Rejects minimized by inline measurement
Long-term experience with non-contact coating thickness measurement in production
For many years, Stobag AG has attached great importance to process efficiency and sustainability. This also includes checking the coating thickness before baking. With a flexible photothermal handheld measuring device, the company succeeds in reducing its scrap in the paint shop to less than one percent.
Stobag AG is an internationally active Swiss manufacturer of sun blinds, awnings and weather protection solutions. The family-owned company is headquartered in Muri, Switzerland, and sells weather protection systems through various subsidiaries and specialized trade partners in 35 countries. The portfolio ranges from the classic sunblind of a conservatory shade to textile pergolas and free-standing awnings to glass roofs. In terms of frame colors and fabric designs, Stobag offers individual configuration options. This requires a high degree of flexibility and efficiency in production in order to be able to realize this individualization wheel economically and with high quality.
When a new powder coating line was to be built at the Wutöschingen site in Baden-Württemberg in 2014 in order to expand capacity, those responsible placed corresponding importance on efficient processes and, above all, sustainability. The company e.Luterbach, a Swiss general contractor for coating systems, was selected. Its concept promised to be able to realize investment and economic efficiency, but also quality and environmental compatibility to the required extent. A good choice, as it turned out, because the plant fulfilled the expectations placed in it and won the ener.CON Award in 2016 for a highly optimized powder coating plant in terms of environmental impact and operating costs. The installed heat recovery and energy supply measures now save 270,000 kilograms of carbon dioxide annually while reducing energy consumption by 30 percent. The plant is also certified to ISO 50001 with regard to energy management and to ISO 9001.
However, resources can be saved not only when it comes to heat, compressed air or booth ventilation. Particularly in powder coating, the coating thickness offers considerable potential for reducing paint consumption and saving energy and process time.
Keeping the optimum coating thickness constant
For this reason, the topic of measuring the layer thickness of a powder coating that has not yet been baked was already high on the agenda at Stobag more than ten years ago. "We do about 250 m²/h with our horizontal line, and the stoving process takes about 45 minutes for us," calculates Daniel Vögele, COO of Stobag Alufinish Gmbh. "This means that if we only discover that we have an undercoating after the powder oven, we can quickly dispose of 500 to 700 square meters of coated material. In the vertical plant, it's even double that in the event of a fault." In this respect, the motivation was high to find a method of being able to check the coating thickness even before baking, so that in the event of a fault, it is possible to remove the powder from the components and recoat them with a manageable amount of effort.
"Back then, we started using ultrasonic measurement technology to measure the coating thicknesses before baking," Vögele describes the beginnings. "Because our products are exposed to heavy climatic stress, undercoating is extremely problematic. On the one hand, we have the GSB Premium and Qualicoat quality marks and naturally want satisfied customers. On the other hand, overcoating can not only cost powder unnecessarily, but also make it difficult or even impossible to fit components together during assembly."
For a while, therefore, up to six ultrasonic measuring devices were in use. "According to our mission statement, we want to be leaders in the industry in meaningful technical optimizations as well as resource-saving investments," explains Vögele. "That's why we are always looking for new possibilities, including in the area of coating thickness measurement. Here, the goal was easier handling as well as increased precision. This led to contact with Coatmaster, a company that specializes in the development of photothermal testing equipment.
Switch to the photothermal measurement method
At this point, the young company had already shown that the photothermal measuring method, in which time is recorded for the dissipation of the heat of a flash of light and converted into a coating thickness, can deliver very precise results - even at a greater distance from the measured object. However, until now only stationary measuring systems were available. But that was too costly and inflexible for the application at Stobag.
"From today's perspective, it was a really good idea to develop the Coatmaster Flex - but without such committed interested parties as Stobag, we probably wouldn't have tackled it so quickly," admits Prof. Dr. Nils Reinke, one of Coatmaster's founders. "We stepped on the gas and developed a first version pretty quickly, but you couldn't seriously talk about a handheld device yet."
The measuring device was mounted on a trolley along with a computer and had to drag a rather long cable behind it for the 220 V connection. The measurement was made with a handheld device on a thick cable.
Nevertheless, Stobag was not afraid to be the first company ever to use the Coatmaster Flex 1.0 in production. "The first version we got in 2017 definitely had its pitfalls," Vögele admits candidly. "The power cable was a real nuisance, the energy consumption was quite high, and the distance between the measuring head and the surface had to be very small. And by no means did every measurement deliver a useful result."
Vögele also reports that the intensity of the flash was still very strong at the beginning and that not only the employees found this not very pleasant - the sensors for the CO2 extinguishing system also proved to be quite sensitive at times. However, Coatmaster quickly and intensively improved the measuring technology and, for example, adjusted the wavelengths of the measuring flash accordingly. In 2018, the trolley finally became a thing of the past and the first battery-powered Coatmaster Flex was launched.
