When working, the electrostatic spray gun or spray cup part 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, the end of the spray gun (or spray plate, spray cup) and the workpiece are formed An electrostatic field.

The electric field force received by the paint particles is directly proportional to the voltage of the electrostatic field and the charge amount of the paint particles, and inversely proportional to the distance between the spray gun and the workpiece;

When the voltage is high enough, the area near the end of the spray gun will form an air ionization zone, and the air will ionize and heat violently, causing a dark red halo around the sharp edge of the spray gun or the pole needle, which can be clearly seen in the dark. When the air produces a strong corona discharge.

Most of the film-forming materials in coatings, namely 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 include organic solvents, cosolvents, curing agents, electrostatic diluents, And other various additives and other substances.

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.

A dielectric composed of polar molecules exhibits electrical properties when subjected to an external electric field; a dielectric composed of non-polar molecules exhibits electrical properties under the effect of an external electric field, thereby generating an affinity for external conductive charges, so that the dielectric is applied Its outer surface can be locally charged in the electric field.

The paint is sprayed out after being atomized by the nozzle. When the atomized paint particles pass through the pole needle of the muzzle or the edge of the spray plate or spray cup, they are charged due to contact. When they pass through the gas ionization zone generated by corona discharge, they will be Increase its surface charge density at one time.

Under the action of the electrostatic field of these negatively charged paint particles, they move on the surface of the workpiece with guiding polarity and are deposited on the surface of the workpiece to form a uniform coating film.

Second, the craft

(1) Surface pretreatment. Mainly degreasing and rust removal, the method is the same as the pretreatment of applying liquid paint.

(2) Scrape putty. Scrape conductive putty according to the degree of defects of the workpiece, and smooth it with sandpaper after drying, and then proceed to the next process.

(3) Protection (also known as covering). 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, preheating is not required. If a thicker coating is required, the workpiece can be preheated to 180-20°C, which can increase the thickness of the coating.

(5) Spraying. In a high-voltage electrostatic field, the powder spray gun is connected to the negative pole, and the workpiece is grounded (positive pole) to form a loop. The powder is sprayed from the spray gun with the help of compressed air, which is negatively charged, and is sprayed on the workpiece according to the principle of opposite sex attraction.

(6) Curing. After spraying, the workpiece is sent to a drying room at 180～200℃ for heating to solidify the powder.

(7) Clean up. After the coating is cured, remove the protector and smooth the burrs.

(8) Inspection. Check the coating of the workpiece, and re-spray any defects such as missing spray, bumps, needle bubbles and so on.

(9) Defect treatment. Repair or re-spray the workpieces with defects such as leaked spray, pinholes, bumps, bubbles, etc., which are detected.

Three, application

The uniformity, gloss and adhesion of the paint layer on the surface of the workpiece sprayed by electrostatic spraying are better than ordinary manual spraying.

At the same time, electrostatic spray paint can be sprayed whether it is ordinary spray paint, oily and magnetic blending paint, perchloroethylene paint, amino resin paint, epoxy resin paint, etc. It is simple to operate and can save about 50% of paint compared with general air spray.

Usually requires high air pressure, fine paint particles and fast speed. But if the air pressure is too high, the effect of electricity will be destroyed. The appropriate paint pressure and air pressure should be selected according to the variety of paints and coatings used, the painting site and the workpiece to be coated, etc.

If the paint contains higher heavy pigments, higher paint pressure and air pressure can be used; on the contrary, 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 company 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.

The development of powder electrostatic spraying technology in my country is relatively late, but it has great potential for development. The powder coating does not contain solvent. The powder coating is sprayed on the surface of the workpiece by electrostatic spraying. The non-adhesive powder particle layer is heated and melted to form a firm coating and tightly bond with the surface of the workpiece.

This coating has excellent anti-corrosion properties and decorative functions. Compared with traditional solvent-based coatings, it has the advantages of safer, less pollution, good adaptability, high efficiency and not relying on petroleum as raw material. But it also has some shortcomings at present: large one-time investment, inconvenient color replacement, etc.

The refrigerator shell produced by our company adopts powder electrostatic spraying technology, using the powder electrostatic spraying equipment manufactured by Nordson Corporation in the late 1990s. Take this set of equipment and its application technology as an example to talk about the author's understanding.

1. Typical process flow of powder electrostatic spraying technology

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

1.1 Pre-treatment

After the workpiece is pre-treated to remove the oil and dust on the surface of the cold-rolled steel plate, the powder can be sprayed. At the same time, a zinc-based phosphate 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 cooled to below 35℃ in order 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 spray booth through the conveyor chain to prepare for spraying operation. The electrostatic generator releases high-voltage static electricity (negative electrode) through the electrode needle of the nozzle of the spray gun to the space in the direction of the workpiece. The mixture of powder and compressed air from the nozzle of the spray gun and the air around the electrode are ionized (negatively charged).

The workpiece passes through the hanger and connects to the ground (grounding electrode) through the conveyor, so that an electric field is formed between the spray gun and the workpiece. 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, pigments, fillers, and various additives (such as leveling agents, moisture-proof agents, edge modifiers, etc.). After the powder is heated and cured, the surface of the workpiece will be formed. Need coating.

