Lesson 11.2CoatChapter 11 · Coatings and surface finishing

Powder coating

Charged dry powder clings to the part, then melts in an oven

Powder coating sprays a dry, colored powder onto metal and bakes it into a hard, smooth skin. Many bicycle frames, garden chairs and washing machines are finished this way. It gives a thicker, tougher coat than liquid paint, and it uses no solvents.

Photo: Hardcoreraveman, Public domain, via Wikimedia Commons

Powder coating · electrostatic spraying in the booth, then curing in the ovenCircled numbers match the steps below. In the booth the powder is still loose grains; in the oven it fuses into one continuous film.

How it works

Clean and pretreat. Parts hang on hooks from an overhead conveyor and pass through washing tanks or sprays. A conversion coating (a thin phosphate or zirconium layer made in a chemical bath) helps the powder stick and slows rust. The parts are then dried.
Fluidize and pump the powder. The powder is finely ground resin, pigment and hardener, roughly as fine as flour. Air blown up through the hopper makes it flow like a liquid, and a pump sends it through a hose to the gun.
Charge the powder. An electrode at the gun tip runs at high voltage, typically tens of kilovolts. It charges the air around it, and the powder grains pick up a negative charge as they pass.
Spray onto the grounded part. The part is grounded (electrically connected to earth) through its hook, so it attracts the charged grains. They follow the electric field lines, even around edges to the back (wrap-around), and cling the way a rubbed balloon clings to a wall.
Recover the overspray. Grains that miss are pulled into a recovery unit (a cyclone and filters) and sent back to the hopper. With recovery, often 90% or more of the powder ends up on parts.
Cure in the oven. At about 160–200 °C for 10–20 minutes, the grains melt, flow together and cross-link (form permanent chemical bonds, like epoxy glue setting). The part leaves the oven, cools and is ready to handle.

Most powders are thermosets (plastics that set permanently when heated): epoxy for indoor parts, polyester for outdoor ones because it resists sunlight. Thermoplastic powders such as nylon simply melt and refreeze. They are often applied by dipping a hot part into a fluidized bed of powder.

Key facts

Materials: Epoxy, polyester, hybrid, polyurethane or nylon powders; applied mostly to steel and aluminum, since the part must conduct electricity and survive the oven
Typical parts: Bicycle frames, garden furniture, appliance panels, car wheels, shelving, window frames
Part size: From small brackets to parts several meters long; the oven sets the limit
Precision: Film typically 50–100 µm thick and less even than liquid paint; coats thinner than about 40 µm are hard to apply
Surface finish: Gloss, satin or matte, plus textured and metallic effects; a slight orange peel is common
Batch size it suits: From a few parts in a job shop to conveyor lines running all day
Tooling and machine cost: A hobby gun and small oven cost hundreds of dollars; an automated line with pretreatment and oven roughly hundreds of thousands to millions
Speed: Spraying takes seconds to a few minutes per part, curing about 10–20 minutes; conveyor lines move parts continuously

Strengths and limits

Strengths

Thick, tough, chip-resistant coat in a single pass
No solvents and almost no VOC (volatile organic compound) emissions
Overspray can be collected and reused
Wraps around edges and onto the back of parts

Limits

Needs an oven, so heat-sensitive parts such as most plastics and wood are hard to coat
Thin coats are hard to apply and tend to show orange peel
Color changes are slow, because the booth and guns must be cleaned
Deep inside corners resist coating (the Faraday cage effect: the electric field cannot reach in)

Where it's used

AutomotiveWheels, chassis parts, springs, brackets
AppliancesWashing machine and fridge panels, oven cavities, dishwasher racks
ArchitectureAluminum window frames, facade panels, railings, fences
FurnitureGarden chairs and tables, office desks, metal shelving
Sports and leisureBicycle frames, gym machines, playground equipment
ElectricalElectrical cabinets, light fittings, transformer housings

How to recognize it

A thick, even coat with softly rounded edges; corners never look razor-sharp.
A slight orange-peel texture, or a deliberate fine-grain texture on outdoor furniture and machines.
A small bare or thin spot where the part hung from its hook.
Very hard to chip; when it does fail, it tends to peel off in sheets rather than flake.

Compare with

Spray painting11.1liquid paint dries at room temperature or low heat and suits plastics and wood, but needs solvents and gives thinner coats.Hot-dip galvanizing11.4dips steel in molten zinc for rust protection; powder is often applied on top for color (a “duplex” system).Thermal spraying11.10also sprays powder, but melts it in a flame or plasma on the way to build metal or ceramic coatings.

Watch it

Xometry · The basic idea: charged powder, grounded part, oven cure

AkzoNobel Powder Coatings · What the powder is made of, and how it is applied and cured

Testrite OEM & Testrite Aluminum · A real conveyor line: pretreatment, spray booth and oven

Coating Equipment Technology, Inc · A hand gun close-up showing powder wrapping around a part

See real photos

Wikipedia: Powder coating — photos of electrostatic spraying and coated parts
Wikimedia Commons photo search

Remember

Powder coating is paint without solvent: charged dry powder clings to a grounded part, then melts into a film in an oven.
It gives a thick, tough coat in one pass, and unused powder can be reused.
The part must conduct electricity and survive about 200 °C, so it is mainly for metal.