- Workpiece after surface treatment;
- Power supply;
- Workpiece;
- Water spray flushing;
- Tank liquid filtration;
- Sedimentation tank;
- Circulating pump
- Using water-soluble coating, taking water as the dissolution medium, saves a lot of organic solvents, greatly reduces air pollution and environmental hazards, safety and health, and avoids the hidden danger of fire at the same time;
- High coating efficiency, small coating loss, and the utilization rate of coating can reach 90% ~ 95%;
- The coating quality of all parts is uniform, such as the coating thickness and depression of the workpiece, which can solve the problem of uniform coating, and the coating quality of other parts is good;
- The production efficiency is high, and the construction can realize automatic continuous production, which greatly improves the labor efficiency;
- The equipment is complex, the investment cost is high, the power consumption is large, the drying and curing temperature is high, the management of coating and coating is complex, the construction conditions are strict, and the wastewater treatment is required;
- Only water-soluble coatings can be used. The color cannot be changed during the coating process. The stability of the coatings is not easy to control if they are stored for too long.
Electrophoretic Coating Equipment
The equipment of electrophoretic coating is composed of electrophoretic tank, mixing device, coating filtering device, temperature regulating device, coating management device, DC power supply device, water washing device after electrophoretic coating, ultrafiltration device, baking device, standby tank, etc. The size and shape of the electrophoresis tank shall be determined according to the size, shape and construction process of the workpiece. Under the condition of ensuring a certain distance between poles, it should be designed as small as possible. The tank is equipped with a filtering device and a temperature regulating device to ensure a certain temperature of the paint solution and remove impurities and bubbles in the circulating paint solution. The mixing device can keep the working paint liquid uniform. The circulating pump is mostly used. The paint liquid circulation is generally 4 ~ 6 times per hour. When the circulating pump is started, the paint liquid level in the tank shall be turned evenly. The function of the coating management device is to supplement and adjust the coating composition, control the pH value of the tank liquid, remove the neutralizer with the diaphragm electrode and remove the low molecular weight components with the ultrafiltration device. The selection of electrophoresis power supply generally adopts DC power supply. Rectifier equipment can be silicon rectifier or thyristor. The current is related to the property, temperature, working area and power on mode of the coating, which is generally 30 ~ 50A / m2. The water washing device is used for washing the workpieces before and after electrophoretic coating. Generally, deionized water is used, but pressurized equipment is required. The commonly used washing nozzle is a washing nozzle with spiral body. The baking device is used to promote the drying and film-forming of electrophoretic coatings. Resistance furnace, induction electric furnace and infrared baking equipment can be used. The design of drying room shall have three stages of preheating, heating and post heating, which shall be formulated according to the variety of coating and the condition of workpiece.Main Process Parameters Affecting Electrophoretic Coating
1. Voltage
Electrophoretic coating adopts constant voltage method, and the equipment is relatively simple and easy to control. Voltage has a great influence on the paint film; The higher the voltage, the thicker the electrophoretic paint film. For parts that are difficult to paint, the coating capacity can be improved accordingly and the construction time can be shortened. However, if the voltage is too high, the surface of the paint film will be rough, and it is easy to produce “orange peel” after drying. The voltage is too low, the electrolytic reaction is slow, the paint film is thin and uniform, and the swimming penetration is poor. The selection of voltage is determined by the type of coating and construction requirements. Generally, the voltage is inversely proportional to the solid content of the coating and the paint temperature, and is directly proportional to the distance between the two poles. The surface of steel is 40 ~ 70V, the surface of aluminum and aluminum alloy can be 60 ~ 100V, and the surface of galvanized parts can be 70 ~ 85V.2. Electrophoresis Time
The paint film thickness increases with the extension of electrophoresis time, but when the paint film reaches a certain thickness, continuing to extend the time can not increase the thickness, but will aggravate the side effects; On the contrary, the electrophoresis time is too short and the coating is too thin. The electrophoresis time should be as short as possible according to the voltage used and under the condition of ensuring the coating quality. Generally, the electrophoresis time of workpieces is 1 to 3 minutes, and that of large workpieces is 3 to 4 minutes. If the surface geometry of the coated object is complex, the voltage and time can be appropriately increased.3. Coating Temperature
The coating temperature is high and the film-forming rate is fast, but the appearance of the coating film is rough, which will also cause the deterioration of the coating; Low temperature, less electrodeposition, slow film formation, thin and dense film. During the construction process, due to the conversion of some electric energy into heat energy during electrodeposition, the mechanical friction in the circulating system will generate heat, which will lead to the rise of coating temperature. Generally, the temperature of paint solution is controlled at 15 ~ 30 ℃ in some aspects.4. Solid Content And Color Base Ratio Of Coatings
The solid content of commercial electrophoretic coatings is generally about 50%. During construction, distilled water is required to control the solid content of coatings at 10% ~ 15%. The solid content is too low, the covering power of the paint film is not good, the pigment is easy to precipitate, and the stability of the paint is poor. If the solid content is too high and the viscosity increases, the paint film will be rough and loose and the adhesion will be poor. Generally, the color base ratio is about 1:2, and the color base ratio of high gloss electrophoretic coatings can be controlled at 1:4. Since the amount of pigment in the coating will gradually decrease in actual operation, the coating with high pigment content must be added at any time to adjust.5. PH Value Of Coating
