Fraunhofer Institute for Industrial Mathematics ITWMOur researchers are developing innovative measurement techniques for determining the thickness of paint coatings – directly during production, accurate down to individual layers and suitable for substrates such as metals and plastics. The goal is automated inline quality control that helps conserve resources, reduce costs, and optimize corrosion protection for complex multi-layer coatings in the automotive industry, aircraft manufacturing, and other industries.
Paint film thickness measurement for maximum quality: Film thickness measurement in multi-layer systems using the automotive industry as an example, Detail Spray Layer
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Lackschichtdickenmessung für höchste Qualität: Schichtdickenmessung in Mehrschichtsystemen am Beispiel Automobilbranche
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It’s All in the Paint! Paint Thickness Measurement for the Highest Quality
Coating Thickness Measurement in Multi-Layer Systems – Precise, Fast, and Inline-Capable
Modern coatings have long since gone beyond fulfilling aesthetic requirements. They protect surfaces from corrosion, scratches, and environmental influences, and must meet the highest industrial standards. The structure is usually complex: elaborate multi-layer systems consisting of primer, filler, top coat, and clear coat are used – not only on cars and airplanes, but also on ship hulls, turbine blades, tablets, and eyeglass frames.
Challenge in Production: Speed Meets Precision
Industrial manufacturing is characterized by highly optimized cycle times – and the measurement technology used to check paint thickness must keep pace. For reliable process monitoring, it is no longer sufficient to determine the total thickness alone. What is needed is precise measurement of the individual layers within a paint system, ideally in real time and inline – regardless of the material used.
Resource Efficiency Through Intelligent Measurement Technology
In automotive production in particular – which consists of pressing, bodywork, painting, and assembly – painting has a major impact on energy consumption and environmental pollution: around half of the energy required in the painting process is used for applying and drying the coatings. In addition, around 99 percent of the solvent vapors in production come from the paint shop.
The thicker the layer, the more energy is required for drying and the more solvent is released. It is therefore crucial to control the use of materials early in the process and to use layer thickness measurement as early as possible in the process. However, wet paints place special demands on the sensor technology. Our research aims to provide robust, reliable, and process-compatible solutions in this area.
Measurement Techniques Used: Non-Contact and Non-Destructive Measurement Methods
Precise adherence to individual layer thicknesses is a decisive quality factor, especially for multi-layer coatings on metal and plastic. In order to reduce material consumption and energy use, layer thickness control should be carried out as early as possible in the process.
For this demanding task, we offer a range of optical and non-contact measurement techniques – individually tailored to the respective application and always non-destructive:
- Interferometric sensors
- Chromatic sensors
- Optical Coherence Tomography (OCT)
- Terahertz-TDS (Terahertz Time-Domain Spectroscopy)
- Terahertz-FMCW (Frequency-Modulated Continuous Wave)
Thanks to the wide range of sensors available, we can select the one best suited to the respective application. Important aspects to consider when selecting a sensor include the layer thickness range, number of layers, color of the layers, and the material of the layers and substrate.
Example of the integration of the terahertz sensor on a robot for paint thickness measurement on car bodies.
Automotive paint with a wedge-shaped clear coat application to demonstrate measurability.
Terahertz Technology With Unique Selling Point
Our terahertz-based measurement methods are particularly noteworthy. Thanks to their unique properties, they offer a clear technological advantage in many cases:
- Non-contact
- Resolution of individual layers in multilayer systems
- Measurements on colored coatings
- Measurements on any substrate
The terahertz testing systems developed by our »Materials Characterization and Testing« department combine all these requirements in compact, robust, and industry-ready solutions. With a reproducibility of less than 1 µm and measurement rates of up to 1,600 measurements per second, they are ideal for inline quality control of multi-layer coatings.
Inline Paint Thickness Measurement for the Automotive Industry
In automotive production, with its high volumes, process reliability through close-meshed quality control is crucial, not least in order to save resources such as raw materials and energy and to avoid quality defects that could jeopardize the brand image.
One key area of application is car body painting. Checking paint thickness at the end of the painting process is particularly efficient if the measurement technology used can detect all applied layers individually. Terahertz sensors can be used to precisely detect the entire layer structure in a single step, without the need for time-consuming multiple measurements at different painting stations. A single measurement system, including a robot, is sufficient to ensure the final paint quality. In addition, the measurement can be carried out outside the explosion-proof area, which reduces integration effort and costs.
