Powder Electrostatic Spraying Process

[I] Principle
During operation, the spray gun or spray cup of electrostatic spraying is connected to the negative electrode, and the workpiece is connected to the positive electrode and grounded. Under the high voltage of the high-voltage electrostatic generator, an electrostatic field is formed between the end of the spray gun (or spray plate, spray cup) and the workpiece. The electric field force on 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, an air ionization zone is formed in the area near the end of the spray gun. The air is violently ionized and heated, so that a dark red halo is formed around the sharp edge or pole needle of the spray gun end, which can be clearly seen in the dark. At this time, 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. Solvent-based paints have organic solvents, co-solvents, curing agents, electrostatic diluents, and other additives in addition to film-forming materials. Except for benzene, xylene, solvent gasoline, etc., most of these solvent substances are polar substances with low resistivity and certain conductivity. They can improve the charging performance of the coating.

The molecular structure of dielectrics can be divided into two types: polar molecules and non-polar molecules. Dielectrics composed of polar molecules show electrical properties when subjected to an external electric field; dielectrics composed of non-polar molecules show electrical polarity under the action of an external electric field, thereby generating affinity for external conductive charges, so that the outer surface of the dielectric can be locally charged in the external electric field. The paint is sprayed out after being atomized by the nozzle. When the atomized paint particles pass through the edge of the gun's pole needle or the spray plate or spray cup, they are charged due to contact. When passing through the gas ionization zone generated by corona discharge, their surface charge density will increase again. Under the action of the electrostatic field, these negatively charged paint particles move toward the surface of the conductive polar workpiece and are deposited on the surface of the workpiece to form a uniform coating film.

【II】Process

1. Surface pretreatment: mainly degreasing and rust removal, the method is the same as the pretreatment of liquid paint.
2. Puttying: Apply conductive putty according to the degree of defects on the workpiece, and smooth it with sandpaper after drying, and then proceed to the next process.
3. Protection (also called covering): If some parts of the workpiece do not require coating, they can be covered with protective glue before preheating to avoid spraying paint.
4. Preheating: Generally, preheating is not required. If a thicker coating is required, the workpiece can be preheated to 180-20℃, which can increase the coating thickness.
5. Spraying: In a high-voltage electrostatic field, connect the powder spray gun to the negative electrode, and the workpiece to the ground (positive electrode) to form a circuit. The powder is sprayed out from the spray gun with the help of compressed air (professional various types of spray lines, paint lines, plastic spray lines/powder spray lines, electrophoresis lines, sandblasting robots, spraying robots, sandblasting rooms, shot blasting machines, paint spraying rooms, spraying equipment, surface treatment equipment and exhaust gas treatment equipment manufacturers, long-term supply of various types of sandblasting room shot blasting machine accessories, paint spraying room accessories, dust collector accessories with negative charge, sprayed on the workpiece according to the principle of opposites attracting each other for curing.
6. Curing: After the sprayed workpiece is sent to the drying room at 180-200℃ for heating to solidify the powder.
7. Cleaning: After the coating is cured, remove the protective material and smooth the burrs.
8. Inspection: Check the workpiece coating. Any defects such as missed spraying, bruises, pin bubbles, etc. should be reworked and re-sprayed.
9. Defect treatment: Repair or re-spray the workpieces with defects such as missed spraying, pinholes, bruises, bubbles, etc.

[III] Application
The uniformity, glossiness and adhesion of the paint layer on the surface of the workpiece sprayed by electrostatic spraying are better than those of ordinary manual spraying. At the same time, electrostatic spraying can spray ordinary spray paint, oily and magnetic blended paint, perchlorethylene paint, amino resin paint, epoxy resin paint, etc. It is simple to operate and can save about 50% of paint compared with general air spraying.

Usually high air pressure, fine paint particles and fast speed are required. However, if the air pressure is too high, it will destroy the effect of electricity. The appropriate paint pressure and air pressure should be selected according to the type of paint and coating used, the coating site and the workpiece to be coated. If the paint contains a higher heavy pigment, a higher paint pressure and air pressure can be used; otherwise, the paint pressure and air pressure can be reduced. Under normal circumstances, the paint delivery pressure is 0.12~0.24MPa, and the atomization air pressure is 0.15~0.20MPa.

The world's first set of powder electrostatic spraying equipment was successfully developed by the French SAMES company in 1962. Since then, powder electrostatic spraying technology has developed rapidly in countries around the world and is gradually replacing solvent-based paint coating technology. my country's powder electrostatic spraying technology developed relatively late, but it has great development potential. Powder coating does not contain solvents. Powder coating relies on electrostatic spraying on the surface of the workpiece. The non-sticky powder particle layer is heated and melted to form a firm coating that is closely combined with the surface of the workpiece. This coating has excellent anti-corrosion performance and decorative functions. Compared with traditional solvent-based coatings, it has the advantages of being safer, less polluting, more adaptable, more efficient and not relying on petroleum as raw materials. But it currently also has some disadvantages: large one-time investment, inconvenient color change, etc.

