Analysis and prevention of causes of body electrophoresis shrink holes

In recent years, with the rapid development of China's automotive industry, people's requirements for automobile quality are getting higher and higher, of which the anti-corrosion performance of automobiles is the focus of attention. The main purpose of automobile coating is to protect, decorate and special functions, in which the excellent electrophoretic paint film can not only provide qualified substrate conditions for medium coating and finish paint, but also enhance the anti-corrosion performance of the body, extend the life of the car and improve its commodity value. Cathode electrophoresis coating using cation resin coating, the conductive coating as cathode placed in the electrophoresis tank liquid, and in the corresponding position of the tank set anode, power between the poles, the surface of the coating will deposit a uniform layer of electrophoresis coating . Cathode electrophoresis coating process is easily affected by process conditions and various adverse factors, resulting in a variety of paint film quality defects, of which electrophoresis shrink hole is one of the most "headache" problems. Shrinking holes are usually continuous, requiring a long time to check each possible change point, it is difficult to solve completely in a short period of time, will form a batch of quality problems. In this paper, a coating workshop in early 2020 in the production process appeared in the batch of small shrink holes as an example, to share the specific inspection process, the improvement of the shrink hole and the later tank fluid maintenance.
1 status quo
1.1 body shrink hole status quo and the principle of production
by the implementation of six emission standards and the dual impact of the epidemic, a coating plant has not been produced for most of the year. In early 2020, the resumption of production, electrophoresis body, the outer surface of the dense hemp diameter of about 1mm of small shrink holes, and paint film surface rough (Figure 1, where the small picture magnified 5 times), the shrink hole distribution is denser than the facade, the appearance of more than the inner table state. When the electrophoresis small shrink hole was polished until it disappeared, it was found that the small shrink hole did not bottom out (Figure 2), and the remaining electrophoresis film thickness at the grinding site was 4 to 8 m. Excessive polishing of electrophoretic paint film will seriously reduce the anti-corrosion performance of the body, resulting in the risk of corrosion of the body, it is urgent to solve the problem of electrophoresis shrink holes as soon as possible.
Figure 1 Electrophoresis small shrink hole
Figure 2 Paint film after grinding small shrink holes
Cathode electrophoresis in the cause of many reasons, mainly due to foreign property in electrophoresis coating film (such as oil, silicone, particles and other impurities);
1.2 electrophoresis process and electrophoresis tank liquid parameters
a coating workshop mainly produces SUV models, the front treatment of electrophoresis transmission equipment for electric hoists, electrophoresis oven using π-type drying room. Pre-treatment electrophoresis process is: white body hanging → hand pre-cleaning→ torrent hot water washing (spraying) → pre-fat (soaking) → degreasing (soaking) → washing 1 (spraying) → washing 2 (soaking) → table tone (soaking) → phosphate (soaking) → water washing 3 (Spray) → Water Wash 4 (soaked) → Pure water wash 1 (soaked) →phosphate film inspection and pure water spray →electric swimming→UF1 (ultrafiltration spray) →UF2 (ultrafiltration immersion) → Pure water wash 2 (soaked) → hanging leachate→ electrophoresis drying.
design beat of the coating production line is 20JPH (workload per hour), and the control range of the electrophoresis tank fluid process parameters is set out in Table 1.
Table 1 electrophoresis tank liquid process parameters
2 shrink holes cause root cause troubleshooting
2.1 reason speculation
According to the number, status, distribution of electric phosphoresing small holes in the vehicle, the preliminary speculation may be due to the long-term use of electrophoresis oven and unsalted pollution. At the same time, according to the field status and personal experience, there may be the following pollution sources or factors, need to check and verify one by one: (1) the main parameters of electrophoresis tank fluid is not qualified; Large clearance rate leads to electrophoretic shrink holes; (5) electrophoretic voltage high paint film is punched through to form shrink holes; (6) electrophoresis tank fluid aging or pollution, such as long bacteria and other factors lead to shrink holes; (7) electrophoresis paint film re-dissolved excessive amount to form shrink holes; (8) UF1 groove surface floating object attachment leads to shrink holes.
2.2 process troubleshooting and verification
2.2.1 electrophoresis tank liquid main parameters testing and testing
according to the day before and after the production of electrophoresis tank liquid test results (Table 2), in addition to pH, all other are in line with the requirements of process parameters.
Table 2 Electrophoresis Liquid Process Parameter
2.2.2 Verify that the electrophoresis oven is contaminated
At the same time for 3 electrophoresis wet film body using tin foil, a package right A column, a package left leaf plate, a package right front door plate, after drying by electrophoresis oven to observe the body is wrapped in tin foil paper, electrophoresis paint film still has a large number of shrink holes, with the body unconstructed parts of the number, form is basically the same, and with the laboratory baking with the car electric test board surface shrinking state is basically consistent.
test results show that electrophoresis oven is not the root cause of small shrink holes.
