Help, I want to know about electrostatic spraying knowledge

In the United States in 1955 began to use the disc type automatic electrostatic coating system for gun butts, toilet seats, chairs, bed parts and other wood products coating. In Japan, the use of elevator-type disc type polyester resin paint coating for wooden TV **** sound box was also started in 1966. Then, the popularization of acid-hardening alkyd resin (AMI-NOALKYDRESIN) coating for dining chairs, tennis racket lacquer (lacquer) coating and other mass-produced products such as coating with a disc-type coating machine, and now it is estimated that about 200 factories use electrostatic coating system, and become the mainstream of mass-produced wood products coating. Since electrostatic coating can greatly save paint and greatly reduce the cost of products, it was applied to the coating of metal products in 1945, and the bell-type was developed in 1951, and the disc-type coating machine was developed in 1953, and the electrostatic coating system was popularized in the following industrial fields. The system was popularized in the following industrial fields: household electrical appliances, automobiles and their parts, locomotives and their parts, bicycles and their parts, steel furniture, office appliances, agricultural equipment, building materials (metal door and window frames and door openings), railroad tracks, high-pressure gas containers, and instrument housings. In addition to the many metal products for which electrostatic coating is used, it is also applied to various resin parts such as acrylene-butadiene-styrene resin (ABS resin). It can be said that electrostatic coating is one of the representative industrial coating methods. In addition, in recent years, the development and popularization of ultra-high-speed rotary disk-type and bell-type coating machines have been adopted for ultra-high-quality paint coating because of the high capacity of these machines.

Second, the principle of electrostatic coating and electrostatic coating machine classification.

(a) the principle of electrostatic coating

Electrostatic coating is to ground the coated object as a positive electrode, paint atomizing device for the negative electrode, and paint atomizing device with high negative voltage, made between the two poles of the electrostatic boundary, so that the atomized paint particles negatively charged, so that the paint is effectively suctioned in the opposite electrode on the surface of the object is the method called electrostatic coating. So why is the object to be coated as the positive electrode and the paint atomizing device as the negative electrode? It is because it is harder and safer to transfer sparks to the opposite electrode. In order to implement electrostatic coating, the grounding of the coated material is an important requirement, and the coated material must be a good conductor of electricity. Wood is usually considered to be a poor conductor of electricity, but when its moisture content is 8% or more, it is possible to apply electrostatic coating, and this has been confirmed empirically. This is because the value of the current flowing between the positive and negative poles of the electrostatic coating is 20~50 microampere, which is very small, so even objects with a large electrical resistance, such as wood, can be electrostatically coated. However, in Japan, during the winter drying period, the moisture content of wood decreases to less than 8%, so the amount of paint adhesion decreases significantly. In this case, it is also effective to increase the water content of the wood surface by using a water vapor chamber to pass through the wood.

(2) Classification of electrostatic coating machine

1. According to the classification of paint atomization

Previously, it was classified as electrostatic atomization and mechanical atomization of two kinds. The so-called electrostatic fog is the aforementioned disc type, paint with high voltage and supply to the center of the disc, and then the disc rotating centrifugal force generated by the expansion of the surface of the disc to form a thin film, and in the edge of the disc from the liquid wire into the paint mist of the way it is called. Another mechanical atomization is the way of paint. However, because of the recent invention of small diameter ultra-high-speed disc or clock, so according to the classification of the atomization of paint slightly complicated, I tried to classify as follows.

(1) electrostatic atomization mode (low-speed rotary atomization head type, knife-edge atomization head type).

(2) Centrifugal force atomization mode (ultra-high-speed rotary atomization head type).

(3) Pressurized air atomization mode.

(4) Airless atomization.

(5) Hydraulic atomization and an auxiliary gas and way (airless and a compressed air atomization mode).

