Paint line part hanging: design, failure modes and the fix

Two parts leave the same powder booth with the same gun settings and the same 70 µm target. One is a 1.5 mm panel hung from two hooks on its top edge; the other is a 6 mm bracket hung from a single hook through its largest hole. The panel cures to a full gloss with 70 µm on both faces. The bracket comes out at 45 µm on the face away from the gun, with a bare ring around the hook and a blister where pretreatment ran out of a closed corner.

Nothing about the powder changed. The hang did. Paint line part hanging sets the angle the part presents to the gun, the path the pretreatment takes to leave it, the rate at which it reaches cure temperature and the point where the current enters it. Each of those is a number, and each can be specified on the drawing before the part reaches the line.

The two that matter most are mass and drainage. A 1.5 mm panel reaches cure temperature in about 4 minutes; a 6 mm bracket on the same carrier needs about 16 minutes, which is longer than a 15-minute oven cycle. And a 12 mm return that holds 14 mL of pretreatment per 100 mm of length will bleed that liquid into the oven, where it becomes a stain or a blister.

What the hang decides

Film thickness. Powder is applied by a charged spray from a gun 200–250 mm from the part. A face square to the gun takes the most powder; a face at a shallow angle takes less, because the particles bounce rather than deposit. Hanging a flat panel truly vertical gives the best drainage but the thinnest film, and it is the first place orange peel shows. A 10–15° lean from vertical is the working compromise.

Wrap. Powder wraps around an edge because the electric field concentrates there. A hook contact within 20 mm of a visible edge steals that wrap and leaves a thin ring. Put the contact on a hidden edge or an internal tab, and keep it at least 30 mm from any face that will be seen.

Drainage. Pretreatment liquid enters every closed section. If the part hangs with a return or a hem at the low point, the liquid stays there through the dry-off and boils in the cure oven. The result is a blister, a stain or a patch of low adhesion that passes a visual check and fails a cross-cut test weeks later.

Earthing. Powder coating needs an earth path through the hook and the carrier. A hook with a 100 µm build reads over 1 MΩ, the part charges poorly, and the film drops on the face nearest the gun while the far side stays thin. Hooks are cleaned every cycle, and the earth resistance is checked before the load goes on.

Cure. The oven sets the metal temperature, not the air temperature. A thick part on a carrier timed for thin parts never reaches the peak metal temperature, and an undercured polyester film looks acceptable on the day and chips within a season.

The numbers that decide a hang

The thermal number is easy to reproduce. For a steel plate heated from both faces, the time constant is τ = ρ c t / 2h, with ρ = 7,850 kg/m³, c = 490 J/kg·K and an effective convection-plus-radiation coefficient h of about 35 W/m²·K. At 1.5 mm, τ = 7,850 × 490 × 0.0015 / 70 = 82 s. A part reaches within 10 K of the oven in about 2.9 time constants, so 4 minutes. At 6 mm, τ = 330 s and the same approach takes 16 minutes — one minute longer than the cycle.

That is why a mixed load undercures. The oven air is at 190 °C after a few minutes; the 6 mm bracket is still at 150 °C when it leaves. Group parts by thickness on a carrier, or run the cycle for the heaviest part and accept the lower throughput.

Paint line part hanging: six failure modes and the fix

Runs on a downward edge

A run is powder that has been applied faster than the film can hold it, usually on a horizontal lower edge where the gun passes twice. Hanging the part so the edge is at 10–15° lets the film stay thin at the boundary and drain away from the face. The other fix is gun technique: shorter passes and a greater distance on the second pass, not a lower gun voltage.

Bare internal corners

A 90° internal corner is a Faraday cage. The powder particles follow the field to the outer face and the corner receives 20–30 µm against 70 µm on the face. The film looks coated but the corner fails a salt spray test first, because it is the thinnest point and the place water collects.

Hang the part so the open side of the corner faces the gun, and specify a second pass for any corner deeper than 25 mm. A corner that cannot be reached by reorienting the part needs a rack that rotates, or a design change to open the corner with a relief.

Pretreatment bleeding out of a closed section

A hem, a return or a box section holds liquid through the dry-off. In the oven it boils, and the steam finds the weakest point in the film: a pinhole, a weld, a cut edge. The result is a blister with a rust stain inside it, often on the face that was perfectly coated.

