Construction variables in powder coating system

Application variables for powder coating system although the worldwide economic development speed slows down in recent years, the powder coating industry in North America still maintains an annual growth rate of 12% - 15% (by weight) (see Figure 1) The rapid development of powder coating requires equipment suppliers to develop more effective powder coating equipment This paper summarizes the powder construction technology and various variables that affect the primary powder loading rate Figure 1 production of North American thermosetting decorative powder coating principle of powder coating electrostatic spraying of powder coating starts from powder fluidization Fluidization is a process of mixing powder and compressed air, which makes powder flow from powder supply barrel to spray gun The powder flow is controlled by adjusting the air flow The powder sent to the spray gun is charged under the action of corona spray gun or friction spray gun The charged powder is moved to the grounded workpiece by the air flow in the spray gun and spray chamber When the powder particles are close to the workpiece, the electrostatic attraction between the charged powder and the grounded workpiece will absorb the powder to the workpiece, and then the workpiece can be solidified through the drying room The overspray powder is collected in a container and sent to the primary bag filter under the action of the fan After treatment, the circulating air without powder is treated by the high efficiency ultimate filter to become clean air and discharged into the workshop The primary bag filter removes the overspray powder by means of intermittent reverse pulse The collected powder is screened and then sent back to the spray gun for reuse Figure 2 shows the whole process with a simple diagram Fig 2 what is the primary powder loading rate in the powder spraying process flow chart? The primary powder loading rate refers to the ratio of the powder attached to the workpiece to the total powder ejected by the construction equipment There are many factors to be considered to improve the primary powder loading rate In the following chapters of this paper, various factors and process conditions affecting the primary powder loading rate will be discussed (see Figure 3) Fig 3 factors affecting powder loading rate design of spray gun and nozzle in the market electrostatic powder spraying equipment, corona charged and friction charged are two very common powder spraying methods At present, the most commonly used electrostatic spraying method in North America is composed of three parts in Figure 4 Fig 4 part of the electrostatic spraying process Electrostatic powder spray gun electrostatic powder spray gun should have the following functions: • shape and direction of powder atomization; • control the size, shape and density of powder spray; • transfer static charge to powder to be sprayed; • control the deposition of powder on the workpiece The function of the charging system is to give the powder particles the ability to adhere to the surface of the bare grounded workpiece (see Figure 5) Fig 5 corona spray gun with corona charged the corona spray gun uses high-voltage cable or low-voltage cable to apply the voltage to the pole pin of the corona spray gun The type of cable depends on the high-voltage generator, which can be built-in or external to the spray gun Generally, the electrode used in the corona spray gun is a negative electrode, which can produce a lot of charged ions The negative potential of the charging electrode is very high, generally between 30000-100000 volts The positive electrode is used when spraying nylon or using automatic friction spray gun system Figure 6 is a schematic diagram of an external powder spray gun and an internal powder spray gun Figure 6 corona charging method the design requirements of corona spray gun are: high electric field strength; low air flow rate; good powder dispersion; reasonable design of electrode geometry to make the ion flow through powder, electrode and grounded workpiece in the same straight line Of course, the shape of powder atomization, the cost of equipment, the difficulty of maintenance and durability must also be considered in the design The word "tribo" is derived from the Greek word "tribone", which means friction Triboelectrification is to let the powder particles flow through the surface of a friction material at high speed and charge transfer occurs during friction (see Figure 7) In fact, there is no external powder supply, and the electric field is located at the front of the spray gun The friction charging method solves the "Faraday cage shielding effect" The powder particles lose electrons in the spray gun and are positively charged to move the powder particles to the substrate surface Under the action of nozzle and air flow, the powder can enter the concave part of the substrate or other parts that are difficult to be coated Because the charging process is carried out in the spray gun, without the electric power line of the electric field, the powder particles will not accumulate on the edge of the spraying workpiece, thus basically eliminating the "fat edge phenomenon" Fig 7 it should be noted that the chemical structure and particle size of the powder required by the friction charged spray gun are more strict than that of the corona charged spray gun, so it is necessary to use the friction charged spray gun to test the powder before use to ensure good coating effect The two main types of powder spray gun are manual spray gun and automatic spray gun The manual spray gun can be used to spray scattered workpieces that do not need automatic spraying, or the automatic spray gun can be used together to paint some workpieces that are difficult to powder and / or complex Because the duster has to hold the spray gun for a long time, the flexibility, weight, efficiency and durability of the spray gun are very important Most dusters will hold the cable and hose with another hand, so the length of the cable and hose should be 3-4 feet In addition, the cable and hose are directly connected to the spray gun, and their weight is directly related to the balance of the whole system The automatic spray gun can be fixed, installed on a movable mechanical arm, or a combination of the two In many cases, the automatic spray gun system does not need auxiliary manual spray gun Here are six questions you need to consider when purchasing a powder spray gun: &; is it easy