According to the development process of automobile whole vehicle and bumper, the control methods of bumper and body color matching are summarized by applying the definition of automobile color and the main detection methods of color difference. Through the control method of body color difference in the production process of automobile bumpers, several common color difference problems and solution measures are analyzed and summarized in order to provide reference for related automobile enterprises.
Color difference refers to the difference between color parts and standard colors in terms of hue, lightness and chroma. Parts such as car body and bumpers can lead to color difference problems due to differences in production batches and production processes of paint suppliers. In the modern concept of automobile customers, judging the goodness of a product is mainly evaluated through the first impression. Similar to the apparel industry, consumers will focus on whether the overall appearance of the car shape and color matching can meet their personal needs when purchasing a car. In order to meet consumers’ visual needs and further increase the market share of their products, automotive companies generally provide a variety of body colors for consumers to choose from for each model. If the product has serious color difference problems, it will cause the product to lag in sales, which will in turn affect the reputation and performance of the enterprise. In the actual production process, the color difference problem has been a problem for the automobile OEMs and exterior trim suppliers. When the product color difference is serious, it will also cause the problem of missing parts, which will lead to production stoppage and affect the normal sales of cars.
1. Definition of color
Lab mode does not depend on light, pigments or specific equipment, and has been widely used in the automotive industry for color perception and measurement because of its fast processing speed for color diversity. The a-axis is the red-green axis, whose value is red when it is greater than 0, green when it is less than 0, and neutral gray when it is equal to 0. The b-axis is the yellow-blue axis, whose value is yellow when it is greater than 0, blue when it is less than 0, and neutral gray when it is equal to 0. The color scale of Lab mode is shown in Figure 1.
Figure.1 Lab mode color scale
The Lab color scale represents the color difference between the specimen and the standard. ΔE is defined as the total color difference of the sample, and a larger value of ΔE indicates a larger color difference. ΔE is calculated by the expression
In the formula: ΔL brightness color difference, the value is a positive value indicates that the brighter, the value is a negative value indicates that the darker; Δa red-green color difference, the value is a positive value indicates that the red, the value is a negative value indicates that the green; Δb yellow-blue color difference, the value is a positive value indicates that the yellow, the value is a negative value indicates that the blue.
2. Color difference detection method
Color difference control is generally used to visually evaluate the standard color card. Because the visual evaluation of color difference can not achieve quantitative evaluation, and each person’s perception of differences, such as the observer in the color difference evaluation in the angle, environment, mood and other factors will affect the color difference judgment, so should use the 比色計 and other convenient data recording equipment to assist judgment and quantitative evaluation.
The colorimeter is an optical measuring instrument that simulates the human eye’s induction of red, green and blue light to detect and judge color difference. The instrument uses a standard light source to perform multi-angle analysis of the sample being measured. The quantification process of the colorimeter is carried out according to the Lab principle of the International Commission on Illumination (CIE) chromaticity space, and the color difference between the measured sample and the standard sample is expressed by the parameter values of ΔE, ΔL, Δa and Δb. These parameter values are conveniently stored and can provide a reference basis for the subsequent optimization work of automotive OEMs.
At present, the main methods of color difference detection are visual evaluation and colorimeter measurement. In order to ensure the accuracy of color difference measurement, the visual evaluation of the color difference of the measured sample is required to be irradiated under natural light conditions, and there is no reflection interference from other colored objects around. If visual inspection cannot determine whether the color difference of the measured sample is within the allowable range of normal color difference, the BYKmac colorimeter (Figure 2) should be used to measure from three angles (i.e. 25°, 45° and 75°) of the measured sample respectively. The standard data used for the colorimeter measurement is the original data published with the master plate, which is also called the master plate data. Figure 3 illustrates the measurement data of the colorimeter. In this case, the ΔEP value is obtained empirically by multiplying the ΔE value by a factor for metallic and non-metallic paints by BYK Chemicals (Germany). This parameter can be used to determine whether the color difference of the tested sample meets the requirements and improve the accuracy of color difference judgment. After testing, if ΔEP≤1.0, the color difference is qualified; if 1.0<ΔEP≤1.7, the color difference can be used as a concession; if ΔEP>1.7, the color difference is judged as unqualified.
Figure.2 BYK mac colorimeter
Figure.3 Measurement data of the colorimeter
3. Development process
When developing a model, the focus should be on matching the body and bumper colors to reduce the influence of color difference. The body and bumper color matching development process which mainly includes the following aspects.
