electrostatic powder coating
With atomization nozzle (paint atomization) and discharge level (electric current) of the coating machine to paint particles, the charge, the electric field formed between the electrode and the coated object, the use of its electrostatic adsorption and coating.
From the point of view of the process itself, has a very high scientific and technological components, the deployment of different colors of paint, produce different color decorative effect, more in line with the needs of interior decoration, is also consistent with the application of aluminum in the construction industry to the trend of indoor development. In particular, the combined use of spray profile and heat insulation broken bridge makes it more fashionable, so that the powder spray profile has a greater space for development, but also the profile produced by other processing methods can not be replaced.
A plant for automatic powder coating normally comprises the pre-treatment, water drier, coating zone, curing oven and conveyor system. Such complete Industrial plants are monitored centrally. According to the layout more booths with corresponding conveying circuits may be necessary. For small production volumes or for special colours a small booth for manual coating is often sufficient, which from the price, and space requirements can be fitted into any production workshop.
Electrostatic powder coating is becoming increasingly more important. The growth rate of this environmentally friendly technology is far above average. Floor space, and personnel costs can be keep low through compact plant design, and the possibility to automate extensively. Since coating takes place in a closed system, without solvents, there are no harmful side-effects. The efficiency of this technology, depending on the recovery system, is up to 99%. This makes powder coating one of the most economical of all surface finishing technologies. With the relatively new development of so-called quick colour change systems, a colour change can be carried out within fifteen minutes. Powder coating is becoming more and more a replacement process for wet coating, which is considerably less environmentally friendly.
1.1 Coating chromatic aberration
Chromatic aberration, also known as chromatic aberration, is a serious defect of lens imaging, which is simply the difference of color , GB 5237.4 requires that the chromatic aberration of powder spraying profile is not more than 1.5. In the electrostatic powder coating of aluminum profiles, the main reasons for chromatic aberration of coating film are powder coating and powder coating process control.
1.1.1 Powder coating
In powder coating color difference, from the powder coating itself, the possible reason is that the heat resistance of the raw materials selected in the formula, such as pigments and curing agents, is not ideal; in addition, there is a gap between the baking and curing conditions in the powder coating factory and the baking and curing conditions in the powder coating production factory when mixing colors may also cause the coating color difference.
1.1.2 Powder coating process
Considering from the aspect of powder coating, the following factors will affect the color difference of powder coating.
(1) Uneven coating thickness, especially light-colored powder coating, the color difference caused by uneven coating film thickness is more obvious. In order to minimize the coating chromatic aberration, in powder coating, must ensure a certain degree of coating thickness, and coating thickness to be uniform.
(2) The temperature control sensitivity of the curing oven is poor, and the coating is also prone to color difference when the temperature distribution is not uniform. In order to reduce the color difference in baking and curing, the temperature distribution of the curing oven is required to be more uniform.
(3) In the baking process of powder coating, the curing time also has an obvious effect on the color difference of the coating. In particular, the thickness of the workpiece material is different at the same time, the workpiece material is thicker than the thin heating time is longer, in the curing oven with fixed conveyor chain speed, the effective curing time after the workpiece reaches the set temperature is short, so that the color difference between workpieces of different material thickness is also easy to occur. Therefore, in powder coating those different materials, thickness, shape and size of the workpiece, it should be classified according to the material, thickness, shape and size of the workpiece, and then set different baking time for baking and curing respectively.
(4) The powder room changes color, the suspension chain stops for too long and the curing time is too long, resulting in excessive discoloration of the profile curing in the oven.
1.2 Poor adhesion of coating
The national standard stipulates that the dry adhesion, wet adhesion and boiling water adhesion of powder spraying profiles reach 0 grade.
1.2.1 Powder coating aspect
Considering from powder coating, if the choice of resin or the variety and dosage of curing agent in the formula design is not reasonable, the quality percentage content or volume concentration of film-forming additives is not appropriate will affect the adhesion of the coating film.
1.2.2 Powder coating process
Considering the powder coating process, the main reasons affecting the adhesion performance of the coating film are as follows.
(1) The surface of the workpiece is not well treated. The surface of the workpiece is not cleaned, or the chromated film quality is not good, and the surface is stained with the residue of the pre-treatment tank, etc. In this case, the adhesion of the coating film to the substrate after powder coating is not good, and the impact resistance of the coating film is reduced, which should be solved by strengthening the surface treatment process of the workpiece.
(2) The baking curing temperature and curing time of the curing oven fail to meet the process requirements, and the curing temperature and time need to be set reasonably according to the size of the profile and the type of powder in the actual production process. In the curing conditions of powder coating, the curing time refers to the time of insulation after the coated object reaches the curing temperature, not the time that starts to be calculated after the workpiece is put into the baking oven. When the workpiece material is thicker, the more pieces are hung, then the heat capacity of the workpiece is bigger, and the time needed for the workpiece to warm up to the control temperature is longer. If the effective baking time is not enough, the curing of the coating film is not complete and the physical and mechanical properties of the coating film will not meet the requirements. Therefore, in powder coating, the curing oven temperature should be measured regularly with the oven temperature tracking meter to determine the reasonable baking and curing temperature and time, so as to ensure the complete curing of the coating film.