2.2.3 verification by the body plate oil pollution caused by
welding before the delivery of the vehicle on the outside surface of the vehicle to focus on cleaning operations, while coating in the manual pre-cleaning station on the 3 white body cover, left and right leaf plate using 400 sludge key polishing, after grinding and then wipe with degreasing liquid, and then into the front treatment and electrophoresis production line. After the body is electrophoresis dried, the shrink holes of the lid and left and right leaf plate are consistent with the number and status of the shrink holes in the undressed body.
test results show that the electrophoresis small shrink hole is not caused by the heavy oil of the body plate.
2.2.4 verify whether it is caused by bad skim
take 12 cold-rolled plate, first soaked with skimmed liquid to wash, and then rinsed with pure water, immediately with waterless ethanol for cleaning, to ensure that each plate without any oil. 12 test plates are with the trailer plate after the pre-treatment of electrophoresis, in the electrophoresis hanging drainage table to remove 6 pieces sent to the laboratory oven drying, the remaining 6 pieces with the car into the oven drying. Results The test plate dried with the car and the test plate dried by the laboratory showed a shrink hole, and the status was the same.
test results show that electrophoresis small shrink holes are independent of the performance of pre-treatment skimming fluid.
2.2.5 verifies that
due to low yields, the table tone has not been replaced for several months, so the table trough fluid is re-provisioned shortly after resumption of work. In the phosphate film inspection and pure water spray work, the use of 600 sandpaper after phosphation of the middle part of the lid to polish, the phosphate film polished until completely dew iron, and then rinsed with pure water, followed by electrophoresis and drying. Results There are still a large number of small shrink holes in the grinding part of the machine cover, and the number and status of the shrink holes in the uncoiled machine cover are consistent.
test results show that electrophoresis small shrink holes are not related to the phosphatation and phosphation status.
2.2.6 verify whether the electrophoresis voltage is high caused
because the pH of the electrophoresis tank fluid has been seriously lower than the lower limit of the process range, and the groove fluid also exists aging phenomenon, electrophoresis tank fluid knock-through voltage drop. By reducing the voltage of 30V in the first and second segments respectively, and using the last verification of production, the results are consistent with the number and status of the electrophoresis small shrink holes of the whole vehicle after drying.
test results show that electrophoresis small shrink holes are independent of the first and second voltages.
2.2.7 verify whether the electrophoresis tank fluid aging, pollution caused by
observation of the anode liquid circulation system, found that some of the roel flow meter inside the filament-like substances, anode water tank floats have similar to the fall of unknown objects, and the anode fluid pH lower limit, infer that the anode fluid has long bacteria. Combined with the electrophoretic paint supplier laboratory test report shows that the electrophoretic tank liquid aging is serious, bacteria grade 3.0, pH is 4.9, acid additive content is 0, conductivity, solvent content are seriously lower than the lower limit of the process, and by doing L-shaped test board, test board has a small number of shrink holes and a large number of particles, test plate paint film rough and film thickness is not up to standard.
test report shows that electrophoresis small shrink hole is related to the aging and contamination of electrophoresis tank fluid.
2.2.8 Verify whether it is caused by resoluble exceeding the standard
6 test plates (test plate skimming degreasing, pure water washing, alcohol drying) in the phosphating membrane inspection and pure water spray work place with the car (test plate skimming degreasing, pure water washing, alcohol drying), 3 of which are in electricity After swimming out of the tank to remove, rinse with pure water, the remaining 3 pieces with the car for UF tank spray, soaking rinse and pure water washing, and then under the electrophoresis hanging leaching table, and then the removed 3 test plates back to the original position with the car into the oven drying. After testing, there was no difference in the thickness of the electrophoresis film of 6 test plates.
results show that the electrophoresis small shrink hole is not related to the re-dissolving amount of the paint film of the UF tank fluid.
2.2.9 verifies whether floating objects on the surface of the UF1 tank fluid caused
to float some yellow substance on the surface of the UF1 tank fluid when it is not produced, as the large cycle of the ultrafiltration fluid is always in the open cycle state (Figure 3).
figure 3 UF1 tank liquid surface floating object
after sampling electrophoresis paint supplier laboratory testing, the floating object was determined to be bacterial excreta. After sampling before production, and then applied to the test plate, after electrophoresis drying, the electrophoresis shrink hole on the test plate slightly large, no shrinkage area of the paint film becomes rough.
But in the actual production process, because UF1 is spray flushed, and UF1 tank fluid from the bottom of the groove is extracted through the filter and sprayed into the body, floating objects basically do not come into direct contact with the body. Therefore, the test results can only prove that electrophoresis small shrink hole and UF1 tank liquid surface floating objects have an indirect relationship.
, the root causes of electrophoresis small shrink holes are related to aging of electrophoretic tank fluid and long bacteria, and indirectly related to yellow floating objects on the surface of UF1 tank fluid.
