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Paint shrinkage mainly refers to the deep recessed defects that appear on the painted surface of a workpiece, resulting from the attachment of substances with low surface tension to the surface of the workpiece to be painted or from their mixing into the paint. Shrinkage is a problem that is difficult to address in the automotive painting process; the process of correcting it is complex, and it cannot be eliminated using ordinary grinding or polishing methods ； Due to the relative complexity of the materials, equipment, and environment used in production, the time required for troubleshooting is long and the task is difficult, making it hard to completely eliminate shrinkage cavities. Based on a case of shrinkage in a painting line for passenger vehicles, this article explores several causes of shrinkage during vehicle painting and proposes solutions. 1 Steps for investigating shrinkage holes: Shrinkage holes may be caused by the introduction of contaminants from one or more sources such as personnel, substrate, coating, equipment, and environment. This issue involves a wide range of factors, and it is not possible to verify and examine each one individually in a short period of time. Drawing on experience in resolving shrinkage defects on site, the author summarizes that the general steps for identifying shrinkage issues are as follows: 4 steps in total. 1.1 Visual identification of shrinkage cavities: After shrinkage cavities occur, it is necessary to determine at which layer of the coating they appear – whether it’s the intermediate coat, the color coat, or merely the clear coat ； Whether there is a core at the center of the shrinkage cavity, and whether the shrinkage cavity is of a relatively regular circular shape – in general, shrinkage cavities with cores are considered to be caused by the base material or by particles falling from the surrounding environment. 1.2 Statistical analysis of the patterns of shrinkage cavity formation: Analyzing the patterns related to color, location, or timing of shrinkage cavity formation can help determine the direction in which to investigate such defects. If only a single color appears, the possibility that the paint material is contaminated is high ； If the shrinkage rate is high only after a certain period of time since line opening, it may be due to unclean compressed air used for spraying ； If the probability of occurrence on horizontal surfaces is much higher than that on vertical surfaces, it is highly likely that the air conditioning supply system is contaminated. 1.3 Determine the approximate source of contamination by observing the patterns of shrinkage associated with different colors, vehicle types, and time periods; by applying a control-variable approach, certain related factors are excluded, thereby identifying the approximate process stage responsible for the shrinkage and the general range of factors that contribute to it. Under the same topcoat and the same production conditions, there is a significant difference in the shrinkage rate between colors A and B. Factors such as substrate contamination, contamination by personnel responsible for cleaning, topcoat contamination, as well as equipment and environmental issues can be ruled out, indicating that the source of contamination lies in the paint itself. 1.4 After using single-factor comparison to identify the likely source of pollution, a single-factor confirmation is required to further verify the pollutant. If substrate contamination is suspected, it can be verified by wiping with alcohol or solvents ； If compressed air pollution is suspected, the frequency of compressed air emissions can be increased for verification ； If it is determined to be contamination of the coating itself, this can be verified by replacing the coating ； If it is a foreign object in the conveying system, the coating can be fed into another conveying system for spraying verification. 2 Introduction to shrinkage cavity cases 2.1 Shrinkage cavities caused by foreign contaminants in coatings In mid-March 2018, shrinkage cavities appeared in a certain gray paint used on the painting line for passenger vehicles; these cavities were regular in shape and appeared on both sides of the vehicle body ； The shrinkage reaches the intermediate coating layer, with no granular core at the center, as shown in Figure 1. https://pic3.zhimg.com/80/v2-1a6c97d9938bbf25e5cc35f07de52c26_720w.jpg This gray color is a tinted paint, applied to the Class B surfaces of the vehicle body. Its manufacturing process differs from that of the other 3C2B products; it involves applying the topcoat in a \"wet-on-wet\" manner under conditions of rapid drying of the intermediate coat. The process flow is as follows: electrophoretic coating of the vehicle body → application of the intermediate coat → application of the gray paint → drying of the intermediate coat → application of the clear topcoat → drying of the topcoat → final finishing touches. Shrinkage only occurred in the gray-painted areas; no shrinkage was seen in the areas painted with other colors. This rules out the influence of factors such as the electrophoretic coating process, personnel, materials used in the intermediate coating stage, compressed air, the environment, and the drying oven ； It is generally presumed to be caused by contamination of the coating supply system or defects in the paint itself. Further verification is shown in Tables 1–2. https://pic2.zhimg.com/80/v2-fb95165a3aaf08840de6a374e0812025_720w.jpghttps://pic2.zhimg.com/80/v2-44891fea626b1acc0b275851e0ccf3e9_720w.jpg Table 1: Both Validation 1 and Validation 2 showed shrinkage defects; the source of this shrinkage remains unidentified. Further on-site investigation revealed that due to fluctuations in the gray paint color, red pigment had to be added during the paint mixing process to adjust the color on site. Since this approach allows for the use of other pigments as substitutes, verification was conducted on these alternative pigments. Through the above verification, it was ultimately determined that the source of gray shrinkage contamination was the red pigment paste. Subsequently, the paint in the system will be replaced, and a new color paste formula will be developed; the original color paste will no longer be used ； The post-coating supplier adjusted the coating; now, no color paste is added, and the problem of cratering in the gray paint has also been eliminated. 