Steel drum electrostatic powder coating technology
Wen/Xin Qiaojuan Yang Wenliang
First, the development of powder electrostatic spraying technology
Since the 1950s, the environmental pollution caused by the rapid development of the world's industrial and transportation industries has become increasingly serious. To this end, many countries have successively enacted strict environmental protection regulations, forcing the coatings and coating industries in the world to commit themselves to pollution-free, energy-saving, resource-saving, and high-quality research. So solvent-free powder coatings and "powder electrostatic spraying" technology came into being.
In 1952, Knapsek Griesheim Company of West Germany studied the successful fluidized bed coating process, and firstly realized the dry coating of the coating. Due to the limitations of this process, it has not been greatly developed in the 10 years after 1952. In 1963, France's Sames Company successfully researched the "powder electrostatic spraying" process and the corresponding spraying equipment, so that the pollution-free dry coating once again attracted the attention of all countries and rushed to study and apply. In 1968, West Germany had 238 “powder electrostatic spraying†production lines.
Following the emergence of thermosetting epoxy powder coatings in 1965, thermosetting acrylic and polyacrylic acid powders were introduced around 1980, and research on powder electrostatic coating technology and equipment continued to yield new results.
In recent years, powder coatings and powder electrostatic spraying technology have been continuously developed and rapidly popularized. The reason for the analysis is determined by its superior characteristics:
1. Since the powder coating contains no solvent, it reduces the possibility of fire and prevents atmospheric pollution caused by solvents.
2. When the powder coating is applied as a top coat, no primer is needed, which simplifies the process and prevents lead poisoning of the operator.
3. Since no solvent is needed, a large amount of vegetable oil and a solvent containing high heat energy (such as toluene, xylene, etc.) are saved, which saves resources and saves energy.
4. The spraying process is simple, and it is easy to realize painting automation after the cold spraying process.
5, according to the needs of the product can be coated 40 ~ 100 microns thick coating at a time to meet various requirements.
6. The overspray coating powder can be recycled and used. The loss during baking and film formation is extremely small, and the coating utilization rate is as high as 99% or more, so the coating cost is low.
7. The powder coatings are all made of high molecular polymer, and the performance indexes of the coating film are excellent.
8. Solid powder coatings are easy to store, transport and manage.
9. The “Powder Electrostatic Spraying†process improves the working conditions of the coating production and civilizes the painting operation.
Therefore, whether it is machinery industry, chemical industry, light industry, national defense industry, this advanced technology is actively used, and it is widely used in automobiles, bicycles, sewing machines, refrigerators, military equipment, petroleum, etc. in insulation, corrosion resistance and decoration. Protective coating and decorative coating for pipes and the like. At present, due to the mature development of the technology and the reduction of coating cost, developed countries have applied the technology to household appliances, packaging products and daily necessities. The application in steel drum coating production was first promoted in the United States and Germany, and quickly became a worldwide hair; Chen Zhan trend.
Since the 1970s, China's Guangzhou Institute of Electrical Apparatus and the Institute of Coatings of the Ministry of Chemical Industry have begun research on powder coating and coating technology. In recent years, the Chengdu Electric Appliance Factory has successfully researched the cold spray process and the corresponding spray equipment, making the “powder electrostatic spray†technology a major step in the practical stage. At present, due to the large-scale introduction of foreign advanced technology, China's "powder electrostatic spraying" technology has maintained a synchronous level with the world, and it is expected that the application in the packaging industry will gradually become popular.
Second, the composition and performance of steel drum powder electrostatic spraying equipment
Steel drum powder electrostatic spraying equipment is mainly composed of powder electrostatic spraying machine, powder recycling device and spraying room. Now take the most common set of spraying equipment in China as an example.
(1) Powder electrostatic spraying machine
The spraying machine consists of three parts: a high pressure generating system, a powder supply system and an electrical control system, which are organically combined in a movable control cabinet.
1. High pressure generating system
The high voltage generating system is composed of a high frequency generator and a high voltage generator, and a high voltage cable. In order to provide sufficient and stable electrostatic field power, and to ensure the safety of the operator and reduce the size of the equipment, in general, the high frequency generator and the high voltage generator are designed with special design and line parameters, and the voltage drops rapidly when overloaded or short-circuited. Zero potential.
