Analysis of electrostitive powder coating technology and influence factors of aluminum alloy.

Author
wen / Wu Guanqun Fujian Wan'an Industrial Group Co., Ltd.
Summary
powder electrostectronic spraying technology was originally used in metal corrosion prevention, electrical insulation and so on, with the continuous development of powder electrostectrical spraying process and the emergence of new powder coatings, in recent years in decorative coating applications are very wide. This paper summarizes the current situation and characteristics of the development of electrostitive powder coating technology of aluminum alloy, analyzes its spraying process, and analyzes the factors of electrostitive powder spraying technology, which provides some help for understanding the electrostation powder coating technology of aluminum alloy profiles.
Aluminum alloy profile is one of the most widely used materials in many industries, the practical application will be different due to different uses of its corrosion performance and other functions have special requirements, which requires a certain treatment of the surface of aluminum, aluminum negative-power properties lead to its very easy corrosion, especially with another type of metal contact, but also increase the degree of aluminum corrosion . Therefore, the main purpose of surface treatment is to prevent aluminum corrosion to extend its service life. Static powder spray aluminum alloy profile is a commonly used surface treatment, this treatment method has good corrosion resistance, after treatment of aluminum acid and alkali salt spray resistance is excellent. The electrostectile powder coating technology of aluminum alloy material has the characteristics of green environment and small footprint, especially its simple process, easy to operate and save material loss.
1 aluminum alloy profile static powder spraying characteristics
(1) process is simple. The instrument used in the whole spraying process has a high degree of automation, some aspects of spraying have been able to achieve computer control, making spray operation significantly less difficult, without the use of too many auxiliary equipment.
(2) yield is high. Under the condition of adequate measures, the success rate of spraying to obtain satisfactory products is high, and the number of non-conforming products is greatly compressed.
(3) energy consumption is significantly reduced. The traditional anode oxidation and electrophoresis process will consume a lot of hydropower resources, especially in the oxidation process, the surface treatment per ton of aluminum alloy profile consumption of electricity up to about 1000 kW/h, electrostitive powder spraying reduced auxiliary facilities also greatly reduced power consumption.
(4) environmental benefits are good, basically do not pollute water and the atmosphere. The spraying process will not use alkali, sulfuric acid and other polluting substances, this improvement of environmental protection benefits will help aluminum alloy profiles in the plastic steel profile market competition, do not use environmentally harmful substances, but also to a certain extent reduce production costs.
(5) effectively reduces the labor intensity of workers. The whole process is multi-use of pipeline automatic operation, feeding mode and fixture use have computer system assistance, greatly improve the production efficiency of aluminum alloy profile spraying.
(6) the surface quality requirements of the aluminum to be treated are not high. After the treatment of powder coating, the extrusion seams and small defects of aluminum surface can be completely covered, which can significantly improve the spray quality of aluminum alloy profile products.
(7) The physical properties of the coating film coated by electrostitial powder are excellent, stronger than other surface treatment films, such as electrostectric spray coating in terms of wear resistance, hardness, acid resistance advantages are very prominent, improve the aluminum alloy profile application range and service life.2 Electrostitive powder spray process analysis
2.1 process analysis
In practice, in order to obtain satisfactory spraying results, the surface of coated aluminum needs to be pre-treated before spraying. Aluminum alloy material contains some alloy elements, to ensure its strong performance, however, its corrosion resistance is not satisfactory, aluminum is easy and oxygen in the air, moisture chemical reaction, the production of loose corrosion products. On the other hand, in the processing of aluminum, due to mechanical contamination of the surface of aluminum alloy parts, affecting the degree of binding between the coating and substrate, so that the surface of the coating pinholes and other defects. The above problems can be avoided by using electrostation powder spraying process, which is shown in Figure 1.
2.2 Main Process Analysis
2.2.1 Pre-treatment
Based on the decorative and anti-corrosion effect, aluminum alloy needs to be pre-treated, with the aim of forming a chemical conversion film with strong corrosion resistance and powder coating binding force, and the pre-treatment process for electrostature powder spraying is: de-oiling →washing→chemical transformation→ washing and drying →.
Aluminum pre-spray chemical conversion, the most common and effective treatment method is chromium and phosphorus chromium, with stable performance, easy to operate, after treatment of color is an important sign to judge its conversion effect, the resulting chrome film is yellow, also known as yellow chromium, and phosphorus chromium film appears green, commonly referred to as green chromium. The temperature and time range required for both processes is wide and can be achieved with spray and groove immersion. The coating thickness is controlled from 0.3 to 0.8 g/m2. However, the disadvantage of chromium is that hexavalent chromium is very harmful to human body, so many countries in the world restrict the use, and vigorously develop chromium-free chemical conversion process. Chromium-free chemical conversion treatment agents that have formed a commodity supply currently have fluorocarbons and fluoroates, which are characterized by thin conversion membranes, only 0.05 to 0.4 g/m2, the disadvantage is that the conversion membrane colorless, difficult to visual identification, to the production operation inconvenience.
chemically converted aluminum alloy parts need to be dried in time to remove surface moisture, because the aluminum alloy parts surface moisture into the powdering process easy to produce bubbles resulting in defects. It should be noted that the drying temperature should be reasonable, too high temperature can easily make the conversion film lose crystalline water, resulting in loose membrane and cause a significant decrease in coating adhesion. Depending on the chrome process formulation, the general drying temperature is controlled at 60 to 120 degrees C.
