Automotive coating pre-treatment process

China Coatings Online News Information: Pre-treatment is the first process of the paint workshop of the whole vehicle factory, a good pre-treatment process can not only guarantee the excellent corrosion resistance of the car body, but also for the paint in the rear channel to provide a high-quality substrate surface. The pre-treatment process can be divided into two parts from the functional point of view, namely, "cleaning the metal body" and "phosphing the metal body", in which "cleaning the metal body" includes cleaning the surface of the plate stamping cleaning oil, rust-proof oil and other types of oil, as well as residual in the body of the solder slag ball, metal grinding chips and other garbage impurities;1. Skimming
degreasing is the use of saponification and surfactant wrap process, to remove all kinds of oil on the surface of the white body, paint workshop skimming has 2 processes, in turn for spray skimming and leaching degreasing. In the spray skimming process through the upper and lower left and right 4 directions of the spray flushing, to remove the white body surface of the oil. During the leaching degreasing process, the vehicle is completely immersed in the degreasing fluid and the oil in the inner cavity of the body, including sheet metal crevices, is dissolved and removed. The control points of the skimming process are as follows.1) skimming temperature: the proper temperature of the skimming fluid can promote the saponification reaction and the dissolution of the oil, the degreasing temperature (55±5) degrees C.
2) skimming time: the leaching degreasing tank is 225m3 rectangular groove body, the vehicle with the swing chain through the leaching degreasing tank time of about 4min.
3) Chemical composition of skimming fluid: In order to remove various types and properties of oil on the surface of the plate, to ensure good degreasing effect, degreasing fluid is composed of a variety of components, including alkali, silicate, phosphate, borate and carbonate, organic components are mainly desulbants, corrosion inhibitors, compounds and surfactants. By detecting the total alkali parameters of the skimming fluid (8 to 10) and the oil content of the tank fluid to monitor the status of the degreasing fluid, regularly reverse the tank, to maintain a certain amount of tank fluid update.2. The main component of
surface adjustment agent is titanium phosphate, which can improve the coarse effect of metal surface due to lye cleaning, so that phosphate crystallization is meticulous. Because there are no filter cylinders and other liquid filtration equipment in the powder table adjustment system, so we should pay special attention to the smooth flow of the table adjustment channel circulation line, so as to avoid the occurrence of unequal concentration due to poor tank fluid circulation, in addition, the spray pressure of the table adjustment process should not be too large, spray pressure may be too large to adsorb on the surface of the car body of titanium phosphate collosus local dispersion, resulting in phosphorylation membrane flowering.3. Phosphating
phosphating film is a layer of inorganic salt crystal between metal substrate (metal) and electrophoresis paint (organic matter), which plays an important role in carrying on and off, and the specific functions of phosphating film include: improving the anti-rust and corrosion resistance of substrate; The shape of phosphate film is an important criterion to judge whether phosphate film is good, and good phosphate film should be closed and dense, uniform and small, no hanging ash, no secondary phosphate. The control points of the phosphation process are as follows.1) Phosphate temperature: tank liquid temperature is a very important process parameters in the phosphation process, phosphate temperature is too low will lead to thin phosphate film, corrosion resistance is weakened, phosphate temperature is too high will lead to a large number of tank liquid slag.
2) Phosphate removal slag: with the phosphation reaction, by-product phosphate slag constantly produced, if not removed in time, easy to lead to hair on the surface of the electrophoresis car (small phosphate slag deposited on the surface of the car body). In addition, high levels of slag in the tank fluid can also cause blockages in circulating lines, stratifier nozzles, and plate heat exchangers.3) Phosphate promoter: The role of the promoter is briefly to depolarize hydrogen, to prevent the formation of hydrogen bubbles on the surface of the work piece that hinder the formation of phosphate film, so that the phosphate film is uniform and dense.
4) Chemical parameters of phosphate liquid: the composition of phosphate liquid is very complex, including nearly 10 kinds of cations and anions, in the daily production process, mainly through the detection of the total acid, free acid, promoter point, slag content and other parameters to monitor the status of phosphate liquid, and according to the test results of feeding.4. The main component of passivation
passivation solution is hexavalent chromate solution, on the one hand, it can dissolve the peak part of phosphate film, reduce the pore rate of phosphate film, so that the surface performance of phosphate film is close. On the other hand, the calcium and magnesium ions brought in by washing are removed and a slightly acidic surface is provided for subsequent electrophoresis paint, thus improving the corrosion resistance and adhesion of the paint film.5. Washing
The role of washing includes cleaning the body of the iron chip solder slag and cleaning residual degreasing liquid, phosphate and passivation liquid, the workshop pre-treatment a total of 6 washing processes, respectively, hot water washing, degreasing after the first Second wash, phosphate after the third fourth wash and the last pure water wash, which hot water wash, the first wash, the third wash for the spray tank, the second wash, the fourth wash and pure water wash for the immersion tank.effect of washing mainly from two aspects, one is how to flush down the white body of garbage particles, and the other is how to remove the waste particles that are flushed down in the tank fluid. For the former is mainly to control the impact of the spray water flow, to get a good flushing effect not only to ensure a certain spray pressure and water flow flow, but also focus on the distance between the nozzle and the surface of the body, because the blow of the spray will increase with the distance rapidly decay. There are many ways to remove garbage particles from the tank fluid, including the use of liquid filter bags, magnetic separators, spin separators, paper tape filters, etc. In addition, according to the characteristics of each washing should also choose the appropriate removal method, for example, for the first washing process of pre-treatment - hot water washing, the particle content of the tank fluid is much higher than other tanks, if only by increasing the accuracy of the liquid filter bag and replacement frequency method, it will greatly increase the production costs, so we have modified the hot water tank, added a combination filter and sedimentation tank, combined filter honeycomb-shaped guide plate can make the groove ball soldering ball. In addition, the combined filter also installed 20 magnetic rods for adsorption of metal chips, only the regular discharge of tapered waste water and cleaning magnetic rods can be in the case of non-consumption of liquid filter bags to achieve the cleaning of the tank fluid, precipitation pool using the principle of static precipitation so that large particles deposited in the bottom of the tank, and then during the weekly tank cleaning can be removed from the sedimentary garbage, simple and efficient.