1. Foreword:
In the United States, the disc-type automatic electrostatic coating system began to be used in 1955 for wood products such as gun stocks, toilet seats, chairs, and bed parts. Painting. In Japan, in 1966, the elevator-type disc type was also used for polyester resin coating of wooden TV sound boxes. Later, the use of disc-type coating machines for coating products such as acid-hardening alkyd resin (AMI-NOALKYDRESIN) coating for dining chairs and lacquer coating for tennis rackets and other products became popular, and now it is estimated that there are about 200 Many factories use electrostatic coating systems, which have become the mainstream for mass-produced wood product coatings. 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. A bell-type coating machine was developed in 1951, and a disc-type coating machine was developed in 1953. Note the popularization of electrostatic coating systems in the industrial field. That is, household electrical appliances, automobiles and their parts, motorcycles and their parts, bicycles and their parts, steel furniture, office equipment, agricultural machinery, building materials (metal door and window frames and door leaves), guard rails, high-pressure gas containers, instrument casings wait. In addition to being used in many metal products, electrostatic coating is also applied to various resin parts such as acrylonitrile butadiene styrene resin (ABS resin). It can be said that electrostatic coating is regarded as a representative industrial coating method. In addition, ultra-high-speed rotary disk-type and bell-type coating machines have been developed and popularized in recent years. Since these machines have high capabilities, they are also used for ultra-high-end ear paint coating.
2. The principle of electrostatic coating and the classification of electrostatic coating machines.
(1) Principle of electrostatic coating
Electrostatic coating uses the grounded object as the positive electrode, the paint atomizing device as the negative electrode, and the paint atomizing device The method of applying a high negative voltage to create an electrostatic boundary between the two electrodes, making the atomized paint particles negatively charged, so that the paint is effectively adsorbed on the surface of the object opposite the electrode, is called electrostatic coating. So why do we use the object to be coated as the positive electrode and the paint atomizer as the negative electrode? It is because on the contrary, it is difficult and safe to transfer it to spark discharge. In order to implement electrostatic coating, the grounding of the object to be coated is an important and necessary condition, and the object to be coated must be a good conductor of electricity. Wood is generally considered a poor electrical conductor, but when its moisture content is above 8%, electrostatic coating can be implemented, and it has been confirmed empirically. This is because in normal electrostatic coating, the current flowing between the positive and negative electrodes is 20 to 50 microamperes (MICRO-AMPERE), which is very small. Therefore, even objects with a large resistance value, such as wood, can Electrostatic coating can be implemented. However, in Japan, during the dry period in winter, the moisture content of wood will drop below 8%, so the amount of paint adhesion is significantly reduced. In this case, it is also an effective method to use a water vapor chamber to pass through the wood to increase the moisture content of the wood surface.
(2) Classification of electrostatic coating machines
1. Classification according to paint atomization method
In the past, it was classified into electrostatic atomization method and mechanical mist Two ways of transformation. The so-called electrostatic mist is the disc type as mentioned above. The paint is charged with high voltage and supplied to the center of the disc. Then the centrifugal force generated by the rotation of the disc expands to form a thin film on the surface of the disc, and changes from liquid filaments to paint at the edge of the disc. The way the fog is called. Another mechanical atomization method is the paint method. However, due to the recent invention of small-diameter ultra-high-speed discs or clocks, the classification according to the atomization method of paint has become slightly more complicated. The author attempts to classify them as follows. Then
(1) Electrostatic atomization method (low-speed rotary atomization head type, blade type atomization head type).
(2) Centrifugal force atomization method (ultra-high-speed rotating atomization head).
(3) Pressure atomization method.
(4) Airless atomization method.
(5) Hydraulic atomization and auxiliary air combined use (no air and pressure air atomization method).
2. Classification according to the charging method of paint
Giving paint mist particles a negative electric charge is an important step in the electrostatic coating process.