2. According to the paint charged way classification

The paint mist particles with negative electric charge, in the electrostatic coating program, is an important part. Including new models, from this interview classification is as follows:

(1) contact charged mode

(2) ion charged mode

Note: The representative models of the above contact charged mode is the rotary atomization head type, while the electrostatic atomization mode, centrifugal force atomization mode belongs to this type of mode. Other still have spray paint gun type coating machine, in the gun inside the paint path to give high voltage, and then to be mechanically atomized. The ion-charged method is used for applicators with short needle electrodes at the tip of the gun. The mechanical energy required to atomize the paint is switched "ON" with the paint switch, and a high voltage is applied to the applicator from the high voltage generator. In the vicinity of the paint tip electrode shown in Fig. 2-(1), there are needle electrodes, so the electric boundary is extremely uneven, and the air is strongly ionized. However, in the vicinity of the tip of the needle electrode, the insulation of the air is partially destroyed, and a very stable glow corona (GLOWCORONA) continuous discharge occurs. Usually, this corona discharge can be seen with the naked eye in a dark room, but it is a violet-blue light discharge point. This part is very strong ionization circle, but the actual effective ionization circle is much larger. As shown in Fig. 2-(2), the paint particles are negatively charged, and they fly in the direction of the electric boundary shown in Fig. 2-(3) and attach to the object. Pressure gas atomization, airless atomization and airless a pressure gas atomization belongs to this way.

3. Classified according to the position of the atomizing head of the electrostatic coating machine in relation to the high voltage generator.

Until recently, the general high voltage generator set in the coating room outside, from the atomization head of 5 meters or 10 meters position for its common sense, but recently the development of the atomization head dirty in the ascending voltage mechanism or rectifier mechanism within the device. The former is called power supply remote control type (POWERSUPPLYREMOTETYPE). The latter is called the PACK-IN type. However, the remote-controlled type of power supply has gradually shifted from the large wet type (which uses insulating oil internally) to the small dry pack (DRY-PACK) type (which uses molding resin). Installation of the type of variable voltage type, high voltage generation type (COCCROFT) or so-called grid (CASCADE) type, electrical power (EGD) type and turbine power generation type and so on.

4. Other classifications

Some are divided into automatic and manual type, or because of the use of robots or automated lifts, and is called robotic, lift type. These classifications are considered to use the distinction on the classification.

Functions of electrostatic coating

The advantages of electrostatic coating are analyzed from three standpoints: cost management, quality management, and production management (engineering management, delivery management).

(1) From the standpoint of cost management: Because of electrostatic painting, when using a pressure air spray gun, the excess paint mist (OVER-SPRAY) or backlash (BLOWBACK) is greatly reduced, and significant savings in paint are realized. For example, in Japan, when polyester resin paint was applied to the door frame of a wooden cupboard, the paint cost was reduced by $400,000 a year. This was achieved by switching from pressure air spray gun coating to elevator-type disc coating with small diameter and ultra-high speed rotation. The reduction in paint helps to keep the paint booth clean. Therefore, although it is an indirect effect, it can also reduce the cost and time needed for maintenance and cleaning.

(2) In terms of quality management position: In the spray paint form of electrostatic coating, the paint particles are negatively charged due to micronization, so they repel each other. This repulsion effect brings the distribution of particles in the paint form more uniform effect, and produce the effect of reducing the uneven coating. Due to the ultra-high-speed rotary coating machine, the degree of micronization of paint particles is high, and the distribution of particle size is also small. So it can significantly improve the uniformity of film thickness.

(3) on the production management position: to reduce the electrostatic force generated by the anti-hopping or circle (LAPAROUND) effect (refers to the electrostatic force of the role of excess paint mist attached to the back of the coated object phenomenon) is not only to reduce the amount of paint consumed, and can shorten the coating time. Especially, if you use an automatic coating machine with a large atomization capacity, the coating speed will be faster. Stable production and easy management. (The ability to paint the end of the material, the edge of the material and other surfaces is also a big function)

Fourth, electrostatic coating and safety

(a) At present, it is estimated that there are more than 10,000 factories in Japan to implement electrostatic coating. Among them, more than 20 years of common use of electrostatic coating of the factory is also a lot, and no harm, no fire. On the other hand, there are several fires in some factories every year, and it is found that the cause is mostly based on the habit of using it for a long time and neglecting the basic management or treatment. Wooden furniture factories are surrounded by a lot of combustible materials. It is not only necessary to neatly organize and tidy up the coated materials, paints and solvents, but also to manage the electrostatic coating machine or paint booth inside and outside in accordance with the stipulated benchmarks at the time of startup. It is a fact that the safety of electrostatic coating machine will be improved with the advancement of technology in the future. For example, it is possible to detect abnormalities in the amount of current flowing between the coater and the object to be coated at high speed, and it is also possible to quickly cut off high voltage and develop various kinds of interlocking (INTERLOCK) devices, automatic safety devices, and other technologies. These technologies make up for what is considered to be unsafe parts of management, processing, or operation.

