Powder electrostatic spraying equipment

1 The typical process flow of powder electrostatic spraying technology is:
Workpiece pretreatment → powder spraying → curing → inspection → finished product
1.1 Preprocessing
The workpiece can only be sprayed with powder after the oil and dust on the surface of the cold-rolled steel plate are removed by pretreatment. At the same time, a layer of zinc-based phosphating film is formed on the surface of the workpiece to enhance the adhesion after powder spraying. The pre-treated workpiece must be completely dried with moisture and fully cooled to below 35 °C to ensure the physical and chemical properties and appearance quality of the workpiece after powder spraying.
1.2 Dusting
1.2.1 The basic principle of powder electrostatic spraying
The workpiece enters the spray gun position of the powder spraying room through the conveyor chain to prepare for the spraying operation. The electrostatic generator releases high-voltage static electricity (negative pole) through the electrode needle of the gun mouth to the space in the direction of the workpiece, which makes the high-voltage static electricity from. The mixture of powder and compressed air ejected from the nozzle of the spray gun and the air around the electrodes are ionized (negatively charged). The workpiece is connected to the ground (grounding pole) through the conveyor through the hanger, so that an electric field is formed between the spray gun and the workpiece. The powder reaches the surface of the workpiece under the double push of the electric field force and the pressure of the compressed air, and forms a layer on the surface of the workpiece by electrostatic attraction. Uniform coating.
1.2.2 Basic raw materials for powder electrostatic spraying
With interior epoxy polyester powder coating. Its main components are epoxy resin, polyester resin, curing agent, pigment, filler, various additives (such as leveling agent, moisture-proof agent, edge modifier, etc.). After the powder is heated and cured, it forms a Coating required. The auxiliary material is compressed air, which requires clean, dry, oil-free and water-free [water content less than 1.3g/m3, oil content less than 1.0×10-5% (mass fraction)]
1.2.3 Construction technology of powder electrostatic spraying
●Electrostatic high voltage 60-90kV. If the voltage is too high, it is easy to cause powder rebound and edge pitting; if the voltage is too low, the powder feeding rate is low.
●Electrostatic current 10～20μA. If the current is too high, it is easy to cause discharge to break down the powder coating; if the current is too low, the powder coating rate is low.
●Flow rate pressure 0.30-0.55MPa. The more flow rate pressure. The higher the powder deposition rate, the faster the coating of the predetermined thickness will be obtained, but the higher the powder dosage and the higher the wear rate of the spray gun.
●Atomization pressure 0.30～0.45MPa. Properly increasing the atomization pressure can keep the thickness of the powder coating uniform, but too high will cause the powder feeding parts to wear quickly. Appropriately reducing the atomization pressure can improve the powder covering ability, but if it is too low, the powder feeding part will be easily blocked.
●The gun cleaning pressure is 0.5MPa. If the cleaning pressure is too high, it will accelerate the wear of the gun head, and if it is too low, it will easily cause the gun head to be blocked.
●The fluidization pressure of the powder supply barrel is 0.04～0.10MPa. If the fluidization pressure of the powder supply barrel is too high, the density of the powder will be reduced and the production efficiency will be lowered. If it is too low, insufficient powder supply or powder agglomeration will occur.
●The distance from the spray gun mouth to the workpiece is 150～300mm. If the distance between the spray gun mouth and the workpiece is too close, it is easy to cause discharge to break down the powder coating, and if it is too far, it will increase the amount of powder and reduce the production efficiency.
●The speed of conveyor chain is 4.5～5.5m/min. Too fast conveyor chain speed will cause insufficient powder coating thickness, too slow will reduce production efficiency.
1.2.4 The main equipment of powder electrostatic spraying
●Spray gun and electrostatic controller
In addition to the traditional built-in electrode needle, the spray gun is also equipped with an annular corona outside to make the electrostatic field more uniform to keep the thickness of the powder coating uniform. The electrostatic controller generates the required electrostatic high voltage and maintains its stability, and the fluctuation range is less than 10%.
●Powder supply system
The powder supply system consists of a new powder barrel, a rotary screen and a powder supply barrel. The powder coating is first added to the new powder bucket, and the compressed air pre-fluidizes the powder through the micro-holes on the fluidizing plate at the bottom of the new powder bucket, and then is transported to the rotary sieve through the powder pump. The rotating sieve separates the powder particles with excessive particle size (above 100μm), and the remaining powder falls into the powder supply bucket. The powder supply barrel will fluidize the powder to a specified level and then supply the spray gun to spray the workpiece through the powder pump and the powder feeding pipe.
●Recycling system
Except for a part of the powder sprayed by the spray gun that is adsorbed on the surface of the workpiece (generally 50% to 70%, our company is 70%), the rest of the powder settles naturally. Part of the powder in the sedimentation process is collected by the cyclone collector on the side wall of the dusting shed, and the powder particles with larger particle size (above 12 μm) are separated by the principle of centrifugal separation and sent back to the rotary screen for reuse. The powder particles below 12pom are sent to the filter element collector, where the powder is vibrated by the pulsed compressed air and falls into the collection bucket at the bottom of the filter element.