Both the high frequency generator and the high voltage generator are installed in the closed cabinet on the left side of the sprayer, and the cabinet is equipped with a door lock to prevent the operator from touching the high pressure part. The high voltage generator is placed in the fuel tank, and the fuel tank is filled with a 25-meter transformer pool. High-voltage cables generally require new products made of new insulating materials and new processes, with a diameter of 8mm, can withstand the power frequency lOOKV, 5 minutes withstand voltage test, and no breakdown flashover phenomenon, belonging to small-diameter high-voltage cable, the whole high voltage occurs The system must be reliably grounded, and all controls are performed on the surface of the sprayer control instrument panel.
2, powder supply system
The powder supply system is composed of a powder feeder, a solenoid valve, a pressure regulating valve, a pressure gauge, a pipe, and the like. The dry and clean compressed air is sent to the pressure regulating valve through the trachea input, and the pressure is adjusted according to the process requirements, and then enters the atomizer of the powder feeder and the spray gun through the solenoid valve. The powder feeder adopts a screw feeding mechanism to accurately control the amount of powder supplied as needed. The powder feeder is a precision powder supply device that meets international standards. The error of repeating accuracy per unit time is not more than 2%. The powder supply volume is independent of the pressure and flow rate of compressed air. It can be continuously and continuously adjusted from 99 to 250 grams per minute. It is an ideal powder supply device.
Because of this characteristic, when the auxiliary air is used to adjust the atomization state and the pattern, the amount of powder supply is not changed, and the adjustment of the powder supply amount is only related to the rotation speed of the powder feeding screw of the powder feeder. Adjusting the rotation speed of the powder feeding screw can accurately change the amount of powder supplied. Due to the structural guarantee, the repeatability error per unit time of the powder supply amount is always maintained at no more than 2%, which provides favorable conditions for automatic spraying while ensuring an excellent and uniform coating.
3. Electrical control system
The electrical control part of the powder electrostatic spraying machine is composed of control panel, instrument, relay, main electric appliance, contactor, solenoid valve, indicating component, etc. It is the heart part of the spraying machine. There are measuring instruments on the control panel, which can measure the voltage and current of the high voltage power supply, the current of the servo motor and the voltage of the common control power supply and the regulated power supply. At the same time, it can be separately controlled by the control knob of the disk surface: a. Turn-on and turn-off of the high-voltage power supply; b. Start-up and stop of the main fan of the recovery system; c. Start and stop of the vibration device of the filter bag dust collector; d. Start stop and adjustment of powder supply; e. start and stop of air compressor; f. start stop and rotation speed adjustment of steel drum rotating mechanism; g. air pressure and flow rate of powder compressed air; h. , power on and off and auxiliary air adjustment; i. automatic spray time control; j. common control power on and off. There are also the positions of the various actuators on the panel, and the light plates are displayed. Each main electrical appliance has a marking frame indicating the purpose and corresponding position. The entire electrical system is connected by cables and cloth wires, and is output from the outlet terminal board or the socket to each. Relevant execution of the motor or electrical appliance, the electrical control system is reliably grounded.
(2) Spraying room
The spray booth is welded with an insulated plastic plate. In order to increase the mechanical strength, the angle is welded into the frame of the spray booth.
The front side of the spraying case is provided with a push-pull door, and the opening degree of the door can be adjusted to adjust the pressure of the air inlet of the spraying room and the air flow speed to enhance the airflow sealing effect. The top of the spraying chamber is equipped with a rotating mechanism of the steel drum, and the bottom of the spraying chamber has an air outlet, and the powder that has been sprayed enters the pipeline of the recycling system. The metal components such as the frame of the spray booth and the rotating mechanism of the steel drum have been mechanically and electrically connected to each other and communicated with the grounding point to prevent the metal components from generating static electricity accumulation in the electrostatic field, so that the operator suffers after being touched. electric shock.
In order to meet the needs of the steel drum production line, the spray booth can also be made "channel type".
(3) Recycling device
The recovery device is composed of a cyclone type dust collector, a centrifugal fan, a bag filter, a discharge valve, a pipe, and the like, and is fixed on a seat welded by the channel steel to form a whole.