The auxiliary material is compressed air, which is required to be clean, 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 technology of powder electrostatic spraying

Static high voltage 60-90kV. Too high voltage will easily cause powder rebound and edge pitting; too low voltage will cause low powder picking rate.

The electrostatic current is 10-20μA. If the current is too high, it is easy to cause discharge breakdown of the powder coating; if the current is too low, the powder loading rate is low.

The flow rate and pressure are 0.30-0.55MPa, the higher the flow rate and pressure. If it is high, the deposition rate of the powder will be faster, which is beneficial to quickly obtain a predetermined thickness of the coating, but if it is too high, it will increase the amount of powder and the wear rate of the spray gun.

The atomization pressure is 0.30～0.45MPa. Properly increasing the atomization pressure can keep the thickness of the powder coating uniform, but too high will cause the powder feeding parts to wear quickly. Appropriately lowering the atomization pressure can improve the powder coverage, but if it is too low, it will easily block the powder feeding parts.

The gun cleaning pressure is 0.5MPa. If the cleaning pressure is too high, it will accelerate the abrasion of the gun tip, and if it is too low, it will easily cause the gun tip to be blocked.

The fluidization pressure of the powder supply barrel is 0.04～0.10MPa. If the fluidization pressure of the powder supply barrel is too high, the density of the powder will be reduced and the production efficiency will be reduced.

The distance between the nozzle of the spray gun and the workpiece is 150～300mm. If the distance between the nozzle of the spray gun and the workpiece is too close, it is easy to cause discharge breakdown of the powder coating, while too far will increase the amount of powder and reduce the production efficiency.

The conveyor chain speed is 4.5～5.5m/min. Too fast a conveyor chain will cause insufficient powder coating thickness, and too slow will reduce production efficiency.

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 to make the electrostatic field more uniform and 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 bucket, a rotary screen and a powder supply bucket. The powder coating is first added to the new powder bucket, the compressed air passes through the micro holes on the fluidization plate at the bottom of the new powder bucket to pre-fluidize the powder, and then is transported to the rotary screen through the powder pump.

The rotating sieve separates the powder particles with too large particle size (above 100μm), and the remaining powder falls into the powder supply tank. The powder supply barrel fluidizes the powder to a specified level and then supplies the spray gun to the spray gun through the powder pump and the powder pipe.

● Recycling system

Except for a part of the powder sprayed by the spray gun that is adsorbed on the surface of the workpiece (generally 50% to 70%, 70% by our company), the rest naturally settles.

Part of the powder during the sedimentation process is collected by the cyclone recovery device on the side wall of the powder spraying shed, and the powder particles with a larger particle size (above 12μm) are separated by the principle of centrifugal separation and sent back to the rotary sieve for reuse.

Powder particles below 12μm are sent to the filter element recovery device, where the powder is shaken by pulsed compressed air into the collection hopper at the bottom of the filter element. This part of the powder is regularly cleaned and packed for sale.

The clean air (containing powder particle size less than 1μm and concentration less than 5g/m3) from the separated powder is discharged into the powder spraying chamber to maintain the slight negative pressure in the powder spraying chamber.

If the negative pressure is too large, it is easy to inhale the dust and impurities outside the powder spraying room. If the negative pressure is too small or positive pressure, it is easy to cause the powder to overflow. The powder settled to the bottom of the powder spraying shed is collected and then enters the rotary sieve through the powder pump for reuse.

The mixing ratio of recovered powder and new powder is (1:3) to (1:1). Using this recycling system, the company's overall powder utilization rate reaches 95% on average.

● Powder spraying chamber

The top plate and wall plate are made of light-transmitting polypropylene plastic material to minimize the amount of powder adhesion and prevent the accumulation of static charges 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, dehumidifiers. The function of the air conditioner is to keep the powder spraying temperature below 35°C to prevent the powder from agglomerating; the other is to maintain the slight negative pressure in the powder spraying chamber through air circulation (wind speed less than 0.3m/s).

The function of the dehumidifier is to keep the relative humidity of the powder spraying room at 45% to 55%. The air with excessive humidity is prone to discharge and break down the powder coating, and if the humidity is too small, the conductivity is poor and it is not easy to ionize.

1.3 curing

1.3.1 The basic principle of powder solidification

The epoxy group in the epoxy resin, the carboxyl group in the polyester resin and the amine group in the curing agent undergo polycondensation and addition reaction to cross-link into a macromolecular network, and at the same time release small molecular gases (by-products).

The solidification process is divided into 4 stages: melting, leveling, gelation and solidification. After 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 and reach the glue point, there is a short period of gelation (the temperature remains the same), after which the temperature continues to rise, the powder undergoes a chemical reaction and solidifies.

1.3.2 The basic process of powder solidification

The powder curing process used is 180°C, bake for 15 minutes, which is normal curing. The temperature and time refer to the actual temperature of the workpiece and the cumulative time to maintain it not lower than 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 related to each other. When the equipment is initially commissioned, it is necessary to use the furnace temperature tracker to measure the surface temperature and cumulative time of the upper, middle and lower 3 points of the largest workpiece, and adjust the curing furnace setting temperature and conveyor chain speed according to the measurement results ( It determines the walking time of the workpiece in the furnace) until it meets the requirements of the above-mentioned curing process.

