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Electrostatic spraying technology and its application

ADD:admin     AddDate:2014-07-30

In the industrial development and people's material and cultural living standards continue to improve, people demand for coating product in appearance is becoming more and more high, requires both beautiful and easy, affordable, reliable and electrostatic spraying technology is quickly developing new technology at home and abroad in recent years, has been widely used in various industrial fields. It can not only save coating, reduce the pollution to the environment, but also can make the spraying process mechanization, continuous, reduce rejection rate, gain excellent coating property, greatly improve the labor productivity, improve labor condition, thereby gaining considerable social and economic benefits.

1 the basic theory of electrostatic spraying

1.1 ordinary coating process

In general, the coated material must be made via pretreatment before coating, the oil removal, rust removal, phosphating film, it can achieve better antirust, better adhesive and coating medium. Due to the coated material specifications, shape differences, if only the common coating technology for coating, spray medium the actual utilization of only about 30% commonly, the other 70% is wasted, so, this not only adds to the cost of the product, but also pollutes the environment.

1.2 the basic principle of electrostatic spraying

Electrostatic spraying on the coated material as the anode, usually grounding; Paint spray for the cathode structure, negative high voltage power connection, so that high voltage electrostatic field was formed at the poles. Due to produce corona discharge at the cathode, can make the coating medium of extrusive charged, and further atomization. According to the principle of "the same, opposites attract", has been charged the coating medium by effect of electric field force (F = qE), along the field lines directional flow, to positively charged the coated surfaces, deposit into a uniform, firm adhesion layer of thin film. Electrostatic spraying is polar corona discharge may be adopted, but the negative polarity critical voltage of the corona discharge was lower than those of normal polarity corona discharge, and more stable, safe and not easy to produce sparks. Therefore, under normal circumstances will be applied as the anode grounding.

1.3 electrostatic spraying and nozzle classification

Electrostatic spraying from the spraying method can be divided into the air or hydraulic pressure atomization electrostatic spraying, centrifugal electrostatic spraying, etc; According to the different features of nozzle can be divided into direct type electrostatic spraying nozzle electrostatic spraying, Y type nozzle, target type electrostatic spraying nozzle, etc. According to the properties of coatings can be divided into solid powder electrostatic spraying and liquid atomization electrostatic spraying. Powder electrostatic spraying are widely used in the coating on the surface of the plate, in the electrostatic spraying of liquid can be according to the characteristics of different liquid itself will oil electrostatic coating, electrostatic paint technology in industrial production. This article focuses on powder electrostatic spraying and liquid electrostatic spraying different characteristics and application situation. In addition, according to the different USES of spraying, electrostatic spraying nozzle can be divided into a variety of, such as SELaw developed embedded electrostatic induction nozzle, Britain's rotating disc electrostatic spray, Japan's miniature cone hole rotating nozzle, mist nozzle, turn our pneumatic cage type droplet spray nozzle, static electricity, and air impact type hydraulic nozzle and double fluid electrostatic spray, etc.

2 powder electrostatic spraying and its characteristics

2.1 powder electrostatic spraying process

Powder electrostatic spraying is through the spray gun or a cup of needle electrode in the direction of the coated workpiece releasing high voltage electrostatic (generally is 60 ~ 100 kv), high pressure electrostatic spray cup or spurting out of the gun powder and mixture of compressed air and the surrounding air ionization, due to the high voltage anode produce corona discharge, has a negative charge, near its powder with negative charge in the electric field intensity high electrostatic field. Powder in the electric field force and dual driven by compressed air pressure to the coated surfaces, formation thickness uniformity of powder layer, heating melting curing coating.

2.2 the characteristics of the powder electrostatic spraying

In general, all have no deformation at 200 ℃, can make the charged powder particles adsorbed on the coated surfaces, the surface coating can be used powder electrostatic spraying. Therefore, powder electrostatic spraying technology can be widely used in instruments and meters, household appliances, electrical equipment, car shipping, light industry building materials and equipment, furniture, machinery, metal parts, such as the surface of the protective and decorative coating. From the perspective of the spraying technology of currently used, powder electrostatic spraying has its unique advantages in the industrial coatings.

(1) powder coating is a solid coating, no solvent, nonvolatile, greatly reducing the pollution problem caused by solvent, is conducive to environmental protection and operation of the workers' health.

(2) in the process of spraying, the excess powder can take advantage of the recovery system centralized collection, achieve the goal of recycling, so the powder utilization ratio is quite high, make the enterprise production cost reduction, gain good economic benefits.

(3) the whole process only after pretreatment, spray coating, curing process, simplifies the traditional means of multistep spraying, spraying line work efficiency is high, both in manual and automatic operation more convenient.

(4) powder electrostatic spraying, after a baking can get excellent performance of the coating surface. Strong and durable, the film to acid, alkali, both friction resistance, impact resistance, high mechanical strength, etc.

But due to the electrostatic spraying powder is in development period, so inevitably there are some disadvantages, such as after spraying coating adhesion between more impurities, shrinkage cavity, color, etc, these are all in the future should focus on to solve the problem.

3 liquid electrostatic spray coating and its application

3.1 liquid electrostatic spraying elementary theory knowledge

Static in uniform, detailing deposition of droplets and sprayed in the target volume, uniformity, adsorption, etc have obvious effect. Electrostatic spraying liquid refers to between the nozzle and the corresponding grounding electrode with high voltage, the nozzle tip flow of liquid due to surface tension, electrostatic force and charge the repulsion between the droplet surface and the gravity force is not stable, and then split after broken, after will eventually be atomized droplets on the coating evenly coated. In the study of electrostatic spraying of liquid, the basic principle of spraying and powder coating is not too big difference, just the stand or fall of droplets atomized degree in high voltage electrostatic field directly determines the effect of spraying liquid.

