Electrostatic spraying utilizes electrostatic attraction of elctrically charged powder particles to the grounded metallic substrate.
The powder particles gain the electric charge when they pass through the ionized air in the spray gun. Air is ionized in the high voltage (40-100 kV) electrostatic field generated by an elctrode located in the spraying gun. This method of electrostatic spraying is called “corona charging”. Friction charging (tribocharge) is another method of charging powder particles. The particles gain positive electric charge when they rub against the gun lining. The charged powder particles are attracted by the grounded part forming a thin coating
Fluidized bed presents a suspension of the coating powder particles dispersed in air. The fluidized bed tank has a porous bottom through which low pressure air is fed. The air stream keeps the powder particles in dispersed state. A preheated part is dipped into the air-powder suspension. The powder particles come in contact with the part surface, melt on the preheated substrate surface and form polymer coating. Thick coating of 0.003 - 0.01 inch (75 - 250 µm) may be obtained by a single dip in a fluidized bed.
Electrostatic fluidized bed differs from the conventional fluidized bed by using ionized air for formation of the air-powder dispersion. The ionized air charges the powder particles, which repel each other by electrostatic force. The charged particles are attracted by the grounded part dipped into the chamber. preheating is not necessary in this method. However the coated parts should be postheated to provide curing of the polymer coating. Electrostatic fluidized bed is used for coating small parts with simple shape.
In flame spray method the air-powder dispersion is injected by a flame gun together with burning combustible gas (eg. propane). The powder particles melt in the flame and form a coating on the cold surface of the part. Preheating of the part is not necessary. The powders for flame spray is prepared from thermoplastic polymers. The method is generally used for coating large objects, which can not be placed in a curing oven.
Advantages of powder coatings:
Disadvantages of powder coatings: