Powder electrostatic spraying process and common problem solving

First, the principle
During the working, the spray gun or the spray cup of the electrostatic spray is connected to the negative pole, and the workpiece is connected to the positive pole and grounded. Under the high voltage of the high voltage electrostatic generator, the end of the spray gun (or the spray cup, the spray cup) and the workpiece are formed. An electrostatic field. The electric field force of the paint particles is proportional to the voltage of the electrostatic field and the charge of the paint particles, and inversely proportional to the distance between the spray gun and the workpiece. When the voltage is high enough, the air ionization zone is formed in the vicinity of the end of the spray gun. Intense ionization and heat generation, forming a dark red halo around the sharp edge of the gun or around the pole, which is clearly visible in the dark, when the air produces a strong corona discharge. Most of the film-forming materials in the paint, such as resins and pigments, are composed of high molecular organic compounds, which are mostly conductive dielectrics. In addition to film-forming materials, solvent-based coatings also contain organic solvents, cosolvents, curing agents, and electrostatic thinners. And other types of additives and other substances. Such solvent-based substances, besides benzene, xylene, solvent gasoline, etc., are mostly polar substances, have low electrical resistivity and have a certain electrical conductivity, and they can improve the charging performance of the coating. The molecular structure of the dielectric can be divided into two types: polar molecules and non-polar molecules. The dielectric composed of polar molecules exhibits electrical properties when subjected to an applied electric field; the dielectric composed of non-polar molecules exhibits an electrode property under the action of an applied electric field, thereby generating an affinity for the external conductive charge, and the dielectric is added. The outer surface of the electric field can be locally charged. The paint is sprayed out after being atomized by the nozzle, and the atomized paint particles are charged by the contact of the needle or the spray plate of the muzzle and the edge of the spray cup. When the gas ionization zone generated by the corona discharge is passed, Increase its surface charge density once. Under the action of the electrostatic field of these negatively charged coating particles, the surface of the workpiece with the polarity of the guide moves and is deposited on the surface of the workpiece to form a uniform coating film.
Second, the process
1
Surface pretreatment
Mainly degreasing, derusting, the method is the same as the pretreatment of liquid paint.
2
Putty putty
The conductive putty is applied according to the degree of defect of the workpiece, and after drying, it is smoothed with a sandpaper to perform the next process.
3
Protection (also called cover)
If some parts of the workpiece are not required to be coated, they can be covered with protective glue before preheating to avoid spraying paint.
4
Preheating
Generally no preheating is required. If the coating is required to be thick, the workpiece can be preheated to 180 to 20 ° C, which can increase the coating thickness.
5
Spraying
In the high-voltage electrostatic field, the powder gun is connected to the negative pole, and the workpiece is grounded (positive) to form a loop. The powder is sprayed by the spray gun with compressed air, which has a negative charge, and is sprayed onto the workpiece according to the principle of opposite-phase attraction.
6
Curing
The sprayed workpiece is heated in a drying room at 180 to 200 ° C to solidify the powder.
7
Clean up
After the coating is cured, remove the protector and smooth the burrs.
8
test
Check the coating of the workpiece. Any leaks, bumps, needles and other defects should be reworked.

9
Defect handling
Rework or re-injection of the detected workpiece with missing spray, pinhole, bump, or air bubble.
Third, the application
The uniformity, gloss and adhesion of the paint layer on the surface of the workpiece sprayed by electrostatic spraying are better than ordinary hand spray paint. At the same time, electrostatic paint can be sprayed by ordinary paint, oily and magnetic blending paint, perchloroethylene paint, amino resin paint, epoxy paint, etc. It is easy to operate and saves about 50% of paint compared with general air spray.
It is usually required to have a high air pressure, a fine lacquer and a fast speed. However, if the air pressure is too high, it will destroy the power. The appropriate paint pressure and air pressure should be selected according to the type of paint coating used, the painting site and the workpiece to be coated. If the paint contains higher heavy pigments, higher paint pressure and air pressure can be used; otherwise, paint pressure and air pressure can be reduced. Under normal circumstances, the paint pressure is 0.12 ~ 0.24MPa, and the atomization pressure is 0.15 ~ 0.20MPa.
The world’s first powder electrostatic spraying equipment was successfully developed by France’s SAMES in 1962. Since then, powder electrostatic spraying technology has developed rapidly in various countries around the world, and is gradually replacing solvent-based coating technology. China’s powder electrostatic spraying technology has developed late, but its development potential is very large. The powder coating contains no solvent. The powder coating relies on electrostatic spraying onto the surface of the workpiece. The layer of powder particles that are not adhered to each other is heated and melted to form a firm coating that is tightly bonded to the surface of the workpiece. This coating has excellent corrosion protection and decorative features. Compared with traditional solvent-based coatings, it has the advantages of safer, less polluting, good adaptability, high efficiency and no dependence on petroleum. However, it currently has some shortcomings: large one-time investment, inconvenient color replacement, etc.
Fourth, the typical process flow of powder electrostatic spraying technology

