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Powder Coaters Manual 1/98 III/3

Powder Application Equipment

1 Fluidized Bed Application

Early in the history of powder coatings, thermoplastic materials wereapplied by the fluidized bed dip process. The fluidized bed is a con-tainer that holds the powder material with an air chamber at the bot-tom referred to as an inlet plenum. The container and the plenum areseparated by a membrane that is porous enough for air to pass throughbut not porous enough for solids to pass through. Compressed air isintroduced into the plenum and up through the fluidizing membrane.As the compressed air passes up through the container, the powderparticles are suspended in the airstream. In this suspended state, re-

ferred to as fluidization, the powder/ air mixture behaves somewhatlike a liquid.

Fluidized bed application is accomplished by preheating a metal partand dipping it into the fluidized bed of powder. The powder materialwill fuse upon contact with the hot part, creating a thick continuousfilm (10-20 mils) on the metal surface. In cases where the part doesnot have sufficient mass to completely fuse the powder, the part willbe put through a short post-cure cycle, typically 3-5 minutes at 400 to500 F (204 to 260 C).

Powder Setting in theBottom of the FluidBed Hopper BeforeFluidization

Pourous Membrane

Powder withCompressed Air Addedfor Fluidization

CompressedAir Inlet

Fluid BedHopper

Figure 3-1 Fluidization of Powder

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Powder Coaters Manual 1/98III/4

Electrostatic Spray Application

Electrostatic Fluidized Bed CoatingElectrostatic fluidized bed application uses the same fluidizing tech-nique and the conventional fluidized bed dip process but with muchless powder depth in the bed. An electrostatic charging media is placedinside the bed so that the powder material becomes charged as thefluidizing air lifts it up. Charged particles of powder move upward andform a cloud of charged powder above the fluid bed. When a groundedpart is passed through the charged cloud the particles will be attractedto its surface. The parts are not preheated as they are for the conven-tional fluidized bed dip process.

2 Electrostatic Spray Application

Electrostatic spray application uses a fluidized bed as a feed hopper tohold the powder and fluidize it so that it can be pumped to the tip of aspray gun using compressed air for transport from the feed hopper tothe gun tip. The spray gun is designed to impart an electrostatic chargeto the powder material and direct it toward a grounded workpiece.

This process makes it possible to apply much thinner coatings with awide variety of decorative and protective features.

The electrostatic charge can be imparted with voltage, called corona

charging, or by frictional contact with the inside of the gun barrel,called tribo charging.

In a corona charging system, a voltage source generates current througha voltage cable that carries it to the powder gun tip. Powder is pumpedthrough the gun and out of the gun tip using compressed air. As thepowder passes through the electrostatic field at the gun tip, it picks upa charge and is attracted to a grounded substrate. The part is thenconveyed into an oven for curing of the powder. In the cure oven, thepowder melts and cross-links to a hard film to complete the process.

Electrostatic spray application of powder is the most common appli-cation method. The parts to be coated are cleaned, dried, and cooled,the coating is applied and cured at the required temperature for therequired time and then cooled for removal from the line.

An electrostatic spray application system includes a delivery systemand a charging system. The basic pieces of equipment that make up asingle application unit are: