Fluidised Bed

Powder coatings can be applied commercially by either fluidised bed or electrostatic spray, the latter being by far the most common.

Fluidised bed

The fluidised bed was originally designed for the application of thermoplastic powders and remains the most common method of application of these products.  In the case of thermosetting powders its use is generally confined to cases where an exceptionally thick, eg. 250-500 micron, coating is required in one application; such as the coating of valves for gas and water pipelines where a tough durable and corrosion resistance finish is essential.

Essentially, the fluidised bed consists of a tank with a porous membrane base, through which a controlled low-pressure air supply is fed.  This serves to fluidise the powder within the tank, giving it properties similar to liquid, thus reducing its resistance to items entering it and ensuring a homogeneous coating.

The objects to be coated are preheated to a temperature above the fusion point of the powder before immersion in the fluidised bed. The coating thickness is determined by time of immersion in the bed and the preheating temperature of the object.  Where the objects are big enough the retained heat may be sufficient to cure the polymer otherwise post-heating will be required.

Advantages

1)    An exceptionally thick and corrosion-resistant coating can be obtained in one immersion and stoving cycle.
2)    With properly formulated powders, uniform films can be obtained.
3)    Low initial plant cost.

Disadvantages

1)    Relatively large quantities of powder are required to charge the tank.
2)    The workpiece must be preheated and, in some cases, post-heated to complete cure.
3)    This method is only applicable to those cases where a high film thickness is required.
4)    This method is confined to articles of relatively simple shape.
5)    Thin gauge metal cannot be coated because of its insufficient heat capacity.