Application of powder electrostatic spraying in refrigerator coating

< br / > Yu Yinghao (Liaoning Yingkou Kelong refrigerator Co., Ltd) < br / > Abstract: the basic principle of powder electrostatic spraying technology and the mechanism of powder spraying curing film are described; the application of powder electrostatic spraying technology in Kelong refrigerator coating line is introduced, and the appearance quality of the film after curing film is analyzed< br / > < br / > 1 refrigerator electrostatic spraying production line < br / > 1.1 basic composition of electrostatic powder spraying system < br / > basic composition of electrostatic powder spraying system is shown in Figure 1< br / >, And from the safety point of view, it should be taken into account that there should be no discharge factorsTherefore, the output power of DC high-voltage generator is required to have the characteristics of high voltage and low current peak valueThe commonly used voltage is 50 ~ 100kV, and the current is less than 300 μ a< br / > (2) spray gun < br / > its function is to produce a good corona discharge, so that the powder sprayed with as much charge as possible, evenly distributed on the surface of the workpieceRequirements for spray gun: ① the powder particles should get the maximum charged effect; ② adapt to different powder spraying amount; ③ have the maximum safety factor< br / > (3) powder supply device < br / > it is to fluidize the powder in the fluidizing bucket, and then send the powder to the spray gun device by air, which is composed of a powder storage device, a powder delivery pump and a powder fluidizing bucket< br / > (4) air supply system < br / > in the process of powder electrostatic spraying, the powder coating should always contact with pressurized air, and the purification quality of compressed air used has strict requirementsThe condition of air source is: the moisture content is less than 1.3g/m3, the oil content is less than 10-7 (volume fraction) < br / > (5) the powder spraying chamber < br / > is the main place for spraying, the main function is to ground the workpiece as the positive pole of the electrostatic field, to form the discharge circuit, to ensure the quality of spraying workpiece, and to recover the powderThe wall panels of nuoxin powder spraying equipment spray room are made of transparent polypropylene material, with little adsorbed powder< br / > (6) the recovery device < br / > catches the powder in the gas coming from the powder spraying chamber and exhausts the clean airThe powder loading rate of the coating line is 70%, and the other 30% is recovered by cyclone separator and core filter< br / > (7) curing furnace < br / > it is the main part of the spraying line, heating and curing the powder on the workpiece to form a film< br / >, If the voltage is too low or the electric field strength is not enough, the powder cannot be fully charged and the coating thickness is uneven< br / > (2) safety issues < br / > powder coating is a flammable organic substanceWhen the powder coating is suspended in the air in the form of powder, and its concentration exceeds the explosion limit (30-50g / m3), it will be charged due to corona discharge, so when the spray gun is close to the workpiece, it will produce a spark phenomenon, and the temperature of the electric spark is as high as 7000 ℃, Powder coating is excited by such a high temperature in an instant, which will cause combustion, and at the same time, chain reaction will occur to cause combustion of surrounding powder In order to prevent accidents, the distance between the spray gun and the workpiece must be kept over 100 mm < br / > < br / > 2 electrostatic powder spraying process and control < br / > the main content of electrostatic powder spraying process control is: how to correctly use the spraying equipment and spray operation, so as to make the deposited coating on the workpiece evenly cover, smooth and flat, reach the required thickness, all physical and chemical indexes are qualified, and ensure the safety and health of the process < br / > 2.1 powder spraying process and parameters < br / > 2.1.1 typical process < br / > self inspection of pre-treatment phosphate parts → powder spraying → solidification → strong cooling < br / > < br / > transfer of qualified products < br / > finished product inspection < br / > rework of unqualified products < br / > 2.1.2 process parameters < br / > are shown in Table 1 < br / > Table 1 powder spraying process parameters < br / > item parameter value item parameter value < br / > electrostatic voltage / kv40-80 flow rate pressure / mpa0.2-0.5 < br / > electrostatic current / μ a10-40 atomization pressure / mpa0.30-0.45 < br / > venturi throat diameter / m m ≤ 8 gas supply pressure / mpa0.7 < br / > distance between spray gun mouth and workpiece / mm150-300 suspension chain speed / m.min-14.7-5.5 < br / > 2.2 powder spraying process control < br / > 2.2.1 coating thickness Control < br / > the workpiece is sprayed with powder electrostatic spraying at room temperature, the coating thickness is generally between 40 ~ 100 μ m; if twice spraying is used, the thickness can reach more than 100 μ m; if thicker anti-corrosion or electrical insulation coating is needed, the workpiece can be preheated before spraying < br / > in the actual production, when the powder spraying process is determined, the following process parameters can be adjusted to properly control the coating thickness < br / > For example, under the current line speed of 4.7-5.5m/min, in order to make the coating thickness meet the process requirements of 60-80 μ m, the powder output of the spray gun should be controlled within the range of 120-150g / min < br / > (2) the electrostatic voltage of the spray gun < br / > will directly affect the thickness of the coating, because the electric field force between the nozzle and the workpiece is the main power of powder coating, and the powder loading rate is different due to its different size For general flat panel spraying parts (refrigerator side plate and back plate), the electrostatic voltage is generally controlled at 55 ~ 70kv, and if it is too low, the coating will be too thin, and if it is too high, it is easy to produce spark breakdown Due to the different degree of old and new spray gun system, its electrical properties are also different After a certain period of use, the spray gun needs to increase the electrostatic voltage properly to ensure the normal coating thickness < br / > 2.2.2 coating uniformity control < br / > the most important factor affecting coating uniformity is electrostatic shielding, so in order to obtain uniform coating, firstly reduce the influence of electrostatic shielding of workpiece, especially