Anode system in the hottest cathodic electrophoret

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Anode system in cathodic electrophoretic coating system

for cathodic electrophoretic coating process, if you want to obtain high-quality electrophoretic paint film in the production process, you must ensure the stability of pH value and conductivity of bath solution. In the cathodic electrophoretic coating process, the anode system plays a role in stabilizing the pH value and conductivity of the bath solution

anode reaction of cathodic electrophoresis

generally, the elements of the electrophoresis anode system include: anode unit, anode liquid tank, anode pump, conductivity meter, flowmeter, pressure gauge, anode liquid supply and return pipeline, anode liquid overflow and discharge pipeline, etc. The anode system of cathodic electrophoretic coating is shown in Figure 1

in the process of cathodic electrophoretic coating, when positively charged resin cations are deposited on the workpiece, organic acid ions (acetate ions, formate ions) and chloride ions will be continuously generated in the electrophoretic tank solution (organic acid ions are from electrophoretic paint, and chloride ions are from curing agent), and will accumulate in the tank solution accordingly. When organic acid ions accumulate too much in the bath solution, it will directly lead to the decrease of pH value and the increase of conductivity, which will affect the quality and appearance of the electrophoretic paint film (for example, strip marks are generated on the surface of the electrophoretic paint, and the paint film is rough). In order to ensure the best coating effect and the stability of the electrophoretic paint solution, these acid radical ions must be continuously removed through the anode system during the coating process

these organic acid radical ions will react with positively charged hydrogen ions enriched on the anode, which we call "anodic reaction"

anode unit

electrophoretic anodes mainly include tube type, roll type, plate type and hollow fiber, among which tube type and plate type are most used. Tubular and plate anodes are usually enclosed in a washable universal testing machine. The specimen (or product) is clamped by a fixture and the specimen is forced into an anode cover, which is made of non-conductive materials

anode unit is generally composed of anode rod (electrode), anode diaphragm, insulated anode cover, anode liquid input pipe, anode liquid output pipe, etc. The anode unit structure is shown in Figure 2

1. Cathode electrophoresis anode rod

the anode rod of cathode electrophoresis is the anode that forms an electric field with the cathode of the workpiece, which shows its importance in the electrophoresis process. The anode rod, bolt and gasket of cathodic electrophoresis usually use stainless steel (such as 316L stainless steel) or titanium alloy plate, and the thickness of the anode rod is preferably not less than 3.2mm. The anode rod of the anode unit directly participates in the electrochemical reaction of electrophoretic coating, so it will be gradually lost. Anode rods have a certain life cycle, and their consumption rate depends on the products passing through the electrophoresis tank and productivity. If the operation is correct, the anode unit usually has a life cycle of 3 ~ 5 years, and the life cycle of the specially treated anode rod will be relatively long. In actual production, 10 ~ 20% of anode units should be disassembled and inspected every year. Generally, each pole cover should be equipped with an ammeter that is easy to observe, so as to continuously monitor the working condition of each anode

2. Cathode electrophoresis anode diaphragm

the anode film is one of the main components of the anode unit. Its function is to remove the acid accumulation generated in the electrophoresis process through electrodialysis, so that excess acid can be removed and the normal pH value of the tank liquid can be maintained

the anode film is a semi permeable membrane, which can selectively allow ions with corresponding charges (negative charges) to pass under the action of an electric field. This semi permeable membrane only allows small molecules such as water molecules and small acid radical ions to pass through, while macromolecules such as resins and pigments cannot pass through. Anode solution and electrophoresis bath solution are separated by means of this selective semi permeable membrane. Only under the action of electric field can acid radical ions migrate from the electrophoresis bath to the anode system. Without the action of electric field, acid radical ions will be evenly dispersed in the bath solution and anode solution under the action of extremely poor concentration. Its working principle is shown in Figure 3

