Inline testing of the wettability of material surfaces
With the new bonNDTinspect® system, Automation W+R enables the reliable assessment of surfaces that are to be bonded, painted or otherwise treated in subsequent process steps. The system is based on a patent held by Fraunhofer IFAM and machine vision components from STEMMER IMAGING.
"Our bonNDTinspect® system for the inline testing of the wettability of material surfaces is suitable amongst other things for testing the readiness of surfaces to be bonded or painted. It offers numerous benefits in comparison with previously known methods", explains Célian Cherrier.
As project manager, the application engineer from Automation W+R has accompanied the development of the system, which recently went into pre-production, from the first steps about two years ago and describes the basic idea as follows, taking bonding as an example: "In order to achieve a good bonding result it is imperative to monitor the state of the bonding surfaces before bonding and to check the surface pre-treatment in order to detect contamination, e.g. due to release agent or fingerprints. Surfaces must fulfil certain criteria in order to enable a secure bonded joint."
Various methods already exist for such testing of the wettability of surfaces, such as test inks, the so-called water-break test or contact angle measurements. However, bonNDTinspect® offers a series of benefits in comparison with these conventional methods. For the first time, for instance, the system allows fast, automatic measurements over a wide area on different surfaces such as plastic, metal or fibre-reinforced composites. In addition, the system from Automation W+R - unlike the other methods - is objective and fully automated, operating with imaging software. Not only that, it enables the immediate processing of the tested component directly following the test and can therefore be integrated seamlessly into the production process.
Ultra-fine droplets for surface inspection
The method is based on a patent held by the Fraunhofer IFAM (Fraunhofer Institute of Manufacturing Technology and Applied Materials Research) for the evaluation of the wettability of surfaces. Automation W+R holds the exclusive licence for the use of this patent. Célian Cherrier explains the mode of operation of bonNDTinspect® like this: "An ultrasonic atomiser generates a defined water aerosol of ultra-high purity water, which is sprayed onto the test surface with a width of approx. 10 centimetres. At the same time the sample is moved, for example by a linear drive, at about 100 mm/s under the atomiser in order to ensure an even application of the aerosol."
According to Cherrier, the necessary relative movement between sample and atomiser can also be implemented, depending on the conditions of use, by a robot that bonNDTinspect® guides over the test component. "In this way 1000 to 2000 droplets with an average diameter of 100 µm are created per cm²."
Depending on the surface energy of the test surface, a characteristic droplet pattern forms during this procedure that is automatically captured by a camera system and evaluated in a matter of seconds by means of image processing. On the basis of the droplet distribution it is then possible to determine whether a surface fulfils the criteria for the subsequent process steps. By specifying target values it is thus possible to evaluate the effectiveness of a pre-treatment, such as a prior cleaning or surface activation, or to detect contamination on the surface.
Such contamination can be caused, for example, by release agent residues, oils, greases or simply by fingerprints and can lead to bonds failing after a certain time or paints peeling off prematurely. Possible consequences are visual or functional defects of the end product such as paint blisters on vehicles or non-adhering printed items, which for example in the case of labels for medicaments in the pharmaceutical industry are not permissable. In extreme cases a bonding fault can endanger life.
Sophisticated image processing
Cherrier says that this test method poses big challenges for the vision system employed: "On the one hand, more than 30,000 droplets are created per second due to the process and these have to be detected and evaluated quickly, because due to the use of ultra-high purity water the evaluated surfaces dry without residues within a very short time so as to enable further processing without delay. On the other, the system is inline-capable and in the extreme case has to run in continuous operation 24 hours a day, 7 days a week."
In view of these difficult boundary conditions Automation W+R fell back on its proven machine vision partner STEMMER IMAGING, with which the company has co-operated for more than 20 years and has had very good outcomes in many applications.
"Following a prior discussion regarding the specific requirements, STEMMER IMAGING made a suggestion during the implementation of this test system for the imaging components that meet these specifications", says Cherrier in retrospect. "At a later stage in the project our partner also assisted us with certain camera adjustments in order to meet the required specification and generally provided outstanding support with all questions regarding the machine vision system employed. With this project too, our collaboration with the experts from STEMMER IMAGING has on the whole saved us a great deal of time in searching for the ideal imaging components and we were able to implement the test system successfully and within the allotted time."
In the current version of the test system the droplet patterns are captured by a monochrome GigE line scan camera of the type Linea from Teledyne Dalsa with a 2k pixel resolution, together with a telecentric lens from Sill Optics which provides for images with a high depth of field. The required light intensity couldn't be achieved with standard lighting, so Automation W+R developed special dome lighting itself. The images are evaluated by Common Vision Blox from STEMMER IMAGING and imaging software from Automation W+R on an Industrial PC from Beckhoff, which also provides the PLC functionality for the system.
The complete test system consists of a PC with monitor and the evaluation software, the test head with ultrasonic atomiser and a control cabinet for control. An interface allows connection to an external PLC and thus to the higher-level system.
The droplet pattern created is automatically captured by a camera system and evaluated by means of image processing in a matter of seconds. In this case a fingerprint has been detected.
Image source: Automation W+R
On the basis of the droplet distribution analysed by the image processing it is then possible to determine whether a surface fulfils the criteria for the subsequent process steps.*
Successful first use
The first users of the system were the bonding specialists at Fraunhofer IFAM, who had used a bonNDTinspect® prototype at their Bremen site for testing the wettability of surfaces following atmospheric pressure plasma activation. It was a resounding success, as Christian Tornow, research associate at Fraunhofer IFAM, reports: "An automated non-contact system for this task has not been available until now. With bonNDTinspect® surface conditions in the bonding process can now be distinguished automatically and reliably. We have been able to consistently detect contaminations such as fingerprints and release agent residues as well as inadequate activations. This is extremely important in particular in bonding technology in order to ensure ideally prepared surfaces for subsequent processes."
Complete system from a single source
Dr Richard Söhnchen, managing partner of Automation W+R, is convinced that the new system is suitable not only for the inline testing of the readiness of surfaces for bonding or painting, e.g. in the automotive industry, or for the coating of electronic components: "These are just the most obvious applications of this exciting technology that we are addressing with our new development. However, there are still numerous other applications for this method, e.g. in aerospace or medical technology, and we are therefore sure that we can make interesting options available to a great many other markets that we haven't even thought about so far."
Söhnchen mentions a further important point that underlines the possibilities of his company: "Automation W+R is part of the Autision Group. In the Autision Group we combine robotics, measuring technology and surface inspection. For each of these areas of expertise there is a separate company within the Autision Group GmbH with special competences. On account of this constellation we can offer our customers all-in-one solutions from a single source to suit their applications and in doing so implement the integration into existing systems as well as the connection to any required robotics", emphasises the managing director.
"If adaptations of the vision system should be necessary, we have the right partner at our side in STEMMER IMAGING to be able to react flexibly to the customer's wishes here, too."
*Image source: Fraunhofer IFAM, taken during the ComBoNDT project. This project was sponsored by the European Union in the programme: "Horizon 2020 research and innovation programme" (Grant Agreement no. 636494).