Correct choice of powder coating equipment

Abstract: there are many ways to apply the powder coating, but the powder coating and the selected coating method must match, for example, the powder used in fluidized bed coating and electrostatic spraying is different< br / > after the powder coating is selected correctly, the coating method is determined by the workpiece structure and production targetThere are two kinds of coating methods, they are suitable for different applications, namely fluidized bed coating and electrostatic spraying< br / > fluidized bed coating < br / > this coating method is the first one for coating powder coating, and it is still used in the situation where the thickness of solidified film is greater than 0.5milThe typical coated workpiece is metal wire, electrical bus, etc< br / > there are two methods of fluidized bed coatingOne method is non-static current bed, which is to preheat the workpiece and dip it into the powder fluidized bed for coating, and make the powder melt and adhere to the surface of the preheated workpieceThe amount of powder coated on the surface of the workpiece is related to the preheating temperature of the workpiece and the residence time of the workpiece in the fluidized bedThis method is not strict enough to control the film thickness< br / > in order to improve the control accuracy of coating film thickness in fluidized bed coating system, the electrostatic principle is introducedAs shown in Figure 1, the workpiece is transported over the fluidized bed, while the powder is adsorbed on the workpiece surfaceThe workpiece does not need preheating before entering the fluidized bed< br / > powder particles are adsorbed on the surface of the workpiece under the action of static charge, which is formed by setting up an electrostatic field above or in the fluidized bed< br / > therefore, the film thickness on the surface of the workpiece in the electrostatic fluidized bed is not only related to the residence time of the workpiece in the fluidized bed, but also related to the electrostatic charge of the powder particlesThis process sometimes uses heating to overcome the Faraday cage shielding problem caused by the workpiece structure< br / > the electrostatic fluidized bed is mainly used to coat the rotor of the motorThis kind of workpiece needs high dielectric strength coating for protection, so the control of the coating thickness is very important < br / > spraying < br / > there are two methods of coating powder coating with electrostatic spraying equipment Each method must use electrostatic to attract the powder to the workpiece, which is different from what is seen in the liquid spraying system There is no mechanical attraction or adhesion to absorb the powder to the workpiece Two types of electrostatic spraying equipment are corona charged spray gun and friction charged spray gun < br / > corona spray gun < br / > this device uses an electrostatic generator to generate an electrostatic field between the spray gun and the grounded workpiece The electrostatic charge on the surface of powder particles is related to the strength of electrostatic field and the residence time of powder particles in the electric field < br / > Figure 1 electrostatic fluidized bed for powder coating < br / > Figure 2 working principle of corona spray gun < br / > another important parameter is the specific surface area of powder particles The following equation (Figure 2) explains this phenomenon well: < br / > electric field strength: e = V / d < br / > electric charge of particles: q = 1 / 2cet2 < br / > the equation shows which parameters are more important For example, the electrostatic field strength is directly proportional to the electrode voltage, while the distance between the workpiece and the spray gun (sometimes referred to as the target distance) directly affects the electrostatic field strength < br / > the amount of charge on the surface of powder particles (generating attraction) is mainly determined by the time the particles stay in the electric field (the square relationship) The time and electric field strength will determine the amount of powder (i.e primary powder loading rate) attracted to the workpiece surface < br / > the process is to electrify the powder coating and deposit it on the conductive (grounded) workpiece This is why it is difficult to spray electrostatic powder on non-conductive materials (such as plastic, rubber, glass, etc.) < br / > in addition, electrostatic charging of conductive materials (metal powder) is also difficult Due to their conductivity, they can form a short circuit between the spray gun and the grounding material, which will dissipate the surface charge of powder particles < br / > there are many kinds of electrostatic generators used in this method Some manufacturers place the generator in the control cabinet and use high-voltage cables to transmit the voltage to the spray gun electrodes Some manufacturers put the generator on the spray gun and use the low-pressure control signal to adjust the electric field strength These two types of spray guns are used in today's spray industry, and play the same function Most manufacturers can provide positive and negative electrostatic spray guns < br / > friction spray gun < br / > friction electrified spraying equipment uses the working principle of friction electricity generation < br / > triboelectrification can be illustrated by the following examples In winter, when you walk slowly on the carpet in slippers, static electricity will be generated, which will be stored on your body When you contact with the ground (such as the switch of the lamp), the charge will usually dissipate This kind of static electricity will only appear in the dry environment That's why we won't be bothered by static electricity in the wet summer, and we often encounter static electricity in the dry winter < br / > friction charged spraying equipment generates static charge on the surface of powder particles by friction between the powder particle beam and the surface of insulating materials Prolonging the path of powder passing through the spraying equipment is the feature of friction spray gun, while the path of powder particle beam can be straight line or curve < br / > the factors that affect the surface electrostatic charge of powder particles are: (1) the action time of friction electrification of powder particles; (2) the specific surface area of powder particles; (3) the dryness of the air used to transfer powder or contact with powder; (4) the type of resin material used to prepare powder < br / > it is very important to control these parameters in order to ensure that the powder particles can carry electricity properly If the powder is not charged, it cannot be adsorbed on the workpiece (unless the surface temperature of the workpiece is high enough to make the powder melt and adhere to it) < br / > the