A method of applying a coating of a thermoplastic resin to an article in which the heated article is immersed in a dense-phase fluidized bed of powdered resin and thereafter heated in an oven to provide a smooth, pin-hole-free coating.
After pre-heating, the item is dipped into a bed of fluidising powder. Beds vary in size, hence a laboratory-sized powder bed may hold as little as a few hundred grams of powder, whilst a large scale production model may hold several tonnes. This bed consists of two compartments, one on top of the other. The upper, larger compartment contains the coating powder. The lower compartment, or “plenum chamber”, is a reservoir for pressurised air. A porous membrane, sometimes called a diffuser, separates the two compartments. Usually the membrane is made of canvas or a high quality filter paper. The porosity of the membrane is critical to the quality of the fluidisation of the powder. Compressed air is forced into the lower compartment. It diffuses through the membrane and moving upwards, still under pressure. It moves between the fine powder particles that are contained in the upper compartment. Hence the powder particles are separated and the mass increases in volume, which can be up to 30%. As a result the bulk density of the powder is reduced and this permits the preheated metal object to be lowered easily, without any resistance, into the now “fluidising” bed of powder. The powder behaves like a liquid and continues to do so, as long as the air is forced into the lower plenum chamber.
- 100% coating efficiency,
- Faster cycle times,
- Thicker coating providing: functional protection, longer life, impact resistance but with higher material usage,
- Superior edge coverage,
- Requires fluidised bed to be full of powder