ASTM F-88: Average Load vs. Peak Load in Seal Strength Test Method
In my previous blog, I talked about how pouch materials impact your test results when an unsupported 90-degree method is used to measure seal strength. I also mentioned that, within the test method and technique selection, you can also do average or peak load. Average load calculates the average of multiple readings collected during the test procedure -- typically programmed to take out from the average calculation the values on the rampup and stepdown of a seal strength curve. The peak load records the highest value during the test procedure.
|Charlie Rivera, CPP|
I've seen that the peak load method is more predominant in the industry than average load method. The more I look into this, the more I see that there is still a lot of test equipment out there that cannot give average load at all. I'm also seeing force gauges that do not even display the seal strength curve when a test run is performed. The test result itself is one thing, but I don't think there is any value in that number if you can't see the seal strength curve. It is extremely important to know how your pouch materials behave based on your test method selection, and the seal strength curve will give you that. Without it, you won't know how much variation is in your test method. When you get different results and standard deviations at the same sealing conditions, besides sealing equipment issues and material lot to lot variation, your test method can be contributing to this, as well.
Remember that tensile testing is all about the tail angle from which the test sample is pulled. As I said in my previous blog, some pouch materials may not maintain the angle as the test is conducted and between test runs when an unsupported 90-degree method is selected. When the peak load method is used, results can be 20-25 percent higher than with the average load method, and standard deviations can be higher, as well. This is because results are based on the maximum number the gauge can register during the test, not the average load required to open the entire seal, which is more representative of how the end user will open the seal. Also, the natural tendency for the pouch material is to go to a higher angle than 90 degrees. A higher angle means higher values.
By the same token, this behavior completely changes on pouch materials that keep and maintain the angle while the test is being conducted and between test runs when an unsupported 90-degree method is selected. The average load values are extremely close to peek values; there is no practical difference between them. The difference may not even get higher than 3 percent.
The average load method has the following advantages over the peak load method.
- It challenges the entire seal width, not just a single point.
- It provides lower standard deviations, since it calculates an average of 1,000 readings, thereby smoothing out any spike in the seal strength curve.
- It detects potential testing issues like test sample slipping on the grippers.
- It detects potential sealing issues.
Understanding this behavior will help you select the test method and technique that will provide more consistent results for the pouch materials you are using. In addition, it will facilitate having the test method validated in accordance with the ISO 11607 part II standard. Maybe it is time to upgrade your test equipment.
Charlie Rivera, CPP