At the end of regular cleaning and packing, waiting for sale. The clean air from the separated powder (containing powder particle size less than 1 μm and concentration less than 5g/m3) is discharged into the powder spraying chamber to maintain a slight negative pressure in the powder spraying chamber. If the negative pressure is too large, it is easy to inhale the dust and impurities in the powder spraying room. If the negative pressure is too small or the positive pressure is easy to cause the powder to overflow. The powder that settles at the bottom of the dusting booth is collected by the powder pump and enters the rotary screen for reuse. The mixing ratio of recovered powder and new powder is (1:3)～(1:1). Using this recycling system, the company's overall powder utilization has averaged 95%.
●Powder chamber body
The top and wall panels are made of light-transmitting polypropylene plastic to minimize powder adhesion and prevent electrostatic charge build-up from interfering with electrostatic fields. The base plate and base are made of stainless steel, which is easy to clean and has sufficient mechanical strength.
●Auxiliary system
Including air conditioners and dehumidifiers. The function of the air conditioner is to keep the powder spraying temperature below 35°C to prevent powder agglomeration; the second is to maintain the micro-negative pressure of the powder spraying chamber through air circulation (wind speed less than 0.3m/s). The function of the dehumidifier is to keep the relative humidity of the powder spraying room at 45% to 55%. If the humidity is too high, the air is prone to discharge and break down the powder coating.
1.3 Curing
1.3.1 The basic principle of powder curing
The epoxy group in the epoxy resin, the carboxyl group in the polyester resin and the amine group in the curing agent undergo polycondensation, addition reaction and crosslinking to form a macromolecular network, and at the same time release small molecular gas (by-product). The curing process is divided into four stages: melting, leveling, gelling and curing. After the temperature rises to the melting point, the surface powder on the workpiece begins to melt, and gradually forms a vortex with the inner powder until it is completely melted.
After all the powder is melted, it begins to flow slowly, forming a thin and flat layer on the surface of the workpiece. This stage is called leveling. After the temperature continues to rise, there is a short-term gel state (the temperature remains unchanged) after reaching the glue point, and then the temperature continues to rise and the powder undergoes a chemical reaction and solidifies.
1.3.2 The basic process of powder solidification
The powder curing process used is 180 ° C, baking for 15 minutes, which is normal curing. The temperature and time refer to the actual temperature of the workpiece and the accumulated time to maintain the temperature not lower than this temperature, not the set temperature of the curing furnace and the walking time of the workpiece in the furnace. However, the two are interrelated. When the equipment is initially commissioned, it is necessary to use a furnace temperature tracker to measure the surface temperature and accumulation time of the upper, middle and lower points of the largest workpiece, and adjust the setting temperature of the curing furnace and the speed of the conveyor chain according to the measurement results ( It determines the walking time of the workpiece in the furnace) until it meets the above curing process requirements. In this way, the corresponding relationship between the two can be obtained, so in a period of time (usually 2 months), only the speed needs to be controlled to ensure the curing process.
1.3.3 Main equipment for powder curing
The equipment mainly includes three parts: heating burner, circulating fan and air duct, and furnace body. The heating burner used by our company is a product of Weishaupt from Germany, and uses 0-35# light diesel oil. It has the advantages of high heating efficiency and fuel saving. The circulating fan conducts heat exchange, and the first-stage opening of the air supply pipe is at the bottom of the furnace body, and there are three-stage openings every 600mm upwards. In this way, the temperature fluctuation in the range of 1 200mm workpiece can be guaranteed to be less than 5°C, and the color difference between the upper and lower parts of the workpiece is prevented from being too large. The return air duct is at the top of the furnace body, which can ensure that the upper and lower temperatures in the furnace body are as uniform as possible. The furnace body is a bridge-type structure, which is not only conducive to preserving hot air, but also prevents the air volume in the furnace from decreasing after production and attracting external dust and impurities.
1.4 Inspection
For the cured workpiece, the daily main inspection is the appearance (whether it is smooth and bright, whether there are defects such as particles and shrinkage holes) and the thickness (controlled at 55 ~ 90μm). If debugging for the first time or the powder needs to be replaced, it is required to use the corresponding testing instrument to test the following items: appearance, gloss, color difference, coating thickness, adhesion (cross-cut method), hardness (pencil method), impact strength, salt spray resistance (400h), weather resistance (artificially accelerated aging), heat and humidity resistance (1 000h)
1.5 Finished product
After inspection, the finished products are sorted and placed in transport vehicles and turnover boxes, and are separated from each other by soft materials such as newspapers to prevent scratches and mark them for use.