Fixturing for Leak Testing
In the abstract, it seems like such a simple thing-figuring out whether or not a product is leaking. However, the reality of performing this task quickly and reliably can be incredibly complex.
Pressure decay systems, mass flow systems, helium or hydrogen accumulation, helium or hydrogen sniffers, mass spectrometers, residual gas analyzers: Life has changed dramatically since when you were a kid and would dunk a bicycle inner tube in a bucket of water to figure out where to patch a slow leak. (Although there are still some manufacturers using “water dunk test” systems to check their products.)
Nonetheless, for all the different technologies on the market today, every leak test system has one thing in common: The products being tested must be effectively sealed and fixtured. Otherwise, as is the case with assembly in general, the most carefully engineered system in the world will fail to provide reliable results.
Unfortunately, as a kind of a corollary, many leak test systems have something else in common: Manufacturing engineers treat them as an afterthought. This state of affairs can lead to all kinds of problems down the road, especially in an automated production system.
Fixturing is often the weak link in a system. In many instances when a customer believes a leak detector is not working, the true culprit lies elsewhere in the process.
Planning ahead is especially important when manufacturing some of today’s cutting-edge products. For example, automakers are using more complex casting designs that have 90 degree and half-moon-type designs that require sealing during a leak test. These can be a real challenge, and require a lot of force and the right seal design to create a repeatable test. Suppliers of leak testing equipment often have to be creative to seal on a seal.
No matter how sophisticated the actual leak detection method, there are a few basic principles that need to be adhered to. For example, engineers should test an assembly in the same "direction" as the pressure gradient the assembly will be facing when it is in use. This means that if a product will contain a liquid or gas when in use--examples, include everything from automotive fuel tanks and radiators to refrigeration coils and medicine delivery systems--then the testing system will need to create a high-pressure region within the assembly and then monitor the product for any escaping gas.
If, on the other hand, the product needs to remain impermeable with respect to outside gasses or liquids, the test will be conducted the other way around. Examples of products in this category include sealed electronic packages, vacuum vessels or systems, and some types of manifolds or valves.
In many cases, the reason for this approach is obvious: Submitting a product to forces it isn't designed to withstand could easily result in its being ruptured or crushed. An example would be a relatively lightweight component with a large surface area, like an automotive fuel tank.
Another important reason for testing in the same direction is that you don't want to "mask" the effects of a component's more subtle, or borderline defects.
Let's say, for example, you are testing the welds on a coil for an air conditioner unit. Pressing "in" on the assembly could also compress the welds, causing them to block the escape of the tracer gas being used, even if they are of questionable quality. Pressing "out," on the other hand, will tend to open up those same welds, quickly exposing a defective part.
Masking is something that also needs to be kept in mind when thinking about fixturing. It's important to ensure that the part or parts are stabilized. But, that doesn't mean clamping the part down so firmly it can't flex and expose any defective joints. Similarly, you don't want the nesting itself to operate as a kind of plug, stopping up, and therefore masking, any potential leaks.
Note that in addition, engineers sometimes need to fixture parts so that their orientation is the same as that which they will be experiencing when in use. Indeed, some automotive customers require "true car position" of components during testing from their suppliers, so they can be confident the part will perform in the field. Some medical devices, like inhalers, also need to be oriented correctly for a leak test to be legitimate.
When leak testing medical devices, it's critical to test the parts in a manner that is consistent with how they will actually be used.
Seal for Approval
Equally important to securing each assembly for testing, is creating an adequate seal between the assembly and the leak detector. This is especially true when performing a pressure or vacuum decay test, or when using a mass flow system to examine an assembly for defects.
In the first case, the test device either pressurizes an assembly or pulls a vacuum to a predetermined setting. It then monitors that pressure to see if it changes-which would indicate a leak. In a mass flow system, the test device pressurizes an assembly to a predetermined level and then holds it there. If any air is required to maintain pressure, the tester will register this fact by quantifying the amount of air flowing by.
In both cases, it is imperative that there be a stable seal between the test device and the product being tested. If the seal shifts, or "creeps," at any time, it will affect the total volume being tested, resulting in an error.
