The electrophoretic coating and the corresponding electrode workpiece into water soluble paint, after the power is switched on, depending on the physical and chemical effects produced by the electric field, the resin, pigments and fillers in paint to be painted as a method of coating on the surface of the electrode uniform deposition to form insoluble film. Electrophoretic coating is a very complex electrochemical reaction process, which includes at least four processes: electrophoretic, electrodeposition, electroosmosis and electrolysis. According to the performance of electrophoretic coating deposition can be divided into anode electrophoresis (workpiece is anode coating is anionic) and cathode electrophoresis (workpiece is cathode coating is cationic); according to the power supply can be divided into DC and AC electrophoresis electrophoresis; according to the process and method of constant voltage and constant current method. At present, the anode electrophoretic method of DC power supply is widely used in the industry.
1, the workpiece after surface treatment; 2- power supply; 3- workpiece; 4- water spray rinsing; 5- tank liquid filtration; 6- sedimentation tank; 7- circulating pump
Electrophoretic coating is compared with other coating methods, with the following characteristics:
(1) using water-soluble coatings and water as the dissolution medium saves a lot of organic solvents, greatly reducing air pollution and environmental hazards, safety and hygiene, and avoiding hidden dangers of fire.
(2) the efficiency of coating is high, the loss of coating is small and the utilization rate of the coating can reach 90% ~ 95%.
(3) the coating thickness is uniform, the adhesion force is strong, and the coating quality is good. All parts of the workpiece, such as inner layer, sag and weld seam, can get uniform and smooth paint film, which solves the problem of other painting methods for coating complex shaped workpieces.
(4) high production efficiency, the construction can achieve automatic continuous production, greatly improve the efficiency of labor;
(5) the equipment is complex, the investment cost is high, the power consumption is large, the temperature required for drying and curing is higher, the management of coating and coating is complex, the construction conditions are strict, and the wastewater treatment is needed.
(6) only water soluble coatings can be used, and the color can not be changed during the coating process. The stability of the coating is not easy to be controlled for long storage.
1. Electrophoretic coating equipment
The equipment of electrophoresis coating is made up of electrophoresis bath, stirring device, coating filter, temperature adjusting device, coating management device, DC power supply device, water washing device after electrophoresis, ultrafiltration device, baking device, standby tank and so on.
The size and shape of the electrophoretic slot body should be determined according to the size, shape and construction process of the workpiece.  Under the condition of ensuring a certain distance between the poles, a little design should be made as far as possible. The tank is equipped with a filter and a temperature adjusting device to ensure a certain temperature of the lacquer and remove impurities and bubbles in the lacquer liquid. The mixing device can keep the paint liquid uniform and uniform, and the circulating pump is used. The circulation of the paint liquid is usually 4~6 times per hour. When the circulating pump is started, the liquid level of the paint should be turned evenly. The function of the coating management device is to supplement and adjust the coating composition, control the pH value of the bath, remove the neutralizer with the diaphragm electrode, and remove the low molecular weight components with the ultrafiltration device. The selection of electrophoretic power supply is generally used as DC power supply. The rectifier device may use a silicon rectifier or a silicon controlled rectifier. The size of the current is related to the properties of the coatings, the temperature, the working area, the mode of electricity, and so on. It is usually 30 ~ 50A/m2. The washing device is used for the washing of the workpiece before and after the electrophoretic coating. Generally, the deionized water is used, but the pressure equipment is needed, and the commonly used is a kind of leached nozzle with a spiral body. The baking device is used to promote the drying and forming of electrophoretic coatings, and can be used in resistance furnace, induction heating furnace and infrared baking equipment. The design of the drying room should have three stages of preheating, heating and post heat, which should be made according to the situation of the variety and the work piece of the paint.
Two. The main technological parameters affecting the electrophoretic coating
1. Voltage
The electrophoretic coating is based on the constant voltage method, and the equipment is relatively simple and easy to control. The influence of voltage on the film is great; the higher the voltage, the thicker the electrophoretic film is, the coating capacity can be improved and the construction time can be shortened accordingly. But the high voltage will cause the rough surface of the film, and the phenomenon of "orange peel" is easily produced after drying. Low voltage, slow reaction of electrolysis, thin and uniform paint film, poor swimmers. The selection of voltage is determined by the type of coating and the requirement of construction. In general, the voltage is inversely proportional to the paint's solid and paint temperature, which is proportional to the distance between the two poles. The surface of iron and steel is 40 ~ 70V, the surface of aluminum and aluminum alloy can be 60 ~ 100V, and the galvanized parts are 70 ~ 85V.
2. Electrophoretic time
The thickness of paint film increased with the prolongation of electrophoresis time, but when the film reached a certain thickness, it could not increase the thickness and increase the side effect. Otherwise, the electrophoresis time was too short, and the coating was too thin. The electrophoretic time should be based on the voltage used, and the shorter the better when the quality of the coating is guaranteed. The general workpiece electrophoresis time is 1 to 3 minutes, and the large workpieces are 3 to 4 minutes. If the surface geometry of the coated object is complex, the voltage and time can be increased properly.
3. Coating temperature
The coating temperature is high, the film forming speed is fast, but the appearance of the paint film is rough, and it will cause the coating to deteriorate. The temperature is low, the amount of electrodeposition is low, the film is slow, and the coating film is thin and dense. In the process of construction, because the electrical energy is converted into heat, the mechanical friction in the circulation system produces heat, which will lead to the rise of the coating temperature. The temperature control of the general lacquer liquid is 15~30 degrees centigrade in some respects.
4. The solid fraction and pigment base ratio of the coatings
The solid content of the electrophoretic coatings sold in the market is about 50%. In construction, the solids of the coatings are controlled in 10% to 15% with distilled water. The solid content is too low, the covering power of the film is not good, the pigment is easy to precipitate, and the stability of the coating is poor. High solids, higher viscosity