OptiSense ensures sustainable coating of window regulator guide rails

The precise coating of window regulator guide rails on both sides is crucial for the function of window regulators. The requirements in the automotive industry are high. An inaccurate coating thickness must not represent a risk of corrosion or a functional risk.

In Upper Franconia, the Metob Group coats many guide rails for car windows every day. If too little powder is applied, there is insufficient corrosion protection. If the coating thickness is exceeded on one of the two sides of the guide rail, the window regulator will later jam. The coating thickness is therefore a functionally relevant factor.

When asked what distinguishes the Metob Group, Managing Partner Marco Jobst immediately answers "our employees". The process technicians for coating technology are real specialists and many of them have already completed their training here.

Martin Kolenda, who has been Head of Quality and Training for many years, can pass on a great deal of know-how to the trainees as a master car mechanic and environmental technician in process engineering. With great practical relevance, the trainees learn how to control the system parameters for a perfect surface. This knowledge, coupled with the team's many years of experience, is a competitive factor that should not be underestimated for the total of eight lines - four powder coating lines, three CDP coating lines and one line for washing, pickling and preserving.

Eight systems for coating tasks

In powder coating technology alone, the spectrum of Metob systems ranges from small manual coating systems and automatic booths for series parts to fully automatic, complex coating systems; the latter with a large pre-treatment section. This means that every single customer order can be produced economically and sustainably. Customers are primarily from the automotive sector as well as the furniture and baby carriage industries. For the automotive clientele, guide rails for car window regulators are coated on both sides every day on the fully automated powder coating system "Number 4".

Once the guide rails have been delivered, the substrates embark on a 200-meter journey through the circulation conveyor system. Production employees hang the parts on specially developed coating devices. The parts then travel upwards. There, they are cleaned of grease and dirt and coated with a conversion layer that serves as corrosion protection and adhesion promoter.

After pre-treatment, the guide rails are rinsed and dried to guarantee optimum surface cleanliness. The conveyor belt now transports the substrates to the heart of the powder application system, the coating unit. This is where the powder coating is applied fully automatically. Guns coat both sides of the component; the powder coating is applied to the A side of the guide rail separately from the B side.

The intelligent gap control recognizes the beginning and end of the hangers. The guns switch off automatically at these component-free gaps. This saves powder. Excess powder is fed back into the circuit.

Challenge in the coating process

The challenge in this automated coating process lies in the rear, averted area of the guide rail. The aim here is to achieve the same coating thickness as on the front of the component. "The guide rails are very small, complex parts with a wide variety of geometries. We also have upper and lower limits on the coating thickness," says Martin Kolenda, describing the challenges of the contract work. "The tolerance range of 40 to 120 μm looks quite comfortable at first glance, but the trick is in the detail: the front and back of the guide rails must have approximately the same layer thickness." If too much powder is applied to the guide rails and the layer thickness is therefore outside the tolerance band, the car windows will jam later when they are opened. "This is because the mechanism can no longer move the window pane upwards. And in the worst case, the electric motor of the window regulator breaks down," says the quality manager.

Metob processes numerous different pairs of rails, which also have completely different geometries. In addition, there are variable external influences such as humidity or temperature, which also affect the coating.

Although the parameters of the coating system can be adjusted to compensate for the disruptive external influences, the coating of the guide rails remains a fragile process. The system coats a large number of such guide rails per day. With the previous testing devices, the coating thickness was measured after curing, i.e. very late in the process. "If it then turned out that the coating thickness was not correct, the entire track was lost after application. This is because the parameters that are set in the system apply to the entire batch. A large quantity would be incorrectly coated and would have to be disposed of at great expense," Jobst calculates.

In order to optimize the process settings and process control, the company group now relies on coating thickness measurement before the powder coating is cured. The Metob Managing Director became aware of OptiSense's in-process coating thickness testing. "Non-contact before curing sounded exciting," recalls Marco Jobst.

PaintChecker tested for three months

OptiSense employee Sascha Schmidt demonstrated the PaintChecker Mobile coating thickness testing system. The hand-held measuring device was then tested by Metob employees during operation for a quarter of a year and compared with the measured values of the contact-based devices after curing. "Ultimately, we wanted to know: Is the system set economically? Because that's the only way we can avoid complaints and incorrect coatings," explains Quality Manager Kolenda. Metob also looked around at competitors. "The measuring spot of the devices was too large and therefore the measured values were not as accurate as we would have liked. That's why OptiSense won the race," says Kolenda. The company initially ordered two of the mobile measuring devices.

Early measurement prevents coating errors

Now a coater from the production team checks the coating thickness directly behind the coating booth when the powder coating has not yet been cured. If the coating is out of tolerance, it is now possible to react much more quickly and correct the system settings.

Each guide rail is checked at six "neuralgic" points, which are marked accordingly. To do this, an employee removes the rail from the production process. He measures the layer thickness and documents it in order to compare the target and actual values. The final results are documented and archived.

Control measurements of the layer thickness are carried out every 15 minutes. Metob relies on extremely intensive testing during series production, in which the coating thickness and gloss level are measured and adhesion tests (cross-cut) are carried out at closely timed intervals. "We are under a lot of pressure these days when it comes to verification requirements. This is due to the special requirements of the different orders. Depending on the customer, there are also other requirements such as tensile shear tests, arc tests, peel tests and corrosion tests. We carry out all these tests in-house and then archive the test reports. We also store retained samples for possible later comparison," says Jobst, explaining the need for complete documentation.

Stable processes thanks to intensive, series-accompanying testing

"Since we started using the PaintChecker Mobile, our processes have become much more stable," says Martin Kolenda, "previously we could only measure the coating thickness after curing; with non-contact testing before curing, the fluctuation amplitudes are significantly lower." In the meantime, the tolerance band of the coating thickness has been harmonized. It has also been possible to equalize the coating thickness values on the front and back. The values now only differ by a few μm and there are hardly any outliers. The process runs more evenly and better."

The new test method pays off

Not only solvent-free paints are used at Metob. Electricity consumption of around 4 million kWh and gas consumption of around 14 million kWh per year are also to be improved in terms of sustainability. A combined heat and power plant was put into operation in 2016. The photovoltaic system was launched in 2020 and is currently being expanded. Since 2020, the company has been supplying its electricity consumption entirely from renewable energies. An offsetting project for natural gas was initiated in 2021.

Metob also gives preference to more environmentally friendly solutions from sustainable partners when it comes to its suppliers. "Thanks to the new coating thickness gauge, we know much more quickly whether our coating is OK or not. And every part that we no longer have to dispose of today means less plant load, energy consumption, use of resources and material loss. So with the PaintChecker Mobile, we save raw materials and energy at the same time.

OptiSense GmbH & Co KG

www.optisense.com