Conventional electrostatic charging(CORONA CHARGING) for powder coating

Conventional Electrostatic Charging (Corona Charging) by passing the powder through a high-voltage electrostatic field.

High voltage (40-100 kV) concentrated at the nozzle of the spray gun causes ionizing of the air passing through the spray gun. Passage of the powder through this ionized air then allows free ions to adhere to a proportion of the powder particles whilst simultaneously applying a negative charge to them.

Between the electrostatic spray gun and the object being coated, the following are present:

Conventional Electrostatic Charging

It is always of vital importance to achieve the highest possible proportion of charged powder particles during the process itself. The method in which the spraying equipment is used also contributes to success.

Non-charged powder particles do not adhere to the object and will be recycled. Even though recycling is common in powder coating, it is always preferable to keep the amount of recycled powder to a minimum.

Free ions are small and far more mobile than powder particles. Excess free ions will move rapidly towards the object transferring, at the same time, large amounts of negative charge to it. The quantity of free ions being totally dependent on regulating the required voltage. Superfluous high voltage produces an oversupply of free ion, which in turn makes good powder coating harder to achieve and, not least, gives poorer flow (back-ionizing). Insufficient earthing of the object will further worsen the situation.

Using high voltages produces electrical field lines between the nozzle of the spray gun and the object, with the powder showing a tendency to follow these field lines. Objects of a complicated structure will have the highest density of field lines on their outer surfaces, particularly on exterior corners. Similarly, a lower density of field lines will occur on interior corner and indentations.

This phenomenon is commonly referred to as the Faraday Cage effect resulting in difficulties with powder application where the field line density is at its lowest, as shown in the following diagram:

Faraday Cage Effect

Higher voltage produces a more intense Faraday Cage effect, leading to a thicker film of powder where surfaces are more easily accessible and a correspondingly thinner coating for areas more difficult to reach. It is important to set the spray gun voltage sufficiently high allowing optimum charging of the powder. However using unnecessarily high electrostatic voltage has many undesired effects. What characterizes a skilled powder coating operator is the ability to achieve the right balance.