Including new models, the classification from this interview is as follows:
(1) Contact charging method
(2) Ion charging method
Note: The above Representative models of the contact charging method include the rotary atomization head type, while the electrostatic atomization method and the centrifugal force atomization method fall into this category. There are also spray gun type coating machines that apply high voltage to the paint path inside the gun and then mechanically atomize it. The ion charging method is suitable for coating machines with short needle electrodes at the tip of the gun. The mechanical energy required to micronize the paint and the paint switch are turned "ON", and at the same time, high voltage is applied to the coating machine from the high voltage generating device. As shown in Figure 2-(1), near the spray-painted tip electrode, there is a needle-shaped electrode, so the electrical boundary is extremely uneven, and the air is strongly ionized. However, near 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 as purple-blue light spots. This part is a very strong ionization zone, but the actual effective ionization zone is larger. As shown in Figure 2-(2), the paint particles are negatively charged, fly in the direction of the electrical boundary as shown in Figure 2-(3), and adhere to the object. Pressure atomization, airless atomization and airless-pressure atomization belong to this method.
3. Classified according to the relationship between the atomizing head of the electrostatic coating machine and the high voltage generating device.
Until recently, it was common knowledge that high-voltage generating devices were installed outside the coating booth, 5 meters or 10 meters away from the atomizer head. However, recently, the internal parts of the atomizer head have been developed to increase the voltage. A device within a mechanism or rectifying mechanism. The former is called the power supply remote control type (POWERSUPPLYREMOTETYPE). The latter is called the PACK-IN type. However, the power supply of the remote control type has gradually shifted from the large wet type (using insulating oil internally) to the small dry-pack (DRY-PACK) type (using molding resin). Internally installed types include transformer type, high voltage generating type (COCKCROFT) or so-called grille (CASCADE) type, electric power (EGD) type and turbine power generation type.
4. Other classifications
Some are divided into automatic type and manual type, or are called robot type, elevator type, etc. because they use robots or automatic elevators together. These classification methods are considered to use discriminative classification.
Functions of electrostatic coating
The advantages of electrostatic coating are analyzed from three standpoints: cost management, quality management and production management (project management, delivery time management) .
(1) From the perspective of cost management: due to the use of electrostatic coating, when using a pressure spray paint gun, excess paint mist (OVER-SPRAY) or rebound (BLOWBACK) is greatly reduced, resulting in significant savings. coating. For example (Japan), when the door frames of wooden cupboards were painted with polyester resin paint, the coating cost was reduced by US$400,000 a year. This is the result of switching from compressed air spray gun painting to small diameter ultra-high-speed rotary elevator-type disc painting. The reduction in paint helps with cleanliness in the paint booth. Therefore, although it is an indirect effect, it can also reduce the cost and time required for maintenance and cleaning.
(2) From a quality management standpoint: In the spray paint form of electrostatic coating, the micronized paint particles repel each other because they are negatively charged. This repulsive effect makes the distribution of particles within the paint shape more uniform, thereby reducing uneven coating. Due to the ultra-high-speed rotary coating machine, the paint particles have a high degree of micronization and a small particle size distribution. Therefore, the uniformity of coating film thickness can be significantly improved.
(3) From the perspective of production management: reduce the rebound or looping (LAPAROUND) effect caused by electrostatic attraction (referring to the excess paint mist adhering to the back of the object to be coated due to the effect of electrostatic attraction) In terms of phenomenon), it not only reduces the amount of paint, but also shortens the painting time. Especially when using an automatic coating machine with a larger atomization capacity, the coating speed is faster. It can produce stably and is easy to manage. (It is also a major function to be able to coat the ends and edges of materials)
4. Electrostatic coating and safety
(1) It is currently estimated that there are more than 10,000 factories in Japan Implement electrostatic coating.
Among them, there are many factories that have commonly used electrostatic coating for more than 20 years, and there is no harm or fire. On the other hand, several fires occur in some factories every year. According to investigations, the causes are mostly due to habituation due to long-term use and neglect of basic management or handling. There are many combustible materials around the wooden furniture factory. It is important not only to tidy up and tidy up the objects to be coated, coatings, and solvents, but also to manage the inside and outside of the electrostatic coating machine or spray booth according to the specified standards at startup. It is a fact that with the advancement of technology in the future, the safety of electrostatic coating machines will also improve. For example, the development of technologies such as high-speed detection of abnormalities in the current flowing between the coating machine and the object to be coated, rapid interruption of high voltage, various interlock devices, and automatic safety devices. These technologies compensate for matters considered unsafe in management, processing, or operation.
