The original paint has a high viscosity, so a large amount of diluent should be added to reduce its viscosity before use, so that it can be atomized well during spraying. The viscosity of the paint used in electrostatic spraying is generally around 20s (Tu-4 meter) (Tu-4 Viscosity Cup measured at 25°C). If the coating with high solid content is used, the viscosity at room temperature can be adjusted to 30~40s when used under heating conditions. There are certain requirements for the volatility, polarity and solubility of the added diluent. According to the different resistivity of the paint, the polarity of the diluent added can be divided into high, medium and low. The paint higher than 300MΩ·cm is added with a diluent made of high polarity solvent, and the resistivity of the paint is 30~300MΩ·cm Use a medium polarity diluent, and add a low polarity diluent to the paint with a resistivity of 10~30MΩ·cm.
Since the paint mist stays in the air for a long time and the solvent volatilizes a lot during the electrostatic spraying process, the selected diluent should have a strong solubility for the paint and a slow volatilization speed. In the process of painting and drying, the thinner volatilizes slowly, which is conducive to obtaining a better paint film. Especially when painting at a higher temperature, or to obtain a thin and glossy coating, a thinner with a slower volatilization speed should be used. And because wet paint mist particles are more likely to acquire and maintain electrostatic charges than dry paint mist particles, the use of solvents with low volatility is beneficial to electrostatic spraying.
(2) Static voltage
Studies have shown that the voltage of the electrostatic field affects the charging rate and adhesion rate of the coating. The charging rate and adhesion rate both increase with the increase of voltage within a certain range, but when the voltage is increased to more than 100,000 volts, the adhesion of the paint will no longer increase significantly with the increase of voltage, and it is easy to cause electric shock. Sparks can cause fires, so the voltage of the electrostatic field is generally controlled below 100,000 volts. The fixed automatic Spray Gun is generally set at 80,000 to 90,000 volts, while the portable Spray Gun is generally set at about 60,000 volts for safety.
The high-voltage electrostatic field converts 220V alternating current into 60,000-100,000 volts of high voltage through a booster transformer, and then turns it into direct current through a selenium rectifier. Connect the negative electrode of the DC electrode to the Spray Gun, the positive electrode to the ground, and connect the workpiece to the ground to form a high-voltage electric field. Due to the high resistance of air corona discharge, the current intensity during high voltage discharge is actually very small, generally in the range of 100µA~5mA, which is very safe for the human body, because under high voltage, the current intensity must be at least 15mA The above will cause electrocution. Due to the low current intensity, there is no feeling of electric shock when touching the negative electrode of the hand Spray Gun. The input power required for portable electrostatic spraying is very small, which is only equivalent to the power consumed by a 60W light bulb. And because the workpiece is well grounded, once the negative end of the Spray Gun touches the workpiece, the voltage will quickly disappear without danger.
A safety protection device is installed in the electrostatic electrode. When the equipment is not operating normally or high-voltage discharge occurs, the protection device will detect the current. When the output current is too high, it will automatically reduce the output voltage or effectively cut off the power supply.
(3) Spraying operation requirements
1. The distance between the Spray Gun and the workpiece
According to experience, the most suitable average electric field strength for electrostatic coating is 30-40kV/cm, so when the electrostatic voltage is below 80-100kV, the distance between the Spray Gun and the workpiece is actually the distance between the positive and negative electrodes of the high-voltage electric field. Under a certain voltage, the electric field strength is inversely proportional to the distance between electrodes. Within a certain electric field strength range, the stronger the electric field strength, the more charge, the stronger the electrostatic attraction, the better the atomization effect and the higher the coating efficiency. high. However, when the distance between the Spray Gun and the workpiece is too close, the electric field strength is too high and spark discharge will be generated. And if the distance is too far, the adhesion rate of paint mist will decrease. Therefore, during electrostatic spraying, the distance between the Spray Gun and the workpiece should be controlled between 250 and 350mm. When spraying small workpieces, take a low value, and when spraying large workpieces, take a high value.
2. Discharge volume
A certain amount of discharge is specified for different types of electrostatic Spray Guns. If the spraying volume is too large, the charge of the paint will be insufficient, and the atomization will be poor, which will affect the coating effect. To improve production efficiency, the method of setting up multiple Spray Guns to spray at the same time is often adopted.
3. Arrangement of the Spray Gun
When arranging Spray Guns in the electrostatic spraying line, in addition to keeping an appropriate distance from the object to be coated, attention should also be paid to the distance from adjacent Spray Guns and other contact objects (including the ground, spraying brackets, wall panels of the spray booth, etc.). The distance from the adjacent electrostatic Spray Guns is based on the principle that the sprays and patterns of the two electrostatic Spray Guns do not interfere with each other, because if the position is improper, the paint particles will collide with each other, and the paint mist will fly randomly due to the repulsion between the same charges, and even fly to the ground. backsplash. When the two electric Spray Guns cannot be on the same plane due to the limitation of the workpiece, the distance between the guns should be kept at least 1m.
4. Hanging of the workpiece
Whether the suspension of workpieces in the production line is reasonable has a great impact on the effect of electrostatic coating. The principle of suspension is that the workpieces do not collide with each other. 0.5m. In the process of electrostatic spraying, it is better for the workpiece to rotate itself, so as to facilitate the uniform thickness of the coating film, and the rotation speed is preferably 3~4r/min. At the same time, attention should be paid to the cleaning and maintenance of the hanger, and the residual paint should be removed in time to ensure that the workpiece is well grounded and prevent electric shock when unloading the workpiece after spraying. It is better to prepare two sets of fixtures and hangers. When one set is used, the other set is sent for cleaning to ensure that the hangers are always in a clean state.