to remove and install? • what type of nozzle is it? • is it easy to clean and change colors? • how many easily worn parts are there? • where is the production plant? • try your selected spray gun on your workpiece According to different application occasions, each type of spray gun has its own market The difference between them lies in the special design and comprehensive charged performance It should be remembered that the most important thing is that the powder spray gun is only one part of the construction chain The design of spray nozzle and spray nozzle is the key to improve the primary powder loading rate As mentioned before, the air flow speed in charging time and space is low, and the powder dispersion is good, which is the purpose of nozzle design The two most commonly used nozzles are partial transformation and flat type, both of which can have different atomization shapes The flat nozzle has more directionality and clearer atomization shape, while the partial transformation nozzle has softer atomization shape, better dispersion and the appearance of liquid paint mist Although there are many other types of nozzles, these two kinds of nozzles account for 90% in the present powder coating system The powder spray gun also has some matching parts, so that you can safely paint some deep sunken workpieces without any effort (see Figure 8) Fig 8 the function of the powder pump and powder flow rate powder pump of the nozzle is to deliver powder to the spray gun at a uniform and constant speed, and the conveying process adopts the principle of windury The flow rate as low as possible is the ideal condition to ensure a good coverage of the workpiece and to achieve a high primary powder loading rate, which is consistent with the requirements of high charged efficiency, that is, to control the powder output of the spray gun This is because the more powder passes through the electrode in the gun in a certain period of time, the less chance for each powder particle to be charged, so a high powder flow rate is very unfavorable to the powder loading rate It is better to increase the number of spray guns to reduce the output of each spray gun than to increase the number of spray guns In any powder coating system, the most easily neglected or underestimated equipment is fluidized powder feeder Let's take a look at several important design parameters of the fluidizing powder feeder • powder quantity control; fluidizing pressure system; exhaust system A good powder control system can maintain a certain powder height and feeding speed The fluidization pressure system is to make the air pass through the gas distribution partition evenly, so as to form a uniform fluidization effect in the powder feeder Too high fluidizing air flow is not good for powder loading rate Too much air or uneven distribution of air will lead to turbulence in some areas, thus reducing the efficiency of powder pump, making powder pump suck air and reducing powder flow The powder feeder is similar to a pressure vessel, which accepts the powder delivered by 3-4 venturi pumps These pumps are ordinary air pumps The air flow from the air pump and the fluidized bed should be decompressed in the powder feeder The best way to remove excess air is to design a direct exhaust system to reduce internal pressure (see Figure 9) Fig 9 the operation efficiency of powder coating system can be improved by the stacking and control process control and monitoring equipment of powder feeder If the spray gun in the system has been spraying powder and the distance between the workpieces is equal to the size of the workpieces, then half of the powder is sprayed into the air, and the highest powder loading rate that the system can achieve is 50% Using a timer to record the empty spraying time of the spray gun, we found that the spraying time of the spray gun accounted for 20% ~ 60% of the total working time of the spray gun By adjusting the spraying time of the spray gun, the primary powder loading rate of the whole system can be increased by 20% ~ 60% In addition, the spray gun which can automatically control the interval time of powder spraying can reduce the phenomenon of fat edge of the workpiece and improve the coverage rate of the workpiece (see Figure 10) Fig 10 the uniformity of air flow in the chamber triggered by the automatic spray gun is one of the key factors affecting the construction quality As mentioned above, for the powder loading rate, too large powder flow is not good Similarly, large air flow will also affect the primary powder loading rate Combined with the factors discussed in this paper, the powder spraying room should meet two conditions: 1) powder recovery; 2) safety The following focuses on powder recovery First of all, the powder coating system is required to be able to recover the powder In order to recover the powder particles that have been sprayed, the powder spraying room should provide an average surface speed of 100 in / S (lfpm) at all the inlet and outlet There are some exceptions, of course: & When the air flow is too high, the powder will be blown away from the spraying workpiece Many people try to compensate by increasing the powder flow rate, but this will reduce the primary powder loading rate The design of powder room should first consider the recovery of powder The room can be likened to a small room composed of four walls, ceiling and ground There is a longitudinal movable groove on the ceiling, and the hanger of the conveyor belt can hang the coated workpiece on it through the groove Because the powder is applied by electrostatic coating and requires high primary powder loading rate, it is always expected that the powder will be adsorbed on the workpiece rather than on the housing In order to achieve this goal, the walls around the powder room or automatic powder spray gun should be made of low conductive or non conductive materials, so that the electrostatic field released by the spray gun can attract the powder to the workpiece instead of the walls of the powder room • the distance from the spray gun to the wall of the spray room will affect the primary powder loading rate and the cleaning of the powder room The powder spray gun shall be at least 12 inches away from the powder room wall to make the electrostatic field work on the workpiece rather than the powder room body If the width of the bottom of the powder room is not enough to place the spray gun, you can add a spray room extension wall on the spray gun mouth to increase the spray gun