2.2 Paint cycle cracking leading to shrinkage defects: On January 21, 2019, batch-wise shrinkage defects occurred during the painting process on the production line; the daily rate of such defects was 10.30%. The distribution pattern of the colors of these shrinkage defects is shown in Table 3. https://pic2.zhimg.com/80/v2-806ac203d30b09f7f105b3ad0724e3e5_720w.jpg More than 80% of the shrinkage defects are black in color, while those that are golden in color account for less than 3% of all shrinkage defects ； In terms of the shrinkage rate for each color, after accounting for the impact of production volumes across different colors, the shrinkage rate of black paint is significantly higher than that of the other colors; therefore, it can be concluded that the shrinkage problem is caused by the black paint. After urgently replacing the paint in the system with paint from the same batch on site, the number of shrinkages decreased significantly on the second day. When measuring the shrinkage rate for each color, the shrinkage rate for black paint dropped to 3%. A comparative spraying test was conducted between the sample of black exhaust paint and a newly opened sample of paint from the same batch; the results are shown in Table 4. https://pic4.zhimg.com/80/v2-936c1216f5b82906b01869b17a87e29f_720w.jpg As can be seen from the control tests in Table 4, the coating in the original system already had shrinkage problems. By measuring the viscosity at different shear rates, it was found that there were significant differences in the viscosity curves between the previously used coating and the newly opened coating (see Figure 2), indicating that the coating had undergone cracking. https://pic4.zhimg.com/80/v2-64bd88374e0e7f1dce88e6908030151f_720w.jpg After consulting paint experts, it was determined that the back pressure in the on-site circulation system was too high, causing the paint to crack due to prolonged exposure to high shear forces. Once the equipment managers in the workshop reduced the back pressure to 0 ~ 0.1 MPa by making necessary adjustments, this problem no longer occurred. 2.3 Paint defects causing shrinkage holes: At the beginning of May 2019, the finishing line reported an increase in defects related to shrinkage holes in the topcoat; the daily rate of defective topcoats due to shrinkage holes was 9.8%, and this figure showed an upward trend. The technician examined the shrinkage cavities on the actual vehicle and found that they occurred only in the clear coat layer, while the color paint layer remained intact ； Statistics show that the shrinkage rate for the main colors is all above 9.3%, with the highest normal shrinkage rate being less than 3% ； The distribution is relatively uniform between the left and right sides, and there is no abnormal increase on the horizontal plane ； No shrinkage was observed in the wet film of the colored paint; shrinkage could be detected in the wet film of the varnish. The number and location of shrinkages in the wet film were essentially the same as those after drying. It was determined that shrinkage was caused by contamination at the topcoat varnish station; the sources of contamination are listed below. 2.3.1 As confirmed by the maintenance personnel, the robot-assisted material contamination issue was resolved; the varnish robot was used only for tasks such as cleaning the cup tops and wiping the surfaces, with a cleaning solvent specific to varnish being utilized. This solvent was used for the first time in mid-March, and no defects related to shrinkage were observed during that period, indicating that there is no risk of contamination posed by the robot. 2.3.2 Investigation into the impact of compressed air used for spraying: The air dew point at the compressed air outlet of the air compressor station is -49.6 °C, which meets the requirement that the pressure dew point be below -20 °C ； Blowing compressed air from the varnish robot side was used to replicate the spraying process on the boards; no shrinkage defects were observed in any of the 10 sets of boards. Only 2 of the boards showed minor dents in their wet film, which were eliminated after leveling and drying. The compressed air quality is deemed acceptable. 2.3.3 Investigation into the impact of coatings: The batch of coating used inside the tank was 1904-0302; it was first used on April 26, and by the time the issue of shrinkage cavities arose, 3500 kg of this coating had been used, with no adverse effects reported ； Actual inspection shows that there are no abnormalities in the color tuner’s addition of materials ； The amount of paint updated within the system is greater than 500 kg; there is no cracking caused by prolonged mixing ； Taking advantage of the downtime at noon, the coating in the system was replaced with a new batch of coating, and the shrinkage rates for different batches on that day were recorded; the shrinkage rate for the 1904-0302 batch was 18.7% ； The shrinkage rate of the new batch is only 0.5%. It is determined that the 1904-0302 batch of varnish added later has a shrinkage issue. The newly opened 1904-0302 batch and the new batch of varnish were subjected to an MS pore reduction test, and the results are shown in Table 5. https://pic1.zhimg.com/80/v2-0a25a2506eb45d897442db528e09c238_720w.jpg Table 5 shows the results: The varnish paint from batch 1904-0302 had a risk of shrinkage defects; the workshop forwarded this information to the paint supplier, who then worked on improving the paint. It was also agreed that the standard for testing for shrinkage defects using the MS method should be increased from 4 hours to 24 hours, and only if no such defects were detected could the paint be used in production. No such problems occurred again in subsequent productions. 3 Conclusion: Shrinkage directly affects the quality of the topcoat and the customer’s perception; it is a common quality defect in coating processes. At present, there is no way to completely eliminate it, and stable control can only be achieved by strengthening daily supervision of operations and quality control of raw materials upon arrival. There are many complex factors that can cause shrinkage cavities, and going through them one by one would take a lot of time. Using the elimination method can significantly reduce the time required to identify the problem, after which controlled experiments can be used to determine the source of the issue. Only by addressing the root causes can we ensure stable quality on site, and this is also the direction in which we need to make sustained efforts. 【Reference】 A Brief Discussion on the Causes of Surface Shrinkage Holes in Vehicle Painting and Corresponding Countermeasures by Lei Jingqin, Pan Shaohua, Tian Qianwei (Dongfeng Liuzhou Automobile Co., Ltd., Liuzhou, Guangxi 545005)