Designing Parts for Powder Coating: A DFM Guide for Engineers

The short version

Powder coating is an electrostatic process followed by a bake. That gives it four habits a designer has to work with:

• The part has to hang and ground. Wherever it touches the hook stays bare.
• Electric fields don’t reach into tight recesses. Inside corners, narrow channels and blind holes coat thin.
• The film is thick. A typical 2 to 4 mils per side is enough to ruin a thread or a slip fit.
• The part goes through a wash and a roughly 400°F oven. Anything that traps liquid or air, or can’t take the heat, causes trouble.

Each can be handled on the shop floor, but that takes labor, and labor shows up in the piece price. Designing around them costs almost nothing.

Hang holes and racking points

Every powder-coated part hangs from a hook or sits on a rack contact, which holds the part and carries the electrical ground. Charged powder only sticks well if the charge can bleed through the part to earth. The contact point ends up uncoated or marked, so where it lands matters.

Design guidance:

• Give the part a hang hole. A through-hole or tab on a non-cosmetic face or edge racks fastest. Without one, the part has to be clipped, wired or fixtured, which is slower and less consistent.
• Place it so the part hangs the right way. Hang orientation decides how wash solution drains, which faces see the guns, and whether a long part swings. Long frames and rails often need two points.
• Don’t count on threaded holes or bores you need clean. A hook in a tapped hole chews threads and leaves a bare spot where you may not want one.
• Tell us where a contact mark is acceptable. If the part has no spare hole, a note such as “contact marks permitted on hidden face B” lets us rack without guessing.

Racks collect cured powder and get stripped to keep a good ground, so a hang hole with a little room around the hook seats better than one sized tight to the wire. Our large batch oven takes parts up to roughly the size of a large cabinet or long frame, so send the dimensions with the drawing and we’ll suggest hang points.

Faraday cage recesses

Corona guns charge the powder and set up an electric field between the gun and the grounded part. Powder follows the field lines, which concentrate on edges and outside faces. Inside corners, narrow channels and recesses are shielded, so powder builds on the lips of a recess instead of reaching its bottom. This is the Faraday cage effect.

As Carl Izzo explains in Products Finishing, through-holes are generally not a problem. Blind holes are, because their bottoms are inside corners. Features that typically give trouble:

• Tight inside corners on formed or welded parts
• Narrow, deep channels and slots
• Boxed gussets and pockets
• Blind holes and closed-end tubes
• Close-spaced parallel fins or ribs

Rodger Talbert, writing in Products Finishing, describes how an operator works into these areas: lower gun current, less air velocity, and coating the recesses before the rest of the part. It works, but it adds gun time per part and can still leave thin film at the bottom, with heavy film on the surrounding edges.

What helps at the design stage:

• Open up channels and gaps where function allows. Wider and shallower coats better than narrow and deep.
• Radius inside corners. Coating company Adelhelm’s design sheet asks for at least 3 mm (about 1/8 in.) fillet radii on parts to be powder coated. A sharp internal bend or weld root is the worst case.
• Turn blind holes into through-holes when the hole doesn’t have to be blind.
• Say whether interiors matter. If the inside of a channel or box only needs light coverage, write that on the drawing. It settles the inspection question before the first article.

Vent and drain holes for tube and hollow weldments

Hollow parts cause two problems.

Wash solution. Parts go through a multi-stage wash before coating. Liquid that gets into a tube, boxed section or lap joint has to get back out, or it can boil or seep out during cure and blister the fresh film.

Trapped air. Air sealed inside a part at room temperature and heated to about 400°F tries to expand by roughly 60 percent (the ideal gas law, using absolute temperature). It escapes through the weakest spot, usually a weld pinhole, and blows a crater through the powder.

Design guidance:

• Vent every sealed cavity. Each closed compartment needs its own opening. A gusset or internal plate can split one tube into two cavities.
• Vent high and drain low, based on how the part will hang. Tell us the hang orientation, or let us suggest one, before you fix hole locations.
• Don’t rely on a single hole in the middle of a cap or base plate. Galvanizers face the same drainage problem in their baths, and the Galvanizers Association of Australia calls a single central end-plate hole never acceptable because liquid can’t drain past it. Put holes near the corners of the plate, as close to the tube wall as possible.
• Seal weld or open up laps and stitch-welded seams. A tight crevice between two plates holds wash solution and bleeds it back out in the oven.
• Tell us if a part must stay sealed, such as a watertight enclosure. It changes how we prep it.

