Color difference control of automobile body and exterior trim

This paper mainly introduces the causes of color difference between car body and exterior trim, color difference measuring instruments and standards, visual matching and judgment of color difference, and the control methods of color difference between car body and exterior trim in the production process.

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Introduction

With the continuous development of China’s automobile industry, consumers’ requirements for the appearance quality of automobiles are getting higher and higher, and the color consistency of automobile body and exterior parts is getting more and more attention and importance from consumers. At the same time, modern car body painting also shows the development direction of diversification and personalization, from the dazzling changing metallic glitter paint to the glamorous pearl paint, a wide variety of paint types and colors give consumers more space to choose, but all car manufacturers will face the same problem, how to control the color consistency between the body and exterior parts.

The principle of color difference generation, quantitative expression of color difference and color difference measurement method

Quantitative representation of color difference

In order to qualitatively and quantitatively express the color, in 1976 the International Color Management Committee (CIE) used L*, a*, b* color space to measure the color parameters of objects. In this color space, L* is the lightness, a*, b* is the chromaticity coordinates. Where: +L is the white direction, -L is the black direction. +a* is the red direction, -a* is the green direction, +b* is the yellow direction. -b* is the blue direction and the center is colorless (as shown in Figure 1 and Figure 2).
The color difference between the parts and the standard color plate that is the color difference △Eab*=[(△L*)2+(△a*)2+(△b*)2]1/2
In the formula:

  • △L*=L*part – L*standard (brightness difference)
  • △a* = a* parts – a* standard (red / green difference)
  • △ b* = b* parts – b* standard (yellow / blue differences)

If △L* is positive, the sample color is lighter; △L* is negative, the sample is darker.
△a* is positive, the sample is red; △a* is negative, the sample is green.
△b* is positive, the sample is yellow; △b* is negative, the sample is blue.
In general, the smaller the value of △E*ab, the smaller the color difference.

Test method of color difference

At present, most automobile manufacturers usually use color difference meter to evaluate the effect of L*, a*, b* color. From 15°, 25°, 45°, 75°, 110°, these 5 angles are accurate, objective and complete to judge the color change of metal and pearlescent paint in different angles. Its color measurement principle and color measurement results are shown in Figure 3 and Figure 4.
The 15° angle measurement is the high light color assessment, the observation direction is close to the mirror reflection, the color is greatly affected by the metal flocculation in the paint. 110° angle measurement is far from the mirror reflection, which can exclude most of the influence of the metal flocculation, the measurement result shows the diffuse reflection color of the sample, that is, the color of the pigment itself.
The color difference measurement results of 25°, 45° and 75° are usually used as the measurement index of color difference. The color difference value of the color standard sample is input into the color difference meter first, and the measurement result △Eab* is the color difference value of a point of the sample and the standard sample.

Color difference identification and determination

Color difference of visual matching and instrument determination

At present, most automobile manufacturers will use visual matching and color difference meter to identify and judge two ways, two ways in the actual color difference control process have advantages and disadvantages, so the common practice is to combine visual matching and instrument measurement, skills subjective simulation of the user’s vision, but also can use the data to quantitatively describe the deviation state, to guide the subsequent adjustment.
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Figure.1 L*a*b* color space
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Figure.2 Simplification model
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Figure.3 Schematic diagram of color difference measurement angle
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Figure.4 Chromatic aberration measurement results
Visual identification: visual identification of color difference is a very direct judgment method, which can directly reflect the naked eye’s perception of color consistency, and can also directly simulate and reflect the user’s perception when purchasing a car, customers will not take a colorimeter to buy a car, so the naked eye’s perception directly determines the user’s judgment of the vehicle’s color consistency. But visual judgment also has a strong subjectivity, different people’s recognition and awareness of color is not the same, for example, red-green color blindness or color weakness is not suitable for color difference judgment, secondly, different people’s psychological feelings about color is not the same, in the process of dealing with the problem we often encounter this situation, part of the people think color difference is not obvious, part of the people think color difference is very serious, another part of the people This situation often occurs when there is a disagreement.
Instrument determination: along with the emergence of color difference meter, each factory can quantitatively measure the difference between the relevant samples and the standard color card, for the subsequent adjustment of color and subsequent control provides data support, through the use of SPC tools, some factories will establish a long-term color difference monitoring system, analysis process fluctuations, a good realization of the consistency of color control.
On the instrument measurement of color difference standards, most automotive suppliers currently use △ E * ab value determination criteria, in general, reference to 25 °, 45 ° and 75 ° three angles of color difference integrated value, △ E * ab ≤ 1 for qualified, 1 < △ E * ab ≤ 1.7 for the section review concession, △ E * ab > 1.7 for unqualified. This standard is only applicable to the same brand and model of colorimeter. However, based on the current SGMW and exterior trim suppliers, there are two situations: First, the body and suppliers use the colorimeter brand differences, different brands of colorimeter because of the difference in the built-in algorithm, will lead to the measurement data between the instruments are not comparable; Second, the same manufacturer and model of colorimeter, in measuring the color difference between the body and exterior trim parts of certain colors, the visual color difference is small, but the data difference Very large, or visual difference is very large, data measurement but the difference is very small, the accuracy and reliability of the color difference meter will also be a problem.
From the above distance can be seen, in the same △Eab* value, the visual difference between the two samples and the standard color plate is very obvious, that is to say, whether using visual identification or instrumentation, can not be very perfect solution to the color difference problem, in the actual color difference control need to combine visual and instrumentation, in order to better control the color difference between the body and exterior trim parts.