While under the effect of electrostatic field liquid atomization mechanism is complicated, but for the electrostatic spraying of liquid research has the very vital significance. Normally can be thought of, the viscous resistance and surface tension of liquid is atomized exist in the process of two main types of resistance. Study found that charged liquid causes the lower surface tension and increased pressure difference inside and outside to liquid atomization, droplets are lotus electricity reaches Rayleigh limit, surface charge repulsion between the droplets continue to split. Liquid electrostatic effect on liquid electrostatic spraying technology was used to improve the effect of atomization, the production efficiency and reduce the production cost.

Used in electrostatic spraying of liquid atomization droplets is evener, by high voltage electrostatic field can effectively reduce the droplet size, improve the uniformity of droplets spectral and deposition efficiency, etc. At the same time, used for liquid spraying coating is of good quality, reliable operation, high utilization rate of equipment, less maintenance, less pollution to the environment, which can obtain good economic benefit. Liquid electrostatic spraying technology was based on this purpose is widely used in industry, can be used for electrostatic spray, electrostatic painting, etc. Here, the author only in the school production of electrostatic oiler, for example to illustrate the application of liquid spraying technology.

3.2 liquid electrostatic spray application

Electrostatic oiler is a successful example of liquid electrostatic spray application, it is a mechanical and electrical and high-voltage electrostatic technology in the integration of high-tech products. It will depend on the role of high voltage electrostatic, liquid spray evenly in the metal plate (with) the rust-proof material on the surface, can be widely used in steel and non-ferrous metal plate (with) material production line, and other processes need high quality oil. Electrostatic oiler spraying atomized droplet work (see figure 2) as follows: the basic principle of knife beam DangTu oil on high voltage dc power supply after the cathode, knife beam and steel plate between the high voltage electrostatic field is established. Because oil knife beam edge is sharp, minimal radius of curvature, thus produce corona discharge phenomenon in knife beam top place, air molecules by ionization become positive and negative ions in this case, the positive and negative ions along the power line fast movement, positive ion flew to injection knife beam by neutralization, negative ions are the increasing electric field to the steel plate (positively charged), will be on the way and the oil droplets collide and make the oil droplets charged. With the gradual increase of oil droplets surface negative charge and the electrostatic repulsion between the electric charge will increase, when the electrostatic repulsion between the charge more than the surface tension of the droplet, the liquid drop will be broken, for atomization phenomenon, namely on the macro will eventually be atomized droplets evenly after coating on the steel strip (steel plate).

4 electrostatic spraying technology development prospects and research direction

Electrostatic spraying technology has a history of more than 50 years of development, now with the wide application of the technology in all walks of life, has developed a controlled by computer to complex operation of automatic spraying device, robot carry with electrostatic spraying device, etc. And as in electrostatic spraying of electrostatic atomization technology continuously explore, using electrostatic atomization technology, has studied the electrostatic inkjet technology, liquid electrostatic atomization of fuel, electric propulsion technology, charged water mist removal technology, electrostatic spraying technology, electrospray ionization mass spectrometry, static modulation technique, electrostatic suspension solution by applying technology, charged water mist suppression ray technology, water mist fire extinguishing technology, etc. The test principle of solid and liquid or powder atomization mechanism also became a focus in the study of electrostatic spraying technology, so the research of electrostatic spraying technology at the same time, the future research should be focus on specific focus on the following respects.

(1) the suitable spray medium and spraying equipment. According to different solid or liquid spraying spraying object to choose the appropriate medium, and for different spray medium and to study the different nozzle and nozzle shape, the structure will affect the intensity of electric field near the nozzle and spray medium load capacity, thus affecting atomization effect and the quality of spraying, so good coating medium and coating equipment is the premise of guarantee superior spraying technology.

(2) we will continue to improve the study of the atomization mechanism of the electrostatic spraying, it is the core of the whole electrostatic spraying technology. Through the establishment of appropriate mathematical model, analysis of the electrostatic field of spraying powder or liquid droplets, thickness, surface tension and adhesion strength of regression mathematical formula, from theory to find out the main factors influencing the electrostatic atomization spray medium.

(3) strengthen the study of the phenomenon of test principle and test of the electrostatic spraying, adopt advanced test method and test method, the thickness and uniformity of spraying particles main atomization and deposition efficiency index to measure, provides the micro test technology for spraying technology, in this way can be better to test the correctness of the theory of argument.

(4) improve the technological process of electrostatic spraying technology, makes the coating line is simple and efficient. At the same time to determine the best effect of spraying parameters, such as voltage, spraying, spraying distance medium flow or pressure, etc., from on macroscopic will be great help for the electrostatic spraying technology.

5 conclusion

In a word, with the continuous development of science and technology, electrostatic spraying technology in all aspects of application is more and more widely. In terms of its painting medium, whether is it powder electrostatic spraying liquid electrostatic spraying, its basic principle is the same. In the research of electrostatic spraying technology at the same time, it is important to note that by test method of the specific circumstances of electrostatic spraying, theory with practice, obtain the best effect of the different medium spraying. At the same time in the study of electrostatic spraying technology will spraying technology with electrostatic dust collection and electrostatic atomization combustion technology, learn from each other, complement each other. With the deepening of the research and technology constantly improve and progress, believe in the near future, electrostatic spraying technology will become more mature and practical technology, meet the needs of the society.