E-Coating of surface treatment

What is E-Coating?

E-Coating is the electrophoretic paint in the Yin and Yang poles, applied under the action of voltage, the charged paint ions move to the cathode, and the alkaline reaction generated by the cathode surface to form an insoluble substance, deposited on the surface of the workpiece.
The E-Coating process is divided into anode E-Coating and cathode E-Coating. If the coating particles are negatively charged and the workpiece is an anode, the coating particles are deposited into a film on the workpiece under the action of electric field force, which is called anode E-Coating. Conversely, if the paint particles are positively charged, the workpiece is a cathode, and the coating particles are deposited on the workpiece into a film called cathode E-Coating.

Principle of E-Coating

The principle of E-Coating is similar to electroplating. The workpiece is placed in the electrolyte, and another electrode in the electrolyte is connected to both ends of the DC power supply to form an electrolytic circuit. The electrolyte is a conductive water-soluble or water-emulsified coating. The dissociated cations in the coating solution move to the cathode under the action of electric field force, and the anions move to the anode.E-Coating along with adsorbed pigment particles onto the surface of the workpiece and loses its charge to form a wet coating, a process called E-Coating.

Process flow

Pickling – washing – pre-degreasing – degreasing – washing – phosphating – washing – washing – ultrasonic cleaning – pure water washing – electrophoretic spray washing – water washing – pure water washing – air drying – baking

Features are as follows

• The use of water-soluble paint, environmental protection.

• High coating efficiency, small paint loss.

• Uniform film thickness, strong adhesion, good coating quality, high production efficiency.

• E-Coating coating equipment is complex, high scientific and technological content, suitable for color fixed production equipment is complex, high investment E-Coating used in automobiles, home appliances, hardware industry, light industry, furniture industry, mechanical and electrical industry.

Disadvantages of E-Coating

• Large equipment, investment in addition to complete pre-treatment equipment, but also need E-Coating tank and the corresponding ancillary equipment, ultrafiltration device and pure water preparation facilities, special DC power supply.

• Applicable coatings and varieties are limited At present, electrophoretic coatings are limited to water-soluble paint and water-emulsified paint colors are limited to dark primer or single-layer bottom dual-use paint, and white and light colors are difficult. The reason for this is that the ionized iron ions and resin anions in the electrophoretic process (such as anodic electrophoretic deposition) are neutralized and deposited on the workpiece to become yellowish brown.

• The electrophoretic paint film needs to be baked after the general baking process is 150’℃, 1h, so the energy consumption is large.

Process anomaly and analysis

★Graininess

Phenomena

• On the surface of the electrophoretic coating after drying, there are rough, hard particles, or fine prickly heat visible to the naked eye, often the horizontal plane of the coating is more serious than the vertical plane, and this paint film is called particles.

Cause

• The PH value of DCED tank is high, and alkaline substances are mixed, resulting in instability of tank, and resin precipitation or condensation.

• There are precipitated “dead corners” and bare metal in the tank.

• After E-Coating, the cleaning solution is dirty, the paint content is too high, and the filtration is poor.

• The entering coated surface and spreader are unclean, and the washing after phosphating is poor.

• In the drying process on the granular dirt.

• The painting environment is dirty.

• The make-up paint or resin dissolves poorly and has particles.

Prevention and treatment methods

• The PH value of the CED tank is controlled at the lower limit, alkaline substances are strictly prohibited, filtration is strengthened, and the renewal of the tank is accelerated.

• Eliminate the “dead Angle” that is easy to precipitate and the bare metal parts that produce the deposition film.

• Strengthen filtration, it is recommended to use a filtration element with a precision of 25um to feed the foam.

• To ensure that the coated surface is clean, there should be no phosphating sludge, to prevent secondary pollution.

• Clean the drying chamber and air filter.

• Keep the painting environment clean, check and eliminate the dust source of the air.

• Ensure that the new coating dissolves well and the fineness of the color paste is within the standard range.

