Powder electrostatic spraying equipment is composed of powder supply device, electrostatic generator, electrostatic spraying machine, spraying room, powder recovery system, drying room and so on.
The output voltage of the high-voltage electrostatic generator should reach 60~100kv, and the current should be lower than 300µA. Generally, transistors have low energy consumption and small size, and should have anti-breakdown safety protection devices.
The electrostatic powder Spray Gun is divided into fixed type and portable type. The fixed type is used in the production line, and the portable type is used for on-site construction. Electrostatic powder Spray Guns are divided into internal charging and external charging according to the charging form (Figure 2_2_18). Internal electrification is charged by the corona discharge between the pole needle and the ring electrode in the gun body. The internal electric field strength is large (6~8kv/cm), which is suitable for coating large amount of powder and complex shape workpieces. External charging is charged by corona discharge between the Spray Gun and the workpiece. The electric field strength of the charging is weaker than that of the internal charging, but not only the electric field strength is large (1~3.5kv/cm), the coating efficiency is high and the application is wide.
In order to effectively coat according to the size and shape of the workpiece and reduce the rebound effect of the powder, the powder diffusion of the electrostatic powder Spray Gun generally includes the impact dispersion method, the air dispersion method, the rotation dispersion method and the agitation dispersion method, etc. (Fig. 2_2_19). The impact dispersion method is easy to operate and widely used.
The powder feeder should continuously and evenly deliver the powder gun powder to the Spray Gun. Generally, there are three types of powder feeders: pressure type, suction type and mechanical type (Figure 2_2_20). The pressure-type powder supply container has a capacity of more than 15~25L, and the powder cannot be fed continuously. It is mostly used for powder supply with a portable electrostatic powder Spray Gun, and is not suitable for automatic production lines. The mechanical powder feeder can accurately and quantitatively supply powder, and is mostly used in continuous production lines. The suction type uses the Venturi principle, so that the powder in the powder hopper is sucked by the air flow to form a powder air flow. Not only is there less powder in the powder hopper, it is easy to clean and change colors, and it has strong adaptability.
Powder recovery equipment is used to recover unadhered powder and prevent powder production from polluting the environment. Generally, the powder adhesion rate of powder electrostatic removal is only 0%-5%, and the powder removal and utilization rate must be recovered by recycling. More than 95%, improve economic benefits, recycling equipment has energy-wind type, bag type and their combination (National 222, energy-wind type is louder, energy consumption is large, and the recycling rate is not high; bag type 1 is small, sound Small size, high recycling rate, but vibration or counter-airflow measures are required to prevent bag clogging. The more professional one is the core-type color-changing chamber, which can achieve rapid color change by replacing the core.
The influencing factors of powder electrostatic spraying process mainly include powder characteristics, spraying voltage and distance height, powder supply air pressure, etc.
Powder properties are mainly powder particle size and powder conductivity. The finer the particle size of the powder, the worse the fluidity of the powder, and it is easy to block in the equipment. The coatability of the powder is improved and can be thinly coated, but the drift of the dust is also increased. The electrical conductivity of the powder coating affects the charging rate and adhesion rate of the powder, and the volume resistivity is generally in the range of 10 10 °~10 14 Ω·cm.
The spraying voltage is generally 60~90kV, and the spraying distance is preferably about 250mm, and the powder adhesion rate is high at this time. The powder supply air pressure affects the charging rate and drift of the powder airflow. With the increase of the powder supply air pressure, the powder adhesion rate will decrease.
Powder electrostatic spraying requires enhanced surface treatment to ensure coating adhesion. An example of the spraying process of the PTFE coating is as follows: surface roughening by sandblasting of the workpiece → 85°C spray cleaning with degreasing agent → 85°C hot water spray washing → 110°C drying for 5-8 minutes → electrostatic powder spraying → 380°C baking for 30 minutes → water spraying Cool → next piece. Sandblasting on the top is to improve adhesion; 85°C hot water spraying is to speed up drying; due to the high crystallinity of the PT_FE coating, after high-temperature baking and fusion, the crystallinity is reduced by forced cooling to ensure coating adhesion.
In addition to being used to coat protective coatings, powder coating can also be used to coat decorative coatings with artistic patterns, and foreign countries have been conducting application tests of thin-layer powder single-light coatings.
