Analysis and prevention of causes of body electrophoresis traces.

Electrophoresis flow marks mainly affect the surface quality of electrophoresis coating, reducing the adhesion of the medium coating. In order to eliminate the above-mentioned effects, the production process generally uses grinding and other methods of repair treatment, the process will produce dust and other secondary defects, and the process will inevitably destroy the electrophoresis coating, affecting the overall corrosion performance, especially for the use of cold-rolled steel plate body, the problem is particularly serious. Experimental verification shows that even after polishing the electrophoresis coating, even with rust-proof spot paint, its anti-corrosion performance is still not up to electrophoresis paint standards, followed by a large amount of dust and noise in the process of treatment, harming the operator's health

. The problem of rework reduces overall productivity and increases production and manufacturing costs. Therefore, the prevention and control of electrophoresis flow marks to improve the overall quality of electrophoresis coating, reduce costs are of great significance.
1 Introduction to electrophoresis flow marks
electrophoresis traces, also known as secondary traces or secondary flow hanging, refers to the electrophoresis wet film is completely normal, but in the drying process, electrophoresis paint residue from the door folds and other gap structures flow out and drip on the outside surface of the body or other visual parts, dried after the residual solid left on the electrophoresis paint film to form electrophoresis defects. There are two main causes, one is the surface pressure of the liquid, and the other is the rapid heating of drying to destroy the surface pressure of the liquid. Specifically, water-inductive electrophoresis paint and treated steel plate immersion is strong, when the gap is suitable, driven by liquid surface pressure, electrophoresis paint will accumulate in each gap. Even after electrophoresis ultrafiltration, pure water and other cleaning is difficult to completely clean the liquid. However, during the drying process, the temperature of the car body increased sharply, resulting in the destruction of the surface pressure of the liquid, the electrophoresis paint in the gap fluid flow out, resulting in flow marks. Figure 1 shows.
Figure 1 electrophoretic flow mark generation principle
2 prone to flow marks
flow marks mainly occur in the presence of gaps in the structural area, to the Company, for example, mainly occurred in the following areas.
2.1 door area
the common flow mark area of the door area in the door folding edge, door accessories and other parts, the impact area has the door outside the door plate, the inner plate visual area, the threshold and so on. The number of defects (PPPH) occurring in this area is around 80. Figure 2 shows.
Figure 2 door area flow mark legend
2.2 inner plate door frame and other sheet metal hitching area
The PPPH that occurs in this area is between 60 and 70. Figure 3 shows.
Figure 3 Inner plate door frame flow mark legend
2.3 bonnet, door edge and other areas
engine cover, five door edge and other areas are prone to produce lighter flow marks, affecting areas such as wing outside the board, skirts near the door, such as the area where the flow marks occurred in about 50. Figure 4 shows.figure
Figure 4 door cover flow drop legend
2.4 front door hinge area
front door, middle door and car body hinge area, easy to produce more serious flow marks, affecting areas such as body B, C column, the area of flow marks occurred in the PPH in about 30. Figure 5 shows.
Figure 5 Hinge Flow Trace Legend
3 Analysis of the Causes of Electrophoresis Flow Marks
As mentioned earlier, electrophoresis traces often arise in the body gaps, so the design and manufacturing process as far as possible to avoid related problems. Specifically, electrophoresis traces are caused by the following reasons.
3.1 body structure design
unreasonable body structure design is easy to cause electrophoresis flow marks, in the coating synchronization project, should be designed reasonable sheet metal clearance. Figure 6 design structure tail clearance is less than 1mm, but the front gap is greater than 3mm, resulting in more residual storage, this position electrophoresis flow marks serious, high frequency, deep flow marks. Therefore, in the subsequent coating synchronization project, it is proposed to change the tail gap to 1.5 to 3mm.
Figure 6 structural design results in flow marks
3.2 stamping process impact
limited by the stamping abrasives and stamping seam link gap, the edge of the body steel plate will produce uneven cutting edges, as well as the presence of the joint, etc. will also lead to the occurrence of coated electrophoresis flow marks. In Figure 7, the inlet of the bonnet plate causes flow droplets, affecting the quality of the outer surface of the wing.figure
Figure 7 bonnet stamping knife leads to flow marks
3.3 welding white body process impact
in the welding white body production process, plate edge crevices, folding glue diameter, coating distance, welding spot line, inside and outside sheet metal wrong edge and other concierge marks have a greater impact. As shown in Figure 2, the flow marks due to the edge of the bag away from the empty 1mm resulting in more fluid accumulation, resulting in serious flow marks. Also shown in Figure 8, the folding glue due to the small diameter of the coating glue, the distance of the coating glue caused by the folding glue edge cover is incomplete, so that more electrophoresis fluid, baking after a large number of flow marks. In Figure 9, due to the existence of production errors, the cutting edge of the inner plate is higher than the outer plate, resulting in fluid flow out of the contaminated outer plate caused by the flow defects.