Pre-treatment of workpieces→dusting→cure→inspection→finished products
1.1 pre-processing
The workpiece can be sprayed after removing the oil and dust on the surface of the cold-rolled steel sheet, and a zinc-based phosphating film is formed on the surface of the workpiece to enhance the adhesion after dusting. The pre-treated workpiece must be completely dried and thoroughly cooled to below 35 °C to ensure the physical and chemical properties and appearance quality of the workpiece after dusting.
1.2 Spraying powder
1.2.1 Basic principles of powder electrostatic spraying
The workpiece enters the spray gun position of the spray booth through the conveyor chain to prepare for the spraying operation. The electrostatic generator releases high-voltage static electricity (negative electrode) through the electrode needle of the gun nozzle to the space in the direction of the workpiece, and the high-voltage static electricity makes the slave. The mixture of powder and compressed air ejected from the lance and the air around the electrode are ionized (negatively charged). The workpiece passes through the conveyor through the conveyor link (grounding pole), so that an electric field is formed between the spray gun and the workpiece, and the powder reaches the surface of the workpiece under the double push of the electric field force and the compressed air pressure, and forms a layer on the surface of the workpiece by electrostatic attraction. Uniform coating.
1.2.2 Basic raw materials for powder electrostatic spraying
Use indoor epoxy polyester powder coating. Its main components are epoxy resin, polyester resin, curing agent, pigment, filler, various additives (such as leveling agent, moisture-proof agent, corner modifier, etc.). After the powder is heated and solidified, it forms on the surface of the workpiece. Need to be coated. The auxiliary material is compressed air, which requires clean and dry, oil-free and water-free [water content less than 1.3g/m3, oil content less than 1.0×10-5% (mass fraction)]
1.2.3 Construction process of powder electrostatic spraying
● Electrostatic high voltage 60-90kV. Excessive voltage can easily cause powder rebound and edge pitting; when the voltage is too low, the powder rate is low.
● The electrostatic current is 10 to 20 μA. Excessive current is easy to produce discharge breakdown powder coating; low current and low powder rate
● Flow rate pressure 0.30-0.55MPa. The more the flow rate pressure. The higher the deposition rate of the powder, the faster the coating of the predetermined thickness is obtained, but too high increases the amount of powder and the wear rate of the spray gun.
● The atomization pressure is 0.30 to 0.45 MPa. Properly increasing the atomization pressure keeps the thickness of the powder coating uniform, but too high will cause the powder feeding parts to wear quickly. Properly reducing the atomization pressure can increase the coverage of the powder, but too low can easily block the powder feeding member.
● The gun pressure is 0.5 MPa. If the gun pressure is too high, the gun head will be accelerated. If it is too low, the gun head will be blocked.
● The fluidization pressure of the powder supply tank is 0.04~0.10MPa. If the fluidization pressure of the powder supply bucket is too high, the density of the powder will be lowered to lower the production efficiency, and if it is too low, the powder supply is insufficient or the powder is agglomerated.
●The distance from the gun mouth to the workpiece is 150~300mm. The distance from the lance to the workpiece is too close to cause a breakdown of the powder coating, which increases the amount of powder and reduces production efficiency.
● The conveyor chain speed is 4.5 to 5.5 m/min. Too fast a conveyor chain can cause insufficient thickness of the powder coating, and too slow to reduce production efficiency.
1.2.4 Main equipment for powder electrostatic spraying
● spray gun and electrostatic controller
In addition to the conventional built-in electrode needle, the spray gun is provided with a ring corona on the outside to make the electrostatic field more uniform to keep the thickness of the powder coating uniform. The electrostatic controller generates the required electrostatic high voltage and maintains it stable with a range of less than 10%.
● Powder supply system
The powder supply system consists of a new powder bucket, a rotary sieve and a powder supply bucket. The powder coating is first added to the new powder drum, and the compressed air is pre-fluidized through the micropores on the fluidized plate at the bottom of the new powder drum, and then transferred to the rotary sieve through the powder pump. The powder particles (100 μm or more) having an excessively large particle diameter were separated by a rotary sieve, and the remaining powder was dropped to the powder supply tank. The powder supply tank fluidizes the powder to a prescribed level, and then supplies the spray gun to the spray gun through the powder pump and the powder feeding tube.
●Recycling system
The powder sprayed from the spray gun is partially deposited on the surface of the workpiece (generally 50% to 70%, 70% of the company). A part of the powder in the sedimentation process is collected by a cyclone collector on the side wall of the dust shed, and the powder particles having a larger particle size (above 12 μm) are separated by a centrifugal separation principle and sent back to the rotary sieve for reuse. Powder particles below 12 pom are sent to the filter recovery unit where the powder is pulsed by compressed air to the bottom of the filter cartridge. This portion of the powder is periodically cleaned and packed for sale. The clean air from which the powder is separated (containing a powder having a particle size of less than 1 μm and a concentration of less than 5 g/m 3 ) is discharged into the dust chamber to maintain a slight negative pressure in the dust chamber. If the negative pressure is too large, it is easy to inhale dust and impurities outside the dusting powder. If the negative pressure is too small or positive pressure, the powder may overflow. The powder settled to the bottom of the dust shed is collected and transferred to the rotary screen through a powder pump for reuse. The mixing ratio of the recycled powder to the new powder is (1:3) to (1:1). Using this recycling system, the company’s overall powder utilization rate reached 95% on average.
●Dusting chamber body
The top and wall panels are made of light-transmissive polypropylene plastic to minimize powder sticking and prevent static charge buildup from interfering with the electrostatic field. The base plate and base are made of stainless steel for easy cleaning and sufficient mechanical strength.
●Auxiliary system
Including air conditioners and dehumidifiers. The role of the air conditioner is to keep the powder temperature below 35 ° C to prevent powder agglomeration; the second is to maintain the micro-negative pressure of the powder chamber through air circulation (wind speed less than 0.3 m / s). The function of the dehumidifier is to maintain the relative humidity of the powder spray chamber from 45% to 55%. If the humidity is too high, the air is prone to discharge breakdown powder coating, and the small conductivity is poor and it is difficult to ionize.