Galvanized tube adds a third issue, outgassing from the zinc. That is covered in our guide to powder coating galvanized steel and castings.

Sharp edges

Powder flows as it melts, and on a sharp edge it pulls back and leaves a thin film. PPG notes that coatings struggle to hold a protective layer on a sharp edge through cure, and that laser-cut edges are harder to pretreat and coat because of the oxide the cut leaves. Those edges rust first.

• Break the edges on parts headed for outdoor or corrosive service. Adelhelm’s sheet asks for a minimum 2 mm (about 0.08 in.) edge radius for powder coating. Even a light edge break helps.
• Remove burrs and dross from punched, sheared and plasma-cut edges. Powder coats over a burr but doesn’t hide it.
• Plan for the laser oxide. If edges are critical, call out how the oxide gets handled. When a spec calls for blasting, we coordinate it as part of the coating job through our prep and masking service.

Film build on threads and tight fits

Powder builds much thicker than plating. A common factory spec is 2 to 5 mils with about ±1 mil tolerance, according to DeFelsko. Cardinal’s data sheets list test film builds from about 1.5 to 3.5 mils for common TGIC, hybrid and urethane powders, and up to 4.0 mils for some superdurables. Plan on 2 to 4 mils per side, and use the data sheet range for the specific powder when a fit is tight.

Remember that film goes on every coated surface:

The thread row uses a plating rule from Quality magazine: on a 60-degree thread, film on the flanks changes pitch diameter by four times the film thickness. The article notes the rule doesn’t apply to paint and heavy coatings. Powder does worse, because it builds unevenly and fills the roots. Mask or recut any thread that has to take a fastener.

Your options for threads:

1. Mask them. High-temperature silicone plugs and caps are standard. Most problems with masking threads come from using the wrong plug or cap, according to Nick Liberto in Products Finishing.
2. Machine after coating. Liberto calls post-machining “the solution that always works” and suggests cutting threads after coating as normal practice.
3. Add threads after coating with inserts or nuts, where the design allows.

If press-in hardware goes in before coating, call out its threads for masking too. Our powder coating masking guide covers plugs, caps, tape and marking masking zones.

Welds and spatter

Powder follows the surface. Spatter, undercut, porosity and deep grind marks all show through, often more clearly than on bare metal. Adelhelm’s design sheet puts it plainly: pores and cracks are not covered, leveled or smoothed out by the coating.

• Grind off spatter on visible and functional surfaces before parts ship to the coater.
• Watch the anti-spatter spray. Fisheyes are small craters where powder pulls away from contamination, and Liberto names silicone as a contaminant to look for when they show up. Some anti-spatter sprays contain silicone; silicone-free versions are sold for parts that will be painted. If you have to use a silicone product, tell us so we can plan extra cleaning.
• Use continuous welds where crevices would trap wash solution, and remove slag and heavy weld scale.
• Expect porous welds to outgas. Trapped gas in weld porosity can escape during cure and leave pinholes, the same way castings do.

Running a weld shop? Our fabricators page covers how parts should look when they arrive.

Part marking

• Ink markers can bleed through. In a Products Finishing case, an aluminum railing maker found black felt-marker cut marks bleeding through its powder coat, especially on light colors, and the marked sites later deteriorated. Use markers sold as paint-over safe, and test them in your color first.
• Stamped and engraved marks must be deep enough to read under 2 to 4 mils of film. A shallow etch can fill in.
• Paper labels and stickers come off in the wash or burn in the oven. Tag the bin or traveler instead.
• Need a part number or legend on the finished part? Mask a spot for an etched mark, or have the legend printed after coating. We coordinate screen printing on coated parts under one PO; our guide to screen printing on painted metal covers artwork and ink callouts.

Put the decisions on the drawing

None of this helps unless it reaches the coater. A good coating callout covers:

• Powder, color, gloss and texture, and the film thickness range
• Masking zones for threads, bores, fits and ground points
• Acceptable hang and contact-mark locations
• Cosmetic faces versus non-cosmetic faces
• Whether interior surfaces need full, light or no coverage
• Vent and drain locations on hollow parts
• Any marking that must survive coating

Our coating callout checklist has wording you can copy into a drawing note. While a part is still in design, send the model or drawing through our request a quote form. We’ll flag hang points, recesses and fits that will cost time or rejects before you release it.