Leaktesting, High Quality Leaktesting in Automation Processes
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Products are leak tested for various reasons. They may be tested to ensure that liquid is not getting out of a container (e.g., syringes or vials containing medicines) or to verify that liquids do not enter the product (e.g., electronic components such as phone or watches). Gas containers or CO2 cartridges should hold the gases (and not leak out) and air is to remain in a product such as a tire.
There are various reasons to test products for leaks but there is no such thing as a completely leak-free component. Therefore it is important to determine the acceptable ‘leak rate’ for a manufactured product, so it is safe and adequate for the customer’s needs.
Since there’s no such thing as a leak-free product, the real question is: What’s the acceptable leak rate?
In this article we start by looking at different (main) leak tests available. A wide range of tests are available, each offering unique properties. But not all tests are suitable to be automated.
Some parts hold expensive liquids or gases which make it hard or expensive to be used for testing. For automated processes, it is easier (or less expensive) to use readily available test gases or fluids. In these cases, a conversion is to be made from the test gases or liquids to the final product. This is discussed in the second paragraph.
The third paragraph looks at specific product & defects properties such as shape and material which may impact the test procedure used. In the fourth paragraph we look at the integration of automated leak testing into a production process as several factors contribute to reliable integration of automated leak testing. In the fifth and last paragraph we look at data collection and processing for leak testing data.
Various types of Leak Testing
A wide range of leak testing procedures and methods is available. Various aspects must be taken into consideration in order to determine the correct leak testing strategy.
The basic approach is similar for all tests, a part is pressurized (positive or negative) and the pressure or flow change is measured. This sounds simple but various aspects such as product shape, material used, level of automation, requirement for repeatability of the tests, test environment, etc. are all to be considered.
We introduce some of the common test methods below. Additional tests are available, these are listed at end of the paragraph.
Water Immersion Bubble Test (with part being under pressure)
This is also called a bubble testing method. A part is held under water, while being under pressure (typically air or nitrogen) and the escaping bubbles are observed. The larger and more frequent the bubbles ocure (in case of water immersion bubble test with part being under pressure), the bigger the leak. This is a relatively primitive test method (difficult to implement in an automated system) and suitable to detect large leaks (hard to observe small bubbles).
Soap Bubble Test
This is a similar test to the previous one but instead of using water, the part (e.g., gas valve connection) is sprayed with a soap solution. The part with the soap is observed to see bubbles forming or escaping. The forming of the bubbles are an indication of the location and size of the leak. The soap applicators may come in a wide range of shapes (spray systems, swaps, brush application, etc.).
Pressure Decay Test
In this test the part is pressurized (e.g., dry air or nitrogen) and then isolated from the air or gas supply and after a stabilization period, the pressure loss (over time) is measured. The amount of pressure loss over time provides an indication for the leak.
Differential Decay Test
This is a similar test as the pressure decay test but now a reference part (under pressure) is used as well. This normally involves two sensors where one sensor measures the test pressure, and a second sensor measures the differential pressure drop from a leak. The pressure is equalized on both sides of the differential pressure sensor during the fill and stabilizing steps.
Vacuum Decay Test
This test works similar to the “Pressure decay test” but then in reverse. The part is now subjected to low pressures and once stabilized, the increase in pressure is measured. This measurement principle is less sensitive to temperature changes but not suitable for thin–walled products.
Other test methods which are available include “High Voltage Leak Test”, “Ultrasonic (Acoustical) Leak Test”, “Tracer Gas Leak” and “Differential Mass Flow”. These test methods are very specific and serve unique testing purposes.
General Overview of Test Accuracy
As described in the previous paragraph; a lot of different pressure tests are available. For reference purposes we include a quick overview of the various test methods, and the leak test rates.
Overview of various Leak Test Values
Reference source: https://www.tqc.co.uk/our-services/leak-testing/useful-information/useful-leak-specification-guidelines/, check this website for more useful information on leak testing data.
With so many test methods available, a wide range of aspects are to be considered in order to select the correct leak testing method. Some of the main considerations to determine the leak testing strategy are described below.
Test Medium & Conversion
As described in the previous paragraph a wide range of leak tests are available. Unfortunately, not all tests are directly suitable for the parts to be tested. Also, some tests are not directly suitable to be used in an automated system (e.g., the soap bubble test will be hard to automate).
Sometimes parts need to be tested that are (eventually) filled with expensive or toxic fluids or gases (refrigerant, air-conditioning, etc.) which make testing very expensive (or dangerous). In this case an alternative medium is to be used.
When alternative fluids or gases are used, a “translation” is to be made. For example, if the leak test has been determined through a water bubble test (manual batch operation) and the process needs to be automated, a different type of leak test may be defined. A conversion from bubbles to pressure then has to be made.
As a reference, the following conversion table can be provided (see below). This is a crucial step in order to determine the correct leak testing strategy and process values. This is not always easy to determine and may require a lot of experience with leak testing procedures and interpretation of results.
Overview of Test Conversion
Cc: Cubic centimeter (also shown as ccm), cm3 or 1.000mm3. 1 cc equals 1ml.
Reference: https://www.tqc.co.uk/our-services/leak-testing/useful-information/useful-leak-specification-guidelines/, check this website for more information on leak testing values.
