Weigh Your Options When Testing Seal Strength
For evaluating package seal strength, testing professionals may employ burst or creep tests, tensile (peel) tests, or some combination. Each method has advantages and disadvantages that testers need to consider, attendees were told at the Medical Design & Manufacturing West 2006 conference.
Restrained package testing more consistently defines minimum seal strengths, as it provides more-uniform pressure distribution along the seal. When packaging is controlled between platens in this method, seals are more likely to fail at their weakest points, Mark Escobedo of Westpak Inc. (Oceanside, CA) told conference attendees.
ISO 11607, “Packaging for Terminally Sterilized Medical Devices,” says that package seal strength as well as integrity must be validated and its conformity to specifications documented. Section 7.3 suggests burst and tensile methods for seal strength evaluation.
Tensile testing uses a defined width sample, such as one inch, for seal perimeter evaluation. An advantage of this method is that it is easy to quantify or determine whether you are meeting process objectives. Peel testing demonstrates continuity in the seal.
“The sample cutting is critical. It has to be parallel to the seal at a precise 90° angle, or you will have variations as you are taking data,” Escobedo said. Since you are looking at a small section of the perimeter, with peel testing you can’t determine seal uniformity.
Burst testing provides a faster method of testing seal integrity and also allows for rapid process correction. “Burst testing purports to test 100% of the seal, but it doesn’t always find the weakest portion of the seal,” Escobedo said.
For instance, with unrestrained burst testing, force distribution will not be uniform when package geometry is not uniform, and packages can twist and torque as they are filled.
Blocking agents such as petroleum jellies, surfactants, and tape may be needed for unrestrained burst testing, such as with large porous packages. In those cases, though, “you may get too much bleed-off through Tyvek, for example, preventing the package from bursting,” he said.
Restrained burst testing is often the solution, but it is recommended usually for pouches only. When this method is used, platens controlling the package need to contact a minimum of 60% of the surface.
In creep tests, packages should be inflated to 80% of the burst value and held at that pressure typically for 30 seconds. “If there are changes in the seal, the package fails, and sealing parameters should be evaluated,” he said.
Escobedo said that though it is possible to predict burst values from seal-strength testing, “the math required is quite complex.” There is a one-to-seven degree variation in values between the tests, and changes in materials can throw off the comparison.
“I think we should use the tests for their intended purposes, instead of trying to compare the two,” he said.