(2) The causes of fires during electrostatic coating vary depending on the type of coating machine, the nature of the coating, the shape of the object to be coated, the method of transportation or the factory environment, and there are many reasons. However, in manual coating machines, since the operator or the solvent tank for cleaning is insulated, discharge occurs when the electrical tape is close to a grounded object, resulting in the most fires. In addition, automatic coating machines cause the most fires due to damage to the insulation of the coating hose.
Safety Countermeasures
1. Most electrostatic painting accidents are caused by those who should be grounded but are not grounded, and form an insulated state, while those who are electrified. Conductive objects in the coating booth must be clearly distinguished as high voltage potential or ground potential. Except for those or parts where high voltage should be applied, they must be grounded. This is the first principle. In addition, regular confirmation of the ground potential can prevent most of the fire accidents. Simply speaking, coating machines are considered to be the most dangerous because they apply high voltage. However, the latest coating machines are equipped with high-level safety measures, so accidents caused by coating machines are very rare.
When the vapor concentration of all general organic solvents is at the explosion limit during painting, the minimum ignition energy of the solvent vapor that can ignite is about 0.2mJ (2/10000 Joules). The electrostatic capacity of the human body is about 100pF. Therefore, if the human body has a potential of about 2KV, it has 0.2mJ of energy. If the operator is not grounded, a dangerous situation will occur. This shows how important it is to keep the human body uninsulated in the electrostatic field.
The following safety measures are based on grounding methods:
(1) Operators must wear electrified shoes (leather-soled shoes are electrified).
(2) Clean the soles thoroughly.
(3) Conductive objects in the coating booth must be firmly grounded.
(4) The solvent tank for cleaning is fixed and must be grounded.
(5) Operators using electrostatic portable paint guns must operate the paint guns with bare hands.
(6) The hanger for hanging the object to be coated should be fully cleaned and the object to be coated should always be kept at ground potential. (Pay attention to the contacts of the hanger)
(7) The ground in the coating room must always be kept conductive.
2. Use and operation should basically prevent the occurrence of discharges that may cause fire hazards. Painting machines or paint tanks that are considered to be electrically charged should be fully ventilated and grounded before use, and residual charges can escape before use. These are the matters that must be followed when replenishing paint or cleaning the paint room.
The following are the key points for use and operation.
(1) When painting is completed (the same applies when painting is temporarily interrupted), the high voltage must be turned off. Do you need to confirm the high voltage before cleaning?
(2) When using conductive paint (rarack and alkyd resin paints are mostly conductive) and the paint supply system is insulated from the ground, apply paint To supply the paint tank, first ground the paint tank [at this time, the ventilation fan must be turned on and the large solvent vapor part (such as the outer bottom of the paint tank) must be grounded.
(3) If a solvent is used to clean the insulating material, confirm that the solvent is dry before applying high voltage.
(4) Regularly clean the paint dust pollution of the coating machine.
(5) Painting machines that can trap solvent vapor are not allowed to be used to prevent contamination and coating.
(6) Ensure that the high-voltage cables or paint hoses are loosened or detached.
(7) For others, be familiar with the instruction manual and follow it thoroughly.
3. Coating machine system and attached equipment
To prevent discharge, an interlock device is installed in the application of high voltage to prevent unnecessary conductive objects and operators from entering. In the realm of tranquility.
Key points on the coating machine system and ancillary equipment:
(1) Provide sufficient locking devices necessary for operation.
(2) Those that require sequential actions will not be able to specify operations other than the sequence.
(3) Provide appropriate interlocking devices for conveyors, entrance and exit doors, air supply and exhaust, fire-fighting devices and coating machines.
(4) Set up safety fences or prevent operators from entering the device.
(5) Use a longer hanger to reduce the contact point between the conveyor and the hanger.
(6) Use automatic cleaning system (automatic coating machine).
(7) For emergencies, install automatic fire-fighting devices.