Bare ring at the hook

The hook contact is the earth path, and it is also an uncoated area. A 4 mm ring on a visible face is a reject; the same ring on a hidden flange is normal. Design a hanging tab on a face that will be covered by a gasket, a bracket or another part, and keep it 30 mm from any visible edge so the wrap is not stolen.

Undercure on a mixed load

A carrier with 1.5 mm panels and 6 mm brackets runs one cycle. The panels reach 190 °C in 4 minutes and spend 11 minutes over-curing; the brackets never reach 190 °C at all. Over-cured polyester yellows and loses impact strength; under-cured polyester chalks and chips. Group by thickness, or run two cycles and accept the cost.

Parts swinging into each other

A 2 m panel hung from one hook swings on the conveyor and touches its neighbour at the bottom. The contact line is uncoated on both parts, and the swing also changes the gun distance. Hang long parts from two points, 400 mm apart, and check that the pair cannot rotate about the conveyor line.

Shadowing from a deep return

A part with a 60 mm return hung with the return toward the gun shields the face behind it. The shadow is a straight band of low film, often mistaken for a gun fault. Rotate the part 90° about the vertical axis so the return is side-on to the gun, or add a second pass from the other side.

Hanging on the Ganesh line

Parts go through seven-tank pretreatment, dry-off and the 18 ft × 14 ft electrostatic powder booth, then the curing oven. Shot blasting, where the drawing calls for it, runs in the 16 ft × 16 ft chamber before pretreatment, and a wet paint booth covers the finishes that powder cannot. Film thickness is checked with a DFT gauge and adhesion with a cross-cut test to ISO 2409 on the first article and on batch samples. The line is covered by ISO 9001:2015, and the pretreatment and effluent by ISO 14001:2015.

Hanging is planned at the same time as the route, not at the booth. The surface treatment and coating hub indexes the related guides. Degreasing mild steel parts before coating covers the stage before the part reaches the hook, and powder coating laser cut edges covers what happens at a cut edge under the film. The paint shop capability page covers the line, and the custom parts page covers the fabricated parts that run through it.

Fixtures and carriers

A coating fixture is a heat sink with a location feature. An 8 kg steel carrier holding a 5 kg part adds 13 kg to the oven load for every cycle, and it pulls current away from the part if the contact face is painted. Aluminium carriers with replaceable stainless hooks weigh less, hold their shape through the oven and keep the earth path clean.

Build the hang angle into the fixture rather than relying on the operator. A carrier with a 12° stop and a marked part position gives the same film on every batch; a carrier adjusted by bending a hook gives a different result after the first repair. Mark each carrier with the part number and the orientation, and retire it when the stops wear or the hooks no longer read under 1 MΩ.

What to put on the drawing

1. The hanging orientation, shown with an arrow on the drawing.
2. Drain holes, diameter and position, in that orientation.
3. Hanging tab or contact point, with the face it must not mark.
4. Masking requirements: threads, Earth bosses, mating faces and hinge bores.
5. Part mass and the maximum thickness on the part.
6. Finish: powder or wet paint, colour, gloss and film build.
7. Whether the part is visible or hidden, and which faces are A-surface.
8. Any internal corner deeper than 25 mm, flagged for a second pass.
9. The adhesion test and salt spray requirement, with the standard named.

Items 1 and 2 are the ones that prevent a rework loop. They cost nothing at the drawing stage and save a full strip and recoat once the part is on the line. Send the drawing with the hang orientation marked to request a quote; we will confirm the drain points and the contact tab before the first batch.

When powder on a hung part is the wrong finish

An e-coat or a zinc-rich primer reaches inside closed sections that powder cannot, so a part that has to survive a salt spray test inside a box section needs a different primer before the powder. Anodising is an aluminium process and is not done on this line; a large fabricated frame beyond the booth envelope is better split into coated sub-assemblies.

A part with a 3 mm hole pattern or a fine mesh cannot be coated without closing the holes, and a part with a mirror finish cannot be hung without a contact mark. A metallic or textured powder also needs a different gun recipe and a different carrier position, so the hang is secondary to the finish choice. Those are design decisions, not line settings. The earlier they are made, the cheaper they are.