Figure 3 working principle of anode film

3 Two precautions for anode film

(1) if there is leakage

if there is leakage or damage of anode unit, stop the anode pump immediately, turn off the rectifier if necessary, and replace the leaking anode unit in time. Before replacing the anode, the power must be turned off first. The specific operations are as follows:

a. turn off the pump of the anode liquid system, which mentions that the rectifier will also be turned off if necessary

b. find out the leaking anode rod

c. isolate the leaking anode unit from the electrophoresis system

d. replace the leaking anode unit

(2) the anode film should be moisturized when the electrophoresis tank is emptied

when the main electrophoresis tank is emptied, the anode liquid circulation system must maintain the circulation state and constantly spray deionized water on the anode film. Otherwise, the electrophoretic paint will dry on the anode film and the anode film will dry out. The dry electrophoretic paint will increase the resistance, and once the anode film is dried out, it will permanently lose the penetration function of the semi permeable membrane

4. Other

vertical pumps must be used in the anode liquid circulation system, rather than horizontal pumps. This can avoid the adverse effect of horizontal pump in case of seal failure. The transmission pipeline of anode liquid must be made of acid resistant stainless steel or plastic. If the return pipe of anode liquid accounts for 20.8%; The equipment with a stroke of more than 1100mm is made of plastic, and the anode must be grounded

in order to obtain a more uniform electrophoretic coating, the top cover anode or the bottom anode can be used for electrophoretic coating. The top cover anode and bottom anode are usually installed in the second voltage zone. If they are installed in the first voltage zone, the effect may be poor due to low voltage

anode solution

the conductivity of the anode solution reflects the acid concentration. In the process of electrophoretic coating, the conductivity of anode solution will gradually increase with the increase of acid concentration. The conductivity of the paint solution can be reduced by regularly discharging the anode solution in the electrode cover and adding deionized water. The conductivity of the anode solution should be maintained within a certain range, such as 500 ~ 1000ms/cm. If the conductivity is high, a certain amount of anode liquid must be discharged and fresh deionized water must be added

the anode liquid must be clean and transparent. In the weeks before the new tank is put into operation, pay special attention to the color of the anode solution. If the color changes quickly, it indicates that there is a problem with the anode system. If the color changes, special attention should be paid. If the anode solution becomes blurred, the anode film may be damaged or contaminated by bacteria. If the color of the anode liquid becomes close to that of the bath liquid, the anode film may also be damaged. If the color of the anode solution becomes darker (brown or black), but it is still transparent, it indicates that the stainless steel anode rod corrodes too fast, or the flow is low, or the conductivity is too high

each anode unit should maintain an appropriate anode liquid flow, and the flow of each anode unit should be able to be monitored independently. The accuracy of the flow valve should not be too large in order to control the flow within the appropriate range. It is very important to maintain the normal flow of anode liquid, which should be checked every day

anode ratio

the ratio of cathode to anode is usually 4:1 ~ 6:1. The anode area can be calculated by the following formula:

sa=jph/60 × T × SC/r

sa - anode area

jph -- production beat (hour-1)

t - single vehicle electrophoretic coating time (min)

sc - single vehicle surface area (M2)

r - cathode area/anode area (usually 4)

single vehicle coating time refers to the effective dip coating time. If the coating time of a single vehicle is longer than 2min, it shall be calculated as 2min. If it is a flexible production line, the surface area of a single car should be calculated according to the maximum design body size

calculate the total anode area required according to this formula, and then determine the number of anode units required according to the area of a single anode unit. The arrangement of anodes should follow certain rules, and more attention should be paid to intermittent Electrophoresis Tanks

installation and arrangement of anode pipe

installation of anode system includes installation of mechanical facilities, circuit connection and tank liquid circulation pipeline connection. The anode unit must be installed before the electrophoretic paint is put into the tank and pass the leakage test. However, generally, the installation time of the anode unit should be as short as possible from the time when the electrophoretic paint is put into the tank, so as to minimize the probability of anode damage. Special care must also be taken to protect the anode film during installation

the anode unit shall be arranged along each tank wall, and the depth shall not be less than 40% of the tank liquid depth. Usually, there is at least one gap between the two electrode sections to prevent the electrophoretic paint from depositing on the anode and the electrode cover in the low voltage area

originally published in AI automobile manufacturing (end)

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