Powder Coating Process

Powder coating is a dry finishing process. A gun charges fine plastic powder and sprays it onto a grounded part, where the charge holds it in place. Then an oven melts the powder into a continuous, tough film. Because there is no solvent, it gives off essentially no VOCs (volatile organic compounds), and overspray can often be collected and reused, so material use is high. It can also reach in one coat a thickness that takes several coats of liquid paint.

The trade-offs are real. Powder needs an oven and a part that can stand the heat. It cannot lay down very thin films. And powder in the air can explode, which is why dust control is a daily job, not a side issue.

What You Will Be Able to Do

• Describe the steps of a powder line, from pretreatment to final inspection.
• Choose a powder type for indoor or outdoor service.
• Apply powder with good grounding and handle recessed areas.
• Prove cure by part metal temperature, not oven setting.
• Run reclaim and housekeeping in a way that prevents fires and explosions.

How the Process Works

1. Pretreat. Clean the part and apply a conversion coating, usually in a multi-stage washer.
2. Dry. Dry the part fully. Water left in seams will spoil the coating.
3. Mask. Plug and tape areas that must stay bare, using products rated for the oven.
4. Rack and ground. Hang the part on a clean hook so it has a good electrical path to ground.
5. Apply. Spray the powder.
6. Check. Look for thin spots and damage before cure, when mistakes are easy to fix.
7. Cure. Bake the part until the powder melts, flows, and crosslinks.
8. Cool and inspect. Let it cool, then remove masking and inspect.

Pretreatment

Powder needs the same clean surface as any other coating. Oil, shop dirt, and rust all cause failures, and the powder will not hide them. Most lines use a spray washer to clean and rinse, then apply iron phosphate, zinc phosphate, or a newer thin-film pretreatment. Heavy rust and scale need blasting first.

Castings and some welded parts can trap air or oil in tiny pores. The oven drives that out, leaving pinholes in the finish, a defect called outgassing. Bake such parts before coating so this happens first. Your setup sheet or powder maker can give the temperature and time.

Powder Types

Most powders are thermosets. They melt, flow, and then crosslink chemically into a film that will not melt again. A few are thermoplastics, which melt and flow when heated and soften again if reheated.

The main selection question is whether the part lives in the sun. An epoxy on an outdoor railing will chalk and fade.

Applying Powder

Corona and Tribo Guns

Spraying a Part

1. Check that hooks are clean and the part is well grounded. Ground is the most important setting on the line. Built-up coating on hooks blocks the path to ground, so strip or replace them on a schedule.
2. Confirm the powder type and the voltage, powder flow, and air settings on the setup sheet.
3. Fluidize the powder in the hopper with air, so it flows evenly.
4. Spray recessed areas and inside corners first, then the open faces.
5. Keep the gun at the distance shown on the setup sheet, moving steadily so the coverage is even.
6. Inspect before the oven. Uncured powder can be blown off and resprayed, so this is the cheapest time to fix a mistake.
7. Move the part gently. Uncured powder is only held on by charge, so a bump knocks it off.

The Two Classic Application Problems

Faraday cage effect. Charged powder heads for the nearest grounded edge, so it stacks up on the outer edges of a corner and does not reach inside it. To fix it, lower the voltage, spray the recess first, move the gun closer or change the angle, or use a tribo gun.

Back ionization. As the powder layer builds, its own charge starts to repel more powder. The result is a rough, starry, or cratered surface. To fix it, lower the voltage, increase the gun distance, or do not keep spraying a part that is already covered.

Film Thickness

A mil is a thousandth of an inch. Measure cured film with a magnetic gauge on steel or an eddy current gauge on nonferrous metals such as aluminum. Too thin, and the coating shows poor coverage and less protection. Too thick, and you get orange peel, chipping, and wasted powder.

Powder cannot make the thin films of 0.5 to 1 mil that liquid coatings can. If a part needs a very thin, precise film, use liquid.

Curing

Powder cures in an oven in stages. It melts, flows out smooth, gels, and then crosslinks into a hard film. Most powders cure in the range of about 350 to 400°F. Typical schedules are 375°F for 15 to 20 minutes or 400°F for 10 to 12 minutes, but your powder's data sheet sets the real numbers, and some low-temperature powders cure well below this.

The clock starts when the metal reaches temperature. The oven's setting and the air temperature do not tell you that. Heavy parts take much longer to heat through than light sheet metal.

Worked example: a powder needs 10 minutes at 400°F metal temperature. A data logger shows a thin panel reaches 400°F in 6 minutes, and a heavy bracket reaches it in 22 minutes.

• To cure the panel, it needs 6 + 10 = 16 minutes in the oven.
• To cure the bracket, it needs 22 + 10 = 32 minutes.
• If both ride a 20-minute oven, the bracket will come out under-cured, even though the panel is fine.
• Run them separately, or set the line for the heaviest part and confirm the panel does not over-cure.