★Shrinkage holes (pits)

Phenomena

Can not be seen on the wet electrophoretic coating film, when dried, the surface of the paint film appears crater-like pits, the diameter is usually 0.5-3.0mm, do not expose the bottom called holes, depressions, exposed bottom called shrinkage holes, particles in the middle called “fish eyes”.
The main reason for this defect is that there is dust in or on the surface of the electrophoretic wet coating film, and the particles that are insoluble with the oil and electrophoretic coating become the center of the pit, which makes the flow ability of the wet paint film in the early drying unbalanced, and the coating film defects.

Cause

• Poor degreasing before being coated or oil and dust falling after cleaning.

• The tank is mixed with oil, floating on the liquid level or emulsifying in the tank.

• The flushing solution after E-Coating is mixed with oil.

• The drying room is not clean, and the circulating air contains oil.

• The face base ratio of the tank liquid is out of proportion, and the low pigment content is easy to produce shrinkage holes.

• The painting environment is dirty, and the air may contain oil mist, paint mist, and organosilicone substances that pollute the coated material or wet coated film.

• The make-up paint has shrinkage holes or the resin is poorly dissolved, and the neutralization is not good.

Prevention and treatment methods

• Strengthen the degreasing process of the coated material to ensure that the phosphating film is not secondary pollution.

• Set oil removal filter bags in the tank liquid circulation system, and check the source of oil pollution, strictly prohibit oil pollution into the tank.

• Improve the cleaning water quality and strengthen filtration.

• Keep the drying room and circulating hot air clean

• Adjust the face base ratio of the tank liquid, and appropriately add color paste to improve the pigment content.

• Keep the painting environment clean, remove harmful material sources for painting, especially silicone material sources.

• Strengthen the management of the make-up paint, unqualified can not be added to the tank, to ensure that the make-up paint dissolves, neutralizes and filters well.

★Pinholes

Phenomena

Electrophoretic coating film in the drying after the production of needle-like small pits or holes, this film disease is called pinholes, it is different from the shrinkage hole (pit) is small aperture, no foreign matter in the center, no paint film accumulation around the bulge. According to the cause of the pinhole there are the following types:

• Pinholes caused by the redissolution resistance of the wet coating film are called redissolution pinholes.

• In the process of E-Coating, due to the intense electrolytic reaction, too many bubbles are produced, poor defoaming, or due to the low temperature of the bath or insufficient agitation, resulting in the paint film wrapped in the drying process of the bubble burst and the pinhole is called the bubble pinhole.

• Charged into the groove step pinhole: occurs in the case of serious defects of the charged into the groove step, the pinhole is exposed along the groove slope, in addition, due to the poor wetting of the groove on the surface of the object, so that some bubbles are closed in the paint film or the foam on the surface of the groove is attached to the surface of the workpiece to form bubble pinholes, generally produced in the lower part of the coated material.

Cause

• After electrophoretic coating, the coated material is not cleaned out of the tank in time, and the wet coated film will dissolve again.

• The content of impurity ions in the tank is too high, the electrolytic reaction is severe, and too much gas is produced on the surface of the coated material.

• The phosphating film has high porosity and is easy to contain bubbles.

• The temperature of the bath is low or the stirring is not sufficient, so that the wet film defoaming is poor,

• When the workpiece is charged into the slot, the transport chain speed is too slow.

• The flow rate of the coated material into the tank surface at the end of the tank is low, and there is foam accumulation.

Prevention and treatment methods

• The coated material should be washed with UF liquid or pure water immediately after leaving the tank, and the time should not exceed 1min.

• Discharge UF liquid and add pure water to reduce the content of impurity ions.

• Adjust the phosphating formula and process to make the phosphating film crystallized and densified.

• Strengthen the tank stirring to ensure that the tank temperature is operated at 28~30℃.

• In the case of too slow chain speed, it is not appropriate to choose the electrophoretic coating process of charging into the groove, and use the power after entering the groove.

• Make the tank liquid surface flow rate greater than 0.2m/s to eliminate accumulated foam.

The difference between E-Coating and electroplating

Electroplating: The process of using electrolysis to attach a metal film to the surface of a metal or other material.
E-Coating is a coating method in which particles such as pigment and resin suspended in the electrophoretic solution are transferred and deposited on the substrate surface of one of the electrodes by an applied electric field.