(ii) electrostatic coating when the cause of the fire is due to the type of coating machine, the nature of the paint, the shape of the coated material, transportation methods or plant environment and so on, and there are a variety of reasons. However, in the manual coating machine, because the operator or cleaning with solvent cylinder for the insulation state, so in the electrical tape and grounded objects close to the discharge and the fire generated by the most. And the automatic coating machine is due to the insulation of the paint tube is destroyed and the fire generated by the most.

Safety measures

1. Most of the accidents of electrostatic coating are caused by the ungrounding of the grounding and the formation of an insulated state, which is electrically charged. The conductive objects in the painting room should be clearly distinguished as high voltage potential or ground potential, and it is the first principle that they must be grounded except for those or parts that should be applied high voltage. It is also important to periodically check that the potential is grounded, and this can prevent most fire accidents. Simply because the coating machine has a high voltage applied, it is considered that the coating machine is the most dangerous, but the latest coating machine is equipped with a high degree of safety measures, so there are very few accidents due to the coating machine.

Coating all the general organic solvent vapor concentration in the explosion limit, can catch fire solvent vapor minimum ignition energy of about 0.2mJ (2/10000 Joule). His side of the human body's electrostatic capacity is about 100pF. So if the human body becomes about 2KV potential, it has 0.2mJ of energy. If the operator is not grounded, the formation of a dangerous state, it can be seen, in the electrostatic field to keep the human body is not the formation of insulation state is how important.

The following safety measures are based on the grounding method:

(1) The operator must wear energized shoes (leather-soled shoes are energized).

(2) Clean the soles of shoes thoroughly.

(3) Conductive objects in the booth must be grounded.

(4) Wash solvent tanks are fixed and grounded.

(5) The operator using the electrostatic handheld spray gun must operate the gun with his bare hands.

(6) Hanging the hanger of the painted object is sufficiently clean, and often keep the painted object for grounding potential. (Pay attention to the contact point of the hanger.)

(7) The ground in the painting room must be kept in a conductive state.

2. Use and operation of the basic to prevent the occurrence of the formation of fire disaster discharge. If the coating machine or paint tank is considered to be electrically charged, it is necessary to ground it after sufficient ventilation and use it after allowing the residual charge to escape, then it is necessary to comply with the items when replenishing the paint or cleaning the painting booth.

The following are the main points for use and operation.

(1) The high voltage must be turned off at the end of painting (the same applies when painting is interrupted for a while). Make sure that the high voltage is turned off before cleaning operation.

(2) When using conductive paints (Raka, alkyd resin paints are mostly conductive) and the paint supply system is insulated from the ground, and the paint is replenished to the paint tank, the first thing to do is to ground the paint tank [at this time, you have to turn on the ventilating fan and ground the part of the giant solvent vapors (e.g., the bottom of the outside of the paint tank)].

(3) If a solvent is used to clean the insulating material, apply a high voltage after confirming that the solvent is dry.

(4) Periodically clean the coating machine from paint dust contamination.

(5) Do not use an applicator that traps solvent vapors to prevent contamination from being coated.

(6) Do tighten or turn around high voltage cables or paint hoses that are loose or detached.

(7) Otherwise, familiarize yourself with the instruction manual and follow it thoroughly.

3. Coating machine system and attached equipment

To prevent discharge, set up interlocking device in the high voltage application to prevent unnecessary conductive objects and workers from entering into the static boundary.

Points to note on the coating machine system and attached equipment:

(1) Provide sufficient locking devices necessary for operation.

(2) Sequential action is required, so that it can not be specified outside the sequence of operation.

(3) Appropriate interlocks are provided for conveyors, entrance and exit doors, air supply and exhaust, and fire fighting equipment and coating machines.

(4) Provide a safety fence or prevent the operator from entering the device.

(5) Use longer hangers to reduce the contact points between the conveyor and the hangers.

(6) Use an automatic washing system (automatic coating machine).

(7) In case of emergency, set up automatic fire-fighting device.

