The secondary flow mark of electrophoretic coating is one of the problems that often occur in the process of automotive cathodic electrophoresis, which seriously affects the appearance quality and anti-corrosion performance of the coating film. In order to eliminate the adverse effect of secondary flow marks on the subsequent middle and top coats, it is usually polished, which will lead to the loss of local electrophoretic coatings, and local additions and repairs are required. However, the corrosion resistance of the added and repaired coating will be greatly reduced. In the article "Analysis and Control of Secondary Flow Marks of Electrophoretic Coatings", the author takes the secondary flow marks in actual production as an example to analyze the causes and control methods of secondary flow marks.
Cause of secondary flow marks
(1) Coating electrophoretic material factors
Coating electrophoretic material parameters are one of the factors that affect the thickness of electrophoretic coatings. The main material parameters are: solid content, ash content, M EQ (milligram equivalent of coating) value, pigment-base ratio, PH value and conductivity. Among them, the influence of the solid content, solvent content and other components of the bath liquid on the secondary flow marks of electrophoresis is particularly important. The high solid content of the bath liquid, the low solvent content or the sedimentation of the electrophoretic coating can easily lead to electrophoresis. The disadvantage of secondary flow marks.
(2) Coating electrophoresis process factors
The pre-treatment is not clean. The body-in-white is formed by stamping and welding various plates. Due to the needs of the process, the plates are coated with various anti-rust oils and stamping oils. Although it will go through multiple pre-treatment processes before it is formally painted , to remove the attachments and oil stains on the surface of the car body, but due to the different components and brands of the surface coatings, it is difficult to effectively remove them. After electrophoresis, it is easy to form a mixture of water, oil and paint in the car body cavity. After that, it flows out from the cracks, resulting in secondary flow marks.
Not enough time to drain. Before the car body enters the drying room, there is not enough time to drain the water. The electrophoretic fluid in the lower part of the body cavity is accumulated and drained. trace ills. Dry too quickly. If the temperature rises too quickly during the electrophoresis drying process, it is easy to cause the electrophoretic liquid to boil and splash out rapidly without having time to dry slowly, forming the disadvantage of secondary flow marks of electrophoresis.
(3) On-site factors of coating electrophoresis production
Clamp installation height or door opening is not appropriate. Generally speaking, the front and rear 4 doors and the front and rear covers will be additionally installed with fixtures during the pre-painting and electrophoresis process, so as to ensure that the doors and front and rear covers are in a good cleaning and painting state. Therefore, in the fixture design stage, it is necessary to test different support methods in order to find a good method that will not damage the paint film of the support points, but also meet the requirements of cleaning and painting, and at the same time, it should have good fluid discharge Function. The layout of the production line is unreasonable. The last process of the production line is pure water washing. Some production lines are set to spray washing. Since spray washing can only wash the surface of the car body, it cannot clean the residual paint hidden in the inner cavity. Therefore, in the case of the same model, the last process The pure water spray cleaning production line has a higher probability of secondary flow marks than the pure water immersion tank.
(4) Body structure design factors
The unreasonable design of the body structure leads to the failure to remove the internal fluid in time, which causes the fluid to boil during the drying process and flow out from the cracks when the surface paint film is not completely cured. This is the root cause of the secondary flow marks of electrophoresis. The secondary flow marks at the A-pillar and D-pillar position of the car are due to the accumulation of water and fluid in the upper cavity of the side wall, which cannot be discharged in time. When the car body is tilted on the uphill and downhill sections of the drying furnace, the fluid is drained from the D-pillar respectively. , A-pillar crevice caused by outflow.
Due to the complex structure of the car body, there are often cavities welded by inner and outer plates, which are prone to water and oil accumulation. Therefore, these cavities are high-frequency parts where secondary flow marks appear, which requires the car body to be designed with good drainage capacity to discharge Effusion. Therefore, in the design of the body structure, in addition to meeting the requirements of strength and stiffness, it is also necessary to meet the requirements of the coating process, rationally optimize the body structure, arrange drain holes at appropriate positions, or add exterior decorations in areas prone to electrophoretic secondary flow marks , keeping the area out of view.
Secondary flow mark control measures
1. Reduce the concentration of effusion in the inner cavity
It is generally easy to understand that it is only necessary to reduce the solid content of UF2 (the second fresh pure water washing). But in fact, after the body passes through the electrophoresis tank, the cavity is full of electrophoretic paint. Generally, it cannot be completely discharged when it enters UF2, so the concentration in the body cavity will be higher than the solid content of UF2; UF2 is almost impossible to be completely clear liquid , as long as the bath liquid is not completely discharged, it will definitely flow out during the baking process, forming electrophoretic secondary flow marks on the car body. Of course, the concentration will be reduced at a lower level, but if it cannot be discharged in time, it cannot be completely eliminated. Therefore, reducing the solid content of UF2 can relieve secondary flow marks to a certain extent, but cannot completely eliminate them.
2. Reduce the amount of fluid in the inner cavity
According to the cause of the secondary flow marks, if the fluid in the inner cavity can be discharged in time before the body-in-white enters the oven for baking, the secondary flow marks can be eliminated. Methods to reduce the amount of effusion in the lumen include:
Method 1: Open a hole in the proper position of the body structure, use the up and down slope of the wire body, concentrate the bath liquid in the inner cavity to the opening before baking, and avoid the accumulation of electrophoretic liquid, so that it will not form on the outer surface of the body. Secondary flow marks.
Method 2: Adjust the installation height of the fixture in order to find a good state that is conducive to body cleaning and painting, and at the same time make the inner cavity fluid drain smoothly.
Method 3: Use compressed air to blow and wipe the corresponding parts of the electrophoresis secondary flow marks, blow off the effusion that cannot be drained during draining, and prevent the effusion from bumping after entering the drying room to form the electrophoresis secondary flow marks. In the actual air blowing operation, it is necessary to explore the air blowing method that is very conducive to the early leakage of the solution, and special attention should be paid to the quality of the compressed air (absolutely free of oil and water, otherwise there will be liquid accumulation in the cavity of the shrinkage body, which will lead to the analysis of secondary flow marks Disadvantages of the liquid discharge port of the liquid cavity) and the pressure and amount of compressed air (it is advisable to blow off the flowing effusion just after blowing off).