Figure 8 crease glue defects lead to flow marks
Figure 9 sheet metal tie-up wrong edge leads to flow marks
3.4 coating process impact
coating process is an important factor in the production of electrophoresis traces, mainly in the electrophoresis paint itself flow marks characteristics, on-site construction solids, electrophoresis after cleaning state, electrophoresis after leaching effect, drying process settings and other factors.
3.4.1 electrophoresis paint flow mark characteristics
different brands of electrophoresis paint affected by the formula, under the same conditions will have different flow mark effect. The following introduces a pre-test method of electrophoresis trace, which can be simulated for different brands of electrophoresis paint, different sandwich sizes and different washing conditions to find the most suitable combination.
select 2 10cm×15cm standard phosphate plates, one of which bends 90 degrees at 5cm from the edges on both sides, and the other 3 1cm×4cm tin foils are evenly distributed to the phosphate plate, sandwiching two treated phosphate plates at 10cm, and the gap between the two plates is adjusted by folding the tin foil. Finally, the conditions required (washing, baking, blowing, etc.) were simulated, and the color, length and appearance quality of electrophoresis paint were compared and judged. Figure 10 shows.
Figure 10 flow mark experimental sketch
3.4.2 electrophoresis system solid score
electrophoresis main tank solid score high will lead to increased flow mark problem, on the one hand, its own solid score is high, difficult to clean, on the other hand, the body carrying electrophoresis tank liquid will aggravate the subsequent cleaning system solid components rise, resulting in subsequent cleaning capacity decline or even failure.
3.4.3 electrophoresis after cleaning
body electrophoresis will have ultrafiltration cleaning, pure water cleaning and other cleaning systems. Ultrafiltration fluid is obtained by ultrafiltration membrane in electrophoresis liquid, such as water molecules, organic solvents, electrolytes, and some small molecular resins. Ultrafiltration cleaning on the one hand can recover most of the body carrying electrophoresis paint, improve the utilization rate of electrophoresis paint, on the other hand, can reduce the surface of electrophoresis paint flocculation. Pure washing further cleanses the body and improves the flatness of the paint film surface.
If the solid content of ultrafiltration cleaning liquid is too high (generally 0.5% to 2%) or the pollution level of pure water washing is high, it will reduce the washing effect and increase the chance and severity of flow marks.
3.4.4 electrophoresis after leaching
electrophoresis after leaching is the last water wash between the entrance to the electrophoresis oven process, its role is to reduce the body carrying water, electrophoresis residue and so on. Too short leaching time, unreasonable leaching design structure will reduce the effect of leaching. This will not only increase the frequency and severity of the flow marks, but also increase the moisture in the electrophoresis oven, resulting in other quality problems and oven discharge problems.
3.4.5 electrophoresis oven process
combined with the electrophoresis flow mark generated by the process, too fast electrophoresis oven heating up speed, will lead to the body temperature heating up faster, body cavity, crevices in the residual fluid is not dried, by the heat expansion after the flow marks. The two coating production lines in which they are located do not have a pre-baking process, resulting in serious flow marks on each model.
4 electrophoresis trace resolution measures
by the previous analysis can be known, electrophoresis flow trace causes are complex, different flow mark location of the specific causes are also different. To solve the problem of body fluid marks is from the design to the system engineering of each process area. Different flow points need to take into account all aspects of the influence factors.
4.1 product
in the product design stage, need to consider the current stamping, welding, coating process conditions. To reduce the generation of gaps and voids, consider the possibility of flow marks, if unavoidable, try to produce flow marks in non-visual areas. Door hair cover edge gap design as far as possible to avoid between 0.5 to 2mm, verified that this area of flow marks produce a larger frequency, such as 3.1 examples.
4.2 stamping, welding process adjustment
stamping mold cut should try to avoid at the structural bend, due to the body wrap edge, glue, welding and other process restrictions, will lead to high frequency of flow marks in this area. Welding process to ensure that the door cover edge clearance <0.1mm, folding glue coating diameter, coating distance should be able to completely cover the edge, not only will reduce the production of flow marks, but also improve the body corrosion performance.
4.3 Coating process adjustment
coating process design stage to verify the characteristics of electrophoresis material flow marks, design and materials most suitable for construction parameters, washing, leaching, baking and so on. After the production of coated electrophoresis equipment transformation is more complex, so the early design is particularly important. Later can adjust the baking temperature, increase the blowing and other processes.
