OCT System Could Improve Quality Assurance of Complex Car Paints

An OCT-based technique could provide a practical solution to managing the number, size and orientation of small metallic flakes in the base coat layer of automotive paints, improving the efficiency of the automotive finishing process.

To image the paint flakes common in the coatings of many cars today, researchers at the University of Liverpool developed a time-domain full-field OCT system to scan automotive panels volumetrically, nondestructively and without contact. The 3D OCT instrument features very high lateral spatial resolution to distinguish miniscule flakes and high-depth resolution to see the position and orientation of each flake. Because a single 3D OCT image can contain thousands of flakes — too many to be measured manually — the researchers also developed an algorithm to automatically distinguish flakes and allow measurement of their properties.

Tests of the technique on nine sections of five panels showed that this integrated approach was able to characterize small flakes in automotive coating systems in 3D, calculating the number, size and orientation accurately and consistently. The OCT system was shown to work well for imaging the top two layers of car paint, which includes the layer with the metal flakes.

According to the researchers, their method could be used to automatically identify and perform 3D measurements of metal flakes that are just 10 µm in diameter and 1 µm thick. The team demonstrated its approach on samples of applied car paint; but say that with further development the technique could be used for in-line monitoring, where it could detect problems that occur during the painting process.

“Using the technique for monitoring in-line processes could also help automotive manufacturers better understand the whole coating process,” said professor Yaochun Shen. “With that better understanding, the car industry may be able to develop new coating processes or new types of coating.”

The automotive industry currently uses ultrasound imaging to examine car paint in quality assurance checks. Although ultrasound instruments are easy to use, they cannot effectively image the tiny metal flakes used in many modern car paints. Ultrasound measurements also require equipment to be in contact with the sample, which means ultrasound cannot be used for in-line monitoring without interfering with the paint job.

“When the car manufacturer approached us to develop a new technique for analyzing the glittery flakes in car paint, we felt OCT could provide a solution,” said Shen. “We use OCT in our ophthalmologic research because of the high resolution and because it can perform measurements without touching the sample. This combination makes it ideal for the analysis the automobile manufacturers needed with production.”

The research team believes that their work represents the first system suitable for industrial use that is capable of measuring such intricate properties of metallic paint flakes in automotive paint.

“We have demonstrated, for the first time, through OCT and our image analysis approach, [that] we are able to quantitatively and automatically measure the size, number and orientation of metal flakes in industrially applied car paint,” said Shen. “This approach could be very useful for quality assurance processes used during car manufacturing.

“The painting step is a bottleneck in the manufacturing process,” he said. “If the finished car paint does not meet requirements, then it must be removed chemically and the car completely repainted. This not only costs time and money but also creates chemical waste and associated environmental issues.”

The researchers say that the current OCT hardware and software is very close to being usable in an industrial setting for analyzing paint that has already been applied. They are currently looking at ways to speed up the measurements so they could be used for real-time monitoring during the application process.

The researchers are also working to apply OCT in other industrial contexts, such as analyzing coatings of pharmaceutical tablets. The quality of these coatings controls drug release rates and OCT could help quantitatively analyze the thickness uniformity of these coatings and check for defects.

“Our research with car paint and with pharmaceutical coatings shows that OCT, which has been used for some time for medical applications, can also be used for industrial applications,” said Shen.

The research was published in Optics Express, a publication of The Optical Society (doi: 10.1364/OE.25.018614).