Color difference control

Separate control according to the characteristics of different colors

At present, the development of body color shows a trend of diversification and personalization, in the face of more and more colors, and even some special colors, the traditional color difference control standards will be broken and overturned, how to develop the body and exterior trim color difference control means and control standards according to the characteristics of different colors, and coordinate the control direction between the two, in order to better control the color consistency of the whole car. The following Table 2 and Figure 6 show examples of the two colors L, a, b values and the range of color difference discernible to the naked eye under different color phase states.
From table 3 can be seen that the color A itself is very high brightness, but a, b color phase is weak, combined with Figure 6 can be seen, when the color phase is getting smaller, the naked eye color difference recognition range (oval range indicates that the color phase in the range when the naked eye can not identify the color difference) is getting smaller, which means that as long as the color phase and the standard color plate between a slight difference, the naked eye can distinguish the difference, in other words, color A in the subsequent color difference In other words, color A in the subsequent color difference control process requires strict control of red / green and yellow / blue phase, brightness control can be relatively relaxed a lot. Then look at color b, brightness L and yellow phase b higher, but the red phase a value is very low, combined with Figure 6 can be seen in color B in the a value of the identifiable number axis above the range is very small, in the subsequent color difference control need to strictly control the red phase a, brightness L and yellow phase b control can be much more relaxed. Through the above example can be seen, each color L, a, b value of the smaller elements determine the color difference control, which is the color difference control in the “barrel effect”.
Table.1 Summary of the advantages and disadvantages of visual identification and instrument identification

Identification method Advantage Disadvantage
Visual identification It can simulate the state of customers when buying cars, and indirectly reflect the evaluation state of color difference when customers choose cars.
  • 1. Different people have great differences in color recognition ability;
  • 2. The psychological factors of personnel have a great influence on the color difference identification, and subjectivity has a great influence on the identification results;
  • 3. Environmental impact: the color difference states seen under different environments are different.
Instrument identification Strong objectivity, not affected by other external factors. The reliability is insufficient, and the proximity between instrument measurement and visual evaluation is only 75%.

Table.2 Comparison of a color data measurement results

Sample1 Sample
OL’ 0.57 0
Oa’ 0.57 0
Ob’ 0.57 1
Eab* 0 1

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Figure.5 Comparison of visual difference under the same measurement value
For the above color characteristics, we will combine the color characteristics in the daily color difference control, the main control elements of the color refinement standard, when the body and exterior trim parts are controlled within this standard, the color difference control of the body and exterior trim parts will get very good results.

Regular color matching

We have already mentioned the advantages and disadvantages of visual matching and instrument identification, in the actual color difference control process, we often use the method of visual and instrument combination. Visual identification to make up for the error in the instrument measurement, instrument measurement to guide the subsequent adjustment and control, to achieve these two purposes must have two conditions: First, there must be a relatively objective identification of color difference team, the second is to effectively identify the measurement characteristics of different colorimeter.
The current practice of most manufacturers is to organize paint shops, exterior trim suppliers, paint suppliers, and quality inspection departments to conduct color matching on a regular basis from the new color project stage. The team members must be able to effectively identify and objectively evaluate the deviations between body and exterior trim parts, examine the color consistency of the whole vehicle from a professional perspective, and guide the subsequent color adjustment of body and exterior trim suppliers. Regular color matching can promote the exchange of color difference control between body and exterior trim parts, facilitate all parties to understand the latest status of their respective colors, and guide the color difference control of all parties to the same direction, so regular color matching is very beneficial to the color difference control of body and exterior trim parts.

Conclusion

Color consistency control of the whole vehicle is a complex work, need to persistently, according to the actual situation to explore and study the control means and control standards, to find the best method of color difference control of various colors. At the same time, we establish a color difference control system based on prevention, so that we can find and identify potential risks in advance, continuously improve the color difference control work, and improve customer satisfaction.
Table.3 Two colors L, a, b values

Colour Angle L a b
Color A 25° 87.09 -0.13 2.77
45° 86.93 -0.11 2.66
75° 87.28 -0.16 2.22
Color B 25° 91.33 7.13 49.24
45° 53.3 4.21 31.25
75° 51.62 4.54 21.21

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Figure.6 Naked eye color difference discrimination range under different color phase states
Authors: Xia Shaowei, Chen Yanjun

Source: China Color Measurement Solutions Provider – www.spectrumgfa.com
www.spectrumgfa.com is one of the leading china Colorimeter & Spectrophotometer Manufacturer, with professional factory. We focus on the colorimeters, spectrophotoemters, haze meters, gloss meters and spectral analysis instruments research and development, manufacturing, sales and service since the company been established. Our products is widely used on Painting,textile,plastic industry,food,building material and other products. 
If you want to have more information about the article or you want to share your opinion with us, contact us at [email protected]
References:

  • [1] Lin Yaopeng . A brief discussion on the color difference control of car body and matching parts . Coating Technology and Digest, 2015-05-31.
  • [2] Hu Meifang, Zhan Jian .Control of color consistency of MG whole car . Automotive Technology and Materials, 2011-01-20.
  • [3] Li W.T., Zhu Y.P., Ma J.Y . Management and control of joint color difference of Zhang Yapu automobile painting. Modern Coating and Painting, 2013-02-20.

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