Current leak testing methods
Currently, there are four main methods to leak test flexible packaging:
1/ CO2 leak detection (automatic or handheld):
There are two kinds of equipment that can be used:
A/ Automatic detection:
A detectable gas (usually CO2) is injected into all the food packages. On a conveyor belt, each package is compressed. A sensing head detects any leaks of CO2. Faulty packages are rejected.
- Test is non-destructive, providing the food is compatible with the gas that is injected in the package.
- It can be installed online and 100% of production can be tested.
- It is fast.
- It does not need consumables (septum).
- No leak smaller than 500 μm are detected.
- It is the most expensive method.
- CO2 has to be injected or present in the package, and the food has to be compatible with it.
- Packaging must be transparent.
- It is a good/no good test. No data is recorded or provided. Therefore, you cannot use it to improve your process.
B/ Manual detection: a detectable gas (usually CO2) is injected into a food packaging. A handheld device is used to detect a CO2 leak.
- Leak location.
- Works without consumables.
- Manual also very time consuming.
- It is a good/no good test only.
- The food packed has to be compatible with CO2.
2/ Immersion test:
It is probably the most commonly used leak test for food packaging in the world today. It follows ASTM F2096–02 = Standard test method for detecting gross leaks in packaging by internal pressurization (bubble test). Method sensitivity is down to 250 µ with an 81% probability.
Operating procedure: A packaging is immersed in a water tank. The leak (s) is (are) detected either by injecting compressed air in the flexible packaging, or by forcing a bubble out of the package with the help of vacuum. There are also some tests where the package is immersed in coloured ink, and the ink is then sucked into the packaging thanks to vacuum. Ink tainted content would indicate that there is a leak.
The only advantage of immersion method is that you can visualize precisely where the leak is (but this is providing the leak is big enough to be seen during the test duration).
The main issue is that it works only for gross leaks: combined, the 4 micro leaks on the left are equivalent to a single hole of 50µ, and we need to detect at least a 20 µ leak to protect the food in the packaging. (Astaara products detect leaks from 5 µ onwards).
The estimated waiting time to see a single bubble escaping from the package above is from 15 minutes up to one hour. The 4 micro leaks will therefore not be detected with an immersion test following the stipulated time according to ASTM F2096–02. The test will be stopped much before a bubble escapes.
If the food is packed under Modified Atmosphere Packaging, the gas recipe will leak as if it was a single leak of 50µ, more than enough to compromise the shelf-life.
Also, humidity will be able to penetrate the packaging.
- The cycle is very long, and you are not sure whether you should wait to see a leak. It will most of the time be stopped before anything happens.
- The test is fully dependent on operator’s skill and attention – no automation.
- There is no measurement of the size of the leak. Data is not available to improve the sealing integrity.
- There is a risk of contamination due to water (water is prohibited in the proximity of production lines in most Countries).
- It needs consumables (septum).
3/ Pressure decay:
80% of Industrial Leak Detection devices are using pressure decay today.
It is very effective to measure leaks for products that are rigid, whose shape does not change when put under pressure.
Operating procedure: A packaging is pressurized with compressed air. Once the pressure reaches the set value, the valve is closed, and a pressure transducer measures if there is a pressure drop in the closed loop. If there is no pressure drop, there is no leak.
There is some data.
Measuring precisely a leak by measuring pressure decay is only precise and reliable if the volume of the sample is not changing during the measurement.
L = (V x P) / ΔT <=======> Leak = Volume x Pressure over a period of time.
L = value of the leak. V = volume of the sample. P = pressure. T = time.
For a precise measurement, the volume needs to be constant while changes in the pressure are being measured.
As far as flexible packaging is concerned, “pressure decay” is therefore not adapted to flexible packaging, as the volume is changing during the test. The method is sufficient to identify whether there is a leak or not, but the measurement will not be very precise.
- This method is sensitive to the volume measured. The same 30 µ leak will be detected on a 0.5 litre sample, but not on a 3 litre sample.
- The volume tested is the volume after the valve shuts off, which would mean that it includes the volume of air in the pipe, which for small samples can be half of the volume tested. This reduces the overall sensitivity.
- It needs consumables (septum).
Conclusion: Products using “pressure decay” have been developed for much larger Industrial applications, for example automotive parts with rigid housings. They can detect leaks in flexible packaging, but are however not suitable for measuring it very precisely.
4/ Flow measurement at a constant pressure:
Flow measurement at a constant pressure is the only reliable method for measuring a leak in a flexible packaging. It follows DIN 55508-1 of 01/02/2018. Leak testing of flexible packaging is a limited application compared to automotive parts leak testing for example, and very few Manufacturers of leak testing equipment are therefore proposing it.
Operating procedure: A sample is pressurized with compressed air coming from a constant air pressure source. Once the sample is fully inflated, the flow stops and the mass flow sensor will indicate zero “IF THERE IS ZERO LEAK”.
However, if the sample leaks, molecules (the flow) will continue through the mass flow sensor after the sample completes inflation, and the value of the flow detected by the mass flow sensor after the sample is fully inflated will be equal to the value of the leak.
It can be used to test any kind of product, solid housing or flexible packaging.
It does not pinpoint precisely the location of the leak.
Out of the four traditional methods above used to identify and measure leaks, only “flow measurement at a constant pressure” is adapted and reliable to test flexible packaging.
This is this method that has been selected to develop the Astaara Range of products, ensuring total precision and reliability.