Predefined Coating Thicknesses for Vehicle Bodies and Wheel Rims
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Predefined Coating Thicknesses for Vehicle Bodies and Wheel Rims The thickness of the coating plays a central role in the coating process, because it has a direct impact on the functional and decorative properties of the coating. For this reason, manufacturers specify tolerance ranges for coating thickness which have to be monitored using appropriate measuring devices.
Professor Nils A. Reinke, Mario Oesterle
The automotive industry sets high standards for process reliability and quality which mean that coating thickness measurements must be repeatable and traceable. Conventional methods, such as measurement system analysis, can be used for the qualification of the measuring device. These methods have highlighted the fact that traditional contact measuring systems are no longer suitable for process monitoring, as a result of the higher industry standards. They are increasingly being replaced by measurement devices based on ATO (advanced thermal optics) technology. The first part of the article illustrates the use of ATO technology for the manual measurement of electrocoatings on vehicle body components. The second part describes how a coating thickness measurement device complements an innovative, automated process for coating wheel rims. Contact measuring devices are typically based on a process involving the distancerelated damping of an oscillating electrical circuit. In the case of flat substrates and smooth coatings, this can be used to calculate the coating thickness. However, varying material compositions and curved surfaces can result in significant deviations. A rough or rippled coating has a negative impact on the repeatability of the measurements. Because contact measuring devices are usually hand-held, the operator too can influence the measuring result. The measurement probe can penetrate soft coatings and distort the measurement. In addition, contact coating thickness measurement devices can only 38
be used with fully cured coatings and are therefore unsuitable for making measurements at an early stage in the application of the coating. As a result, they do not help to improve the reliability of the process. Non-contact coating thickness measurement with ATO technology is based on a simple and robust principle. The coating is heated briefly by a few degrees Celsius using a pulse of light and the lifetime of the wave of heat in the coating is measured by infrared sensors. This measurement is then converted into the coating thickness. As the heat always travels vertically through the coating, the measurement is largely independent of the orientation of the measuring device and therefore of the operator.
Manual measurement of electro coatings within tight tolerances Electrocoating is an electrochemical process that involves coating components in an immersion bath. It is ideal for parts with complex structures that are being coated in large quantities. Electrocoating is the standard process used to provide vehicle bodies with corrosion protection. Many of the properties of electrocoatings are directl
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