Coating thickness measurement of wet paint on chrome before baking

Checking the coating thickness of chrome-plated components coated with wet paint was a particular challenge for a paint technology company. With a mobile coating thickness measuring device, this is now possible even before stoving.

Whether piano lacquer look, metallic effects or anti-slip coatings: Stoz GmbH from Rottenburg am Neckar paints components that are primarily used in sanitary technology, the two-wheeler and automotive industries and electronics. These industries have one thing in common: they have extremely high requirements in terms of visual and tactile quality, durability and resilience.

Paintwork on chrome is a particular challenge - especially when it comes to testing the precise coating thickness while still wet. The management quartet of Managing Director Maximilian Stoz, Production Manager Holger Salzmann, Quality Assurance Manager Jürgen Andrusch and Technical Manager Steffen Fessele report on how this is now achieved.

Stoz GmbH offers many products and processes with unique functions under its own brand name. These are high-performance coatings that focus on their technical character. These include corrosion protection, anti-friction coatings, graffiti protection and anti-slip coatings. Each system is perfectly matched to the substrate - this applies in particular to coatings for chrome-plated components. It goes without saying that the appearance of the surfaces is not neglected, as an excursion into the world of kitchens and bathrooms shows.

Taps have a special place in the modern home. They are among the most stressed objects. "Innovative technologies, futuristic design, lasting value and sustainability are standard today. The demands on the surface properties and therefore on the coating are correspondingly high," says Managing Director Maximilian Stoz, describing the demanding requirements of his sanitary technology customers.

Initially only in isolated cases, but now also in the mainstream of the world of fittings, a new variety of surfaces has developed, which is particularly impressive due to its colorfulness. Manufacturers and consumers had probably had enough of the eternal chrome - everything should be more individual and at the same time bring a luxurious touch to the bathroom and kitchen. Colors such as copper, rose gold, various shades of black, anthracite or even silky matt white are all the rage.

However, there was one fly in the ointment: it was extremely difficult to determine the coating thickness. "The component first had to be baked and only tested once it had hardened - often this was only possible with a destructive micrograph," says Salzmann.

User Andrusch confirms: "There were also clear limits to the measuring capability of the eddy current device used to date. Although the coating thickness test works well on steel, we were unable to obtain reliable measurement results on a chrome-plated substrate." To make matters worse, these components often had complex geometries with small measuring surfaces, tight radii, grooves and cavities.

While searching for a testing device that can determine coating thicknesses on chrome-plated substrates, Holger Salzmann came across a technical article in an industry magazine about the PaintChecker from OptiSense, which was described as a miniature handheld device specifically for measuring small areas in holes, recesses and hard-to-reach places.

After the Stoz management team had convinced themselves of the quality of the PaintChecker at the leading industry trade fair PaintExpo in Nuremberg, they wanted to test the coating thickness measurement system extensively in their own company. "The manufacturer from Halten provided us with a loan unit for a month. We wanted to find out whether the PaintChecker could really check the coating thickness of the wet paint-chrome material combination precisely," says Managing Director Stoz, describing the solution to the measurement problem: "At the same time, we had tried out a Swiss brand for the 'paint on chrome' task. However, we aborted the test phase of the competitor's device because it never really worked. As a result, even employees who were initially rather critical of the PaintChecker were quickly won over. The OptiSense measuring system really is the only one on the market that can precisely test this combination of materials. This was confirmed in all comparative measurements," says Salzmann, summarizing the results of the test phase.

PaintChecker coating thickness testers use the photothermal measurement method to determine the thickness of coatings in a non-contact and non-destructive manner. The different thermal properties of the coating and substrate are used to determine the absolute coating thickness. The surface of the coating is heated up by a few degrees with a short, intense light pulse and then cools down again by dissipating the heat into deeper areas. The thinner the coating, the faster the temperature drops. The temperature curve over time is recorded from a distance using a fast, highly sensitive infrared sensor and converted into a corresponding coating thickness. The photothermal measurement method thus provided a fast, quantitative method of determining coating thickness that delivers precise, reproducible results.