In this way, the corresponding relationship between the two can be obtained, so within a period of time (usually 2 months), only the speed of control is required 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 product of German Weishaupt, using 0～35# light diesel oil. It has the advantages of high heating efficiency and fuel saving.

The circulating fan performs heat exchange. The first-stage opening of the air supply pipe is at the bottom of the furnace, and there are three-stage openings every 600mm upwards. This can ensure that the temperature fluctuation within the range of the 1 200mm workpiece is less than 5°C, and prevent excessive color difference between the upper and lower parts of the workpiece.

The return air pipe is on the top of the furnace body, which can ensure that the upper and lower temperatures in the furnace body are as even as possible. The furnace body has a bridge structure, which not only helps to preserve the hot air, but also prevents the reduction of the air volume in the furnace after the end of production and attracts external dust and impurities.

1.4 Inspection

After curing the workpiece, the main daily inspection is the appearance (whether it is smooth and shiny, whether there are defects such as particles, shrinkage, etc.) and thickness (control at 55-90μm).

If you need to debug for the first time or need to replace the powder, it is required to use the corresponding testing instrument to detect the following items: appearance, gloss, color difference, coating thickness, adhesion (cross-cut method), hardness (pencil method), impact strength, salt spray resistance (400h), weather resistance (artificial accelerated aging), heat and humidity resistance (1000h)

1.5 Finished product

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

2. Common problems and solutions for powder electrostatic spraying operations

2.1 Coating impurities

Common impurities mainly come from particles in the powder spraying environment and impurities caused by various other factors. They are summarized as follows:

(1) Curing impurities in the furnace. The solution is to thoroughly clean the inner wall of the curing furnace with a damp cloth and a vacuum cleaner, focusing on the gap between the hanging chain and the air duct. If it is a large black particle impurity, you need to check whether the air supply pipe filter is damaged, and if there is any damage, replace it in time.

(2) Impurities in the powder spraying room. Mainly dust, clothing fibers, equipment abrasive particles and dust deposition system. The solution is to use compressed air to purge the powder spraying system before starting work every day, and thoroughly clean the powder spraying equipment and powder spraying room with a damp cloth and a vacuum cleaner.

(3) Pendant chain impurities. Mainly the products of the suspension chain oil baffle and the primary spreader water pan (material is hot-dip galvanized sheet) after being corroded by the pre-treatment acid and alkali vapor. The solution is to clean these facilities regularly.

(4) Powder impurities. The main causes are excessive powder additives, uneven pigment dispersion, and powder spots caused by powder extrusion. The solution is to improve powder quality and improve powder storage and transportation methods.

(5) Pre-treatment impurities. Mainly are large particles of impurities caused by phosphating slag and flakes of small impurities caused by yellow rust of phosphating film. The solution is to clean up the slag accumulated in the phosphating tank and spray pipeline in time, and control the concentration and proportion of the phosphating tank liquid.

(6) Water impurities. It is mainly the impurities caused by the excessive sand content and salt content in the water used in the pretreatment. The solution is to add a water filter and use pure water as the last two stages of cleaning water.

2.2 Coating shrinkage

(1) Shrinkage caused by residual surface active agent caused by poorly degreasing pre-treatment or poor washing with water after degreasing. The solution is to control the concentration and proportion of the pre-degreasing tank and the degreasing tank, reduce the amount of oil on the workpiece and strengthen the washing effect.

(2) Water shrinkage caused by excessive oil content.

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 of compressed air. The solution is to discharge compressed air condensate in time.

(4) Shrinkage caused by damp powder. The solution is to improve the storage and transportation conditions of the powder and increase the dehumidifier to ensure the timely use of the recovered powder

(5) Shrinkage caused by the oil on the suspension chain being blown off by the air-conditioning wind to the workpiece. The solution is to change the position and direction of the air outlet of the air conditioner.

(6) Shrinkage caused by powder mixing. The solution is to thoroughly clean the powder spraying system when changing the powder.

2.3 Coating chromatic aberration

(1) Color difference caused by uneven distribution of powder pigments. The solution is to improve the quality of the powder to 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 runs well to ensure uniform coating thickness.

2.4 Poor coating adhesion

(1) Incomplete pre-treatment water washing results in poor adhesion caused by residual degreasing agent, phosphating residue on the workpiece or pollution of the washing tank with lye. The solution is to strengthen the 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, blooming or partial absence of phosphate coating. The solution is to adjust the concentration and ratio of the phosphating bath solution and increase the phosphating temperature.

(3) Poor adhesion caused by poor moisture drying in the corners of the workpiece. The solution is to increase the drying temperature.

(4) Poor adhesion to large areas of the coating caused by insufficient curing temperature. The solution is to increase the curing temperature.

(5) Poor adhesion caused by excessive oil content 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 still many powder electrostatic spraying technologies and their application methods, which need to be used flexibly in practice.