Solution to the bubble problem of door cover sealant in automobile coating
Wang Tongxing, Shen Min
(coating production department of Nanjing Automobile Group Co., Ltd., Nanjing 210061)
0 Introduction
In the process of coating and putting into production of several new model projects of our company, it is found that the bubble problem of four door two cover edge sealant (hereinafter referred to as the bubble) is the primary problem affecting the process quality and JPH climbing in the process of project implementation, which seriously affects the smooth start-up of the coating project. Taking the production of Rongwei 360 model as an example, more than 11 rubber bubbles were found in a single set in the early stage of the project. See Figure 1 for the statistics of the proportion of rubber bubble defects in the first few months of the project.
1 description of sealant bubble
1.1 phenomenon description
After the middle coat drying and the finish coat drying, it will be found that there is a bubble on the edge coating adhesive of the body door cover, and there is a bubble in the middle of the sealant, which is still exposed after the assembly workshop is assembled as a whole vehicle, which can be seen by customers and cause complaints of different degrees, so it needs to be handled and eliminated before leaving the coating workshop.
1.2 production mechanism
The mechanism of the bubble problem of the sealant for four doors and two covers is: when the edge of the BIW board is not tight or the edge folded adhesive is not filled enough, there is a gap at the local defects. When the sealant is applied at the weld on the outside of the board, the whole gap will become a closed space with air. When the body enters the drying room for baking, the air will expand by heat, and the expanded gas will not be discharged smoothly. Compared with the plate and fully cured welding flanging adhesive, the sealant is always in the weakest and most vulnerable state to external influence and damage in the early stage of complete curing. With the extension of baking time and the continuous rise of body temperature, the sealant changes the original flat state under the action of expansion gas pressure, and finally makes the sealant surface appear bubbles.
1.3 distribution of rubber bubble
To solve the problem of rubber bubble, first of all, it is necessary to understand the location where the frequency of rubber bubble is high, which is easy to analyze and solve pertinently. Therefore, first of all, make statistics on the location and quantity of rubber bubbles, and form a statistical table after a period of time, as shown in Table 1. It can be clearly found that the location with more rubber bubbles is the left rear door and the right rear door, and the specific location is in the middle of a single door strip.
2. Analysis of the root cause of sealant bubble
From the perspectives of human, machine, material, method and ring, the paper analyzes the possible causes of sealant bubbles by using fishbone diagram tools, as shown in Figure 2.
The main possible causes are:
1) The viscosity of sealant material is not enough to ensure that the sealant can resist the pressure of thermal air expansion in the process of temperature rise.
2) The thickness of sealant strip is not enough to ensure that the sealant can resist the pressure of thermal air expansion during the heating process.
3) The temperature of the temperature rise area and the heat preservation area of the drying room in the bridge is too high, which leads to serious air expansion and excessive pressure, so as to jack up the sealant.
4) The temperature of the temperature rise area and the heat preservation area of the bridge type drying room is too low, the sealant can not be effectively cured, and it is unable to ensure that the adhesive strip can withstand the pressure of thermal air expansion.
5) The gap between the four doors and two covers of BIW is too large, which leads to the large volume of the cavity body, and the air expands when heated, thus jacking up the sealant.
6) The diameter and position of the body in white flanging adhesive are improper, which leads to the large volume of the cavity body, and the air is heated and expanded to jack up the sealant.
7) Due to the improper selection of the break point of the body in white folding adhesive, the air can not be discharged smoothly, which is gathered in the binding adhesive, and the sealant is jacked up in the process of thermal expansion.
3. The fundamental solution of sealant bubble
In view of the above reasons that may cause sealant bubbles, measures shall be taken one by one to verify and solve them.
1) Adjust the viscosity of sealant material. By adjusting the viscosity of different batches of sealant materials, it was observed that the best viscosity range of the materials was 100-130 Pa.s, while blindly increasing the viscosity would affect the workability of the field application of sealant, and the number of sealant bubbles did not improve after tracking the viscosity adjustment of the dried body, so it can be seen that adjusting the viscosity of sealant did not greatly help to solve the problem of sealant bubbles.
2) Adjust the thickness of the bead. According to the requirements of PDM, the thickness standard of sealant strip is 2-3 mm. In order to verify this influencing factor, we deliberately test a batch of vehicles to increase the thickness of the strip to 4-5 mm, track the number of bubbles after drying, and no improvement is found. It can be seen that the thickness of sealant has no significant impact on the bubbles.
3) Adjust the temperature of each zone in the drying room. ① Adjust the set temperature of the first zone in the heating zone of the intermediate coating drying room, reduce 10 ℃, 20 ℃, 10 ℃ and 20 ℃ respectively, track the temperature curve of the furnace, ensure that the temperature of the intermediate coating furnace conforms to the baking window (140 ℃, 20 min), track the number of sealant bubbles, and no improvement is found. ② Adjust the setting temperature of the two and three zones in the heat preservation zone of the middle coating drying room, reduce 5 ℃ and 10 ℃ respectively, and increase 5 ℃ and 10 ℃, and then track the furnace temperature curve to ensure that the middle coating furnace temperature conforms to the baking window (140 ℃, 20 min), and track the number of sealant bubbles, and no improvement is found.