Proving the Cure

1. Attach thermocouple probes from a data logger to the thickest and thinnest parts on a rack.
2. Run the rack through the oven and download the temperature curve.
3. Compare time at temperature against the data sheet.
4. Repeat when the parts, line speed, or oven change, and on a regular schedule.
5. Back it up with film tests, such as a solvent rub test, an impact test, or a bend test, as your specification requires.

Warning: Cure ovens and fresh parts are hot enough to cause serious burns. Let parts cool, and wear the heat-resistant gloves your site requires. Lock out and let the oven cool before any work inside it.

Reclaim and Color Changes

Overspray that misses the part gets pulled into a collection system, usually a cyclone or cartridge filter, then is sieved and fed back to the hopper. This is where powder's high material use comes from.

• Follow the setup sheet for the mix of virgin and reclaimed powder. Reclaim has fewer fine and coarse particles than new powder, so too much hurts the finish.
• Sieve reclaim to remove dirt and clumps.
• Keep powder cool and dry. Moisture and heat cause clumping and poor flow.
• For a color change, clean out the gun, hose, hopper, and booth. Any leftover powder can show up as specks in the next color.

Dust Fire and Explosion Control

Powder is fine plastic dust. Suspended in air and given a spark, it can burn fast enough to explode. A cloud needs fuel, oxygen, an ignition source, and a dispersed concentration above the minimum explosive concentration (MEC). In an enclosure, the pressure has nowhere to go, which is what makes booths and ducts dangerous.

OSHA 1910.107(l) requires exhaust ventilation that keeps the air below the lowest explosive limit, and requires all areas to be kept free of accumulated powder. NFPA 33 adds detail for powder systems, including enough airflow to keep concentration in the ducts at no more than 50 percent of the MEC. Your site's fire authority decides which edition applies. Plants with larger dust systems may also fall under NFPA 660, the consolidated combustible dust standard.

Daily controls that matter on the floor:

• Ventilation. Keep booth exhaust running, and do not spray if it fails. Equipment should be interlocked so it cannot run without it.
• Grounding. Conductive objects in the spray area must be grounded. NFPA 33 sets a limit of no more than 1 megohm to ground. Check booths, hoppers, floors, and hooks. A floating part can spark.
• Operators. Hand-gun operators must stay grounded too, using the footwear and gun contact the maker requires.
• Housekeeping. Do not let powder build up on floors, ledges, beams, pipes, hoods, or light fixtures.
• Cleaning method. Use a vacuum approved for combustible dust. Do not blow powder off with compressed air, which makes the exact dust cloud you are trying to avoid.
• Ignition sources. No sparks, open flames, or smoking near the booth. Keep hot work away unless it has been cleared and permitted.
• Fire detection. Automatic systems commonly use flame detectors that shut down powder flow, conveyor, and air and sound an alarm. Do not bypass them.

Warning: If a fire starts in the booth, shut the system down and raise the alarm the way your site's emergency plan says, then get out. Do not blow, sweep, or fan powder near a fire, because stirring up dust can turn a small fire into an explosion.

For your own health, wear the PPE on the SDS. Powder can irritate your lungs and skin, and some powders contain sensitizers such as TGIC. Your respiratory protection is set by the SDS and your site's program.

Common Powder Defects

Common Mistakes

Check Your Understanding

1. An outdoor bench made of steel needs a finish that keeps its color. Which powder would you start with?

A polyester, or another outdoor-rated powder. Epoxy and hybrids chalk and fade in sunlight.

2. Parts from the oven show low gloss and scratch easily, though the oven reads the right temperature. The load is heavier than normal. What do you suspect, and how do you check?

Under-cure, because heavy parts did not reach metal temperature long enough. Run a data logger on the thickest part, compare time at temperature with the data sheet, and slow the line or adjust the oven.

3. Inside corners of a welded box keep coming out thin, while the outer edges are thick. What is going on?

The Faraday cage effect. Lower the voltage, spray the corners first, change the gun angle or distance, or try a tribo gun.

4. At the end of the shift, powder has settled on the ledges around the booth. A coworker grabs the air hose. What do you say?

Do not use it. Blowing powder creates a dust cloud. Use the approved vacuum, and do it before dust builds up.

Key Takeaways

• Powder is sprayed dry, held on by charge, and melted and crosslinked in an oven, usually at about 350 to 400°F.
• It has essentially no VOCs and allows high material use through reclaim.
• Grounding drives quality and safety, so keep hooks clean.
• Choose powder by exposure. Epoxy stays indoors, and polyester goes outdoors.
• Cure is proven by metal temperature and time, measured with a data logger.
• Keep powder from building up, and vacuum it. Never blow it around.