In order to obtain a good quality coating, in addition to controlling the above-mentioned important process parameters, the following construction techniques should usually be paid attention to.
1. Control the thickness of the coating
The workpiece is electrostatically sprayed at room temperature, and the coating thickness is generally between 40 and 100 µm. To reach 100µm, generally two sprays can be used. If a thicker anti-corrosion or insulating coating is required, the workpiece can be preheated and then sprayed. The powder output of the Spray Gun has a direct impact on the thickness of the coating. The main factors affecting the coating thickness of sprayed workpieces on the automatic production line are the speed of the conveyor chain and the powder output of the Spray Gun. When the speed of the conveyor chain is determined, the powder output of the Spray Gun is an important factor. For example, the transmission speed of the spraying line on the surface of a refrigerator is generally 4.7~5.5m/min, and the coating thickness of the refrigerator is required to reach 60~80µm. At this time, the powder output of the Spray Gun should be controlled at 120~160g/min.
2. Control the uniformity of the coating
In the high-voltage electrostatic powder spraying, the important factor affecting the uniformity of the coating is the electrostatic shielding, so to obtain a uniform coating, the influence of the electrostatic shielding of the workpiece must be reduced first. Due to the uneven distribution of power lines in the high-voltage electrostatic field, it is easy to cause the distribution of power lines to be concentrated at the tip or edge of the workpiece, and the phenomenon that there are few power lines in the concave part is called Faraday shielding phenomenon. Its direct effect is that during electrostatic spraying, the coating particles concentrate on the tip and edge of the workpiece and the concave part is coated with a small amount of unevenness. Secondly, it is necessary to ensure the stable and uniform diffusion of the powder during the spraying process. For this reason, the following points should be done.
(1) Correctly hang the surface of the workpiece with a large area or have an important impact on the appearance. It should be perpendicular to the ground and perpendicular to the powder discharge direction of the horizontally placed Spray Gun.
(2) The workpiece should be well grounded. Poor grounding is an important reason why the workpiece is not easy to absorb paint powder and cause uneven coating. Therefore, the workpiece and the grounded hanger should be in good contact. If there is a powder coating film on the hanger, it should be Remove promptly.
(3) Prevent reverse ionization of the powder layer. The reverse ionization phenomenon refers to the phenomenon that when the sprayed powder layer reaches a certain thickness and then continues to spray, the coating will not thicken, but will appear many pits, making the surface rough. The main reason for this phenomenon is that the negative charge in the accumulated powder layer accumulates more and more during the powder application process, and the electric field strength is also increasing until it rises to the strength of being broken down, resulting in local insulation being broken down. , The phenomenon that the coating deposition layer is destroyed. In order to prevent the occurrence of reverse ionization, the spraying thickness should be controlled, and the powder layer should not be too thick.
(4) Try to remove the dirt and debris in the powder coating. The powder coating used must be screened to remove impurities, and the recycled powder coating needs to be screened. In order to reduce the impact of impurities, recycled powder should be used in combination with new powder, and cannot be used alone. Generally, the ratio of recycled powder to new powder should be controlled at (1:1)~(1:3), and mixed before use.
(5) To reduce adverse effects in the environment On the one hand, the temperature and humidity of the environment must be well controlled. The suitable ambient temperature for powder spraying is about 25°C, and the relative humidity is ≤70%, otherwise the powder coating is prone to agglomeration, which will affect the screening ability of the rotary sieve and the coating quality; on the other hand, the quality of the compressed air supplied must be well controlled. The input compressed air should be clean and dry, free of oil and moisture. It is used for the compressed air of the powder spraying system. The maximum input pressure is 1.0X105Pa, the maximum water vapor content is below 1.3g/m3, and the maximum oil content is 1.0X10-7( volume fraction) below.
3. Control the Spray Gun well to make the powder output stable and the atmosphere uniform
During automatic spraying, attention should be paid to adjusting the up and down strokes of the Spray Gun according to the height of the workpiece before construction, so as to ensure that the workpiece is fully sprayed and minimize the deposition efficiency caused by the emptying of the Spray Gun. When spraying manually, the atomized powder should be evenly sprayed on the surface of the workpiece. When spraying large workpieces, try to keep the distance between the Spray Gun and the surface of the workpiece as constant as possible according to the shape of the workpiece. Continuous reciprocating spraying should be carried out, and the moving speed of the spraying should be as uniform as possible. For workpieces with complex shapes, the grooves and edges are generally sprayed first, and the main surfaces are sprayed last. When spraying starts, the Spray Gun can be closer to the workpiece. When the workpiece absorbs a certain amount of powder layer, the Spray Gun should be farther away from the workpiece. This can not only increase the powder feeding speed, but also prevent the counter ion flow from penetrating the powder layer due to the Spray Gun being too close to the workpiece, causing pinholes and hemp holes in the coating film.
When spraying the interior of housings such as valves, instruments, and instruments, the Spray Gun head should be extended into the housing for spraying, and the powder supply air pressure should be properly adjusted to a small level, and attention should be paid to prevent excessive powder from accumulating on the corners and steps. . For the coating film with decorative anti-corrosion effect, the thickness should be about 60~80µm. When the anti-corrosion performance is required, the thickness can be increased appropriately, but one spraying should not be too thick, otherwise the coating film will easily produce pitting and sagging. It can be improved by spraying multiple times or spraying after the workpiece is preheated.
There may be the following reasons for the unstable powder output and uneven atomization of the Spray Gun.
① The diameter of the Venturi pump nozzle in the powder supply device is too small or the delivery pipe from the powder outlet of the Venturi pump to the Spray Gun is too long, resulting in blockage of the powder delivery pipe. If this happens, in addition to possible changes to the equipment, the air supply pressure should be increased to increase the rate of powder feeding airflow.
② The hole of the diaphragm breathable plate on the fluidized bed in the powder feeder is blocked, resulting in uneven ventilation. At this time, it is necessary to dredge the holes of the diaphragm breathable plate. Sometimes it may also be caused by the powder level in the fluidized bed being lower than the powder suction port and the air supply of the fluidized bed being too small to interrupt. At this time, attention should be paid to increasing the supply of powder coating or adjusting the compressed air volume of the fluidized bed.
③ Caused by the powder coating itself. If the powder coating itself has poor fluidity or agglomeration, it will also cause uneven powder output from the Spray Gun. At this time, measures such as screening and refining the powder coating and strengthening the vibration of the fluidized bed should be adopted.