4. Safety system of electrostatic coating machine
The safety system of electrostatic coating machine can be divided into two categories. One is a coating machine in which the needle electrode is installed at the tip of the spray gun. The body of the coating machine is made of insulator to reduce the electrostatic capacity of the needle electrode. A high resistance is inserted next to the electrode and connected to the high voltage generator. The needle-shaped electrode is easy to discharge. As it approaches the ground object, the high voltage applied to it decreases rapidly. The relationship is shown in Figure 3 below. If it is a paint with a high resistance value, it will be a very safe coating machine. Figure 3 shows an example of this type of distance-voltage and current change. The other is an electrostatic atomization or centrifugal force atomization coating machine. Its electrode capacity is extremely large, so it has large ignition energy. Therefore, it is very dangerous to use it in its original condition, so safety devices such as so-called spark guard (SPARKGUARD) or overcurrent detector are installed as preventive measures. That is, before spark discharge is generated, if the passing high voltage exceeds the set inspection current value, the machine will be shut down due to its own weight, and its safety is lower than the former (a coating machine with a needle electrode installed at the tip of the spray gun) and requires higher Performance. However, the test current value changes di/dt and the residual charge of the coating machine is grounded with an ultra-high-speed high-voltage interrupter. We have developed such a very safe system, so we have reached the point where we can use the electrostatic coating machine with confidence.
Process conditions that affect the effect of air electrostatic spraying
(1) Relative position of spray guns: In places where multiple spray guns are operated at the same time, attention should be paid to their relative positions when using them, and the spray should be The paint flows do not interfere with each other and prevent paint mist with the same charge from repelling each other, especially for places where small and medium-sized parts are sprayed.
(2) Spray room wind speed: During electrostatic spraying, the wind speed and exhaust volume of the spray room are smaller than those for general air spraying. The wind speed is 0.5-0.7m/s.
(3) Resistance of coating: The electrical properties of coating directly affect electrostatic atomization and electrostatic effects. If the resistance value is too high, it will be difficult to charge, and the electrostatic effect will be poor; if the resistance value is too small, it will easily leak electricity, endangering the spray gun and not conducive to safety. Electrostatic spray guns of various specifications have their own specific resistance value ranges, which can be determined based on field tests and the recommendations of the spray gun supplier.
(4) Coating viscosity: The construction viscosity of coatings used for electrostatic spraying is generally slightly lower than that of air spraying, in order to improve the atomization effect and make the paint mist easy to surround and deposit along the direction of the power line.
(5) Air pressure: Under the same pipeline paint delivery system conditions, the pressure required by an air electrostatic spray gun is lower than that of an ordinary air spray gun. Excessive air pressure will affect the normal operation of the turbine generator on the one hand, and also affect the electrostatic effect on the other hand.
(6) Paint delivery pressure: Compared with ordinary air spray guns, the paint pressure required for electrostatic spray gun spraying is also slightly lower.
A preliminary discussion on the air electrostatic spray gun spraying process
The electrostatic spraying process has been increasingly used in the machinery industry, especially in the automobile manufacturing industry. It is increasingly favored by people for its extremely high paint utilization rate, efficient painting productivity, excellent painting quality, substantial reduction of waste paint, waste paint mist, and reduction of environmental hazards. According to the shape and installation method of the electrostatic atomizer, electrostatic spraying equipment can be divided into two categories: fixed and manual mobile.
For the design of a new painting production line and the renovation of some old production lines of a certain scale, the former is often used. However, for old production lines with smaller original design scale and more crowded painting operation sites, portable electrostatic spray guns are used because of their flexibility and portability. , does not occupy the working space and has unique advantages in uniform coating of workpieces with complex shapes and recessed parts. This article conducts a preliminary discussion on the process based on the batch trial of air electrostatic spray guns on the production line.
1. Spraying tools and usage
The spraying tool is the PRO3500 air electrostatic spray gun produced by GRACO. It consists of a gun handle, a barrel, a trigger, an air cap assembly, and various pipelines. , gas circuit system and other components. This type of spray gun has been used in batches by many automobile and motorcycle manufacturers in my country and has certain practicality and representativeness. Its electrostatic principle is that compressed air drives a built-in turbine generator and is continuously amplified and rectified to form a 6.5X104V high-voltage direct current. , the paint liquid particles are charged and further atomized for electrostatic coating. The difference in the external structure of an air electrostatic spray gun from an ordinary air spray gun is that in addition to the electrode needle at the nozzle, it is also equipped with an exhaust pipe for removing excess exhaust gas from the turbine generator and cooling the generator. Additionally, its air tube is electrically conductive and is safely grounded when used in spray gun systems. What you must pay attention to when using it is that as a safety guarantee procedure, when inspecting, repairing, installing, cleaning and interrupting spraying operations, you must relieve the pressure, that is, turn off the electrostatic switch, cut off the air and paint supply, pull the trigger, and spray the *Ground releases residual paint and air. In addition, the maintenance of the air electrostatic spray gun is also very important. The cleaning solvent must be a non-conductive solvent. Conductive solvents will cause the spray gun to malfunction. It is also not allowed to soak the spray gun in the solvent, otherwise it will easily cause a short circuit and reduce the electrostatic effect.
