Powder coating

Powder coating is a finishing process in which dry powder paint is applied to a part and then melted and cured in an oven into a continuous coating. Unlike liquid (wet) painting, the powder contains no solvents, most of the overspray can be captured and reused, and a single coat reaches a thickness for which liquid paint would need several passes.

The process has three steps. Pretreatment removes grease, mill scale and oxides and creates a base for adhesion, usually by degreasing and a conversion coating (phosphating or newer phosphate-free treatments), and for steel often also by blasting. During application, the powder is electrostatically charged in the spray gun and attracted to the grounded part. Finally, the coating is cured in an oven, typically at 160 to 200 °C for roughly 10 to 20 minutes (measured as part temperature). A typical single coat is 60 to 120 µm thick.

Powders differ by binder. Epoxy powders have excellent adhesion and chemical resistance but chalk and change shade in sunlight, so they suit indoor use or serve as a primer. Polyester powders resist UV radiation and weather and are the standard for exterior use. Hybrid epoxy-polyester powders are a compromise for interiors. Powders come in RAL colors with various gloss levels, textured or with a metallic effect.

Parts can be powder coated if they withstand the curing temperature and are electrically conductive: steel, stainless steel, aluminum, galvanized steel and zinc die castings. Plastics and wood-based materials require special low-temperature powders and are not usually powder coated. Outdoor steel structures are often protected with a duplex system, meaning hot-dip galvanizing plus a powder coat.

Powder coating is a good choice for sheet metal enclosures and covers, machine frames, brackets, railings, light fixtures, aluminum extrusions and castings, in other words anywhere a durable colored surface in a specific shade is needed. Compared with liquid paint, a single coat gives higher abrasion and impact resistance and more consistent quality across a production run.

Liquid paint is often the better option for structures that do not fit in the oven, parts that cannot tolerate the curing temperature, and applications that need a thin film or easy local touch-up. For aluminum parts with a metallic look and precise dimensions, consider anodizing.

The coating adds tens of micrometers to every surface. Mark threads, fitted holes, mating faces and grounding points on the drawing for masking (plugs, high-temperature tape), or tap and machine them after coating. Expect a mark where the part hangs and design a hole or a spot for it on a non-cosmetic side.

The layer is thinner on sharp edges, so round them. Powder has trouble reaching deep pockets and inside corners because of electrostatic shielding (the Faraday cage effect). Closed sections and weldments need holes to drain pretreatment liquids and to let air escape in the oven. Porous castings and hot-dip galvanized steel can release gases in the oven that leave bubbles in the coating, so they are outgassed by preheating.

In the RFQ, state the RAL shade, gloss level and texture, coating thickness, service environment (indoor, outdoor, or the corrosivity category), masked surfaces and cosmetic sides. Approve a sample for a new shade or a new production run.