the Faraday effect of workpiece tip or edge, and secondly ensure the stability and uniform dispersion of powder In production and application, the following points should be paid attention to: < br / > (1) the workpieces should be hung correctly on a large area or a surface that has a major impact on the appearance, which should be perpendicular to the ground and keep perpendicular to the direction of powder discharge of the horizontal spray gun < br / > In the process of production, it is mainly for this purpose to finish the hook and paint off the hanger < br / > (3) to prevent the anti ionization phenomenon of the powder layer < br / > the anti ionization refers to the phenomenon that when the powder layer reaches a certain thickness and then continues to spray, the coating does not thicken, on the contrary, there are many pockmarks and the surface becomes rough The main reason for this phenomenon is the local insulation breakdown on the stacked powder layer The prevention countermeasure is to properly control the spray gun to prevent the powder layer from being too thick < br / > (4) there should be no dirt and sundries in the powder < br / > the recycled powder should be screened before reuse, and the new powder should be screened as much as possible if possible; the recycled powder and the new powder should be mixed in a certain proportion (1:1-1:3) before reuse < br / > (5) control of environmental factors < br / > the temperature (about 25 ℃) and relative humidity (not more than 70%) of spraying environment shall be well controlled, otherwise the powder is easy to agglomerate, which will affect the screening capacity and coating quality of rotary screen; compressed air shall be supplied stably, clean and dry, free of oil and moisture, The specific quality index is generally determined as (take nuoxin ck05sl powder spraying system as an example): < br / > maximum input air pressure: 1.0 × 105Pa; maximum water vapor content: 1.3g/m3; optimum input air pressure: 6.0 × 105Pa; maximum oil content: 1.0 × 10-7 (volume fraction) < br / > (6) the distance between the spray gun and the workpiece is generally set as 150 ~ 300 mm On the premise of ensuring the thickness of the coating, the distance between the spray gun and the workpiece is better, which is conducive to the uniform distribution of the coating < br / >, In addition, the flow pressure should be increased to increase the air flow rate of powder feeding; secondly, the problem of fluidizing plate failure should be solved due to the blockage and uneven air permeability of the ventilating plate of the fluidized bed; or because the powder level in the fluidized bed is lower than the powder suction port and the air supply of the fluidized bed is too small or even interrupted, attention should be paid to adding powder or regulating the fluidizing air flow; thirdly, the fluidity of the powder coating itself is too poor or there is caking phenomenon, At this time, the powder or additional vibration measures of the fluidized bed should be replaced < br / > 2.2.3 coating defects and control < br / > in the process of electrostatic powder spraying, sometimes due to subjective or objective reasons, the coating will have defects, some of which have already appeared on the powder layer after spraying the workpiece, some of which will be revealed only after the coating is solidified < br / > the main reasons for the defects of the coating are as follows: those related to the coating, such as unstable batch performance, dampness, dirt, etc.; those related to the surface treatment process, such as oil removal, unclean rust removal, poor quality of the phosphating film, poor quality of the black blank itself, etc.; those related to the machine and equipment, such as oil and water content in the compressed air, powder accumulation at the nozzle of the spray gun, failure of the powder feeder, and failure of the curing furnace, And other reasons such as improper operation The following is a further analysis on the causes of several common coating defects in actual production and the prevention and control measures < br / > (1) the most fundamental reason for the shrinkage defect of the powder coating is the contamination in the base material or powder, which makes the local surface tension of the powder layer inconsistent The specific analysis has the following several possible factors: < br / > &; the pretreatment of coating is not complete, including oil removal is not clean, there is no phosphating film, etc < br / > < br / > &; environmental factors, such as the pollution of lubricating oil sludge and rust ash of suspension chain, the poor effect of oil-water separation and freeze-drying of compressed air, air exhaust and strong cold air supply after phosphating, etc < br / > how to prevent the formation of shrinkage cavity during spraying? In addition to the selection of stable and reliable powder coating and pre-treatment, the most important thing is to prevent environmental pollution For example, the suspension chain adopts spray oiling and is equipped with a cleaning brush dust collecting device; the compressed air purifier is frequently inspected and regularly maintained; the air collecting direction of the air conditioner in the spray room is reasonably arranged, and the strong cold air supply after phosphating and the air inlet of the spray room door hole are filtered and dedusted; the air inlet and exhaust of the curing furnace and the drying furnace channel are equipped with filter screens, and the drying channel is cleaned comprehensively and carefully on a regular basis < br / > (2) sand < br / > sand appears in the powder coating, which is often brought by the powder material itself Due to the unstable quality of resin, the basic raw material for powder manufacturing, the powder quality is often unstable In this respect, in addition to requiring powder material suppliers to strengthen their quality control, it is more important to strictly check and do a good job in the incoming inspection of powder In addition to the common screen filtration method, the solvent method is an effective method for the detection of powder sand This method uses the principle that the powder can be dissolved in a certain solvent (cyclohexanone) but the sand particles are insoluble The sample is first dissolved, then filtered with 49 μ m (300 mesh) screen, and the constant weight is 2H at 60 ℃, so as to obtain the content of sand impurities in the sample At present, the control range of the index of dissolved filter is no more than 40 mg / 100 g powder < br / > in order to effectively prevent the occurrence of sand particles in the powder coating, all recovered powder and new powder must also be screened by rotation in production.