Early in the history of powder coatings, thermoplastic materials were
applied by the fluidized bed dip process rather than by electrostatic guns.
The fluidized bed is a container that holds the powder material with an air chamber at the bottom referred to as an inlet plenum.
The container and the plenum are separated by a membrane that is porous enough for air to pass through but not porous enough for solids to pass through. Compressed air is introduced into the plenum and up through the fluidizing membrane.
As the compressed air passes up through the container, the powder
particles are suspended in the airstream. In this suspended state, referred to as fluidization, the powder/air mixture behaves somewhat
like a liquid.
Fluidized bed application is accomplished by preheating a metal part
and dipping it into the fluidized bed of powder. The powder material
will fuse upon contact with the hot part, creating a thick continuous
film (10-20 mils) on the metal surface. In cases where the part does
not have sufficient mass to completely fuse the powder, the part will
be put through a short post-cure cycle, typically 3-5 minutes at 400 to
500 °F (204 to 260 °C).
Other articles on fludiized beds:
Fluidized Bed Powder Coating (Innotek pdf)
Fluidized Bed Chart (PCC)
Fluidized Bed (The Powder Coating Center)
Understanding Fluidized Beds (PFonline)
June 14, 2012