The application of automatic spraying system has become an important factor to measure the level of the coating production line. Automatic electrostatic spraying is one of the widely used spraying systems. The setting of process properties directly affects the coating quality. This article takes automatic electrostatic turbo bell coating as an example to find the influence of various process parameters in coating application.
1 – high voltage value of atomizer
The principle of electrostatic turbo bell coating is by atomized paint thrown out through the edge and center of bell rotating at high speed, forming a cone-shaped atomized substance under the action of shaping air and high voltage, the workpiece is grounded, and the paint particles are charged with an electrostatic charge. Under the action of the electrostatic field between the device and the workpiece, the paint particles are uniformly adsorbed on the surface of the workpiece to form a uniform film. The increase of the high voltage value will strengthen the electric field force of the electrostatic field, increase the paint lacquering rate, and thicken the film layer, but the high voltage value is not as high as possible. When the voltage is set too high, the spray will Spark discharge is generated, and at the same time, at the corners and edges of the workpiece, sagging, air bubbles, and the like are likely to occur.
When the high pressure value is too low, it will cause a huge flow of paint back during the spraying process, and the surface of the vehicle body will have obvious eddy current, and the paint covering the surface of the workpiece is too small, resulting in a low painting rate. The high voltage value is generally set in the range of 50 to 80V.
2 – atomizer air flow
The shaping air is a main parameter affecting the shape of the jet flow and the moving speed of the paint. The larger the volume of shaping air, the smaller the area formed by the conical jet flow, and the thickness of the coating on the central part of the jet flow will increase significantly. The smaller the amount of shaping air, the larger the area of the conical spray paint flow, resulting in a thinner film thickness in the area. The shaping air parameters are generally set at 50~200. When it is too low, it tends to cause low paint lacquer rate, and it also causes paint mist pollution of the rotating cup. When it is too large, due to the large flow of compressed air, airflow interference occurs, so that the paint is attached to the atomizer, which will lead to the paint film. The quality of the surface is ill, and the amount of plastic air affects the effect of the rotating cup.
3 – atomizer paint flow
The paint in electrostatic bell is sprayed from the bell gap by rotating, and the paint is sprayed from bell center hole: the paint flow rate of automatic spray painting machine is generally in the range of 50 to 250, and the precise control is achieved by metering paint pump. If the paint flow rate is set too big, it will not only affect the atomization effect but also cause the paint to be coarse, and it will produce paint defects such as dripping paint, sag, and bubbles. Its parameter setting is related to the following factors:
- Paint component: When the required film thickness is the same, the solid content of the paint is high, and the flow rate of the paint should be appropriately reduced.
- Chain speed: When the chain speed increases, the paint flow should also increase accordingly.
- Requirements for paint film thickness: Generally, the film thickness of varnish is required to be 35~50. The thickness of metal paint film is required to be between 12 and 18. The thicker the paint film, the larger the paint flow rate is required.
4 – Bell rotation speed
The main function of electrostatic bell rotation speed is, sprayed paint passes through a speed of up to 25 000 – 50 000 ^ 1.1 to achieve a certain atomization effect.
The setting of the bell rotation speed depends the paint requirements: the speeds required for the primer, base paint and varnish are different. For solvent type paint, for example, the speed of the varnish is generally controlled at 35 000-40 000, the middle and bottom coat is slightly lower at 30 000-35 000.
Under normal condition, the greater the rotation speed, the thicker the film thickness, and the better the atomization effect. However, if it is operated for more than 45,000 for a long time, it will cause damage to the atomizer, such as: wear of the bearing unit, increase The cost of equipment spare parts. Compared with other paint parameters, the adjustment of the rotation speed of the rotor is relatively easy. Therefore, sometimes the spray quality is controlled by adjusting the rotation speed of the rotor, for example, the temperature in the winter is suddenly lowered, and the viscosity of the paint cannot be adjusted in a short time. The rotation speed of the rotating cup can be appropriately increased to avoid the occurrence of defects such as sagging.
