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The Appearance Standard Of Automotive Coating
- Dec 13, 2017 -

This Artical is provided by BYK,the core supplier of Jinwei Chemical.


The paint finish of a car has two main requirements: protect the surface underneath and enhance the quality of the overall product. Eye catching finishes should not only have a beautiful and rich color, but look like a mirror “high gloss and perfectly smooth” - and of course not diminish over time.

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Therefore, automotive companies define color and appearance standards to specify an acceptable finish. Especially important is the uniformity - it must be consistent over the entire vehicle. Critical is the uniformity within the horizontal and vertical panels and the harmony between car body and add-on parts. That means how well do the add-on parts – such as bumpers, spoilers, door handles, fuel doors and decorative trim - match the adjacent body panels. Differences are especially obvious on models with zero gap styling. In the past the appearance was described with a variety of terms such as glossy or brilliant or dull – wet look or snap – orange peel – smooth. Today the wave-scan is the standard to objectively quantify appearance of the painted body and off-line painted parts at all major car companies.

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By assigning an objective value to the appearance quality lengthy and heated discussions between automotive producers and their suppliers do no longer exist. In addition, the measured data can be used for trouble shooting to improve the quality. The wave-scan DOI and micro-wave-scan measure various structure sizes to evaluate the perception of the appearance quality – close up and at a great distance. The obtained measurement results, the so-called structure spectrum, support process and material engineers to optimize material properties and application parameters. This achieves the highest possible quality.

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Structure Spectrum – a Diagnostic Tool for Trouble Shooting The structure spectrum clearly shows what structure sizes are on the surface finish – it can be compared with the fingerprint of the surface appearance. Analyzing the entire structure spectrum and evaluating the ratio of the various structure sizes helps to explain the visual perception and the possible cause. For example, the substrate quality can influence the final appearance. As an automotive finish is a multi-layer system, the quality of each layer could possibly telegraph through the final clear coat layer. Very often substrate quality issues can be seen in increased shorter waves (SW or Wa – Wb). In the following graph an increase of flash-off time caused a “rougher” basecoat, which could be mainly seen in a higher shortwave value.

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Another example for influence of substrate quality is described in the following pictures. Initially the sample surface shows a high amount of shorter waves (Wa, Wb) dominating our visual perception. By sanding the primer surface the shorter waves were drastically reduced resulting in a very glossy appearance. Due to the high gloss the longer waves (Wd) are much more apparent. This is a good example to illustrate the importance of the ratio between long waves (Wd) and short waves (Wb) in regards to the visual perception.

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In order to achieve a more smooth looking paint finish it is preferable to have a slightly higher amount of Wb than Wc (ratio 1.5:1 or 2:1).

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On the other hand a high long wave could for example be caused by an insufficient amount of clear coat.

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The following table shows some possible causes for the various structure sizes and can be used as a guide to get started analyzing surface appearance.

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