Powder Coating

Take a container with a porous plate across the bottom — sintered plastic, or a tight cloth over a perforated sheet. Feed low-pressure air underneath it. Fill above the plate with powder coat paint to about a third of the container depth.

Turn the air up slowly. At the right flow the powder surface rises slightly and begins to churn like a simmering liquid. That is fluidization.

Too little air and it stays a packed heap. Too much and it erupts and blows powder out. The window between them is the process.

Degrease the steel bar and remove rust. Powder coating hides nothing — every fingerprint and scale patch shows as a crater or a bare spot.

Phosphating the part first, as in the phosphate blueprint, is what industry does: the phosphate layer both keys the coating and stops rust creeping under it from a chip.

Heat the part in the oven to 200–250 °C. Check it with the infrared thermometer, not the oven dial.

The heat is in the part, not the bed. This is Gemmer's condition — the article is preheated above the sintering or melting point of the coating material, so powder melts on contact with the metal and nowhere else. The bed itself stays cold.

Lower the hot part into the fluidized bed for 3–8 seconds, moving it to and fro the whole time — the patent says so explicitly.

Holding it still lets the powder immediately around it melt and locally chill the bed, giving a thick lumpy patch and a thin one next to it. Movement keeps presenting fresh cold powder.

Longer dip and hotter part both mean a thicker film. This is the only thickness control you have.

Return the part to the oven at 200 °C for 10–15 minutes.

Watch the surface change: the powdery texture flows out to a smooth gloss. That flow-out is the resin melting fully; curing continues after it looks finished, so do not cut the time short.

Cool in air. The coating is hard when cold, not before — do not handle the face while warm.

Measure the film if you can: expect 200–400 µm from a single dip, against 25–50 µm for one coat of wet paint.

Then abuse it. Strike an edge with a hammer, and scratch it with a key. Powder coat resists both far better than an equivalent air-dried paint — it is a thermoset plastic, not a dried film.

Now weigh what is left in the bed. Nothing was lost. Powder that missed the part is still in the container, ready for the next one. That single fact — no solvent released, no overspray discarded — is why the process took over.

The patent, from the document. US 2,844,489, "Fluidized Bed Coating Process", inventor Erwin Gemmer, assigned to Knapsack-Griesheim Aktiengesellschaft near Köln. Filed 20 December 1957, granted 22 July 1958, German priority the same day. ⚠ Secondary sources widely give "filed in Germany May 1953, issued 1955" — those dates do not match this document, and if an earlier German filing exists it is a different patent with a different number. Cite what you can see.

Why a chemical company invented it. Knapsack-Griesheim made resins, not coatings. The fluidized bed came out of powder handling, which is why the patent is written as a general process for applying any meltable powder rather than as a paint invention. Nylon and other engineering plastics were coated this way before decorative finishes were.

What replaced the dip. Fluidized-bed dipping only suits parts you can heat and immerse, and it gives thick films. Electrostatic spray — charging the powder and earthing the part, so powder is attracted to a cold workpiece — arrived in the 1960s and is what almost all powder coating uses now. It gives thin films on complex shapes and needs no preheat. Gemmer's bed survives for thick functional coatings: dishwasher racks, pipeline coatings, electrical insulation.

The environmental claim, honestly. No solvent is emitted and overspray is recovered, so it genuinely beats wet paint on both counts. But it is not free: the oven consumes real energy, and thermoset powder that has cured cannot be remelted and reused. It is a large improvement, not a closed loop.