4. Electrostatic coating machine safety system

Electrostatic coating machine safety system can be divided into two categories. A needle electrode mounted on the tip of the paint spray gun type of coating machine, the insulating material manufacturing coating machine body, narrow the needle electrode electrostatic capacity, high resistance inserted in the electrode evening and connected to the high voltage generator. The needle electrode is easily discharged, and the applied high voltage decreases rapidly as it approaches the grounded object, and the relationship is shown in Figure 3 below. If it is a coating with high resistance value, it is a coating machine with very high safety. Figure 3 shows an example of the change in voltage and current at a distance of this type. The other type is electrostatic atomization or centrifugal force atomization type coating machine, which has a large electrode capacity and therefore has a large ignition energy. Therefore, it is very dangerous to use them in their original state, so safety devices such as the so-called spark prevention device (SPARKGUARD) or overcurrent checker are installed as a preventive measure. That is, before the spark discharge, the high voltage that passes through exceeds the value of the set check current, then the self-weight blocking, and its safety degree is lower than the former (needle electrode installed at the tip of the spray gun coating machine), and it is required to be high-performance. However, the test current value is changed to di/dt and the residual charge of the coater is grounded by the ultra-high-speed high-voltage interrupter. And has developed such a very safe system, so it has reached the realm where you can use the electrostatic coating machine with peace of mind.

Process conditions affecting the effect of air electrostatic spraying

(1) the relative position of the spray gun: in the place of simultaneous operation of a number of spray guns, the use of its relative position should pay attention to, should be made to spray the paint flow does not interfere with each other, to avoid the same sex of the charge of the paint spray repulsive, especially for spraying small and medium-sized parts of the place.

(2) Paint booth wind speed: electrostatic spraying paint booth wind speed, exhaust volume is less than the general air spraying. Wind speed of 0.5-0.7m / s is more appropriate.

(3) paint resistance: the electrical properties of the paint directly affect the electrostatic atomization and electrostatic effect. Resistance value is too high charged difficulty, electrostatic effect is poor; too small is easy to leakage, jeopardize the spray gun and is not conducive to safety. Various specifications of different electrostatic spray gun have their own specific resistance value range, can be combined with the field test and gun supplier recommendations and determine.

(4) the viscosity of the paint: electrostatic spraying with paint construction viscosity is generally slightly lower than the air spraying, in order to improve the atomization effect, so that the paint mist is easy to deposition along the direction of the power line ring.

(5) air pressure: in the same conditions of the pipeline paint delivery system, air electrostatic spray gun pressure is lower than ordinary air spray gun. Too high air pressure on the one hand will affect the normal operation of the turbine generator, on the other hand, also affects the electrostatic effect.

(6) Paint delivery pressure: Compared with ordinary air spray gun, electrostatic spray gun spraying required paint pressure is also slightly lower.

Air electrostatic spray gun spraying process preliminary discussion

Electrostatic spraying process has been more and more widely used in the machinery industry, especially in the automotive manufacturing industry. It is with its high paint utilization, high efficiency coating productivity, excellent coating quality and waste paint, waste paint mist of a substantial reduction and the reduction of environmental hazards more and more times people's favor. According to the shape of the electrostatic atomizer and the different ways of installation, electrostatic spraying equipment can be divided into two categories: fixed and manual mobile. For the design of a new coating line and part of a certain scale of the old production line renovation, often the former mostly, but for the original design of a smaller scale, more crowded coating site of the old production line, portable electrostatic spray gun is flexible, lightweight, does not take up the workplace and the shape of the complexity of the workpiece and the uniformity of the concave part of the coating unique advantages. In this paper, the air electrostatic spray gun in the production line of the batch trial on the process of preliminary discussions.

1, spraying tools and the use of

Spraying tools for the GRACO PRO3500 air electrostatic spray gun, which is produced by the handle, barrel, trigger, = air cap components, piping, air system and other components. This kind of spray gun has been in our country's automobile, motorcycle manufacturing plant batch use, has certain practicality and representativeness, its electrostatic principle by compressed air driven by the built-in turbine generator by constantly amplified and rectified into a 6.5X104V high voltage direct current, so that the paint particles charged and further atomization and electrostatic coating. Air electrostatic spray gun and ordinary air spray gun external structure of the difference in addition to the nozzle part of the electrode needle, but also equipped with a turbine generator for the elimination of excess exhaust gas and cooling generator exhaust pipe. In addition, its air hose is electrically conductive and can be used to ground the gun system*. It is important to note when using the gun that, as a safety assurance procedure, it must be depressurized, i.e., the electrostatic switch must be turned off, the air and paint supply must be cut off, the trigger must be pulled, and the residual paint and air must be released to a *groundable object, in the case of inspections, repairs, installations, cleanings, and interruptions of the spraying operation. In addition, the air electrostatic spray gun maintenance is also very important, cleaning solvent must be non-conductive solvent, conductive solvent will cause the gun malfunction; also do not allow the spray gun immersed in solvent, otherwise it is easy to cause short-circuiting and electrostatic effect is reduced.

