Application of Multi-angle Spectrophotometer in Color Detection of Automotive Painting

With the continuous development and progress of automobile consumption concepts, consumers have increasingly high requirements for automobile appearance. Therefore, automobile manufacturers must commit themselves to solving the problem of color difference between auto parts and the whole vehicle, which is their key work. To this end, automobile manufacturers need to conduct strict color management and inspection to ensure the accurate handling of automobile appearance colors. This article will introduce the application of multi-angle color difference meters in the color difference detection of automobile painting.

A multi-angle color difference meter is a measuring instrument that can evaluate the color difference of objects from multiple angles.

There are many reasons for color differences in automobile painting. First, differences in the formula and quality of paint may lead to inconsistent colors. Secondly, environmental factors such as temperature and humidity during the coating process will also affect the color performance of the paint. In addition, improper processing techniques and manual operations may also cause color differences. Finally, the vehicle's use environment and daily maintenance habits will also have an impact on the color of the paint. In short, the causes of color differences in automobile painting are multifaceted, and attention should be paid to the impact of each link.

The causes of color differences in automobile painting are various, but they can be mainly summarized into the following aspects:

1. Factors influenced by the type of paint

According to the classification of pigment properties, paints can usually be divided into solid-color paints and metallic paints. The pigments in solid-color paints are arranged uniformly and densely, but some solid-color paints have high color sensitivity, which makes them prone to color differences, such as white and bright yellow. The aluminum powder in metallic paints has directional arrangement, and the reflection values at different angles are different, with large differences in brightness, which is an important factor affecting color differences.

2. Factors affecting paint batches

Before mass production, the formula for each paint batch needs to be determined and fine-tuned according to test data before batch production. There will be slight color differences between different batches of paint. For example, if two batches of light gray metallic topcoats for plastic parts are used to spray standard test panels under the same process environment, there may be a risk of color difference.

3. The material of plastic parts has an important impact on their performance and use. Different types of plastic materials have different characteristics, such as strength, heat resistance, chemical resistance, and wear resistance. Therefore, choosing a suitable plastic material is crucial to ensuring that plastic parts can meet specific needs. In addition, the choice of material also affects the cost and processing method of plastic parts. Therefore, when designing and manufacturing plastic parts, the influence of materials must be fully considered.

There are many types of plastic parts, and each type of plastic part varies greatly in terms of painting construction parameters. For example, ABS material has good adhesion and can be used without a primer. However, ABS plastic parts are divided into black and white according to color. If no primer is used, the applied color paint is difficult to cover, which will have a great impact on color difference.

4. Expired paint can bring a series of impacts. First, expired paint may lose its original quality and performance. This may lead to slow drying, poor adhesion, and inaccurate color of the paint. Secondly, the use of expired paint may have a negative impact on the construction surface. Expired paint may not effectively cover the construction surface or provide good protection. Finally, the use of expired paint may also have an impact on health. Expired paint may contain harmful substances, and long-term contact may cause allergic reactions or respiratory problems. Therefore, it is recommended to avoid using expired paint as much as possible and regularly clean and update the stored paint.

After the paint expires, various performance indicators will be abnormal. For example, if we spray a plastic part with unexpired metallic paint and then spray another plastic part with expired metallic paint, we will find that the shape of the aluminum powder is completely different, and the color reflectivity will also change.

5. How does the geometric state of plastic part assembly affect the assembly process and results?

An important factor leading to paint color difference is the geometric state when different painted plastic parts are assembled with the car body. When the interface between the painted plastic parts and the car body assembly is inconsistent, the reflection angle of the light source will also be different, resulting in different visual color difference effects. For example, if the color transition between the painted plastic bumper and the car body fender on the same plane is not good, obvious color difference will occur. Therefore, the assembly geometric state of plastic parts is also an important factor causing color difference.