1. Typical process flow of powder electrostatic spraying technology

Workpiece pretreatment → powder spraying → curing → inspection → finished product

1.1 Pretreatment

The workpiece can only be sprayed with powder after the oil and dust on the surface of the cold-rolled steel plate are removed by pretreatment. At the same time, a layer of zinc phosphating film is formed on the surface of the workpiece to enhance the adhesion after powder spraying. The workpiece after pretreatment must be completely dried and fully cooled to below 35°C to ensure the physical and chemical properties and appearance quality of the workpiece after powder spraying.

1.2 Powder spraying

1.2.1 Basic principles of powder electrostatic spraying

The workpiece enters the spray gun position of the powder spraying room through the conveyor chain to prepare for spraying. The electrostatic generator releases high-voltage static electricity (negative electrode) to the space in the direction of the workpiece through the electrode needle at the spray gun nozzle. The high-voltage static electricity ionizes the mixture of powder and compressed air sprayed from the spray gun nozzle and the air around the electrode (negatively charged). The workpiece passes through the hanger and the conveyor link to the ground (grounding electrode), so that an electric field is formed between the spray gun and the workpiece. The powder reaches the surface of the workpiece under the dual push of the electric field force and the compressed air pressure, and forms a uniform coating on the surface of the workpiece by electrostatic attraction.

1.2.2 Basic raw materials for powder electrostatic spraying

Indoor epoxy polyester powder coating is used. 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 cured, the required coating is formed on the surface of the workpiece. The auxiliary material is compressed air, which is required to be clean, dry, oil-free and water-free [water content is less than 1.3g/m3, oil content is less than 1.0×10-5% (mass fraction)]

1.2.3 Construction process of powder electrostatic spraying

• Electrostatic high voltage 60-90kV. Too high voltage can easily cause powder rebound and edge pitting; too low voltage has a low powdering rate.
• Electrostatic current 10~20μA. If the current is too high, it is easy to produce discharge and break through the powder coating; if the current is too low, the powder coating rate is low.
• Flow rate pressure 0.30-0.55MPa. The higher the flow rate pressure, the faster the powder deposition speed, which is conducive to quickly obtaining a coating of a predetermined thickness, but too high will increase the amount of powder used and the wear rate of the spray gun.
• Atomization pressure 0.30~0.45MPa. Properly increasing the atomization pressure can maintain the uniform thickness of the powder coating, but too high will cause rapid wear of the powder feeding parts. Properly reducing the atomization pressure can improve the coverage of the powder, but too low will easily cause the powder feeding parts to clog.
• Gun cleaning pressure 0.5MPa. Too high gun cleaning pressure will accelerate the wear of the gun head, and too low pressure will easily cause the gun head to clog.
• Fluidization pressure of powder supply barrel 0.04~0.10MPa. Too high fluidization pressure of powder supply barrel will reduce the powder density and reduce production efficiency, and too low pressure will easily cause insufficient powder supply or powder agglomeration.
• The distance from the spray gun mouth to the workpiece is 150~300mm. If the distance between the spray gun nozzle and the workpiece is too close, it is easy to produce discharge and break through the powder coating. If it is too far, it will increase the amount of powder and reduce production efficiency.
• Conveyor chain speed 4.5 ~ 5.5m / min. If the conveyor chain speed is too fast, the powder coating thickness will be insufficient, and if it is too slow, the production efficiency will be reduced.

1.2.4 Main equipment for powder electrostatic spraying

❈ Spray gun and electrostatic controller

In addition to the traditional built-in electrode needle, the spray gun is also equipped with a ring corona on the outside to make the electrostatic field more uniform to maintain the uniform thickness of the powder coating. The electrostatic controller generates the required electrostatic high voltage and maintains its stability, with a fluctuation range of less than 10%.

❈ Powder supply system

The powder supply system consists of a new powder barrel, a rotary screen and a powder supply barrel. Powder coating is first added to the new powder barrel, and compressed air pre-fluidizes the powder through the micropores on the fluidizing plate at the bottom of the new powder barrel, and then it is transported to the rotary screen through the powder pump. The rotary screen separates powder particles with too large particle size (above 100μm), and the remaining powder falls into the powder supply barrel. The powder supply barrel fluidizes the powder to a specified degree and then supplies it to the spray gun for spraying the workpiece through the powder pump and the powder delivery pipe.

