From the dictionary – Plastic: capable of being molded or modeled (plastic clay) : capable of adapting to varying conditions : PLIABLE
Products made from plastic create some of our biggest challenges in leak testing. That’s not to say all plastic products are difficult to test. Many plastic products are made from “rigid” polymers and are tested at low pressure where they do not flex or deform. However, some products are made from very flexible materials and are designed to change shape or conform to contours.
For example, consider medical bags for the containment of body fluids or delivery of drugs. This type of product is extremely flexible, in fact a medical bag is not far removed from a balloon. The flexible nature of the product can create false leaks or mask real leaks if not properly fixtured.
Before leak testing a product like this we need to take a little time to consider its geometry and any constituent components that are included such as valves, stopcocks and ports for the connection of tubes. Areas where these features are to be found are commonly the site of leaks to due seal failures at the site of inclusion.
A practical method of leak testing bags like this is to restrain the bag between two rigid plates whilst allowing the seams and connected parts to be exposed at the edges or through the plates. It’s unusual to encounter leaks such as pin holes and tears in the core material, but just in case they do exist we can cover the contact side of the plates with a porous material such as Porex™. This will allow air to escape from the bag through its open cell structure. Toggle clamps, screws or cylinders can be used to hold the restraining plates in place while the bag is pressurized and then tested using the standard format pressure decay test.
This is a very effective and simple to set up test and we have tested probably millions of bags in thousands of formats like this. Nevertheless, there are design considerations to be observed:
For a different approach we can use the pressure vs. force relationship to measure the force exerted on the restraining plate by using a strain gauge or load cell. Leak testing in this way is conducted under vacuum condition in the Qualipak leak tester for packages that are sealed and can be adapted for medical bags or other flexible products which have a port that can be used for internal pressurization. Bags which leak will exhibit a decay in the force measurement as the internal pressure decreases through the leak after the product has been isolated in the leak test step.
After the best test method is established and the next step is to prove out the system by checking repeatability one thing to keep in mind is that any test part that has a dynamic characteristic may themselves not repeat. In other words, virgin parts may have a slightly different pressure change signature vs. one that have been previously tested due to the parts have stretched slightly. This is how a balloon reacts.
Try blowing up a balloon for the first time. It has a greater resistance to start inflating the first time you blow it up vs. once it has been blown up it no longer has the initial resistance. There are a few things we can do to insure a proper repeatable setup for these applications as well.
Our Applications Engineering group is on hand to answer your questions on this subject and to help with any others you may have concerning your leak testing application. Call us on 281 671 2000 (USA), +44 (0)1284-760606 (Europe) or +86 -21-58365859 ( China) and ask to speak with one of our sales or application specialists.