Analysis of the solution to the defects of the color difference of the car body paint film

1. Color difference phenomenon confirmation and analysis

A 2020 Chery Ruihu 8 car, due to a cut accident, caused damage to the right rear door paint film, and the body paint film needed to be repaired to restore the paint film to its original color. The body color was rhineblue metallic paint, and the paint film was repaired, and it was found that there was a color difference defect between the right rear door and the right front door, and the car could not be delivered (Figure 1).

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Figure.1 Right rear door and right front door have color difference
Initially, the reason for the color difference was the inaccurate construction color formula. The reasons for the inaccurate formula are: different batches of color masterbatch were used; the color masterbatch was not fully stirred before use, and the pigments in the color masterbatch were not evenly dispersed; the color masterbatch in the color paint formula was not evenly stirred after adding and mixing; the viscosity of the color paint was too large, and the pigments or silver powder were not evenly dispersed; the gun caliber was wrongly selected or incorrectly adjusted; the wrong construction process (incorrect spraying method), etc.
The body color of the car is Rhineblue metallic paint, and the paint film contains metal particles inside, which can produce front and side effects in the light. When analyzing the color difference of metallic paint, we should first observe the size difference of metallic particles, and then analyze the three attributes of color – hue, brightness, and color from front and side observation respectively. The frontal observation angle of metallic paint is that the surface of the paint film is not reflective; the side observation angle is that the surface is matte. 2.

2. Reasons for color difference and solutions

According to the understanding, the technician in the Chery Ruihu 8 body paint repair, because of the inaccessibility of the body color formula, according to personal experience and the ability to identify the color, using the body color formula of the previous repair to allocate spray, which led to the color difference after repair. The color difference was mainly on the sides, and the color was not purple enough. Metallic paint is mainly the side color difference is large, because the metal particles are not the same arrangement, affecting the side color effect.
For general maintenance companies, because of the wide variety of brands and body colors of repair vehicles, it is difficult to find the car color color formula, all can only rely on the experience of the technician and the ability to identify the color for color mixing. For the color difference of the car, this paper uses two solutions to solve the color difference defects, one is the color card comparison method, and the other is the spektrofotometer color retrieval method.

2.1 Color card comparison method

Paint company in order to make the maintenance station convenient to check the color formula of the repair vehicle paint film, reduce the color difference of the paint film after repair, made a car color color card (COLOR PROFL SYSTEM, abbreviated as CPS), each color card corresponds to a car paint film color, in the back of the color color card can see the car color code, which is the color ID number. The color card increases with the increase of body color, and since the number is too much, barcode is made to make the formula inquiry faster (Figure 2).

The solution to solve the color difference defects by using color swatch comparison method is as follows.

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Fig.2 Color swatch (screenshot)
(1) Clean the panels adjacent to the discrepancy color using a cleaner to remove contaminants from the body surface.
(2) To restore the shine of the body paint film when it was new, polish the cleaned body using high gloss wax.

(3) Select the blue color card inside the CPS system that is close to the body color and place it in the body polishing position for color comparison (Fig.3), and the steps for comparison are as follows.

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Figure.3 Color comparison

  • 1) Particle size. When comparing, firstly observe the size of metal particles inside the paint film, the metal particles of this car belong to medium size particles.
  • 2) Frontal effect. The color of the car belongs to blue hue, darker and higher color.
  • 3)Side effect. The color of the car body belongs to the blue hue with purple phase, darker and higher color.
  • 4) Through comparison, the color card number MB918.80 was selected.

(4) After inputting the number MB918.80 into Color Online Mobile (cell phone color online formula query software). The system will give the color recipe of the corresponding color card.
(5) According to the formula provided by the color system to mix the paint, spray the color sample, and compare the color with the body again. In order to avoid color differences from spraying color samples, the following principles need to be followed when spraying color swatches.

  • 1) The technician who sprays the color swatch should be the same person who repairs the car’s paint film.
  • 2) The spray gun used for spraying color swatches should be the same as the sprayed body paint film.
  • 3) The spraying method of spraying color samples should be the same as spraying body paint.

(6) According to the comparison of spraying color sample, it is found that the color sample is not enough for the body side purple, at this time the color formula needs to be fine-tuned, color fine-tuning is an important link in auto paint film construction, in order to ensure the accuracy of the color formula, the following principles need to be followed when doing color fine-tuning.