❈ Recovery system

Except for a part of the powder sprayed by the spray gun being adsorbed on the surface of the workpiece (generally 50% to 70%, 70% for our company), the rest of the powder settles naturally. Part of the powder in the sedimentation process is collected by the cyclone collector on the side wall of the powder spraying booth, and the powder particles with larger particle size (above 12μm) are separated by the centrifugal separation principle and sent back to the rotary screen for reuse. Powder particles below 12μm are sent to the filter element recovery device, where the powder is shaken down by pulse compressed air into the collection bucket at the bottom of the filter element. This part of the powder is regularly cleaned and boxed for sale. The clean air (containing powder particles with a particle size of less than 1μm and a concentration of less than 5g/m3) separated from the powder is discharged into the powder spraying room to maintain a slight negative pressure in the powder spraying room. Too much negative pressure can easily inhale dust and impurities outside the powder spraying room, and too little negative pressure or positive pressure can easily cause powder overflow. The powder that settles to the bottom of the powder spray booth is collected and then fed into the rotary screen for reuse through a powder pump. The mixing ratio of recycled powder to new powder is (1:3) to (1:1). Using this recycling system, the company's overall powder utilization rate averages 95%.

❈ Powder spray booth body

The top plate and wall panels are made of light-transmitting polypropylene plastic to minimize the amount of powder adhesion and prevent static charge accumulation from interfering with the electrostatic field. The bottom plate and base are made of stainless steel, which is easy to clean and has sufficient mechanical strength.

❈ Auxiliary system

Including air conditioners and dehumidifiers. The function of the air conditioner is to keep the powder spraying temperature below 35°C to prevent powder agglomeration; the second is to maintain a slight negative pressure in the powder spraying room through air circulation (wind speed less than 0.3m/s). The function of the dehumidifier is to keep the relative humidity in the powder spraying room at 45% to 55%. If the humidity is too high, the air is prone to discharge and breakdown of the powder coating. If the humidity is too low, the conductivity is poor and it is not easy to ionize.

1.3 Curing

1.3.1 Basic principles of powder curing

The epoxy groups in epoxy resin, the carboxyl groups in polyester resin and the amine groups in curing agent undergo polycondensation and addition reaction to crosslink into a macromolecular network, while releasing small molecular gases (byproducts). The curing process is divided into four stages: melting, leveling, gelling and curing. When the temperature rises to the melting point, the surface powder on the workpiece begins to melt, and gradually forms a vortex with the internal powder until it is completely melted.

After the powder is completely melted, it begins to flow slowly, forming a thin and flat layer on the surface of the workpiece. This stage is called leveling. After the temperature continues to rise to the glue point, there is a short-term gelling state (the temperature remains unchanged), and then the temperature continues to rise and the powder undergoes a chemical reaction and solidifies.

1.3.2 Basic process of powder curing

The powder curing process used is 180℃, baking for 15 minutes, which is normal curing. The temperature and time refer to the actual temperature of the workpiece and the cumulative time that it is maintained at or above this temperature, rather than the set temperature of the curing furnace and the walking time of the workpiece in the furnace. However, the two are interrelated. When the equipment is initially debugged, it is necessary to use a furnace temperature tracker to measure the surface temperature and cumulative time of the top, middle and bottom points of the largest workpiece, and adjust the curing furnace set temperature and conveyor chain speed (which determines the walking time of the workpiece in the furnace) according to the measurement results until the above curing process requirements are met. In this way, the corresponding relationship between the two can be obtained, so within a period of time (generally 2 months), only the speed needs to be controlled to ensure the curing process.

1.3.3 Main equipment for powder curing

The equipment mainly includes three parts: heating burner, circulating fan and air duct, and furnace body. The heating burner used by our company is a German Weishaupt product, using 0~35# light diesel. It has the advantages of high heating efficiency and fuel saving. The circulating fan performs heat exchange, and the first-level opening of the air supply duct is at the bottom of the furnace body, and there is a level opening every 600mm upward, for a total of three levels. This can ensure that the temperature fluctuation within the range of 1200mm workpiece is less than 5℃, and prevent the upper and lower color difference of the workpiece from being too large. The return air duct is at the top of the furnace body, which can ensure that the upper and lower temperatures in the furnace body are as uniform as possible. The furnace body is a bridge structure, which is conducive to preserving hot air and preventing the air volume in the furnace from decreasing after production, which in turn causes the inhalation of external dust and impurities.

1.4 Inspection

After curing, the workpiece is mainly inspected for appearance (whether it is flat and bright, whether there are particles, shrinkage holes and other defects) and thickness (controlled at 55-90μm). If it is the first time to debug or the powder needs to be replaced, the following items are required to be inspected using the corresponding testing instruments: appearance, gloss, color difference, coating thickness, adhesion (grid method), hardness (pencil method), impact strength, salt spray resistance (400h), weather resistance (artificial accelerated aging), and moisture and heat resistance (1 000h).