4) Optimize the hemming clearance of BIW.
After taking the above measures, it is found that it is not very helpful to improve the sealant bubble, so we turn our eyes to the BIW structure, and the first thing we think about is to adjust the BIW edging gap.
The standard of hemming clearance of BIW is ≤ 0.15 mm. In many places of left rear door and right rear door where there are many rubber bubbles, the gap is measured with feeler gauge according to the inspection method of hemming clearance of BIW. It is found that there are many gaps exceeding the standard, between 0.2-0.3 mm. After communication and discussion with the body engineer, these parts with large gaps were optimized, and the gap was reduced to within 0.15mm, then the problem of rubber bubbles was tracked. It was found that the problem of rubber bubbles was improved in a considerable part, from the previous average of 11 single units to about 8 single units, down nearly 27.3%.
5) Optimize the diameter and position of the body flange rubber. After the clearance optimization, the body production department and the body Engineer shall check the folding adhesive coating of the left and right rear doors together. The process standard of the diameter and position of the folding adhesive of the body is: diameter (2 ± 1) mm, the center of the rubber column is 6-8 mm away from the flanging. After the inspection, it is found that the data are basically within the range. Although within the scope, after discussion with the body engineer, the diameter of the flange rubber is adjusted to 3 mm, and the distance is adjusted to the lower limit of 6 mm, so as to squeeze the cavity and reduce the volume of the air in the chamber. Then, the number of the rubber bubbles after the coating of these bodies is tracked. It is found that the number of the rubber bubbles has been reduced to a certain extent, from the previous average of 8 single units to about 4 single units, down nearly 36.4%.
6) Optimize the position of the break point of the body flange rubber. After the implementation of the above measures, the problem of rubber bubble has been greatly improved, but the proportion is still a little high, which affects the production line's capacity climbing. After discussion with the body engineer, continue to adjust the folding adhesive. The breaking point of the folding adhesive is basically at the corner of the sheet metal surface, and the adhesive strip with too long distance will be broken from the middle. See Fig. 3 for the broken rubber of the body flange.
The requirements of the process instruction manual for the body flanging adhesive are as follows:
1) A, B, e, F, h are the process breaking points, and the breaking length is (20 ± 5) mm; C is the process breaking point, and the breaking length is (30 ± 5) mm.
2) D, G is the process breaking point, symmetrical with respect to the reinforcement line, and the breaking length is (40 ± 10) mm.
3) All glues shall be applied along the edge of the sheet metal of the outer plate. The center of the glue column shall be 6-8 mm away from the flanging (except for sections A2 and A3), and the center of the glue column of sections A2 and A3 shall be 4-6 mm away from the flanging.
4) The total length of glue is (2160 ± 100) mm.
We consider increasing the break point between the A4 and A5 sides, and adjust the break point for many times to find a way to exhaust the air in the edge cavity and reduce the air volume. Due to the structure of different models, this breaking point is not the same, so it needs to be tested many times. After tracking the sealant bubbles from the car body to the coating, it was found that the rubber bubbles had been greatly improved, from the previous average of 4 single units to about 1.5 single units, down nearly 22.7%.
4 on site repair measures for sealant bubbles
When the rubber bubble problem is reduced to 1.5 per unit on average, in order to improve the production capacity of the production line, it needs to be repaired quickly under the premise of ensuring the quality. Due to the repair after finishing coat, it is necessary to polish off the multi-layer paint such as intermediate coat, color paint, varnish, etc., the repair time is long, and the quality is not necessarily guaranteed. Therefore, first of all, it is determined that the most economical process is to polish and repair the intermediate coating.
The first step is to cut off the bulged part of the rubber bubble with an art knife to expose the cavity.
The second step is to fill the cavity with filler. After testing the sealant itself, thumb glue, putty and other materials, it is found that the quality of filling the cavity with putty is the most stable and the speed is the fastest. The putty material used on site is DuPont 779r, and the quality ratio of putty and curing agent is 100:1. The putty is scraped into the hole by the putty scraping method, and after the putty surface is dry, it is polished with 800 mesh sandpaper until it is smooth.
Finally, in order to ensure that different color of the car body will not appear color difference in the area of putty repair, the part of the repair bubble shall be covered with white repair paint to ensure that there is no color difference.
After the implementation of the above measures, the rubber bubble problems found in the finishing line were reduced from the initial average of 11 single units to 0.5, with a reduction rate of 95.5%, basically reaching the acceptable state of the production line.
5 Conclusion
In the process of automobile coating, the bubble problem of door cover edge sealant occurs in many models, and the repair is more time-consuming, which often becomes the primary problem that restricts the new model's capacity climbing. The root cause of this problem is in the car body workshop, so in order to solve this problem, the process quality personnel of the car body and the painting workshop need to work together to ensure the stability of field personnel or robot operation, so as to better control this problem.
(for details, please refer to modern coating and coating 2020-2)
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