Mesure et contrôle de la différence de couleur dans la carrosserie d'un véhicule

Harmony and uniformity of color are one of the main appearance characteristics of passenger cars. With the progress of society and the improvement of people’s living standards, cars have become one of the travel necessities. Therefore, in addition to gradually improving the performance requirements of cars, the requirements for car appearance and color are also gradually increasing. The color difference is an important factor affecting the appearance of scars, and reducing the color difference of car bodies is increasingly attracting the attention of manufacturers. Therefore, color difference measurement and control have become research focuses.

1. Preface

The automotive color difference is a very complex and difficult-to-control problem usually encountered in the automotive production sector, with a large number of bumpers, door handles, rearview mirrors, gas tank lids, and other decorative parts, requiring the body and plastic parts without deviation in color, the entire car color difference control has put forward higher requirements. Currently, most car factories use a combination of instruments and visual measurement methods to control the color difference. The usual method: the first development of standard color samples, the completion of the car lap parts of the color either visual inspection or instrumentation measurement should be close to the standard plate. The color difference is affected by various factors, such as color paint material, spraying process, spraying equipment, etc. are closely related to it.

2. The principle of color difference

Color meters are made of color principle; now, most automotive companies use multi-angle color meters to assess the color difference. Lab color space is the color space for measuring the color of an object (Figure 1), color space, L* for lightness, +L* for white direction, -L* for black direction. a*, b* for chromaticity coordinates, +a* for red direction, -a* for green direction, +b* for yellow direction, -b* for blue direction, the center is colorless. l*, a*, b L*, a*, b* can be calculated from the three-phase excitation values X, Y, Z. The smaller the value of Eab*, the smaller the color difference.

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Figure.1 L*a*b* color space

3. Measurement of color difference

Our company uses a combination of instruments and visual methods of a color difference measurement, first visual judgment, as shown in Figure 2, such as more than the visual limit, for the convenience of management, quantitative description, the use of a color meter can be obtained from multiple angles of the accurate color difference value.

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Figure.2 Color difference phenomenon

Currently, there is no uniform standard for color difference measurement; according to the actual situation and experience, each passenger car factory develops its own color difference benchmark. Some manufacturers use Eab as the evaluation standard, Eab<1.7, is qualified, and Eab>1.7 is considered unqualified. According to the different types of paint, the evaluation angle is different; monochrome paint only needs 1 angle (i.e., 25 °) test results to evaluate the color; metal paint (containing aluminum powder and pearl powder), its arrangement has directional, the angle value is different, need to use multi-angle color meter for measurement, the experiment proves: color meter can get, and our naked eye feel similar to the objective data. Most plastic parts, such as bumpers, spoilers, and wheel heads that match the car body, are the key for color difference measurement.

4. The causes of color difference

Many factors affect the color difference, which is difficult to control, mainly caused by the material’s defects and the flexible process control. The problem of aluminum powder accumulation in the material metal paint is often the main reason for visual chromatic aberration. In addition, uneven film thickness, poor substrate condition, drying temperature, construction temperature and humidity, paint supply system, etc., greatly impact the color difference.

5. Monitoring of color difference

The data measured by the color meter only objectively reflects the amount of color difference change. The judgment standard must still be formulated for various colors, combined with practical experience. Take white pearl paint as an example; the color benchmark adopts the standard color plate value, determines the color control range, and makes the trend chart according to the measurement value and visible result in the spraying sample and actual vehicle debugging. In mass production, the color difference problem sometimes shows gradual, regular changes; the field instrument measurement records, color difference control, and analysis are particularly important. Perfect records and charts can help us avoid color difference problems or find the root cause of color differences as soon as possible. First of all, according to the actual on-site production set, the number of daily tracking units, the development of a color difference trend chart to find the trend of the distribution of points beyond the control line to analyze and pay attention to. Different color paints have different benchmarks and monitoring range intervals, such as in Table 1, the value of L, a, b, to ensure the consistency of the lap parts color difference, measurement of the same body part to be measured at multiple points. The upper and lower limits in the table are the measurement value monitoring area (value confidential); each angle and each color has the corresponding interval; under normal circumstances, the measured value should be stored in the interval; if there is beyond, that is, abnormal, to the same color car this part of the measurement for many times, the measured value is beyond the limit, need to combine the standard plate and visual for the final decision, to develop the corresponding improvement program, to develop a practical method to control color difference.

Table.1 Measurement results of the white pearl paint part of the car body

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The table only reflects the color difference value, but the graph cannot visually reflect the trend. Figure 3 shows the changing trend of the L value of each part of the car at a certain stage of 25°. Initially, the L value of both sides of the rear side surround exceeded the upper limit. Still, after improving the coating and spraying process, the value was stabilized between the measurement and control line in the middle and later stages, which means that the change of each parameter in the production process has been smooth.

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Figure.3 White pearl paint car body of a period of 25 ° L value measurement trend graph

The graph can also reflect the changing trend of the L value of each lap part, such as in Figure 4. regularly the appropriate deployment of color assessment is also very important; in outdoor natural light, the application of visual and color meter measurement combined method to monitor the assembly of the whole car color difference problem, to visual after no color difference as a standard, the instrument to do a quantitative review, to find the direction of adjustment, to reduce the color difference played a great role.

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Figure.4 Trend of 25°L value of white pearl paint body and bumper in a certain period

6. Summary

Body color difference control is a complex project; color difference control and optimization is a process of continuous exploration and continuous improvement; only the relevant departments, in the work of continuous efforts, sum up the lessons learned to do a better job of color difference so that the user more satisfied.

Authors: Zhang Zhiqin, Ma Xiuming

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