The Powder Coating Process

Powder coating processes may vary by the type of coating material being applied. However, the processes can generally be divided into four functions:

Preparation of the surface starts with a process to insure that the surface is properly cleaned and free of “soils” such as manufacturing oils. Subsequent surface treatments might include blasting, if required and iron phosphate or zinc phosphate depending on the long term corrosion performance desired Thermoplastic powders normally require a primer to promote adhesion. Thermoset powders do not need a primer because they have an adhesion promoter built-in. However, primers may be used with thermosets to enhance certain properties. The priming material used must be compatible with the curing temperatures required for powder coatings.

Application of the powder is a function with many variations of two basic techniques. These techniques are fluidized bed and electrostatic spray. The fluidized bed is the original powder coating technique. It is still the primary technique used for the application of thermoplastic powders. The fluidized bed is also used for the application of some thermoset powders where high film build is required or components are very small. Thermoset powders designed for electrical insulation often use the fluidized bed technique. The parts are preheated to a temperature significantly higher than the melting point of the powder. The parts are then immersed into a "fluidized bed" of the coating powder where the plastic powder is melted onto the part.

Electrostatic spray is the primary technique used for thermoset powders. The particles of powder are given an electrical charge in the powder coating gun. The target part is attached to a fixture that is grounded. The electrically charged powder particles are attracted to the grounded part and attach themselves like little magnets to the part. The particles are actually repelled by the charged particles on the part and the coating process stops. This provides an even film thickness.

Heating of the surface is normally accomplished using ovens fueled by natural gas. Thermoplastic powders normally require the part surface to be heated prior to the application of the powder via the fluidized bed technique. Thermoset powders are normally applied via the electrostatic spray technique while the part is at room temperature. The part surface and the powder are then heated at the same time.

Curing of the coating applies to thermoset powders. (Thermoplastic powders normally are not cured.) After the part surface is heated to the desired temperature (180º-200º C) it is held at that temperature for a specified period of time, normally 10 - 15 minutes, while the curing agent develops the desired film properties.