.

4.1.1 The maximum seal force is important information, but for some applications, average force to separate the seal may be useful, and in those cases also should be reported.

4.2 A portion of the force measured when testing materials may be a bending component and not seal strength alone. A number of fixtures and techniques have been devised to hold samples at various angles to the pull direction to control this bending force. Because the effect of each of these on test results is varied, consistent use of one technique (Technique A, Technique B, or Technique C) throughout a test series is recommended. Examples of techniques are illustrated in Fig. 1.

4.2.1 Technique A: Unsupported-Each tail of the specimen is secured in opposing grips and the seal remains unsupported while the test is being conducted.

4.2.2 Technique B: Supported 90° (By Hand)-Each tail of the specimen is secured in opposing grips and the seal remains hand-supported at a 90° perpendicular angle to the tails while the test is being conducted.

Note 4: Excessive lateral forces applied via hand may impact results. Actual gripping of samples is not intended and will influence results; contact is intended to be loose, only preventing tail movement up or down.

4.2.3 Technique C: Supported 180°-For flexible to flexible applications, the least flexible tail is typically supported flat against a rigid alignment plate held in one grip. The more flexible tail is typically folded 180° over the seal and is held in the opposing grip while the test is being conducted. Alternatively, in rigid and semi-rigid applications, the package structure may be maintained for the least flexible side; with this structure gripped or fixtured.

Note 5: Properties of some flexible materials may cause movement or flipping of the tail throughout the course of the test; this has potential to impact the measured strength and should be reported with results.

Note 6: Test method validation should account for use of fixtures or alignment plates, as well as determination of which material is placed into which grip as these factors are known to impact results, and feasibility of each approach may vary depending on design features. Examples of optional fixtures and equipment with built in fixturing are included in Appendix X4 for reference. Refer to Guide F3263 for guidance on test method validation.