Coating thickness measurement for large surfaces and moving components

A newly developed scanning photothermal coating thickness measurement process enables the surface inspection of endlessly long surfaces and moving objects without having to move along with the components.

The new PaintChecker Move scanning coating thickness measurement system from measuring system manufacturer OptiSense has a completely modular design so that the measuring device can always be optimally adapted to the respective application: The excitation and detection modules are self-contained and can be modified independently of each other. The innovative measuring system can inspect endlessly long surfaces and moving objects without having to move along with the components. The new technology actively uses the movement between the workpiece and the sensor instead of compensating for it, as is the case with conventional devices.

Cost-effectiveness, precision and surface quality have been among the dominant requirements in the coating industry for some time now. At the same time, however, it is still hardly possible in many companies to monitor a very decisive parameter in this context at an early stage and permanently: the amount of paint applied. It makes sense to check the coating thickness as a relevant parameter before it hardens, i.e. while it is still liquid or soft, as incorrect coatings can still be easily corrected at this stage.

A typical example is the coating of flat substrates such as films for battery production. The strip is coated with liquid slurry on a copper or aluminum foil. The inspection must be contact-free and seamless, as the production and subsequent use of battery modules and packs are associated with numerous safety risks that require special protective measures.

To date, coating thickness testing of moving objects has been problematic, as the object is no longer within the detector's field of view after a short time. Of course, it is possible to move the entire measurement setup, but this approach is often technically complex and is not expedient where space is limited. In addition, no checks can be carried out while the coating thickness inspection system is moving back to the starting point in order to detect other moving objects. This "return gap" therefore does not allow seamless measurement.

To avoid this dilemma, the heating area is enlarged for measurements in motion. This gives the moving component sufficient time to absorb the necessary amount of heat. During the test time, the heated workpiece passes through the detector's field of view and the temperature curve is detected. The result is a coating thickness value - however, the spatial resolution then no longer corresponds to the size of the detector's field of view, but to the distance covered during the measurement time. Even at relatively low speeds, this is already several centimeters. The layer thickness determined then corresponds to the average value over this range, but small imperfections can no longer be detected.

The situation is no better when checking the coating thickness of extensive surfaces: One current idea for inspecting large surfaces with spatial resolution sounds simple at first: the heating region is enlarged for this measurement task and an IR thermal imaging camera is used as a detector. However, this approach quickly becomes very complex. The larger the area, the greater the required excitation power, which often requires several energy-intensive light sources. In addition, the larger the area, the greater the demands on the camera and therefore the higher the costs. Another legal hurdle is that high-resolution IR cameras are often classified as dual-use goods, which leads to trade restrictions.

This means that reliably and continuously monitoring the coating thickness of large surfaces or moving objects before curing can only be achieved with enormous effort using existing approaches.

Dr. Fabian Gaußmann from the development department of measuring system manufacturer OptiSense was therefore looking for a solution that could check both large and moving objects for correct coating without contact, precisely, continuously and even before curing.

The result is a scanning measurement process that makes use of the movement between the component and the sensor instead of compensating for it at great expense. The object is not scanned once as a whole, but continuously as it passes through the measuring device. "The movement of the workpiece is therefore no longer an obstacle, but an elementary part of the process. The testing system can be guided along the stationary workpiece, the workpiece can move past the fixed sensor, or both the sensor and the component can move relative to each other," says Dr. Gaußmann, explaining the basics of the new measuring principle: "The process is patent pending and represents a milestone in the measurement of moving and large-area workpieces."

The new, scanning OptiSense system is called PaintChecker Move and has a completely modular design so that the measuring device can always be optimally adapted to the respective application:

The excitation and detection modules are stand-alone and can be modified independently of each other. They are connected via an element that also contains the data cable connection and provides the mechanical connection to the customer's system. The angle of the connecting element is adapted to the respective test situation.

The excitation module contains the optical components for beam shaping. Depending on the measurement situation, different shapes of the excitation area on the workpiece surface are possible. In a standard configuration, a homogeneously illuminated rectangle with an edge length of up to 100 x 10 mm² is generated. The edge lengths can be configured independently of each other.

Lasers of different power classes and wavelengths are used as excitation sources. What all variants have in common is that the laser itself is installed in the external controller housing and is connected to the excitation module via an optical fiber. This means that the excitation module does not require cooling, as it only contains passive optical components. The glass fiber is protected from mechanical influences by a 5-meter-long metal tube with plastic sheathing and is suitable for drag chains.

The detection module essentially contains the IR sensor matrix including imaging optics. Data communication takes place via cable. Power supply and communication take place via a 5 meter long cable.

The excitation and detection module have a footprint of 55 x 55 mm². The height is approx. 150 mm (excitation module) and 115 mm (detection module). The sensor weighs around 1.5 kg. The external controller housing has the same dimensions and type as the models currently used by OptiSense for industrial applications. The system is controlled via a PC connected to the controller via a network cable.

The PaintChecker Move is flexible and can also be easily integrated into existing production lines at a later date. In particular, the condition of the coating (wet, moist, soft, dry, baked) and the dimensions of the workpiece hardly play a role. It is even possible to accurately inspect objects of any size in a non-contact and repeatable manner with several measuring systems mounted next to each other.

The economic advantages of the OptiSense measuring system are considerable. The PaintChecker Move reduces material waste, optimizes production processes and increases product quality. Coating companies can also achieve significant cost savings. For example, reducing the reject rate by just 0.5 percent saves a medium-sized contract coater annual costs in the five-digit range. These savings result not only from the reduced material consumption, but also from less reworking or reduced rejects and the associated savings in labor costs.

At the same time, the system helps to increase production efficiency. Continuous monitoring guarantees more stable and efficient production, which minimizes downtime and increases the overall productivity of the systems. PaintChecker Move also significantly reduces the risk of complaints.

Furthermore, the PaintChecker Move's continuous data acquisition and analysis enables comprehensive process and quality monitoring. The data obtained can be used to identify weak points in the production process and take targeted improvement measures. Interest in the market is already very high. The evaluation phase with several pilot customers has already got off to a very good start and the measuring system will be available shortly.

The PaintChecker Move marks a significant advance for the coating industry as a scanning 3D coating thickness inspection method. The precise and continuous measurement of coating thickness significantly increases both production quality and efficiency. The resulting material and cost savings as well as the increase in quality make this technology a worthwhile investment.

Permanent and reproducible monitoring of the application ensures a uniform and consistent coating. This is particularly important to improve the durability and optical properties of the products and to meet the increasingly demanding quality standards and all necessary safety regulations. The latter are particularly crucial for products that are subject to liability, such as battery modules or highly stressed parts from the transportation industry, such as in the aircraft and automotive industries, railroads and bicycle production.

The new, scanning 3D PaintChecker Move is flexible and user-friendly. It is easy to integrate into existing coating lines - even in the most difficult production environments. The inspection system thus offers coating companies a powerful solution for optimizing their production processes and ensuring and increasing their product quality. (OM-10/24)