1.5 Finished products

After inspection, the finished products are classified and placed in transport vehicles and turnover boxes, and separated from each other by soft materials such as newspapers to prevent scratches and mark them for use.

2. Common problems and solutions for powder electrostatic spraying

2.1 Coating impurities

Common impurities mainly come from particles in the powder spraying environment, as well as impurities caused by various other factors, which are summarized as follows:

1. Impurities in the curing furnace. The solution is to use a wet cloth and a vacuum cleaner to thoroughly clean the inner wall of the curing furnace, focusing on the gaps between the hanging chain and the air duct. If it is a large black particle impurity, it is necessary to check whether the air supply duct filter is damaged, and replace it in time.
2. Impurities in the powder spraying room. Mainly dust, clothing fibers, equipment abrasive particles and scale accumulation in the powder spraying system. The solution is to use compressed air to blow the powder spraying system before starting work every day, and thoroughly clean the powder spraying equipment and powder spraying room with a wet cloth and a vacuum cleaner.
3. Impurities in the hanging chain. Mainly the product of corrosion of the hanging chain oil baffle and the primary hoist water tray (made of hot-dip galvanized plate) by pre-treatment acid and alkali vapor. The solution is to clean these facilities regularly.
4. Powder impurities. Mainly excessive powder additives, uneven pigment dispersion, powder points caused by extrusion, etc. The solution is to improve the quality of powder and improve the way of powder storage and transportation.
5. Pre-treatment impurities. Mainly large particles caused by phosphating slag (professional various types of spraying lines, paint lines, plastic spraying lines/powder spraying lines, electrophoresis lines, sandblasting robots, spraying robots, sandblasting rooms, shot blasting machines, paint spraying rooms, spraying equipment, surface treatment equipment and waste gas treatment equipment manufacturers, long-term supply of various types of sandblasting room shot blasting machine accessories, paint spraying room accessories, dust collector accessories and small impurities caused by yellow rust of phosphating film. The solution is to clean up the slag in the phosphating tank and spray pipeline in time, and control the concentration and proportion of the phosphating tank liquid.
6. Water quality impurities. Mainly impurities caused by excessive sand and salt content in the water used in pre-treatment. The solution is to add a water filter and use pure water as the last two levels of cleaning water.

2.2 Coating shrinkage cavities

1. Shrinkage cavities caused by residual surfactants due to incomplete degreasing in pre-treatment or incomplete water washing after degreasing. The solution is to control the concentration and proportion of the pre-degreasing tank and degreasing tank liquid, reduce the amount of oil on the workpiece and enhance the water washing effect.
2. Shrinkage caused by excessive oil content in water. The solution is to add a water inlet filter to prevent oil leakage from the water supply pump.
3. Shrinkage caused by excessive water content in compressed air. The solution is to discharge the condensed water of compressed air in time.
4. Shrinkage caused by moisture in the powder. The solution is to improve the storage and transportation conditions of the powder and add a dehumidifier to ensure the timely use of the recovered powder.
5. Shrinkage caused by the oil on the hanging chain being blown onto the workpiece by the air conditioning wind. The solution is to change the position and direction of the air supply port of the air conditioning.
6. Shrinkage caused by mixed powder. The solution is to thoroughly clean the powder spraying system when changing powder.

2.3 Color difference in coating

1. Color difference caused by uneven distribution of powder pigment. The solution is to improve the quality of the powder and ensure that the L, a, and b of the powder are not much different and the positive and negative are uniform.
2. Color difference caused by different curing temperatures. The solution is to control the set temperature and conveyor chain speed to maintain the consistency and stability of the workpiece curing temperature and time.
3. Color difference caused by uneven coating thickness. The solution is to adjust the powder spraying process parameters and ensure that the powder spraying equipment operates well to ensure uniform coating thickness.

2.4 Poor coating adhesion

1. Poor adhesion caused by residual degreasing agent, phosphating slag or alkaline contamination of the washing tank on the workpiece due to incomplete pre-treatment water washing. The solution is to strengthen water washing, adjust the degreasing process parameters and prevent the degreasing liquid from entering the washing tank after phosphating.
2. Poor adhesion caused by yellowing, flowery or partial absence of phosphating film. The solution is to adjust the concentration and proportion of the phosphating tank liquid and increase the phosphating temperature.
3. Poor adhesion caused by incomplete drying of moisture at the corners of the workpiece. The solution is to increase the drying temperature.
4. Poor adhesion of large areas of coating caused by insufficient curing temperature. The solution is to increase the curing temperature.
5. Poor adhesion caused by excessive oil and salt content in deep well water. The solution is to add a water inlet filter and use pure water as the last two cleaning water.

In short, there are many powder electrostatic spraying technologies and their application methods, which need to be used flexibly in practice.