Electrostatic powder spraying is an advanced coating technology, which atomizes powder coating fine and even under the action of compressed air, adsorbs it on the surface of metal workpiece uniformly by virtue of the action of high-voltage electric field and solidifies it into film at high temperature. It was first developed by France's sames company in 1962. In 1965, with the introduction of a complete set of electrostatic powder spraying equipment in Britain, France, Germany, the United States, Japan and other countries, it has been widely used in industrial production.
Powder electrostatic spraying belongs to post-treatment of coating, which must be carried out after the workpiece has been strictly pre-treatment (oil removal, rust removal and phosphating). The surface of the workpiece is free of oil, rust, flying dust and dust hanging, and there is absolutely no high temperature (180-200 ℃) decomposition products. Sand blasting or zinc phosphating (the effect of iron phosphating is very poor) shall be used as far as possible. After oil removal and rust removal is not allowed, the electrostatic powder spraying shall be directly used for passivation and rust prevention, such as nitrite, which is easy to cause poor adhesion and large area falling off of the coating. This has been a painful lesson in many manufacturers.
The main equipment required for electrostatic powder spraying are:
(1) spraying host (including a set of high-pressure generator, spray gun, powder supply barrel and control system);
2. Spray chamber
(3) recovery device
(4) conveying system (including conveying chain and freight car, etc.)
(5) curing device (including automatic temperature control device and ventilation device)
(6) air compressor (including oil-water separation system).
The main functions and important parameters of each equipment are briefly introduced as follows:
1. Spraying host
Some of the main spraying machines used in the market are decentralized and some are combined. No matter what method is adopted, the function is the same: to make the powder coating atomized evenly and adsorbed on the surface of the metal workpiece. The main function of the high-voltage electrostatic generator is to generate high-voltage charge, generate potential difference with the workpiece with zero potential, and form the main power of powder coating particle adsorption. Generally, the input voltage of the high-voltage generator is 220V (or 24V or 36V) and the output voltage can be as high as 50-100kv after multiple high-frequency oscillations and voltage doubling amplification, but the output current under normal conditions (non short circuit state) is only 10-20ua, which has no damage to human body. In normal operation, adjust the high pressure of the spray gun to 45-50kv to make the powder coating well adsorbed (the suspension device of the workpiece should be well grounded, R ≤ 4 Ohm). If the second supplementary spraying, the voltage can be adjusted to 60-70kv (or spraying after preheating the workpiece), otherwise it is not easy to powder.
The spray gun is the key part for powder coating to arrive at the workpiece from the powder supply barrel. The gun body must have good insulation performance. It is easy to keep the distance between the workpiece and the nozzle between 120-180mm when working. According to different workpieces, different powder spray diffusers shall be selected and fixed on the gun head to achieve uniform atomization. If it is too close to the workpiece, it is easy to short-circuit and ignite, resulting in the breakdown point on the coating surface, affecting the coating effect; if it is too far away, it is not easy to absorb the powder. During spraying, it is necessary to keep the gun perpendicular to the workpiece plane, with uniform forward and backward movement speed of about 0.1 M / s. there is no gap between the upper and lower spacing, but spraying and missing spraying are not allowed. The powder conveying pipeline is generally 3-5M long, and the high-voltage cable shall be well insulated and shall not butt joint with each other. Operators shall wear conductive shoes during the production process, and it is forbidden to pad insulating board for operation.
The powder supply barrel is the main equipment that affects the powder output and atomization in the process of powder spraying. Generally, fluidized boiling powder supply bucket is used in the market. Before production, powder coating should be loaded to two-thirds of the bucket volume. Adjust the boiling air pressure (also known as fluidized air pressure) to 0.05-0.08mpa, adjust the air pressure of powder supply to 0.08-0.12mpa, so that the powder supply is generally between 80-150g / min (depending on the proficiency of workers and the shape of workpieces and the difficulty of adsorption). If the recycled powder is added to the powder supply barrel, it must be added after passing 180 mesh sieve. It is easy to mix one recycled part and three new parts. When the production is stopped, the powder coating in the powder supply barrel must be cleaned to prevent the powder from being damped or the microporous plate from being blocked by moisture. The control system mainly includes solenoid valve and pressure reducing valve, which controls the powder supply and air on and off through the handle switch.
The spray chamber is the main equipment for the workpiece to receive the surface coating inside. It is required that the height and opening of the spray chamber are easy for the workpiece to enter and exit and the operation of the production workers. The less the opening, the better.
3. Recovery device
At present, there are generally two kinds of recovery devices on the market, the old traditional recovery is divided into one-stage recovery and two-stage recovery; the first stage recovery is the cyclone recovery, which is recovered by the cyclone dust collector, leaving a large amount of recovered powder at the bottom, and discharging the air containing the powder into the secondary recovery device. The secondary recovery device is composed of a filter bag and a rapping device. The air is discharged through the wall of the filter bag, and the ultra-fine powder (only a small amount, accounting for about 2-5%) is left at the bottom of the filter bag. The old recycling device has the disadvantages of large area, difficult cleaning and powder changing.
The new recovery device is improved on the basis of the old one. Return the secondary recovery and primary recovery to use, let the air containing the powder be pumped to the filter bag or core wall, take the air out and leave the recovered powder (left in the spray room), form the air flow back blowing by the back pulse device every two to three minutes and complete it in a few seconds, shake the powder adsorbed on the filter bag or filter element into the spray room, and then repeat its initial working state. When pulse back blowing, several filter elements are alternately carried out to prevent dust from overflowing. At present, most of the filter elements are made of paper, coated with organic resin, with smooth surface, not easy to stick powder, strong filter paper, good permeability and metal mesh protection, which can be used for a long time.
No matter what method is adopted for recycling, it is necessary to ensure that the indoor dust does not overflow. Generally, a negative pressure of about 0.05-0.09mpa should be formed in the spray chamber, and the air flow rate at the opening of the spray chamber should be controlled at 0.5-0.6m/s to achieve this purpose.
Contact Person: Mr. Don