2. Safety regulations and safety inspections
The most important issue that requires special attention in electrostatic spraying is safety. For this reason, the country has specially promulgated relevant safety standards. For PRO3500 and other similar air electrostatic spray guns, the following safety measures and safety inspections need to be taken before each use.
(1) Spray gun system safety: The system must be safely grounded, and the resistance of the gun body to ground is less than 1-2 megohms.
(2) Human safety: Operators and all personnel entering the work area must wear conductive shoes. Operators are not allowed to wear insulating gloves. The human body resistance to ground is less than 1-2 megohms.
(3) Workpiece status: The ground chain trolley or guide rail can be directly grounded and in point or line contact with the workpiece. The resistance of the workpiece to ground is less than 2 megohms, otherwise it will directly affect the electrostatic effect and even cause paint dance back. Hug the operator.
(4) Operation area safety: All flammable liquids in the operation area must be placed in safely grounded containers, and all conductors should be grounded.
(5) Operation guarantee: All operators must be trained to use the electrostatic spraying system safely and master pressure relief techniques.
3. Process conditions and operations
Compared with ordinary air spraying, the following process conditions directly affect the coating effect:
(1) Relative position of the spray gun: In places where multiple spray guns are operated at the same time, attention should be paid to their relative positions when using them, so that the sprayed paint streams do not interfere with each other and avoid paint mist with the same charge being repelled, especially for places where small and medium-sized parts are sprayed.
(2) Spray room wind speed: During electrostatic spraying, the wind speed and exhaust volume of the spray room are smaller than those of general air spraying. On the one hand, electrostatic spraying itself has a small amount of paint mist and solvent volatilization, so there is no need for excessive air exhaust volume. On the other hand, excessive wind speed directly affects the adsorption force of paint mist in the electric field and affects the electrostatic effect. For general air electrostatic spray guns, the recommended wind speed is 0.5-0.7m/s.
(3) Resistance of coating: The electrical properties of coating directly affect electrostatic atomization and electrostatic effects. If the resistance value is too high, it will be difficult to charge and the electrostatic effect will be poor; if the resistance value is too small, it will easily leak electricity, endangering the spray gun and being detrimental to safety. Electrostatic spray guns of various specifications have their own specific resistance value ranges, which can be determined based on field tests and the recommendations of the spray gun supplier.
(4) Viscosity of coating: The construction viscosity of coatings used for electrostatic spraying is generally slightly lower than that of air spraying, in order to improve the atomization effect and make the paint mist easy to surround and deposit along the direction of the power line.
(5) Air pressure: Under the same pipeline paint delivery system conditions, the pressure required by an air electrostatic spray gun is lower than that of an ordinary air spray gun. On the one hand, excessive air pressure will affect the normal operation of the turbine generator, and on the other hand, it will also affect the electrostatic effect.
(6) Paint delivery pressure: Compared with ordinary air spray guns, the paint pressure required for electrostatic spray gun spraying is also slightly lower.
4. Painting effect
Compared with ordinary air spraying, air electrostatic spray gun spraying has the following advantages:
(1) The thickness is increased and uniform: in Under the same operating method, due to electrostatic adsorption, the film thickness of air electrostatic spraying is slightly higher than that of air spraying (about 3-5μum), and due to the surrounding effect, corners, depressions and other parts of the workpiece are easy to paint. The film is more uniform and less prone to leakage.
(2) Good leveling properties. Since the wet film is smooth and uniform, under the conditions of appropriate viscosity, temperature and humidity, the paint film is easy to level, the orange peel is significantly reduced, and the paint film is flatter< /p>
(3) Sharpness of image: Due to the reduction of appearance defects such as orange peel, the sharpness of the paint film is directly improved.
(4) Gloss: The gloss is slightly improved (about 1%), and the gloss improvement is more obvious for areas that are difficult to paint such as inner surfaces and corners.