static charge of the friction lance is generally less than that of the corona Lance The polarity of friction charge is related to the type of powder coating and friction material If the two materials are the same, the polarity will be the same < br / > powder conveying < br / > rotating cup < br / > there is a new powder spraying technology called powder rotating cup The device uses air turbine to turn the rotor to atomize powder coating < br / > pump the powder to the rotating cup, which atomizes the powder with the rotating force < br / > the feeding device of the rotating cup spraying system is similar to that of the spray gun spraying system, and it is also sprayed by the corona charged spray gun < br / > the powder rotating cup can spray a large amount of powder coating on the surface of large substrate, so the typical application of this spraying equipment is large and flat workpieces, such as electrical appliances and automobile body < br / > a feeding system is required for all spray equipment (Figure 3) The feeding system is composed of a feeding hopper, a powder pump and a powder feeding hose < br / > Figure 3 powder feeding system < br / > there are two forms of feeding hopper (Figure 4) The first type is called the gravity feed hopper, which, as the name implies, uses gravity to move the powder to the powder pump at the bottom This hopper is usually tapered and collects the powder into the pump < br / > Figure 4 hopper design < br / > agitators or vibrators can be used to keep the powder flowing evenly Usually, without the help of mechanical devices, the powder will be bridged at the bottom of the funnel, resulting in uneven feeding to the pump < br / > because there is no air in the hopper to mix with the powder, the system sends dense powder materials to the powder pump; large powder output is the characteristic of this kind of system < br / > the second type of powder feeding hopper is fluidized bed It is similar to the fluidized-bed system described earlier Connect the compressed air source to the plenum under the fluidizing bed plate The air shakes the powder in the hopper through the fluidizing plate to form a fluid state like water, which is convenient for powder pump to extract powder Because the powder is mixed with the air from the plenum chamber, resulting in the reduction of powder density, the powder output of this feeding system is lower than that of the gravity feeding system < br / > if the powder pump is installed on the top of the hopper, use an extraction pipe to extract the powder from the hopper The extraction pipe is generally located two-thirds of the distance from the fluidized bed powder pump (Figure 5), which is to prevent the turbulence on the surface of the fluidizing plate from being pumped into the powder Turbulence will destroy the uniformity of the feed to the spray gun < br / > Figure 5 structure of powder pump < br / > powder box feeder < br / > powder equipment manufacturer has invented a new method, that is, directly take powder coating from powder box and send it to spray gun, this method is called powder box feeding, it uses an inclined vibration table to support powder box and insert the extraction pipe connected to powder pump into the deepest part of powder box Sometimes, a compressed air nozzle is arranged at the end of the extraction pipe to help the powder flow into the pipe < br / > as the powder is directly extracted from the powder box to the spray gun, only the extraction pipe, pump and hose need to be cleaned and the powder box can be replaced < br / > the powder pump is designed according to the Venturi principle The compressed air is perpendicular to the suction direction of the venturi, resulting in a differential pressure or vacuum that draws the powder out of the hopper The powder enters the compressed air stream and is sent forward along the hose to the spray gun < br / > some powder pumps introduce additional compressed air at the entrance of powder into the air flow (Fig 5) Since the amount of powder extracted is directly proportional to the amount of compressed air used to produce venturi suction, it is beneficial to introduce a second gas source The more the first air source is used, the more powder is pumped < br / > if the feeding hose is too long or the path through it is bent, the powder may pulsate at the spray gun If the powder stays at the bottom of the hose and does not move forward, enough air pressure will be accumulated to push it forward abruptly, which is the pulse of the powder < br / > the second air flow helps to prevent powder pulsation, pushing the powder through the hose without adding more powder to the fluid Since both air sources have a one-way valve, it can control both the amount of powder and the speed of powder flowing through the feeding hose < br / > powder hoses can be made of a variety of materials Several typical materials are urethane, vinyl resin and some rubber compounds Hose diameter and length are key parameters The hose diameter is determined by the powder pump used and must match the manufacturer's recommendations The length of the hose shall be as short as possible to reduce the delivery resistance of the powder pump and the surge of powder fluid on the spray gun, and avoid the bending and discount of the hose path < br / > if the more powder is delivered to the spray gun, the faster the powder will pass through the electrostatic field When the feeding speed is fast, the powder loading rate will decrease < br / > spray gun movement < br / > automatic spray system is usually equipped with auxiliary equipment for spray gun movement, which can be divided into four categories: oscillator, reciprocator, multi axis device and manipulator < br / > Figure 6 spray gun motion device < br / > each spray gun motion system has different design and is used to meet a certain kind of special painting requirements; however, these systems have one thing in common, that is, they are used to make the spray gun move on one or more planes, and the spray area is larger than the spray gun motion area, so the number of spray guns required for spraying a given area can be reduced < br / > oscillator and shaker < br / > there is a kind of gun moving device called oscillator Different from other movers, the oscillator usually has a fixed stroke and speed Some devices can adjust these parameters, but they cannot be adjusted when the machine is running < br / > the main components of the oscillator are the eccentric wheel and the eccentric rod (Fig 6) The motor drives the eccentric wheel to rotate, and the connecting rod is fixed at a certain distance from the center point of the wheel, so as to change the rotation into a vertical movement up and down < br / > the stroke length can be adjusted by fixing the connecting rod on the different radial positions of the eccentric wheel, and it can be installed by the motor and the speed regulator