(3) The air compressor provides auxiliary air for the coating process. Insufficient air drainage and oil removal will have a direct impact on the coating quality. Therefore, the oil-water separator, dryer and drainage device need to do daily maintenance work and develop a daily dedicated maintenance system.
1.3 Coating orange peel
Orange peel is uneven and uneven coating, visually like orange peel , the causes of orange peel mainly include the following aspects.
(1) powder quality problems, powder leveling agent and other raw materials of substandard quality or formulation problems.
(2) the coating film is too thin, the leveling is blocked can not be well unfolded, at this time can see the texture of the profile substrate; coating film is super thick, the surface of the coating film is speckled orange peel.
(3) Powder coating curing speed is too fast, and its own leveling is poor.
(4) The baking temperature is too low, and the melt fluidity is poor.
(5) powder atomization is not good when spraying powder.
Its solution measures should also be considered from the following aspects.
(1) readjust the curing speed and leveling of powder coating, or choose new powder coating.
(2) do a good job of coating process film thickness control, to avoid film thickness is too thin or ultra-thick.
(3) Adjust the process parameters of electrostatic spraying, which should mainly control the high pressure electrostatic and the distance between the surface of the workpiece and the gun to achieve the effect of controlling the size of the film thickness and the uniformity of the coating.
1.4 Coating shrinkage
A shrinkage is a circular depression formed on the coating surface starting from a central point.
1.4.1 Powder coating aspect
Consider from the powder coating itself.
(1) The quality of the powder coating itself, such as the mixing of oil and water in the manufacturing process of powder coating, and the quality of the resin itself, etc..
(2) the resin composition of different powder coatings mixed, poor compatibility between powder coatings.
1.4.2 Powder coating process aspect
From the perspective of powder coating, there are several reasons that can easily produce the drawbacks such as shrinkage of the coating film as follows.
(1) shrinkage caused by unclean pretreatment degreasing, oil on the surface, or residual treatment solution caused by unclean water washing after degreasing.
(2) powder coating electrostatic spraying used in the compressed air in the oil, water content exceeds the standard, resulting in powder coating contaminated.
(3) hanging chain of oil falling to the surface of the workpiece caused by shrinkage.
(4) When the powder coating is seriously damp, it is also easy to make the coating film produce shrinkage. Powder coatings should be stored at room temperature and dry conditions. For those materials that have been opened but not yet used up, make sure to tie the plastic pockets to prevent moisture. Pay special attention to recycled powder coatings in the recycling process is easy to absorb moisture and bring in impurities and other problems, recycled powder should be mixed with new powder coatings in a certain proportion and used immediately.
1.5.1 For powder coating
Considering from the powder coating itself, the impurities brought into the powder or the dust in the environment into the powder coating during the powder coating manufacturing process may cause surface particles .
1.5.2 Aspects of powder coating process
Considering from the powder coating process, the following problems are likely to cause coating film particles.
(1) Defects such as burrs and pits on the surface of the workpiece can easily cause particles to appear in the coating film. This is because when the powder coating is baked and cured, the powder coating melts and flows flat so that the surface of the workpiece is closed, and as the temperature of the workpiece rises, the air pressure in the burr and pockmark increases, thus arching the molten and flowing coating film. The fundamental measure to solve this problem is to control the surface quality of the profile extrusion process.
(2) The surface treatment of the workpiece is not clean, and impurities such as residues in the surface treatment fluid are still attached to the surface of the workpiece, which can easily produce coating particles when powder coating is carried out. This problem can be solved by cleaning up the residue of the pre-treatment in time and cleaning the workpiece in the water washing process.
(3) In electrostatic powder coating, the production environment around the powder spraying room is not clean, especially some dust and particles in the air are brought into the powder coating or powder spraying room, in the powder coating melt flow level, become particles in the coating film. (b) This problem is solved by isolating the powder spraying room from the production plant in order to prevent dust and dirt from the production plant from being brought into the powder spraying room.
(4) In the process of recycling powder coating, the recycled powder is not sieved or the mesh of the screen is too small can cause coating particles. The solution to this problem is to choose the appropriate mesh sieve, recovered powder by automatic or manual vibrating sieve, sieve the powder and then mixed with the new powder in proportion to the use.
(5) powder volatiles in the curing oven into flocculent or conveying chain pendant particles fall off, will produce coating particles. The solution to this problem is to promptly clean up the curing oven and the impurities attached to the conveyor chain.