4.3.1 electrophoresis paint construction parameters adjustment
electrophoresis paint construction parameters are the most critical solid points, to maintain solid points stability, reduce fluctuations can reduce the frequency of body traces, for electrophoresis paint film thick stability is also beneficial. By optimizing the feeding method to reduce the fluctuation of solid points, the general electrophoresis using automatic drip-addition method, control the feeding flow can achieve stability. In addition, the management can be controlled by the method of statistical process control (SPC), and the mobile polar graph control chart can be used for management.
4.3.2 electrophoresis after washing adjustment
electrophoresis after washing contains ultrafiltration liquid cleaning and pure water cleaning, the main control point is the cleaning of solids (pollution level) and nozzle flushing angle. Ultrafiltration liquid per hour design supply ratio to body area is 0.5 to 1L /
m
2
, fresh ultrafiltration liquid solids are generally less than 0.3%, all levels (three-stage cleaning) solids are controlled at 0.7%, 0.9%, 1.5% or less. Mainly to maintain the site use of ultrafiltration membrane, inlet pressure, regular back-washing, replacement can maintain a stable state, generally 2a replacement 1 time. In addition, through all levels of spray flow and overflow flow adjustment to reduce the solid points at all levels, in its coating line through calculation shows that the reduction of ultrafiltration cleaning 1 (UF1) of fresh ultrafiltration rehydration liquid, increase the ultrafiltration cleaning 3 (UF3) rehydration can reduce the solid score of all levels of ultrafiltration tank body, the site UF1 (volume of 8
m
3
3) decreased by 1.6
m
3
, UF3 (volume of 8
m
3
) increased by 1.6
m
3
, and the solid score of slots decreased by 0.3%, respectively.
The key control point of pure water washing lies in the quality of pure water and fresh pure water replenishment setting, pure water quality lies in the membrane group backwashing and regular replacement, the production line due to industrial hydropower conductivity is too high (>1200 s/cm), water quality is hard, generally 1a replacement 1 time. Reasonable fresh pure water set flow can reduce the severity of the flow marks, the production line speed up 10%, fresh pure water replenishment is still using 8
m
3
/h, resulting in a deepening of the flow marks color, adjust the overflow of 10
m
3
/h, the flow marks color fade, part of the flow marks can not be treated.
In addition, the appropriate increase in the temperature of pure water washing can reduce electrophoresis traces, the production line in the electrophoresis after pure water immersion tank to increase the heat exchange device, water temperature control at 40 to 42 degrees C, the body around the electrophoresis trace frequency reduced by 30% to 40%.
4.3.3 electrophoresis after leaching adjustment
electrophoresis after leaching area is generally designed to have humps, increase leaching capacity, leaching time is generally set to more than 10min. As the production line drainage time is 8min, the current models have more flow marks, can only be reduced by other means. Some of the more advanced process equipment manufacturers to increase the lifting body, vibration to increase the effect of leaching. As shown in Figure 11.
Figure 11 electrophoresis after leaching equipment
4.3.4 electrophoresis oven adjustment
general passenger car factory in the electrophoresis oven design will be left 10min pre-baking time, so that the body slowly heat up to 100 degrees C, to ensure that the internal cavity residue can be dried in advance. Due to the production line does not have a pre-baked segment, resulting in electrophoresis oil shrink hole and electrophoresis flow trace frequency is higher. After the late transformation, increase the 5min pre-baked section, the frequency of flow marks everywhere in the body decreased by 20% to 30%.
4.3.5 electrophoresis after blowing water and diversion workwear
for difficult to eliminate electrophoresis flow marks, some car factories use artificial compressed air blow-off or fixed point compressed air blow-off, reduce the residual fluid at the point of flow marks, thereby reducing the frequency of flow marks. As shown in Figure 12.
Figure 12 compressed air blow-off transformation
part of the electrophoresis flow marks can be used to design the flow of workwear program to eliminate, the flow of workwear can avoid the flow marks drop to the body, thus avoiding grinding, but will increase the cost of dismantling the tooling and the production of workwear, the need for specific balance. In addition, the design of reflow workwear to consider the problem of paint film injury. As shown in Figure 13.
Figure 13 Diversion Workwear
5 Conclusion
Electrophoresis secondary flow marks cause complex, involving cumbersome process, the best solution is prevention. In product design and process design, as far as possible to consider the need to apply electrophoresis flow marks to solve the needs, to avoid easy to produce flow marks of the gap design, process design to consider effective cleaning, longer leaching time and leaching equipment, suitable electrophoresis oven pre-baking, reduce the transformation after production and problem solving. If it is unavoidable, you can start with the adjustment of factors in the process of stamping, welding, coating, etc., and verify them item by item. Reducing electrophoresis traces protects the integrity of electrophoresis paint, thereby reducing the body's corrosion resistance.
source: 2020 "Coating Industry" No. 4
Li Liang, Wang Lei SAIC GM.