Engineers should be aware that, because test pressures are often higher than the pressures a product will be subjected to when in use, additional measures may be required to create a good seal. For example, even though a product employs a flat seal when in use, it may require an O-ring seal during quality assurance. Engineers also need to be careful that the materials used to fabricate a seal are tough enough to remain stable under test conditions. If the seals move around, that will cause problems. Engineers should seal for the test, not just the real world.
Assemblers need to be able to create an effective seal without using undue pressure, especially on less robust components. Anyone can clamp down with a huge amount of force. The trick is avoiding damaging the assembly.
Finally, sealing test parts can be especially difficult given some of today's assembly technologies. A lot of modern products use a liquid gasket seal material in the final assembly and thus have features that work against the automation seal.
In terms of the different options available, engineers can create a seal around the outside or the inside of a port leading into the assembly. Or, if there is no port, they can create a "face seal" along the same surface where the component will mate with the rest of an assembly. The latter arrangement may require the use of hydraulic clamps, because larger sealing areas require higher clamping pressures. Inner-diameter (ID) or outer-diameter (OD) seals often employ pneumatic clamps, although a hybrid "air-over-oil" system may be necessary if the sealing mechanism is operating within the confines of a vacuum test chamber.
The decision to go with an ID or OD seal is often dictated by part geometry. Nonetheless, all other things being equal, an OD is preferable in that it frees up the entire orifice for either pumping in a tracer gas or pulling a vacuum. This translates into shorter cycle times.
In either case, engineers can go with standardized sealing systems or, if necessary, a custom sealing device or devices to ensure efficient and consistent sealing.
In addition, it's important to create a system that will hold up under the rigors of repeated use. Most manufacturing processes are designed to provide maximum uptime and can ill afford to stop the assembly process every few hours to change seals. The seal design and material type are very important considerations.
Sealing surfaces must survive thousands of uses, while mimicking final application seal. A reliable seal that has a reasonable life span is key to limiting the number of false negatives, especially in an automated system.
Along these same lines, it's important to ensure that the sealing material is compatible with the tracer gas being used. Running an air-based leak test doesn't pose any undue restrictions. However, silicone is highly porous to tracer gasses like helium and hydrogen. Instead, engineers should use materials like Viton and BunaN. Teflon seals are also poorly suited for use with hydrogen and helium tracer gasses.
Finally, when thinking about seals, don't forget the role played by the fixturing. Proper fixture-to-part tolerance helps maintain a stable condition and eliminates part movements. These tolerances are also important to maintain proper seal contact to the part. Any variances in sealing angle will affect the repeatability of the test by causing changing conditions in the sealing process. Proper sealing sequence can sometimes also affect proper mating of the part to the tooling fixture.
In the end, the key to effective, trouble-free leak testing is to not wait until the last minute. Design-for-assembly is all the rage these days-at least in theory-and these same lessons apply to leak testing.
Creating an effective, efficient leak-testing arrangement requires a particular skill set. In maybe a fifth of all cases, a general knowledge of engineering may be enough to get the job done. But a certain level of expertise is necessary in the overwhelming majority of applications.
Implementing a leak test system is a lot easier if manufacturing engineers begin thinking about how they are going to test for leaks before the rest of the assembly process is in place. Many assemblers are surprised to find that the same pallets and fixturing they use to build their products may not work when it comes time to perform their leak-testing operations. But, the requirements of, say, an automated screwdriving station, are much different than those of a helium-based, multi-site accumulation system.
Then there are a number of ancillary issues that can be much more easily addressed before the rest of an assembly process has been set in stone. For example, a leak test instrument should be placed as close to the test part as possible to eliminate line test volume. Assemblers can also employ filler blocks to reduce the volume being evacuated or charged with test air or tracer gas.
Finally, why not think about leak testing during product design? Granted, creating a product that is easy to hook up to a leak detector is probably the last thing on many engineers' minds. But, many engineers do just that--and are glad that they took the trouble to do so afterwards.