2.3 Root Confirmation Verification
For further verification of the root cause of electrophoresis small shrink holes in the electrophoresis tank liquid, through cross-hanging plate comparison test, that is, a coating plant in the first phase of the phosphating plate in the second phase of electrophoresis line electrophoresis, the second phase of the silane plate in the first electrophoresis line electrophoresis, test board are drying in the laboratory oven. The results showed that the test plate of the second electrophoresis had no shrink holes, and the test plate of the first electrophoresis had a shrink hole, i.e. it was confirmed that the root cause of the small shrink hole of electrophoresis was in the electrophoresis tank fluid.
3 shrink hole solution measures
according to the real car and trailer verification, combined with electrophoretic paint supplier's tank liquid sample test report, to determine that electrophoresis small shrink hole is due to electrophoretic tank fluid aging, long bacteria caused. To this end, according to the production plan to develop the following solutions.
(1) after pouring the electrophoresis tank, rinse the groove wall and anode tube film with pure water to remove the electrophoresis paint deposited on the bottom of the tank.
(2) electrophoresis tank liquid is poured back into the electrophoresis tank, add a new electrophoretic resin to the upper limit of the solid partiality process.
(3) first drain the anode liquid, then re-provision the anode liquid, and then add a fungicide cycle 2d after exhaustion, add pure water cycle to the day before production, and then re-provision the anode liquid.
(4) electrophoresis tank fluid and ultrafiltration circulation system to add fungicide cycle, replace the oil removal filter bag once a day.
2 barrels of electrophoresis solvent to the process range (120m3 electrophoresis slot) is added to the process range.
to this point to complete the above measures, the workshop arranged several production, the result of the electrophoresis body of the number of small shrink holes decreased, but not completely resolved. The sample was then taken again and sent to the electrophoresis paint supplier laboratory for testing, which showed that the electrophoresis tank fluid and UF1 were free of bacteria (as shown in the results on both sides of the Bacteria Test Stick in Figure 4).
4 Bacterial test results
but solvent content, conductivity, pH and partial acid additive content is still lower than the lower limit of process requirements, MEQ value is higher than the upper limit of process requirements. In consultation with the electrophoretic paint supplier, it was decided to add 2 barrels of anti-shrinking hole aids and 1 barrel of solvent to the electrophoretic tank fluid. After the addition of the potion, the 2d workshop arranged production, resulting in a significant reduction in the number of small shrink holes in the electrophoretic body.
week, there were no small shrink holes on the outer surface of the electrophoresis body produced again, and only a few shallow disc-shaped shrink holes on the inside. After adding 1 barrel of solvent to the electrophoresis tank fluid two days before each production, after 2 productions, the small shrink holes in the inside and outside of the body completely disappeared, and the roughness of the electrophoresis paint film decreased significantly (as shown in Figure 5)
Figure 5 paint without shrink holes Membrane appearance
4 measures to delay the aging of the groove liquid
Although the small shrink hole of the electrophoresis paint film has been resolved, but the electrophoresis tank liquid a number of parameter indicators still do not meet the process requirements, and the electrophoresis film thickness <18 m. Due to the low production volume, in order to prevent the tank fluid parameters continue to deteriorate and membrane thickness decline, delay the speed of aging electrophoresis tank fluid, in addition to normal maintenance, the workshop is required to implement the following measures.
(1) non-production time to reduce the electrophoresis tank liquid temperature to 25 to 28 degrees C.
(2) UF large cycle is opened, 1 to 2t ultrafiltration fluid is discharged daily to prevent pH from continuing to drop.
(3) add 1 barrel of solvent before each production, maintain the content of slot solution solvent, increase the thickness of the film and improve the appearance of the paint film.
(4) UF large cycle after the end of production cycle 2d, the UF tank liquid drain, pour into the new ultrafiltration liquid cycle, if the next 1 week or more is not expected to produce, close the UF and electrophoresis tank cycle, daily white night shift scheduled cycle 1 to 2h can be.
(5) electrophoresis tank after the temporary addition of new electrophoretic resin, in the first two days of production of a one-time supplement, improve the tank liquid update rate.
(6) regularly check the electrophoresis tank fluid, UF tank fluid, anode water tank is long bacteria.
(7) drain the anode fluid at the end of each production, cycle with pure water, and reconfigure the new anode fluid the day before production.
(8) in accordance with the material supplier testing report recommendations, the purchase of relevant additives and as required to add, to maintain the stability of the tank fluid.
5 conclusion
in the last 3 months, through electrophoresis tank, supplement electrophoresis resin and solvent, sterilization, add anti-shrink agent and other means, electrophoresis hole problem has been completely resolved, and in the follow-up production did not re-emerge. Although some of the process parameters of the tank fluid are still not qualified, but greatly reduced the work intensity of the polishing staff and improve the anti-corrosion performance of the product.
there is no complete non-aging cathode electrophoresis paint, only through some appropriate means and measures to delay the rate of aging tank fluid. For the aging tank fluid, there is no special agent to quickly restore the performance of the tank fluid, can only be improved by the update of the tank fluid. The above method of delaying tank fluid aging provides reference value for production lines with low long-term non-production or production volume and inevitable tank fluid aging.