What is electrostatic spraying & How it work?

Electrostatic spraying is a kind of coating method which uses corona discharge principle to make atomized coating negatively charged under the action of high-voltage direct current electric field and adsorb on the surface of positively charged substrate for discharge. Electrostatic spraying equipment is composed of spray gun, spray cup and high voltage power supply.

How it works

Electrostatic spraying is based on the principle of electrostatic attraction, that is, the grounded coating material is used as the positive electrode, and the fuel atomizer (i.e. spray cup or spray plate) is connected with high voltage electricity as the negative electrode.

The steel strip to be oiled passes through the upper and lower spray beams, which are made of special aluminum alloy and fixed on the adjustable insulating arm. The gap on the fuel injection beam can be adjusted to 0.2mm evenly. There is an oil groove inside the beam. The oil inlet pipe is connected to the middle part, and two oil return pipes are connected at both ends. There is a certain distance between the upper and lower fuel injection beams and the strip steel, and the fuel injection beams are designed to flow oil to the fuel injection port under zero pressure. When the fuel injection beam is electrified, a high-voltage electrostatic field is generated between the fuel injection beam and the grounded strip steel, and the corona discharge is generated at the negative electrode, which makes the spray oil mist negatively charged. Under the action of electrostatic field, the charged particles can be absorbed to the steel strip efficiently and evenly along the power line direction, showing a unique "encircling effect". However, it is not easy to produce corona at the positive electrode.

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The effect of electrostatic spraying not only depends on the equipment and management level of electrostatic spraying, but also depends on whether the type of coating used meets the requirements of electrostatic coating. In electrostatic coating, the resistivity of the coating is required to be low, and its value is 5-50m Ω· cm. The resistivity of the coating is related to not only the resin in the coating, but also the solvent composition. Therefore, in the actual coating. It is often used to adjust the resistivity by adding solvent.

The resistivity of coatings for electrostatic spraying is low, but the resistivity of common coatings is relatively high. In order to reduce the resistivity of coatings, solvents with appropriate polarity are often added to the coatings.

The wind speed in the electrostatic spraying room is also important. The exhaust air in the spraying room is mainly used to eliminate the solvent vapor generated in the spraying process, so that the content of the solvent vapor in the room is below the lower explosion limit concentration of organic solvent, so as to ensure the construction safety. The wind speed of electrostatic spraying room should be controlled at 0.3 ~ 0.7m/s, and the spraying effect will be affected if the wind speed is too high. The wind speed regulating mechanism should be set in the exhaust device.

Type editing

Electrostatic spray can be divided into two types: electrostatic spray and electrostatic spray.

1) Pure electrostatic atomization

It is represented by rotary cup type electrostatic spray gun. The rotary spray gun is simple in structure, not easy to block and easy to clean; due to its mechanical centrifugal electro atomization, it has low requirements for the conductivity of coating and solvent (of course, the conductivity is also good); the effective area is large, the adsorption efficiency is high, and the coating uniformity is greatly improved; after atomization, the coating is fine, and the surface is smooth and smooth. It is suitable for workpiece with simple shape.

Its defect is that the sprayed coating has a central hole, and it is difficult to spray on the workpiece with complex shape, and the coating is uneven or the pit can not be sprayed; due to the different charging ability characteristics of various pigments, the phenomenon of uneven color will appear when spraying the coating made of various pigments.

2) Electrostatic atomization with additional energy

According to the types of additional energy, it can be divided into two types: air atomization method and hydraulic atomization method. Both whirlwind spray gun and portable electrostatic spray gun belong to air atomization. They atomize the coating with the help of compressed air and electrostatic force, so they can spray objects with complex shape or large area. The three serpentine nozzles on the cyclone spray gun can be adjusted, and it is convenient to change the coating diameter. It can reduce or even eliminate the phenomenon of coating center hole, and obtain more uniform coating easily. However, due to the volatile solvent in the process of air atomization, the coating film is easy to produce orange peel and other defects, so the requirements for the solution are very high, such as low viscosity and high solid content, good covering power, slow solvent evaporation speed and good flow performance. In addition, due to the pressure flow during air atomization, the charged paint particles can not be coated on the workpiece, thus increasing the loss of coating.

Hydraulic atomization is a combination of high pressure airless spraying and electrostatic spraying equipment. With the help of pressure, the paint liquid is pressurized to a higher pressure range, and then it is ejected from the nozzle hole. The coating under high pressure is sprayed into the atmosphere and then rapidly expands and atomizes.

Compared with air atomization, hydraulic atomization has better atomization state, larger spray volume and higher coating efficiency, while the requirements for coating are similar to air atomization. [3]

Feature editor

(1) The construction environment and working conditions are good. The fixed electrostatic spraying equipment, which is widely used, is usually matched with suspension conveyor to form a continuous painting production line. In this case, the operator's work is limited to the preparation, loading and unloading of the finishing workpiece, as well as the regulation and care of the equipment. Therefore, the chance of direct contact with the coating and the intensity of physical labor are greatly reduced. Moreover, when spraying in high-voltage electrostatic field, the diffusion of paint mist is far less than that of air pressure spraying or airless spraying, so that the pollution of finishing environment is obviously improved.

(2) The utilization rate of coating is high. In the high-voltage electrostatic field spraying, the coating particles with negative charge are painted on the surface of the workpiece along the direction of the power line. Therefore, there is basically no phenomenon of coating jet rebound and paint mist flying, the loss of pain

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