  • 1) The color masterbatch needs to be fully stirred before use.
  • 2) The amount of paint blending must be greater than the minimum amount of the formula, usually 100g-150g.
  • 3) The color masterbatch added during fine tuning is the color masterbatch within the paint formula, and no color masterbatch outside the formula can be added.

Color fine-tuning generally uses the exclusion method to analyze the color formula, and usually add silver powder or white masterbatch for lighter tones, but the white masterbatch may cause the paint to become cloudy. Adding black masterbatch for darker tones may reduce the color of the paint. Add color masterbatch according to the actual color difference. Find out the biggest color difference and adjust it.
By analyzing and excluding the characteristics of each masterbatch in the formula, it was found from the color masterbatch wall chart that A527 masterbatch in the formula can make the color formula lateral purple, add 30% of A527 masterbatch in the formula for fine-tuning (30% of the masterbatch quality of the original formula), and spray the color sample again to compare with the body color. It was found that the color sample still could not reach the purple phase on the side of the body, but the color had a tendency to be close to it.
(7) After determining the color formula, the right rear door with difference color was sanded and repainted with top coat to reach the delivery standard.
The above is the solution to the color difference defect using color swatching method. If the color formula queried by color swatching is close to the original body color, you can repaint the difference color door directly without color fine-tuning, but the color formula in this case needs to be color fine-tuned before repainting the body, depending on the actual situation.

2.2 Spectrophotometer color retrieval method

With the development of science and technology, digital color measuring instruments (Figure 4) have emerged, making the automotive paint film color formula query becomes simple and fast, color measuring instrument to measure the color accurately, simple and efficient, color measuring instrument using two light sources, respectively blue and white LED lights, 6 angles 12 light paths without dead ends measurement. At the same time, the built-in color camera can take clear pictures, read the color of the car surface, analyze the size of the particles and their texture details. The spectrophotometer needs to be used in conjunction with the color system software.
The solution to solve the color difference defect using spectrophotometer color retrieval method is as follows.

(1) Clean the panels adjacent to the discrepancy color using a cleaner to remove contaminants from the body surface.

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Figure.4 Spectrophotometer
(2) After the spectrophotometer is turned on, the spectrophotometer needs to be calibrated, and then the cleaned body is tested for color, and the test is operated according to the spectrophotometer prompts.
(3) After the work order is saved, connect the spectrophotometer with the computer equipped with color system with data cable, and also open the wireless option of the spectrophotometer, enter the wireless user name and password for wireless connection. After that, you can import the test data of the spectrophotometer, and the color system interface will show the measured color data transmission.

(4) The color system automatically searches and adjusts the recipe. The color system will analyze the test data according to the spectrophotometer, automatically search for color formulas, while the color system will automatically adjust the formula to match the best formula, generally select the closest after adjustment, then select the first adjusted to “98%” of the formula (Figure 5).

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Figure.5 Recipe selection interface (screenshot)

Click “Details”, and you can see the original, target and adjusted color curves in the “Optical Peak” screen (Figure 6). The front and side color curves are analyzed by the front and side color curves, and the effect of the adjusted formula is very close to that of the target formula.

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Figure.6 Color detail interface (screenshot)
In the “color visualization” interface (Figure 7) you can see the original, target, adjusted color effect, color formula for the final construction formula. The powerful function of the software greatly reduces the dependence on the paint technician, reduces the difficulty of color mixing, and solves the problem of color difference defects caused by color formulas.

(5) According to the formula provided by the color system to mix paint, spray color samples, and color comparison with the body, from the size of silver powder particles and front and side effects are very close to the original color, to determine the color formula, the right rear door with differences in color grinding treatment, re-spraying top coat, to meet the delivery standards.

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Figure.7 Color visualization interface (screenshot)

The above is the solution to solve the color difference defect by using spectrophotometer color retrieval method. The spectrophotometer will automatically search the color formula according to the test result and adjust the color formula to provide the closest color formula. In order to ensure the consistency with the body color, it is still necessary to spray the color sample and compare it with the body again. The color formula given by the spectrophotometer in this case is close to the body color and can be directly sprayed on the body, but it also depends on the actual situation, and if there is a color difference, it still needs to be fine-tuned.

Sumber: Cina Produsen Spektrofotometer - www.spectrumgfa.com

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