Fluidized Bed

The fluidized bed coating process is a simple dipping process that can be either conventional or electrostatic. In the convention fluidized bed process, the fluidized bed is a tank with a porous bottom plate. The plenum below the porous plate supplies low pressure air uniformly across the plate. The rising air surrounds and suspends the finely divided plastic powder particles, so the powder-air mixture resembles a boiling liquid as shown in Figure 1. Products that are preheated above the melt temperatures of the powder are dipped in the fluidized bed, where the powder melts and fuses into a continuous coating. A high transfer efficiency results from little drag out and no dripping.

Figure 1: Illustration of the fluidized bed process.

The fluidized bed powder coating method is used to apply heavy coats in one dip, 3 - 10 mils (75 - 250 µm), uniformly to complex shaped products. It is possible to build a film thickness of 100 mils (2500 µm) using higher preheat temperatures and multiple dips. An example of film build-up is presented in Figure 2.

Figure 2: Effects of preheat temperature and dipping time on the film build in coating a steel bar with epoxy resin.