Determine styling → Select color scheme → Release standard color card → Body/bumper matching → Determine color control range → Production process monitoring → Continuous improvement
(1) Determine the styling.
According to the market positioning, determine the shape of the car shape, body and bumper, and clarify the matching relationship between them.
(2) Select color system.
Determine the color of the body and bumper according to the comprehensive factors of car shape, market positioning and user groups.
(3) Release the standard color card.
The standard color card is issued by the department in charge of automobile styling in the technical center of the automobile OEM, and the standard color card is usually valid for 2 years. If the standard color card exceeds the validity period, the relevant supplier should purchase it through the official website designated by the automobile OEM or directly from the automobile OEM.
(4) Body and bumper color matching.
The paint shop and bumper supplier of the automobile OEM should debug the sample parts close to the standard color card according to the requirements of the standard color card. First, visual evaluation should be conducted under natural light conditions; then, the sample parts should be evaluated by the auto OEM’s appearance evaluation team to determine if they meet the requirements. Based on past project experience, the body and bumper color matching process should be carried out for at least 3 rounds of evaluation, with the evaluation cycle lasting 3-4 weeks.
(5) Determine the color control range.
During each sample production commissioning, relevant data should be recorded using a colorimeter. According to the matching results to determine and publish the ΔE, ΔL, Δa, Δb and other parameters range, and at the same time clarify the various colors need to focus on the direction of color control.
(6) Production process monitoring.
According to the production control plan requirements, the paint shop and bumper supplier of the automobile OEM shall use colorimeter to detect color difference and keep records when the first, middle and last products are produced daily, and continuously monitor the production process by using statistical process control (SPC) method to ensure the consistency of products. By using the WeChat-JanDao cloud system for information sharing, each work station in the painting workshop and the bumper supplier can keep abreast of each other’s color difference data and color phase direction, and assess in advance whether there is any risk of mismatch between body and bumper colors.
(7) Continuous improvement.
According to the results of production process consistency monitoring, we continuously upgrade and improve the process control methods, and at the same time, according to the results of vehicle matching and consumer feedback, we continuously optimize the color phase control standard and scope.
4. Production process control
In order to ensure the color of the bumper matches the bodywork, the automobile OEM and bumper suppliers are required to strictly control the color difference during the production process. The bumper production process control mainly includes the following five aspects.
(1) Paint incoming material inspection.
This stage must be controlled by: ① check the paint manufacturer’s factory inspection report; ② check and confirm the original paint viscosity, resistivity, cleanliness, adhesion and other incoming inspection items; ③ in the case of new batches of paint, a sample test spray should be conducted to determine whether the paint is qualified before use.
(2) Paint mixing process control.
The control content of this stage includes:
- ① Implement and track the standardization process of paint mixing;
- ② Monitor the operation of the paint mixing and delivery equipment and key parameters within a reasonable range;
- ③ Inspect the viscosity of the paint, check the fluctuation of the amount of relevant additives (fast and slow drying);
- ④ If the metal paint has been stored for more than 3 weeks, a sample test spray should be arranged before the next spraying to avoid cyclic shearing of aluminum powder in the metal paint and precipitation phenomenon.
(3) Spraying process control.
This stage of control includes:
- ① Check the consistency of the fixture, do a good job of fixture maintenance;
- ② Check the accuracy of the relevant parameters in the water washing process before treatment to ensure that the parts are clean and free of contamination;
- ③ Flame treatment should increase the activity of the body and reduce the dust on the surface of the body by means of electrostatic dust removal;
- ④ Check the control process parameters of the spraying robot to ensure that the workpiece completes spraying according to the specified parameters. When abnormal stoppage occurs, the unqualified products should be isolated and controlled;
- ⑤ In the process of color change, it is very easy to cause parts to string colors, the color change process should be strictly enforced, and the empty, scratch and warp should be well controlled.
(4) Parts off-line inspection.
This stage control content includes:
- ① Use standard color card for comparison check.
- ② Use colorimeter to measure the part color difference data and record, upload the data to the Jane Dao cloud system and compare with the body data of the host plant. The production site uses the SPC method to monitor the color phase direction and adjust it in time by means of a chart. Regularly calibrate the colorimeter to ensure accurate data.
- ③ Strictly control the repainting process of reworked parts, in principle, bumpers can only be repainted once and must be repainted with the same base color.