2.2.2 Electrostitive Spraying
This process requires the powder to be atomized in the air, to keep compressed air dry and clean, if there is moisture and oil in the compressed air, resulting in powder charge due to moisture, reduce the atomization properties, and thus make the coating thickness and uniformity are not satisfied. In addition to dust, some solid impurities can make the appearance of the coating worse, so purifying compressed air is necessary.
In spraying, it is necessary to recover unsorbented powder in a timely manner, the spilled spray powder will affect the environment to some extent, the spilled powder is affected by air moisture, and pollution, limiting its recycling.
2.2.3 Leveling and curing
spraying work requires that aluminum alloy parts be sent to the oven in time to complete the leveling and curing process, in order to prevent the adhesion of dust in the air and affect the properties of the coating. Baking coating needs a uniform furnace temperature, aluminum alloy parts to maintain a certain distance between aluminum alloy parts to prevent collisions, in practice, according to the characteristics of powder coating to control its leveling, curing temperature and time.
3 Major factors affecting electrostectrical powder spraying
3.1 powder conductivity
The electrostectrical field force between electrostectrical powder and aluminum alloy parts used in spraying is one of the sources of spray power, and the volume resistance of powder coatings has a great influence. Spraying performance, if the volume resistance is too small, the charged powder will impact the powder already attached, resulting in its shedding, and too large volume resistance will make the powder charge is not easy to release, so that the aluminum alloy part surface charge accumulation is too much, increased repulsion limits the production of thick film. Based on this, the volume resistance of powder particles needs to be strictly controlled in electrostectrectectral spraying, usually the volume resistance can be controlled in the range of 1010 to 1014 Ω.cm to ensure construction performance, but also to obtain a thick film coating.
3.2 powder particle size
powder itself has the characteristics of mobility, coating, etc. , which affect the spraying process, especially easy to block the spray gun and film thickness aspects. The powder material is absorbed by electrostectrical gravity to the surface of the aluminum alloy part, the electrostectrical gravity between the coating particles is positively related to its charge, and the formula for charge of the electrostectrical coating particles is as follows: in the
formula, Q is the charge charge of the particles;
from the formula, the charge of powder particles is positively related to the particle radius size. If the powder particle size is smaller, the less charge the particles have, the less static gravity, the smaller the electrostectrulation gravity particles are easily thrown out of the electrostation field or suspended in the electrostation field, resulting in a lower spray powder rate. When the powder particle size is too large, although the charge is large, but the particles themselves are significant, the paint particles also have a greater rebuke force, reducing the powder rate, too large particle size coating is difficult to get a thinner coating. In practical applications, powder particle sizes typically used range from 30 to 50 m.
3.3 Spray voltage
the formula in 3.2 shows that the electrostectrectrative field strength is directly compared to the powder charge. Located at low voltage, if the spray voltage continues to increase, the coating powder's charge increases accordingly, the amount of attachment will also increase, however, static voltage has a saturation value, once greater than the saturation value, static rejection will occur resulting in less increment of attachment, too high voltage increases the likelihood of powder breakout, harm the quality of the coating, commonly used spray voltage control in 60 to 80kV.
3.4 spray distance
the main impact of coating thickness and powder deposition efficiency, spray distance and electric field strength are closely related. In practical application, the spray distance usually controlled is 200 to 300mm, which has a good powder deposition efficiency. If the spray distance is too small, it is easy to break through the powder, affecting the quality of the coating. If the spray distance is too large, the deposition efficiency will be too low, the powder adhesion is reduced, increasing the work of recycling powder.
3.5 Gas Supply Pressure
Based on the principle of adsorption process for electrostation powder spraying, the air pressure and the amount of air delivered by the powder need to be carried out in accordance with minimum requirements as far as possible during spraying. Too much gas supply pressure allows the powder to gain greater momentum, making it easy to bounce off the surface of the aluminum alloy and difficult to absorb. In practice, there are three types of parameters of gas supply pressure, namely, powder pressure, atomized air pressure and fluidized air pressure, the changes of the value of these three pressure parameters will lead to significant changes in spray efficiency and quality.
4 conclusion
the use of electrostectric powder spraying technology in aluminum alloy surface treatment makes it anti-corrosion performance is good, and its resistance to acid-alkali salt spray is also superior. Static powder spraying technology has: simple process, high yield, energy consumption significantly reduced, water and air pollution level decreased, workers' labor intensity significantly reduced, the surface quality requirements of wool standards have been significantly reduced, and some physical indicators of coating film than other surface treatment film has significantly improved. The factors affecting electrostitive powder spray are powder conductivity, powder particle size, spray voltage, spray distance and gas supply pressure.
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