6. Construction technology has a great impact on the project. It involves technologies, materials, equipment, and methods used in the construction process. Reasonable selection and application of construction technology can improve the quality and efficiency of the project, reduce costs and risks. Changes in construction technology will also affect the progress and safety of the project. Therefore, it is necessary to fully consider the characteristics and needs of construction technology and make appropriate decisions in the project.

At present, the main spraying technologies are electrostatic spraying and manual air spraying. Generally, the whole vehicle painting line adopts electrostatic spraying because the arrangement of metal aluminum powder is relatively neat. Supporting plastic parts (such as bumpers, decorative strips, spoilers, rearview mirrors, side skirts, etc.) are generally sprayed by manual air spraying due to their small spraying area and complex shape. The arrangement of metallic paint and aluminum powder is relatively disorderly, which leads to differences in the arrangement of metal aluminum powder, and then causes varying degrees of color difference.

7. Impact on process parameters

Process parameters are important factors affecting color difference, such as paint viscosity, spray booth temperature, humidity, drying time, film thickness, over-baking, etc. Changes in these parameters may lead to varying degrees of color difference after spraying.

8. Human factors have a significant impact on things.

Factors affecting paint color difference include manual touch-up painting, inadequate stirring of paint, wrong spraying techniques, uneven spraying film thickness, and incomplete cleaning of paint delivery equipment. In addition, the difference in everyone's judgment of color difference is also one of the important factors affecting color difference.

We use multi-angle color difference meters to detect color differences in automobile coating.

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Application of multi-angle color difference meters in automobile painting color difference detection:

Color difference refers to a physiological sensation produced by the brain's recognition of information formed when light from two objects is projected onto the retina under the same light source. There are three conditions to satisfy color difference: at least two objects, the same light source, and normal visual senses. Generally, there are two commonly used color difference measurement methods: visual inspection method and color difference meter measurement method.

The visual inspection method is to visually judge between the painted plastic parts and the painted standard panel (or the whole vehicle assembly) with one's own eyes under the condition of a standard light source (natural light source). The visual inspection method has high sensitivity. When the wavelength changes by 1-2nm, the color change can be detected. Therefore, the judgment of color difference is mainly based on visual inspection, but it is difficult to quantify the color difference measured by the human eye. Therefore, when there is a dispute over color difference in the visual inspection method, a color difference meter can be used for quantitative measurement. A commonly used color difference meter is the 3nh colorimeter MS3005, whose light source is usually set to D65 daylight source, and the observation angles are 15°, 25°, 45°, 75°, and 110°. The quantitative measurement of paint color usually adopts the CIE color system of the International Commission on Illumination. The measurement principle is that the monochromator irradiates the measured sample, the reflected light is converted into an electrical signal through a phototube, amplified by an amplifier, and the current intensity is measured by a galvanometer, so as to realize the measurement of the absolute value of the reflected light color of the measured sample, i.e., the CIELAB color space. Using a multi-angle color difference meter, the chromaticity values (L*, a*, b*) of the coating, as well as the chromaticity differences (△L*, △a*, △b*) and color difference value (△E) between the color plate to be measured and the standard color plate, can be measured at 5 viewing angles: 15°, 25°, 45°, 75°, and 110°.

Generally, automobile factories use standard color plates to control the color of the coating. If the color difference △E between the car body coating and the standard color plate is within a certain range, it is regarded as a qualified coating. If the color difference exceeds the range, it is regarded as unqualified. However, there is no unified standard for the △E range in the industry.

Some domestic automobile factories usually judge whether the color of the coating is qualified by comparing the car body with the standard color plate. When △E ≤ 1.5 (at a 45° viewing angle), it indicates that the coating color is qualified; when △E > 1.5, it indicates that the car body coating color is unqualified. In the actual production and new color development process, the color of the car body coating should be consistent with that of the standard color plate, or when measured with a multi-angle color difference meter, the △E values at three angles should not exceed 1.5 (the smaller the better) to meet the user's color requirements.