Principle and process of electrophoretic painting
Principle and process of electrophoretic painting
The principle of electrophoresis is similar to electroplating. The workpiece is placed in the electrolyte, and the other electrode in the electrolyte is respectively connected to both ends of the DC power source to form an electrolytic circuit. The electrolyte is a conductive water-soluble or water-emulsified coating. The dissociated cations in the coating solution move toward the cathode and the anions move to the anode under the action of an electric field force. These charged resin ions are electrophoresed together with the adsorbed pigment particles The surface of the workpiece loses its charge and forms a wet coating. This process is called electrophoresis.
(A) advantages of electrophoresis:
1. High production efficiency. Among various painting methods, electrophoretic painting has the highest production efficiency. As long as the workpiece is immersed in the coating, the electrophoretic painting process can be completed within minutes, so it is suitable for mass production and easy to realize automated production.
2. Good coating quality. As long as the electrophoresis equipment and process are normal, the surface of the electrophoretic paint layer is uniform, the paint film is tight, and the adhesion to the workpiece is good, and defects such as flow marks and unevenness will not occur.
3. Save raw materials. Electrophoretic painting, the material utilization rate can generally reach more than 85%, saving 40% than spray painting. Raw materials can be fully utilized.
4. Good working conditions. Electrophoretic paint electrolyte solvent is water, there is no flammable and explosive problem, and it does not pollute the air, so the working environment is good.
(B) the disadvantages of electrophoresis:
1. Large equipment and investment. In addition to complete pre-treatment equipment, an electrophoresis tank and corresponding ancillary equipment, ultrafiltration devices and pure water preparation facilities, and dedicated DC power supplies are also required.
2. Applicable coatings and varieties are restricted. At present, electrophoretic coatings are limited to water-soluble paints and water-emulsified paints; colors are limited to dark primers or single-layer undercoats, and white and light colors are difficult. The reason is that during the electrophoresis process (such as anodic electrophoretic deposition method), the ionized iron ions and resin anions are neutralized and deposited on the workpiece into a yellow-brown color.
3. Bake after electrophoresis paint film. The general baking process is 150 ° C, 1h, so it consumes large energy.
3. Electrophoresis process:
The electrophoresis process is divided into anodic electrophoresis and cathodic electrophoresis: if the paint particles are negatively charged, the workpiece is an anode, and the paint particles are deposited on the workpiece under the action of an electric field force, which is called anodic electrophoresis; otherwise, if the paint particles are positively charged, the workpiece is a cathode The deposition of coating particles on the workpiece is called cathodic electrophoresis.
The characteristics of anodic electrophoresis are: cheaper raw materials (generally 50% cheaper than cathodic electrophoresis); simpler equipment and less investment (generally 30% cheaper than cathodic electrophoresis); low technical requirements; poor corrosion resistance of coatings compared to cathodic electrophoresis ( (Approximately 1/4 of the life of cathodic electrophoresis).
The reason for the high corrosion resistance of the cathodic electrophoretic coating is that the workpiece is a cathode, no anodic dissolution occurs, and the surface of the workpiece and the phosphating film are not damaged; the electrophoretic coating (generally a nitrogen-containing resin) has a protective effect on the metal and the paint used is expensive High quality.
The general process of anode electrophoresis is:
Workpiece pretreatment (oil removal → hot water washing → rust removal → cold water washing → phosphating → hot water washing → passivation) → anode electrophoresis → workpiece posttreatment (water washing → drying).
1. Remove oil. The solution is generally a hot alkaline chemical degreasing liquid with a temperature of 60 ° C (steam heating) and a time of about 20 minutes.
2. Wash in hot water. The temperature is 60 ° C (steam heating), and the time is 2min.
3. Remove rust. Use H2SO4 or HCl, such as hydrochloric acid rust remover, the total acidity of HCl is ≥ 43 points; the free acidity is> 41 points; add a cleaning agent 1.5%; wash at room temperature for 10-20 minutes.
4. Wash in cold water. Rinse in cold water for 1 minute while flowing.
5. Phosphating. Phosphating at medium temperature (10min at 60 ° C), commercially available finished phosphating solution can be used.
The above steps can also be replaced by sandblasting → washing.
6. Passivation. Use the medicine matching the phosphating solution (provided by the manufacturer of the selling phosphating solution), and it can be 1-2 minutes at room temperature.
7. Anodic electrophoresis. Electrolyte composition: H08-1 black electrophoretic paint, solids mass fraction 9% -12%, distilled water mass fraction 88% -91%. Voltage: (70 ± 10) V; Time: 2 ～ 2.5min; Temperature of paint liquid: 15 ～ 35 ℃; PH value of paint liquid: 8 ～ 8.5. Note that the work piece must be powered off and in and out of the slot. During the electrophoresis process, the current will gradually decrease as the paint film thickens.
8. Wash with water. Wash in running cold water.
9. Drying. It can be dried in an oven at (165 ± 5) ℃ for 40-60 minutes.