Parts, Shape & Defects
The parts themselves (and the defects) that require detection may also provide an interesting challenge. Some parts have complex shapes that make them hard to test. The material used for the part may also have an influence on the final test results.
Specific defects may be present in the product which can influence the final results. We take a look at some of these aspects in the next paragraph.
The shape of the parts may offer the first challenge. Medical devices, for example, often have unusual shapes that may make it difficult to reach the required test pressure in time. They also may require special fixtures in an automation system to accurately hold the parts in place. Especially for high-speed automation applications, this may become a point for consideration.
Dedicated (unusual) shapes may provide challenges to have them completely filled (in time) with a tracer gas (or have them fully pressurized).
This image shows an example of an unusual shape in Leak Testing that may require additional time to be completely filled
The part (used as an example) shown above may require additional time to be completely filled for accurate testing due to its shape. This aspect is to be evaluated in the leak testing strategy to ensure reliable and accurate leak test results.
Part volume becomes especially important when very large products or flexible products are to be tested. A change in volume may indicate a leak while this may only be due to the product changing volume under pressure. For example, IV bags are hard to test in a reproducible way (meaning: getting the same results for each test cycle). Also, for thin-walled products attention is needed to ensure the change in volume (expansion under pressure) is not regarded as a leak test result.
Plastics (especially) can behave in strange ways when being tested. The elasticity of plastics may make repeatable testing a challenge.
Some parts may continue to expand once the required test pressure is reached. Additional testing time should then be reserved for the parts to stabilize (increasing the test time). Dedicated fixturing (holding the parts in place) is to be designed to securely keep the part in place.
Special cavities present in the fixtures may be needed to allow controlled expansion of the parts. This requires dedicated engineering and experience with mechanical design and leak testing in general. Information from experienced partners is crucial here.
When the tests are performed under high pressure or in a short amount of time, the elasticity of plastics plays an increasingly significant role in the test results.
Type of Defect
When selecting a leak test procedure or when leak test results are evaluated, the type of defects is also to be considered. When an exceptionally large defect is present, the test medium may already have escaped before the test interval has been completed (thus giving a false reading).
When a defect consists of capillary-like corridors, this may also influence the results. The test medium cannot enter the leak (or is trapped). One example is a so called “stringer leak” where the time it takes for the test medium to distribute and emerge from the defect is important to consider (when this exceeds the test interval, the results may not be reliable).
For illustration purposes, two defects are shown below each with different flow behavior which needs to be considered (“pinhole” leak on the left and a “stringer” on the right).
This image shows both a ‘Pinhole’ and a ‘Stringer’ defect
Automation & Leak Testing in a Production Environment
When leak testing is introduced in automated equipment or in a manufacturing process, additional aspects are to be considered to achieve stable (and reproducible) measurements. Various aspects to be considered are dealt with below.
The production area in which the leak testing unit will be installed is important to evaluate. Large temperature differences, which may occur due to e.g., opening a door, can influence the final results. Temperature differences (between a reference pressure and a product) need to be avoided as these may indicate a test value which is mainly caused by the temperature alone.
This image shows an example of Leak Testing technology implemented in an automated application
Cycle and Test Time
When we look at the automated process and the leak testing process, the cycle time of the machine and the leak test time have to be considered. To test a product, it needs to be filled, then the pressure needs to be stabilized before the part can be tested.
The full test sequence needs to be less than the cycle time of the automation equipment. This is presented graphically in the image below.
This image depicts a graphically presentation of the full test sequence
Position (location) in the Production Process
The position (location) at which the leak testing is performed in the manufacturing process is also important to be verified. It may prove useful to test individual components to eliminate bad parts from the process early before they are assembled into a final (expensive) product.
This may also be a useful approach when the final product is difficult to test.
Test Procedure and Fresh Products
Parts can be tested at various pressures, depending on the requirements. But a part will behave differently under high pressure than under regular pressure (e.g., 10 bars pressure applied vs 1 bar pressure). Especially when using high pressure, accurate and stable handling becomes more important. The pressure applied should be included in the design of the handling (product holder) and leak testing unit.
When the parts are tested directly after manufacturing (e.g., when they are still warm), this may impact test results and repeatability. The part may provide different results when it has cooled down.
To achieve accurate and reproducible leak test results the design of the assembly machine is to be considered when determining the leak testing strategy. Both the leak testing equipment and the machine design are to be considered carefully.
Data and Process Storage
Leak testing results can be provided as a “go” or “no go” result (exceeding a dedicated value). But for more accurate results, a signature analysis can be performed. Signature analysis tools capture the leak test waveforms in real time and then apply algorithms to automatically analyze the entire leak curve instead of just measuring two points in time.
Using this data during the leak test process provides great ways to further improve and optimize the test sequence. With this data, the cycle times can be optimized thus increasing set up times. The data also provides good means for traceability of the parts and processes.
Stay tuned for more, coming up next! In the meantime, please consider following Grauel, a brand of AAE, on LinkedIn for weekly updates and extra content.
Leak Rate Units
Leak Rate Units (2021). Available at: https://www.cincinnati-test.com/leak-rate-units (Accessed: 8 June 2021).
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CapEx, 2019, Intro to Pressure Decay Leak Testing. August 07, 2019, Available at: https://www.youtube.com/watch?v=T7c7pzhafxU (Accessed: 8 June 2021).
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Leak Specification Guidelines and Pressure Conversions | TQC
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