Maintaining the correct measuring distance in particular is very intuitive, adds the production manager: "The Paint Checker has three LEDs that visually indicate the perfect measuring distance. The exact correct distance is achieved as soon as the three positioning aids converge at one point. That's a very clever solution." Coating thickness measurement on wet paint offers many advantages for the company, as most of the Stoz painting systems are robot-operated. Job-specific painting sequences are programmed for this purpose. In the first step of programming, it is particularly important that the same coating thickness is applied everywhere. The actual layer thickness can be adjusted later via the quantity of paint applied.

"We use the PaintChecker to check the uniformity of the coating while it is still wet - on the very first component before actual series production. With the PaintChecker's photothermal testing method, our programmer knows immediately whether the coating thickness is identical everywhere or whether it needs to be readjusted in order to achieve a completely uniform coating application," says Technical Manager Fessele.

When asked how many points are tested per component, Fessele replies: "It depends on the chrome-plated test specimen. There are visible surfaces on a painted part where the coating thickness must be maintained. There are also surfaces, for example, that may be coated but do not have to be. Ultimately, the programmer decides how many measuring points he needs to optimally adjust the system." Every tap manufacturer has its own designers and there are countless different designs of lever mixers, kitchen spouts, etc. The variety of parts is enormous. The variety of parts is enormous, which means that the individual series are relatively small, but the total quantity is quite high. Stoz coats around 20,000 sanitary components every week.

The PaintChecker saves the environmentally conscious coating company time and resources, as the first component is tested non-destructively and therefore no longer represents a reject. "The topic of sustainability is generally very important to us," emphasizes Stoz. This includes, but is by no means limited to, environmentally friendly materials and processes.

The company's approval under the Federal Immission Control Act and certification in accordance with the international environmental management standard ensure that the environment is protected - even when processing solvents and other hazardous substances. Thanks to modern plant technology and the exclusive use of electricity from renewable energy sources, Stoz also makes a contribution to reducing CO2 emissions. The company also reduces material costs, as the layer thickness test immediately shows how thick the coating will be after curing. Excessive "safety margins" are therefore a thing of the past.

The PaintChecker is of course also used for jobs in other sectors, such as the motorcycle industry. Stoz paints brake levers for motorcycles, for example. The coating thickness is functionally relevant here. Although the tolerance range for coating thickness is relatively wide at 30 and 110 μ, the coating thickness can easily be undercut or exceeded with multiple coatings despite this wide tolerance range. The brake lever then jams if the coating is too thick or can no longer be fitted. And these aluminum components are difficult to measure with a contact measuring system due to their geometry. The paint checker has to be used again," laughs Salzmann.

Another Stoz specialty is decorative surfaces for a wide variety of substrates with special visual and tactile properties. High-gloss coatings in all colors are just as much in demand as metallic effects or soft paint with a particularly pleasant surface. These paint finishes are now also tested with the PaintChecker, e.g. veneers for car interiors. They should be functional and look modern and elegant.

However, fingerprints often spoil the appearance. To ensure that these remain invisible in car interiors, the component is coated with the special clear coat, the thickness of which is also precisely checked with the PaintChecker. "Thanks to the particularly fine paint structure, the surface achieves optimum anti-fingerprint properties and very good gloss behavior," reports Andrusch. In addition to a non-stick effect and optical suppression of fingerprints, the paint applied by Stoz is particularly abrasion-resistant and largely transparent.

Soft-touch coatings give components a certain feel. Depending on the finish, the surfaces feel velvety soft and silky or almost rubbery. Normally hard and smooth product surfaces are given an effect that resembles the feel of leather. Soft-touch finished products are therefore often referred to as hand flatterers. In the automotive industry, soft-touch coatings are mainly used in interiors.

The soft-touch effect coating is colorless, so the final color depends on the base material or primer. For this reason, the visual character of wood or metal surfaces is retained. By adding pigments, it is even possible to produce colored soft-touch lacquer. The different haptic perception of soft-touch lacquers is caused by the lacquer composition or special drying processes. The light is completely scattered on the microstructured surface, which creates the distinctly matt effect. And here too, Stoz achieves excellent test results with the PaintChecker.

"The application examples could go on and on," summarizes Managing Director Stoz, "We are optimally positioned for our purposes with the PaintChecker." (OM-1/24)