What impacts cycle time in leak testers?
A short or reduced cycle time is often touted as a key differentiator in many instruments, because a reduction in cycle time can lead to improved efficiency and optimized profit potential. Leak testers are no different, and as a manufacturer of leak testers, one of our goals in the R&D process is to keep our cycle times as short as possible.
However, there is often a trade-off between a short cycle time and reliability and repeatability. You never want to reduce cycle time to the point that reliability and repeatability are out of specification. There is a sweet spot between the shortest cycle time and highest reliability and repeatability measurements, and it’s important to understand the variables that go into identifying this ideal point.
Part volume can have a tremendous impact on the achievable cycle time, as well as on the sensitivity to detect the leakage at specified levels. The larger the part volume, the longer it might be necessary to stay connected to the part and measure for pressure changes. Larger part volumes also require a greater number of channels, or even a second tester. Because medical devices within the same manufacturing process can vary from extremely small and rigid to very large and elastic, it is important to select a tester that is flexible enough to accommodate different volumes accurately, without sacrificing sensitivity.
It is not unusual for medical devices to have unusual geometries. Examples of these include tubes, valves, stopcocks, transducers, enclosures, and components within components. This not only makes leak testing challenging, but also emphasizes the critical nature of fixturing and control. The geometry of the part often determines how repeatable the parts are manufactured. For instance, if the part to part volume varies then the leak tester may start rejecting good parts because the internal volume is different from one part to the next. Also some leak test setups see differences in part performance depending on what cavity of a multi-cavity mold is being tested. Part geometry can also influence the parts’ capability to transfer heat due to the parts’ internal surface areas. Parts that transfer heat well will typically stabilize faster, which then means a faster cycle time.
Polymers and plastics can behave in strange ways. It can take a much longer time for pressure to stabilize when testing materials with elastic properties, as with products like balloons, IV bags, mixing bags, and anything with a bit of give. Elasticity can make repeatable testing very difficult, as some parts continue to expand after reaching the desired test pressure, which leads to a long time to stabilize. Sometimes this effect is referred to as “compliance.”
One of the ways to minimize the effect of elasticity is to minimize part movement through fixturing or part control. However, caution should be exercised in the design of such fixtures because the parts can seal themselves either through inherent elasticity or under pressure against the fixture. For this reason, fixtures often have a porous surface or are textured in such a way that air may escape from the product into what is a small cavity or cavities built into the fixture.
Accurate determination and measurement of leaks is vital for medical devices, which have a critical role in patient safety and comfort. Because of this, the medical device industry requires tighter leak test specifications than any other industry. Even so, the leak rate specifications for medical devices can vary tremendously from one device to another. One of the biggest considerations is avoiding cross contamination (air or gas ingress/egress) to minimize the chance of infection or improper medication.
Some medical devices present greater challenges when it comes to testing for leaks. If a leak rate is extremely small, a part is highly elastic and flexible, and the volume is large, testing a leak to customer specifications can be complex. It’s important to work with a leak test expert who has the facilities to lab-test the medical device product to actually hook it up to a tester to analyze the physics of the part to see how it reacts. This can ensure the right leak tester configuration for the specific application.
Test pressure can have a significant impact on the performance of a leak tester, so it is very important that the pressure used within the test process is at the right levels for the intended application. Cycle time and sensitivity can both be impacted by the test pressure. When the pressure is higher, more adiabatic heat is generated, which means a leak tester will require more time to stabilize. High pressures may cause parts to flex, which is a situation that also requires a longer time to stabilize. Higher pressures require a wider range sensor, which can reduce sensitivity to the leak and lead to a longer period of measurement to get an accurate result. Higher pressures can also lead to increased leak rates in holes and can weaken welds or glue joints.
Most medical devices are manufactured in a clean-room environment, which means the leak tester and fixturing must comply with clean-room conditions. Leak testers must not release contaminates into the part or clean room environment. Components inside should be of non-corrosive material. Fittings that require sealant should not have any off-gassing into either part or room.
Many clean rooms have constant air flow and filters, which can impact the leak testers. Air flow means temperature change, and temperature change can impact the pressure change in relation to temperature. It is necessary to minimize rapid temperature changes, so pressure remains as constant as possible.
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