Electrostatic Painting Principle and Composition
Electrostatic painting uses the workpiece as the positive electrode(usually grounded). The paint atomization device is the negative electrode which is connected to the negative high voltage power supply. Between is a high-voltage electrostatic field with the workpiece. The paint is squeezed into the nozzle at high speed under compressed air then hits on the nozzle, here it’s negatively charged and forms micron-sized atomized particles then spray through the circular outlet of the nozzle. Due to the effect of corona discharge, the paint particles repel each other and further atomize. According to the principle of “opposites attract each other”, the negatively charged paint medium forms an embracing effect on the workpiece under the action of the electric field force, along the power line directionally flow to the surface of the positively charged workpiece, then accumulate into a layer of uniform, firmly adhered film.
From the analysis of these six factors of product quality – human, machine, material, method, environment and measurement, electrostatic painting quality issues are categorized by the following.
1- Operator Skills
Statistical analysis was carried out on the operation qualification and working years of the painting operators. All personnel participated in the training and examination organized by the training center as required, and obtained the operation qualification certificates, and the working years were more than 8 years with sufficient spray painting experience. After analyzing the feedback of uneven spraying failure, it is believed that there is no obvious corresponding relationship between the failure and the operator. Insufficient skill of the faulty operator is not the main factor.
2- Raw material quality
Paint, thinner and curing agent have all been tested before using, and are used only after passing the test. During the spraying process, there was no failure of large-area uneven paint spraying, so it is considered that the possibility of uneven paint spraying due to unqualified raw materials such as paint, curing agent, and thinner is relatively small.
3- Compressed Air Pressure Analysis
Insufficient compressed air pressure can easily lead to insufficient atomization of paint or insufficient paint spraying power, resulting in uneven spraying. The production site generally requires the spray gun pressure to be 0.35~0.5MPa. There is a pressure gauge of the electrostatic spray gun on paint pump body which can monitor the pressure value of the compressed air at any time. On-site feedback, the spraying operation was not carried out when the compressed air pressure was insufficient, so insufficient compressed air for spraying was not the main factor.
4- Environment Temperature and Humidity Analysis
When painting, the ambient temperature and humidity have an impact on the painting quality. If the the construction site relative humidity is too high, exceeding 80% or reaching saturation, due to the adsorption of water vapor on the metal surface, a very thin layer of water curtain that is eye invisible will adhere to the sprayed surface, thereby affecting the paint film-forming quality. Therefore, it is stipulated that the temperature of the construction site should be 18~25℃, and the relative humidity should be 45%~65%. The ambient temperature should not exceed the range of 12~30℃, and the relative humidity should not exceed 80%. By checking the on-site temperature and humidity record sheet and work records, not work being done when the temperature and humidity did not meet the requirements, so the influence of spraying environment temperature and humidity is not the main reason.
5- Electrostatic spray gun analysis
During the electrostatic spray gun maintenance procedures, it is required to cycle and clean the spray gun system immediately after spraying. But it does not require separate cleaning and inspection of nozzles. According to the electrostatic spray gun nozzle structure diagram, the nozzle diameter is 0.13mm. There is a fan-shaped groove along the circular hole, which can make the paint mist sprayed by the nozzle form a uniform oval paint mist. If the nozzle is not cleaned thoroughly, the nozzle hole will be blocked, and the paint material will be blocked when it is sprayed from the nozzle at high speed. The paint flow rate will change when it passes through the nozzle hole. The shape and uniformity of the atomization will eventually lead to the formation of uneven paint film on the sprayed workpiece.