Under the action of the airflow of the spray booth, the oversprayed powder coating cannot escape, and the cyclone dust collector is sucked through the air outlet of the spray booth to be recovered as the first part. The dust collection efficiency of the cyclone dust collector is not less than 80%. The air outlet of the cyclone dust collector is connected to the air inlet of the fan through a pipe. Part of the powder coating that has not been collected in the cyclone dust collector is pressed into the filter bag dust collector after passing through the fan. Under the filtering effect of the filter bag, the powder coating is left in the hopper of the dust collector, and cleaned. Air is vented to the atmosphere from the air outlet. The efficiency of the filter bag dust collector is not less than 95%, the total efficiency of the recovery device is not less than 99%, and the recovered powder coating is discharged from the discharge valve.
Third, the application of steel drum powder electrostatic spraying technology
(1) Equipment installation requirements and precautions
1. Powder electrostatic spraying equipment should be installed in a workshop with good lighting and ground with water grindstone or ceramic tile. The workshop does not allow dust to fly.
2. The equipment should be arranged reasonably before installation and installed according to the layout drawing. The spraying room and the recycling device should be placed smoothly during installation, and the anchor bolts can be omitted.
3. The pipeline between the spraying room and the recovery device should be properly connected. No air leakage is allowed. The outlet of the dust collector should be extended out of the workshop and should be higher than the roof of the workshop. The outlet of the outlet should be protected from rain. Cover to prevent rainwater from falling into the filter bag dust collector.
4. The equipment should be reliably grounded, and the grounding point should be no less than three (ie spraying machine, spraying room metal frame, recycling device). The gun should also be reliably grounded.
5. The power supply, incoming line, outgoing line, high voltage cable and control cable of the spraying machine should be protected by a serpentine tube.
6, the operator should not wear insulated shoes or plastic and rubber soles, should wear cloth shoes, so as not to produce contact inductance.
7. When installing, connect the power cable and control cable to the corresponding socket or terminal board according to the wiring diagram. When the inspection is correct, the electrode test operation can be turned on.
(2) Application methods and precautions:
1. Check whether the connecting cable is disconnected or loose before the equipment is running. If it falls off or loose, it should be removed first, then close the total power supply of the sprayer, pay attention to whether the display data of the power supply indicator and the power supply voltage meter are normal, and the meta switch trips and blows. After everything is normal, turn on the common control power supply, start the air compressor, adjust the pressure regulating valve, and read the readings of the powder air and auxiliary air from the pressure gauge separately, and adjust to the specified value of the process parameters.
2. The powder feeder does not add powder coating first, starts the powder feeder feeding mechanism, adjusts the knob of the powder supply, from low speed to high speed, monitors whether there is friction or the like, and monitors whether the servo motor DC power supply meter is ambiguous. Overload phenomenon. After all the normal, add powder coating to stop the machine, add 5 kg of powder coating at a time, and tighten the sealing cap of the feeding port after feeding.
3. When working, first check whether the door of the spraying room is flexible to open and close, whether the moving mechanism of the spray gun can rotate freely along the plane, whether the positioning block that moves up and down the spray gun is fixed firmly, and whether the rotating mechanism of the steel drum rotates or not. After all the normal conditions, start the drum rotation mechanism, rotate the adjustment knob, and observe whether the steel drum rotation hook follows and evenly changes the speed.
4. Before the high-voltage generating system is put into operation, it should be strictly checked whether the grounding points are well grounded; whether the non-grounding poles and high-voltage cables are off, loose and grounded short-circuited; whether the gun head (negative electrode) is too close to the grounding part (generally Keep a distance of 250mm or more). After everything is normal, turn the high voltage power supply voltage adjustment knob to the zero position counterclockwise, then press the high voltage power start button to turn on the high voltage system power. At this point, you should pay attention to the regulated voltage indicator on the instrument panel. The reading is stable at 24V. The reading of the ammeter exceeds the normal load value. If there is any abnormality, the high-voltage system power should be disconnected to find out the cause. After normal, slowly rotate the high-voltage power supply adjustment knob clockwise, and pay close attention to the high-voltage power supply voltage indicating instrument and current indicating instrument with or without jump, or no indication or overload phenomenon. If the high-voltage power supply adjustment knob is turned, the voltage and current are both rising smoothly and slowly, indicating that the high-voltage system is working properly. When the voltage rises to about 50KV, you can hear a slight click of the gun gun. When the current does not exceed 100μA, the voltage can be raised to the rated value (100KV). If the gun head is from the grounding electrode (steel barrel) 30Omm, the gun head can emit glow discharge, accompanied by a heavier hum, so the current does not exceed 150 ~ 200μA, which proves that the high-pressure system works normally, and can be used for powder loading.