- ④ Strengthen the control of unqualified products and end-of-life products to ensure that only qualified products can leave the factory.
(5) Complete vehicle color review.
The control content of this stage includes: ① the bumper supplier and the host plant to conduct weekly review of the color match between the whole car body and the bumper, the review should be conducted under natural light conditions, if abnormalities are found, timely adjustment should be made; ② monthly confirmation of the colorimeter alignment to ensure that the colorimeter data of both the bumper supplier and the host plant are consistent; ③ the host plant should continuously optimize the control range of color phase and the release of the standard color card.
5. Problem analysis and improvement
The following is an analysis of common color difference problems combined with several cases of body and bumper color mismatch in an automobile OEM, and also proposes corresponding improvement measures.
5.1 Color difference between pearlescent white bumper and body of CN202S model
The reason for the color difference in this case is that the bumper of this model missed the pearlescent powder spraying, and the spraying program needle of the automatic spraying line failed to recognize the pearlescent white parts and did not start the spraying robot spraying pearlescent powder program. Therefore, for the color parts with special spraying materials, such as pearlescent white, nebula purple, aurora silver and other color parts, continuous production should be implemented in the scheduling of each production shift, in which the first and last products and the total number of products should be confirmed by the team leader, and the team leader should confirm the spraying quantity and whether the signal point of the pearlescent powder robot has been activated again before spraying. In addition, the correlation between the special color parts program and the special material spraying robot should be increased, and the system should automatically correlate whether the special material spraying robot program is activated after the color of the product is selected.
5.2 Color difference between green apple green bumper and door of CN202W model
The reason for the color difference in this case was that the color paint was too thick in the edge area where the bumper and the side surrounds meet, and the thickness was 45μm, which did not meet the standard thickness requirement for color paint (15-25μm). The root cause of the color difference problem was that the program origin of the spraying robot was not calibrated in time after the failure, resulting in excessive edge spraying flow. In addition, when the part was inspected off-line, the colorimeter only inspected the flat area and did not check the film thickness at the edge of the part. Therefore, when the equipment fails, it is important to reconfirm that the program and origin are calibrated, and that mass production should be done only after a small sample verification is completed. In addition, a film thickness inspection process for the edge area of the part should be added to the area where the part is polished off-line.
5.3 Color difference between bumper and body of CN730S model in milk tea curry color
The reason for the color difference in this case is that within the brightness limit (a±0.5), the brightness value of the bumper and the brightness value of the body were chosen to be the opposite of the a value, i.e. the brightness value of the bumper was taken as the upper limit and the brightness value of the body was taken as the lower limit. Under natural light conditions, the visual effect of the difference appeared. And the root cause of the color difference problem is the bumper and body pre-color in matching, did not make color boundary limit samples for comparison, did not match to the lower limit of brightness value and the upper limit value car condition, and the bumper and body brightness value respectively in 2 limit value when the matching verification is not sufficient. Therefore, the brightness control range of milk tea curry color should be readjusted, and the brightness value should be adjusted to 0-1.0, and brightness is required. In the subsequent project development and matching verification, the confirmation link of visual matching effect should be added to the limit samples at both ends.
5. 4E50 model candy white bumper and body color difference problem
The reason for the color difference in this case is that the yellow phase (b value) of the body of this model does not meet the requirements, the design requirements should be in the range of 0-1.00, but the actual b value is -0.75. It is confirmed that although the b value of the body of this batch of products meets the standard of 0-1.00 when it is painted off the line by the colorimeter, due to the spraying of the candy white color of the E50 model using the 3C1B process, and this process will make The body of the car after spraying the phenomenon of yellow phase reduction. For the 3C1B process, and combined with the actual car yellowing verification data found that the candy white body sprayed off the line 3 days, its b value will be reduced to 1.20-1.50. Therefore, the candy white paint off the line when the yellow phase control standard range should be changed to -1.50-0.20, while adjusting the spraying process to meet the color difference control standards for off-line models.
To sum up, the color difference quality management of automobile is a complicated system work, and the color difference control of paint is also a technical problem that is difficult to control accurately. Only through strict control of the production process of the automobile OEM and exterior trim suppliers, and scientific management of materials, process flow and production monitoring, as well as continuous optimization of testing methods, can we finally achieve effective control of the color difference problem and make the car body color meet consumer demand.
Author: Mingzhan Lai