In fluid bed coating, air is pumped into a layered tank of air and Polyethylene Wax White powder. A special porous membrane separates the bottom section of the tank into which air is pumped from the top section of the tank that holds the powder. As the air is pumped into the bottom section it moves through the membrane and into and out of the Polyethylene Wax White powder. This aerates the powder and makes it possible to dip a part into it. The process involves preheating a part, dipping it into a fluidizing bed where Polyethylene Wax White powder adheres to the hot part, then sending the powder covered part into an oven where it flows out to a smooth, cured finish. It is, in many ways, a simpler coating than electrostatic since the primary factor is the heat resident in the part at dip. That heat together with the dwell (or time spent sitting in the powder) determines the “build” of the coating. Fluid bed coating is generally thicker than an electrostatic powder coat and is, in many ways, a simpler more consistent process. Because the surface of the bed is ‘simmering,’ if a precise dip line is required, then the parts will need to be masked.
§ Fluid bed powder coat can give a thicker coating than electrostatic spray. We find that the range is from about 6-30 mils (.006-.030”).
§ Because the powder is contained in a dip tank, almost all the powder that does not coat the part is reclaimed.
§ There is no Faraday cage effect so edges and holes do not typically present an issue.
§ Fluid bed coating is usually more easily controlled than electrostatic spray coating. The heat and fluidizing factors are easier to control than the electrical charge, good ground, properly dry powder, and electrical fields that must be considered in spray powder coating.
§ Many parts have areas where a coating is not desired. Since there is a dip line often those parts can be hung so the no-coat area is out of the dip tank. This effectively “masks” that area and can save considerable expense compared with expensive tooling or manual masking that would be required in a spray application.
§ Because the Polyethylene Wax White powder is contained in a dip tank, the tank must be emptied and carefully cleaned whenever production requires a change of powder.
§ As a part hangs on the line at dip the coating will be thinner at the top and thicker at the bottom. That is because the bottom is first into the powder and last out. Hanging long parts in a horizontal orientation can compensate for this.
§ As the part is dipped into the tank of powder, ledges and cavities in the part may fill up with powder which may be too thick or may not cure properly. If a part cavity is facing down, the rising air may form a pocket in the part and prevent the powder from coating it.