How do you document minimum seal strength?

By Pharmaceutical & Medical Packaging News staff in Testing on May 03, 2013

We have all been asked for the minimum seal strength of our packages at one time or another. EN/ISO 11607-1 (2006) 5.1.8 (c) requires you to demonstrate a minimum.

Hal Miller

There was a time when there seemed to be a universal minimum seal strength everyone would cite for their packages, regardless of size, shape, material, or contents. That was the 1 lb./in. seal width (~2.6N/15mm). This all started over 30 years ago with a heat seal coated material supplier that would guarantee that their coating would provide a minimum seal strength. Thus, a new industry standard was born. Since then, I hope, we have gotten smarter.

Using an "industry standard" can be limiting to say the least. In these days of demonstrating conformance to requirements, how does one provide documentation for an industry standard? The minimum seal strength of a package or package family is that seal strength that will withstand the rigors of manufacture, sterilization, distribution, and storage up to the time of use. Minimum seal strengths may vary by package type or product categories. For instance, a lightweight suture package does not need the same as an orthopedic hip package. Also, seal strengths may vary for the same package and material from process to process. So, what is a packager to do?

Ideally, packages should be tested to failure during the design and development phase. This includes minimum seal strength to maintain sterile package integrity. However, most of us have neither the time nor the resources to test to failure.

Another course of action starts with the sealing process. A validated process will yield a specific range of seal strengths from the lowest process parameters -- typically temperature, pressure, and dwell time for heat sealing -- to the highest set of parameters. Seals tend to be weaker at the lowest parameters. Your validated process is therefore allowed to run at the lowest settings even though you may never operate there.

At those settings, there will be an average seal strength with a variance. This is the minimum seal strength for the process, and the package seal strength that needs to be performance tested with shipping and handling tests.

There are a couple of options for producing packages for minimum seal strength testing. One is to produce packages at the lowest process sealing parameters, and use these for performance testing. The supposition is that the variation in seal strength at these conditions will produce some packages at the very lowest limit. However, a better and more definitive method is to produce packages below the lowest limit that will still provide seal integrity and acceptable visual attributes.

The seal strengths produced here become the minimum with demonstrated integrity after performance tests. An additional benefit is the package failure limit will be further away from the process limit, thus allowing for a more robust process.

Whichever way you choose to demonstrate minimum seal strength, remember to provide a rationale and document, document, document.

Focused on medical packaging design and technology?

By Hal Miller, President, PACE Solutions

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