"That was already an enormous change; suddenly there was a handheld device that could be used with similar flexibility as a cordless screwdriver," Vögele enthuses. "The measuring precision was much better and also the handling through the luminous points, which showed exactly whether you were keeping the right distance, proved to be very helpful in practice." Nevertheless, there were still a few minor teething problems that were gradually cured. For example, the housing was made more stable over time and the wi-fi antennas were also improved to stabilize the connection to the cloud, where the measured values can be stored and evaluated. Furthermore, the measurement precision improved. Whereas in the early days, coating thickness measurements were subject to uncertainties of between ten and 15 µm, in daily measurement practice the figure for the current models is only 4 to 5 µm. "This is perfectly adequate for our applications," explains Vögele. "We coat a wide variety of profiles, some are wider and some are narrower, and the geometries also differ. This inevitably results in certain measurement inaccuracies, but they are not problematic on this scale."
Vögele also has words of praise for the recently introduced new Blue technology, which reduces the flash intensity perceived by the human eye to about one percent. "Especially with the vertical system, where you measure relatively high up, this makes a particularly positive difference for employees," Vögele says.
Excellent result: only 0.8 percent rejects
Stobag's willingness to be at the forefront of Coatmaster Flex development as a user has paid off. The average reject rate in surface technology is currently just 0.8 percent. The Gema powder coating booths, each with 20 guns, consistently apply coating thicknesses between 70 and 90 µm. Without control measurement, coating thicknesses well above 100 µm would have to be run.
"In order to estimate the coating thickness on a geometrically demanding component with the naked eye with some degree of reliability, you need a lot of experience," Vögele explains another advantage. "Since most of our components also require precoating, the know-how of the coaters is of great importance. From our experience, we can say that an inexperienced coater will normally produce 50 to 60 percent scrap at the beginning. By allowing beginners to check their coating results before baking, this figure improves dramatically and we can reduce the learning time by up to 60 percent. Once a gauge needs to be serviced, it quickly becomes apparent how accustomed coaters are to the ability to check coating thickness - they are much more tense and anxious because they don't know if they really coated correctly until after the bake."
Customers' options for customization make themselves felt at Stobag in up to 100 color changes per day. These take about seven minutes when recovered, depending on the color. If the press is run on loss, the color change is completed in three minutes, and the MES system always shows the coater in advance whether the next step is to run on loss or whether recovery is possible. The manufacturer of weather protection solutions also leaves nothing to chance when it comes to powder consumption. For this reason, the coater weighs the cartons before coating begins and also immediately afterwards.
The difference is automatically recorded by the system control and fed into the ERP system for recalculation. In principle, the coating thickness measured by the Coatmaster can also be used to infer powder consumption, if required, or, if the total powder consumption is known, as is the case at Stobag, the coating efficiency can be estimated.
Up to 100 color changes a day
At Stobag, customers customisation options are reflected in up to 100 colour changes per day. Depending on the colour, these take about seven minutes for recovery; if driven to loss, the colour change is completed in three minutes.
The MES system always indicates to the coater in advance whether the next colour change has to be done on loss or whether recovery is possible. The manufacturer of weather protection solutions also leaves nothing to chance when it comes to powder consumption. That’s why the coater weighs the boxes before coating begins and immediately afterwards. The difference is automatically recorded by the system control and fed into the ERP system for recalculation. In principle, the coating thickness measured by the coatmaster can also be used to determine the powder consumption or, if the total powder consumption is known, as is the case at Stobag, the coating efficiency can be estimated.
Since the coating thickness is measured not only after each colour change but also every hour and even every half hour on the vertical system, the coatmaster Flex is in frequent use. A battery charge lasts between three and four hours in this application. Changing the battery is similarly unproblematic as with a screwdriver and the batteries can be purchased in specialised shops.
The properties in terms of working distance, distance tolerance and angle tolerance and the measuring time in the millisecond range enables robust measurements even by untrained personnel, which helps to ensure efficient use of materials and consistently high quality. The data is stored centrally on a cloud and can be accessed from any browser. In addition, it is possible to download the data in appropriately visualised form in tables and diagrams. In the meantime, it is also possible to automate the coatmaster Flex through the system control, so that the measurement results are automatically assigned to specific products or product carriers, so that manual settings when saving the measurement results are no longer necessary and no errors can occur in this respect.
The successful application of non-contact coating thickness measurement at Stobag shows very clearly that anyone who wants to achieve particularly resource-efficient and high-quality coating cannot avoid considering the option of coating thickness measurement before curing. This is the only way to optimise the coating thickness and at the same time avoid expensive and environmentally unfriendly material waste - not to mention the additional costs in the form of incorrect coatings.
coatmaster AG
www.coatmaster.com
Stobag Alufinish GmbH
wwww.stobag-alufinish.de