2, safety specifications and safety inspection

Electrostatic spraying is the most important issue that requires special attention is safety, for this reason, the state specifically promulgated the relevant safety standards. For PRO3500 and other similar air electrostatic spray guns, the following safety measures and safety checks are also required before each use.

(1) gun system safety: must be systematically grounded, the gun body to ground resistance is less than 1-2 megohms.

(2) human safety: the operator and all personnel entering the work area must wear conductive shoes, the operator is not allowed to wear insulated gloves, the human body to ground resistance is less than 1-2 megohms.

(3) the state of the workpiece: ground chain trolley or rail can be * grounded, and the workpiece is a point or line of contact, the workpiece to ground resistance is less than 2 megohms, otherwise directly affect the effect of static electricity, or even paint dance back to hold in the operator.

(4) work area safety: all combustible liquids in the work area must be placed in a safe grounded container, all conductive body should be grounded.

(5) Operation Assurance: All operators must be trained to safely use the electrostatic spray system and master the unloading technique.

3, process conditions and operation

Compared with ordinary air spraying, the following process conditions have a direct impact on the coating effect:

(1) the relative position of the spray gun: in the place of simultaneous operation of multiple spray guns, the use of its relative position should be noted, should be made to spray the paint flow does not interfere with each other, to avoid the repulsion of the paint mist of the same charge, especially for the spraying of small and medium-sized parts of the place.

(2) Paint booth air speed: electrostatic spraying paint booth air speed, exhaust volume is less than the general air spraying. On the one hand, the electrostatic spraying itself paint mist, solvent volatilization is small, do not need too high air volume, on the other hand, the wind speed is too large, directly affect the paint mist in the electric field of adsorption and affect the electrostatic effect. For the general air electrostatic spray gun, the recommended wind speed of 0.5-0.7m / s.

(3) the resistance of the paint: the electrical properties of the paint directly affect the electrostatic atomization and electrostatic effect. Resistance value is too high charged difficulty, electrostatic effect is poor; too small is easy to leakage, jeopardize the spray gun and is not conducive to safety. Various specifications of different electrostatic spray gun have their own specific resistance value range, can be combined with the field test and gun supplier recommendations and determine.

(4) the viscosity of the paint: electrostatic spraying with paint construction viscosity is generally slightly lower than the air spraying, in order to improve the atomization effect, so that the paint mist is easy to deposition along the direction of the power line ring.

(5) air pressure: in the same conditions of the pipeline paint delivery system, air electrostatic spray gun pressure is lower than ordinary air spray gun. Too high air pressure on the one hand will affect the normal operation of the turbine generator, on the other hand, also affects the electrostatic effect.

(6) Paint delivery pressure: compared with ordinary air spray gun, electrostatic spray gun spraying required paint pressure is also slightly lower.

4, the coating effect

Compared with ordinary air spraying, air electrostatic spray gun spraying has the following advantages:

(1) the thickness of the increase and uniformity: in the operation of basically the same conditions, due to the electrostatic adsorption effect, air electrostatic spraying film thickness is slightly higher than the air spraying (about 3-5?um), and due to the effect of the embracing the corners of the workpiece, depressions, etc., easy to paint, coating film more than the air spraying (about 3-5?um), and due to the embracing effect of the corner, depressions and other parts. And because of the ring effect, the corner of the workpiece, depression and other parts of the paint is easy, the film is more uniform, not easy to cause leakage spraying.

(2) good leveling, due to the wet film flat, uniform, in the appropriate viscosity, temperature and humidity conditions, the film is easy to level, the orange peel is significantly reduced, the film is more flat

(3) fresh reflection: due to the reduction of the appearance of the orange peel and other defects, which directly improves the film's fresh reflection.

(4) gloss: gloss slightly improved (about 1%), for the inner surface, corners and other difficult to paint parts, the gloss is more obvious.