5 – Main nozzle control parameters
The main nozzle control refers to open or close the atomizer during the spraying process. For example, when painting on automotive top window, let’s say it is between 252 and 276 position. The machine runs to starting point 252. Set the main needle to be closed. When it reaches 276, the main nozzle is turned on. Its function is equivalent to the nozzle in the manual spray gun, the needle is opened, the paint is sprayed, and the main nozzle can also control the atomizing. Start point parameters for long and short cleaning and paint spraying procedures.
6 – Overload percentage setting
The percentage of overload is a percentage value that controls the parameters of the conveyor chain speed, paint flow, forming air volume, and high voltage value. This parameter setting value can make the setting of the parameter quite simple in some cases.
For example, the existing chain speed is 2.6m/Min. Due to the demand for production line capacity, it needs to be upgraded to 2.86. Then, the paint consumption per unit time must be increased accordingly, otherwise the paint defects of paint, orange peel and the like will occur in the body. If the adjustment is made in the modified parameter window, it is necessary to increase the amount of paint and the amount of forming air in each area up to 90. All areas and more than 200 parameters should be re-adjusted.
7 – Profiling and positional parameters
During the operation of each atomizing device of automatic spraying machine, the moving point of the space is determined by the three point axes and atomizer rotating axis. Taking automobile body spraying as an example, the side sprayer is mainly used to spray the vertical surface of the body side, and each side is equipped with 2 (or 3) side sprayers. The side sprayer has three axes of motion: the V axis (the horizontal movement of the cantilever is 1 080-1 550); the vertical movement of the 2 axis 1 cantilever, the range is 350~2 160; the eight axes (fog) The rotary motion of the chemical device) ranges from 25 to 90.
The top sprayer is mainly responsible for the paint spraying of the front cover, the top and the tail of the car body, and there are five movement shaft systems. When it is found that some parts of the side of the painted body are always painted, the paint rate is relatively poor. At this time, the coordinate point of the V-axis of the side machine can be modified to make the distance of the atomizer close to the vehicle body to solve the problem; There are too many curved surfaces in a certain area. By adding contour points, the spray profile curve is closest to the surface of the body, and the defects are constantly found in the production process. The contour points are modified to make the paint of the automatic spraying machine. The best results are achieved by the utilization rate, the spraying effect, and the electrostatic effect of the corners.
8 – Procedures of short cleaning, long cleaning and pre-painting
Automatic sprayer atomizer shall be subjected to long cleaning or short cleaning after each spraying. Long cleaning procedures, short cleaning procedures, and pre-painting procedures are mainly used for solvent cleaning and paint pre-drying for color change or after each spraying process. Note that their respective purposes are as follows:
Long cleaning procedure: After color change or same color is sprayed 10 times, all the pipes, valves and bell in spraying system are automatically cleaned with solvent, the time is 8~108, and the solvent flow is in the range of 100-450.
Short cleaning procedure: After each object is sprayed, electrostatic bell is automatically cleaned with solvent for 4~6, and the solvent flow is in the range of 100-450.
Pre-painting procedure: After bell is cleaned, the pipeline and valve in the spraying system are painted, the time is 4~6, can be adjusted.
Application example: After object is sprayed, the dark red color paint is generally difficult to clean, especially when the base color is red but followed by white. If the red paint is not cleaned in the pipeline and valve, it is very likely to cause rework. At this time, two parameters in cleaning program can be adjusted: 1- extend the long cleaning program time 1~ 3; 2- increase the solvent cleaning flow rate 100-150.
The cleaning procedure can be modified by above method, and different cleaning procedures can be set for the paints of different colors, thereby eliminating rework by cross-color issue.
Conclusion
Based recent years experience in the coating production line, the application of automatic electrostatic spraying has greatly improved the paint shop production level. Compared with traditional manual air powered spraying, not just production efficiency but also paint efficiency has been greatly improved in painting workshop. Quality of coating is stable and the finishing is good. To meet with competition on the market, coating lines is becoming more flexible, multi-variety and intelligent. Therefore, automatic electrostatic spraying machines and bells are used. Robotic spray coating is next stage for most applications so product quality can be guaranteed and the production line can meet the market demand.