The pH value of electrophoretic coating directly affects the stability of bath solution. If the pH value is too high, the newly deposited film will dissolve again, the film will become thinner, and the film will be removed after washing after electrophoresis. The pH value is too low, the surface gloss of the workpiece is inconsistent, the stability of the paint solution is poor, the dissolved resin will precipitate, the surface of the paint film is rough and the adhesion is reduced. Generally, the pH value shall be controlled between 7.5 and 8.5 during construction. In the construction project, due to continuous electrophoresis, cationic ammonium compounds are accumulated in the coating, resulting in the rise of pH value. Add stock solution with low pH value, replace distilled water of cathode cover, remove ammonium ions with ion exchange resin, and reduce pH value by anode cover. If the pH value is too low, ammonium ethoxide can be added to adjust it.6. Coating Resistance
The impurity ions of the coated object brought into the electrophoresis tank from the previous process cause the decrease of the coating resistance value, resulting in the defects of rough and uneven paint film and pinholes. In the coating construction, the coating needs to be purified. In order to obtain high-quality coating, cathode cover equipment can be used to remove ammonium, calcium, magnesium and other impurities and positive ions.7. Distance Between Workpiece And Cathode
Close distance and high deposition efficiency. However, if the distance is too close, the paint film will be too thick, resulting in sagging, orange peel and other disadvantages. The general distance shall not be less than 20cm. For large and complex workpieces, when the external film has been deposited with a very thick film, but the internal film is still thin, the auxiliary cathode shall be added at the part far away from the cathode.The Methods And Skills Of Electrophoretic Coating
- The process flow of electrophoretic coating on general metal surface is as follows:Pre cleaning → on-line → degreasing → water washing → derusting → water washing → neutralization → water washing → phosphating → water washing → passivation → electrophoretic coating → on tank cleaning → ultrafiltration water washing → drying → off-line.
- The substrate and pretreatment of the coating have a great influence on the electrophoretic coating. The castings are generally derusted by sand blasting or shot blasting, the floating dust on the surface of the workpiece is removed with cotton yarn, and the residual steel shot and other sundries on the surface are removed with 80#~ 120# sandpaper. The steel surface shall be treated with oil and rust removal. When the surface requirements are too high, phosphating and passivation surface treatment shall be carried out. Ferrous metal workpieces must be phosphated before anodic electrophoresis, otherwise the corrosion resistance of the paint film is poor. During phosphating treatment, zinc salt phosphating film is generally selected, with a thickness of about 1 ~ 2 μ m. The phosphating film is required to be fine and uniform.
- In the filtration system, primary filtration is generally adopted. The filter is of mesh bag structure with pore diameter of 25 ~ 75 μ m。 The electrophoretic coating is delivered to the filter by vertical pump for filtration. Considering the comprehensive replacement cycle and film quality, the aperture is 50 μ M filter bag is the best. It can not only meet the quality requirements of paint film, but also solve the blockage of filter bag.
- The amount of circulation in the circulating system of electrophoretic coating directly affects the stability of bath solution and the quality of paint film. Increase the circulation volume and reduce the precipitation and bubbles of tank liquid; However, the aging of tank liquid accelerates, the energy consumption increases, and the stability of tank liquid becomes worse. It is ideal to control the circulation times of tank liquid 6 ~ 8 times / h, which can not only ensure the quality of paint film, but also ensure the stable operation of tank liquid.
- With the extension of production time, the impedance of anode diaphragm will increase and the effective working voltage will decrease. Therefore, the working voltage of the power supply should be gradually increased according to the voltage loss in production to compensate for the voltage drop of the anode diaphragm.
- The ultrafiltration system controls the concentration of impurity ions brought into the workpiece to ensure the coating quality. During the operation of this system, it should be noted that the system should operate continuously after operation, and intermittent operation is strictly prohibited to prevent the ultrafiltration membrane from drying up. The dried resin and pigment are attached to the ultrafiltration membrane and cannot be cleaned completely, which will seriously affect the water permeability and service life of the ultrafiltration membrane. The effluent rate of ultrafiltration membrane decreases with the operation time. It should be cleaned once for 30 ~ 40 days to ensure the ultrafiltration water required for ultrafiltration leaching and flushing.
- The electrophoretic coating method is suitable for the production process of a large number of assembly lines. The renewal cycle of electrophoresis tank solution shall be within 3 months. Taking an electrophoresis production line with an annual output of 300000 steel rings as an example, the scientific management of tank liquid is very important. Various parameters of tank liquid are tested regularly, and the tank liquid is adjusted and replaced according to the test results. Generally, the parameters of tank liquid are measured according to the following frequency:PH value, solid content and conductivity of electrophoresis solution, ultrafiltration filtrate and ultrafiltration cleaning solution, cathode (anode) solution, circulating cleaning solution and deionized cleaning solution once a day; Color base ratio, organic solvent content and laboratory small tank test twice a week.
- For the management of paint film quality, the uniformity and thickness of the film should be checked frequently. The appearance should be free of pinholes, sagging, orange peel, wrinkles and other phenomena. The physical and chemical indexes such as adhesion and corrosion resistance of the film should be checked regularly. The inspection cycle is based on the inspection standard of the manufacturer. Generally, each batch needs to be tested.
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