The technology has been successfully established as an end-of-line inspection and is now used worldwide by automobile manufacturers for inline quality control.
Reliably Testing Plastic Components and Radar Systems
Exterior and interior plastic parts such as bumpers are also subject to strict quality requirements – especially since they often perform safety-related functions. For example, radar or assistance systems are often located behind painted bumpers, and their function is significantly influenced by the thickness of the material and paint.
Since terahertz measurement technology can measure on any surface, it can also be used here, and the experience gained with car body paint can be directly transferred. A particular advantage is that the wall thickness of the bumper can be measured at the same time as the paint thickness – both of which are important for the correct functioning of the integrated radar sensor.
Not only must the paintwork on car bodies meet high quality standards in the automotive industry, but so too must the various exterior and interior plastic parts. Take bumpers, for example.
Coating Thickness Measurement Directly During the Painting Process – Even With Wet Paint
In many industrial applications, it is not sufficient to check the baked-on paint first. Instead, early inspection of the coating thickness while it is still wet is required.
Challenge: Measuring in a Changing Medium
Measuring wet paint films is particularly challenging: during drying, solvents or water evaporate, causing the material composition to change continuously. To enable a precise prediction of the final dry film thickness, the measurement method must be able to reliably detect the current composition and solid content in the film.
Conventional measurement methods quickly reach their limits here:
- Magnetic inductive, eddy current, and ultrasonic methods require mechanical contact and are therefore unsuitable for wet films.
- Photothermal methods are contactless, but often deliver insufficient or incorrect results when the layers are still wet.
Terahertz Technology for Wet Film Measurement
Terahertz measurement technology has currently proven to be the only suitable method for measuring wet films non-destructively, contactlessly, and precisely – both on metallic and non-metallic substrates in the industrially relevant range from 10 µm to 500 µm.
A particular breakthrough: the measurement of wet film thickness directly during painting – even through the paint mist – has been successfully demonstrated. This opens up completely new possibilities for real-time monitoring and process control in painting technology.
Illustration of the layer thickness distribution on a painted test sheet.
Temporal development of layer thicknesses along the marked line during drying. After drying and baking, the layer thickness was determined by contact using a magnetic induction sensor.
Comparison of weight loss and terahertz measurements during the curing process.
Coating Thickness Measurement for Enameled Wire – Precise Even at High Production Speeds
Enameled wire is a thin metal wire, usually used in coils – often between 0.01 and 1 mm in diameter – that is coated with an insulating varnish. This varnish layer performs an essential electrical insulation function and must therefore be applied precisely and evenly.
The challenge: High wire speeds are achieved in production, while at the same time the wire moves slightly perpendicular to the direction of movement, which rules out the use of classic point sensors such as chromatic or interferometric sensors.
OCT image of a thin coated wire with two opposing measuring heads. Assuming a circular wire, the wire diameter can also be determined.
OCT image of a thin enameled wire: The lateral measurement range is larger than the wire to enable measurement despite lateral movement of the wire. The multiple reflections are used to increase measurement accuracy.
OCT System Measures Coating and Wire Thickness Simultaneously
An Optical Coherence Tomography (OCT) system provides a reliable solution to this problem. The technology makes it possible to
- accurately determine the coating thickness on the wire
- measure the wire thickness itself
- and check the even distribution of the coating around the wire – all inline and in real time.
This enables complete quality control of the enamel wire during production – even under demanding conditions.
Reliably Testing Painted Eyeglass Frames
Eyeglass frames are also delicate components that require precise coating thickness measurement – both for metal and plastic components. The focus here is on high surface quality and aesthetic appearance as well as protection against wear and tear.
OCT technology can be used to achieve reliable measurement results on such fine structures in a non-contact and non-destructive manner.
The Challenge of Wood: Visually Measuring Textured Surfaces
OCT image of a painted wooden panel. Both the smooth painted surface and the wood grain are visible. This allows the thickness distribution of the paint to be determined.
Measuring paint thickness on wooden surfaces is particularly challenging. This is due to the natural grain of the wood, which, depending on the type and processing, can result in a highly textured surface. Many conventional optical methods cannot be used here – they either provide inaccurate results or no results at all.
OCT technology, on the other hand, has proven itself even on textured substrates such as wood. However, the definition of the paint thickness to be measured is crucial for the measurement strategy: Should the minimum, maximum, or average layer thickness value be evaluated? The selection influences both the evaluation and the interpretation of the results – and should be tailored to the specific application.