Main control indicators for powder electrostatic spraying The main indicators to be controlled during powder electrostatic spraying of insulated aluminum profiles are film thickness, film thickness uniformity and coating efficiency. GB5237.4-2004 requires a film thickness between 40 and 120, and the range is very large. If a company can control the film thickness to a lower limit within the required range, the cost reduction will be large. It is beneficial.
Powder supply and its influencing factors
At a given conveyor speed, in order to ensure the coating thickness required by the process, the amount of powder discharged from the gun should be controlled. The concentration of the powder/air mixture, ie the powder output (the amount of powder supplied) depends on the pressure of the conveying air and the pressure of the supplementary air, the characteristics of the powder, the length and diameter of the powder tube, the number of coils of the powder tube, the spray gun and the ejector The difference between the height and the type of nozzle. Generally, the amount of powdered powder increases as the pressure of the conveying air increases, the diameter of the powder tube increases, and the length of the powder tube decreases.
When the inner diameter of the Swiss-made Golden Horse and Lier powder gun is 11mm, the US Nordson powder gun and powder pump also use a powder tube with an inner diameter of 11mm. In order to achieve the effect of **, Nordson suggests that the length of the powder tube is not Should be greater than 8m, the height of the rise should not be greater than 2.7m.
Experience with pneumatic material handling techniques has shown that the delivery medium requires a defined volumetric flow rate when delivering a small solid material such as powder in a hose. For a hose with an internal diameter of 11 mm, this determined volume flow value is 4 ~5m/h. Therefore, when a small amount of powder is required for some process reasons, the pressure of the conveying air becomes smaller, so that the gas flow rate in the powder conveying pipe is lowered to a value of 4 to 5 m/h. Therefore, the powder delivery becomes unstable, and the phenomenon of "puffing powder" occurs. In order to prevent this from happening, a supplemental air inlet is generally provided on the sub-pump so that the air flow rate in the powder tube reaches 4~5 m/h again. . The flow meter on the gun controller generally determines the total flow of the delivered air and the supplemental air. Therefore, during operation, the [float" in the flow meter should be controlled to float in the green section of the scale. The use of supplemental air is an important condition for continuous and uniform powder supply.
In addition to the above influencing factors, the amount of powder supplied can also be adjusted by changing the gap between the nozzle and the powder collecting nozzle in the sub-pump. However, this method is troublesome and rarely used in the field.
In the process, if the powder supply is insufficient, and the requirements are still not met after adjusting for the above factors, the reason should be found from the following aspects:
(1) The venturi tube that may be divided into pumps is too small, and should be replaced at this time;
(2) The fluidized plate may be clogged and the ventilation is uneven;
(3) The powder level in the fluidized barrel is lower than the suction port and the fluidization pressure is too small or even interrupted. At this time, the powder should be added or the fluid pressure should be adjusted;
(4) Since the fluidity of the powder coating itself is too poor or agglomerated, replace the powder or check whether the vibrator is normal.
Coating uniformity control
The important factors affecting the uniformity of the coating are electrostatic shielding, anti-ionization, diffusion uniformity, repeated spraying times, workpiece grounding conditions, and the distance between the gun and the workpiece. Pay attention to the following points in the application:
Properly suspend the workpiece and select the nozzle. A large area or surface that has a major impact on the appearance should be perpendicular to the ground and perpendicular to the direction of the horizontal gun. When the vertical dusting line adopts a 4×90° rotating spreader, the profiles are coated on the same side with the same powder, with uniform coating uniformity. In addition, different nozzle nozzles are used for different profile sections. Flat nozzles should be used for complex profiles and nozzles with horn guides for flat profiles.
Prevent the reverse ionization of the powder layer. Anti-ionization means that when the powder layer reaches a certain thickness and then continues to spray, the coating not only does not thicken, but instead produces a lot of honeycomb pits, which makes the surface rough. The main reason for this phenomenon is that the negative charge accumulated in the surface layer powder of the workpiece is very resistant due to the large resistance of the powder coating itself, and it is not easy to lose the charge. The more the surface powder is accumulated during the powdering process, the higher the surface potential is, until it rises to Partial discharge occurs, causing damage to the applied deposit layer and partial insulation breakdown. The precaution is to control the spray moderately and not to make the powder layer too thick.
Ensure that the spray gun is stable, and choose the spray gun speed and the number of repeated sprays.
The workpiece is well grounded, otherwise the workpiece is not easy to adsorb the powder and the coating is not uniform.
The distance between the spray gun and the workpiece is better than the distance between the workpiece and the uniformity of the coating. At present, the distance required for commercially available spray guns is generally between 100 and 300. However, for different profiles, the distance between the spray gun and the workpiece is different. Generally, it should be adjusted according to the supplier's opinion.
The control of environmental factors should control the temperature and relative temperature of the spraying environment, otherwise the powder is easy to agglomerate, affecting the screening capacity and coating quality in the powder supply box. The compressed air needs to be stable, clean and dry, and ensure the quality of the gas supply.
2019 03/13