5. The powder supply system works normally, and the working condition of the recycling system can be checked. Firstly, open the powder air solenoid valve and the auxiliary air solenoid valve, then start the powder feeder mechanism, and start the ventilation device of the recovery device. The spray gun does not add DC high voltage, the powder input amount is from small to large, and the spray chamber door opening distance is one. At this time, it is observed with the naked eye whether there is dust leakage from the air outlet of the fan to the various parts of the bag filter. If the first test after installation, the vehicle should also be sampled and measured in the working part of the worker, the workshop and the exhaust vent of the recovery device according to the relevant environmental protection regulations. The dust content of the above parts should meet the national standards, and the recovery system should be considered normal. Start the vibration device before the recovery system fan starts, then turn on the fan to put the prison device into operation. When the machine stops, turn off the fan first, then turn off the vibration device after l0 minutes.
6. The high-pressure system, the powder feeder, the recovery device, and the electrical control system have been inspected and confirmed to be working normally. The complete set of equipment is available for powder belt loading operation, and the steel drum is officially sprayed. Before starting the machine, the operator should check the cleaning of the equipment. The high-voltage cable, the spray gun, and the powder for the bitterness should be wiped clean with dry gauze. The smudge on the high-voltage electric flying cable and the spray gun can be used to wipe the dry gauze with carbon tetrachloride. The machine body and its components should also be wiped clean with a dry gauze. Operators should wear cloth shoes to avoid the electric shock caused by the large difference in ground potential.
7. The procedure for starting the spraying is as follows:
(1) Check the high voltage power supply adjustment knob and turn it to the zero position;
(2) Check the speed control knob of the rotating mechanism of the steel drum and screw it to the middle position;
(3) Check the knob of the powder feeding mechanism of the powder feeder and rotate it to the middle position;
(4) Closing the main power switch;
(5) causing a moving air compressor;
(6) Start bag type dust collector vibration device (the fan can be started after l0 minutes);
(7) Adjusting the powder delivery air pressure regulating valve to stabilize the pressure to the pressure required by the process;
(8) Adjusting the auxiliary air pressure regulating valve (generally greater than the powder feeding air pressure);
(9) Open the powder feeding air solenoid valve;
(10) opening the auxiliary air solenoid valve;
(11) Hanging steel drums;
(12) Turn on the high-voltage system power supply and adjust the voltage to the value required by the process;
(13) causing a dynamic recovery system ventilator;
(14) Start the powder feeder and the powder feeding device (start with the spray coating);
(15) adjusting the auxiliary air pressure regulating valve to adjust the dusting pattern;
(16) The spraying is terminated (after the specified time or the naked eye has been sprayed), the high pressure adjusting knob is adjusted to the minimum position in the counterclockwise direction, and the feeding mechanism of the high and pressure system power supply and the powder feeder is cut off;
(17) closing the powder feeding air solenoid valve and the auxiliary air solenoid valve;
(18) The steel drum is taken out and sent to the heating polymerization process.
8, matters needing attention
(1) When starting spraying, the powder feeding and auxiliary air first solenoid valve must be opened first, so that the compressed air can blow the powder in the pipe and the spray gun before the powder supply mechanism can be opened to avoid snoring; when the spraying is stopped In contrast to this, the powder supply is stopped before the solenoid valve can be closed. The time interval between stopping the supply of powder and closing the electric and magnetic valves depends on the accumulation of powder in the pipeline at the construction site.
(2) Spraying with a hand-held spray gun, the booting procedure is exactly the same. Before powering on, the power socket of the hand-held spray gun is placed on the end of the fixed spray gun and connected to the ground wire, and the spray gun moving mechanism is horizontally moved to the side of the spray booth, and the fixed spray gun is oriented. Tilt down to fix.
(3) If the powder air mixing valve of the powder feeder is clogged, the sprayer can be opened, the mixing valve can be removed, and the plugging powder can be removed with copper.
(4) Before and after the shift, clean the high-voltage cable, spray gun, high-voltage generator, etc., and clean the dust on the ground with dry and clean gauze.
(5) Before the shutdown, the recycling system first turns off the fan, and the vibration device can be stopped after 10 minutes of operation.
(6) After the machine is shut down, the sprayer should be covered with a dust cover and the main power supply should be cut off.
(China Packaging News)
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