Corner thin spots forced a plant to recoat 15 percent of its steel brackets, and each recoat doubled the labor. Corners read 40-50 μm while flats ran 80 μm.
This junior case guide sets the electrostatic voltage, gun distance, powder flow and first-pass technique that build 60-80 μm on corners the first time.
Common Mistakes and How to Avoid Them
| Mistake | Why It Happens | Practical Fix |
|---|---|---|
| 1. Gun too close | Back-ionization thins corners | Hold 15-25 cm |
| 2. Voltage too high on corners | Corona repels powder | Drop to 60-70 kV near corners |
| 3. Poor part grounding | Powder falls off | Check hook contact and resistance |
| 4. Fast passes | Light film everywhere | Slow the first pass |
| 5. Flats coated first | Corners starved | Cover corners in the first pass |
| 6. Film never measured | Thin spots unnoticed | Measure corners with a gauge |
| 7. High atomizing air | Powder bounces off | Reduce atomizing air |
| 8. Gun distance varies | Patchy film build | Keep distance steady |
| 9. Recoat without cure | Film defects pile up | Cure before a second coat |
Best Practices
- Check grounding before the first part
- Set voltage 60-90 kV by part shape
- Keep gun distance steady at 15-25 cm
- Cover corners and edges on the first pass
- Measure film on corners, not just flats
- Cure fully before any recoat pass
Implementation Roadmap
| Step | Frequency |
|---|---|
| Check grounding | Hook contact, below 1 MΩ |
| Set the gun | Voltage, distance, air |
| First pass | Corners and edges first |
| Measure | Film on corners and flats |
| Cure and re-check | 60-80 μm after cure |
First-pass powder flow
- Ground - Resistance below 1 MΩ. (Poor grounding drops powder.)
- Set - 60-90 kV, 15-25 cm. (High voltage repels corner powder.)
- First pass - Corners and edges first. (Flats-first starves corners.)
- Measure - Corner film per part. (Flats-only readings lie.)
Corona guns repel powder at very short range. | A slow first pass beats two fast ones.
Powder gun setup data
Reference values for corona gun powder application.
| Parameter | Reference | Why It Matters |
|---|---|---|
| Voltage | 60-90 kV | Lower near corners and recesses |
| Gun distance | 15-25 cm | Too close causes back-ionization |
| Film target | 60-80 μm cured | Below 50 μm coverage fails |
| Atomizing air | 1-3 bar | High air bounces powder off |
| Grounding | Below 1 MΩ to hook | Weak ground drops first-pass yield |
| First-pass speed | Slow and even | Corners first, then flats |
| Film check | Corners plus flats | Gauge after cure |
Thin spot fault map
| Symptom | Cause | Fix |
|---|---|---|
| Thin corners | High voltage repulsion | 60-70 kV near corners |
| Thin flats | Fast passes | Slow the first pass |
| Patchy film | Distance varies | Steady 15-25 cm |
| Powder falling off | Poor ground | Clean hook contact |
Voltage by part shape
| Part | Voltage | Distance |
|---|---|---|
| Flat panels | 80-90 kV | 20-25 cm |
| Brackets with edges | 70-80 kV | 15-20 cm |
| Recessed parts | 60-70 kV | 15-20 cm |
Case 1 - 15 percent corner recoat on steel brackets
Scenario. A bracket plant recoated 15 percent of parts for thin corners; corners measured 40-50 μm while flats passed at 80 μm.
Action. Voltage was lowered to 70 kV near corners, gun distance was fixed at 15-20 cm, and the first pass started on corners with a film gauge check.
Result. Corner film reached 65-75 μm on the first pass, and recoat dropped below 2 percent.
Case 2 - powder falling off hooks with weak grounding
Scenario. An Australian fabricator saw powder falling off parts at the booth exit; hooks carried cured powder and grounding was intermittent.
Action. Hooks were stripped to bare metal, contact resistance was checked below 1 MΩ, and grounding was verified before every rack.
Result. First-pass yield rose from 78 to 95 percent within a week.
Frequently Asked Questions
Why are corners thin?
Corona guns repel powder at short range, and corners curve the electric field.
What voltage should I use?
60-90 kV by shape; use lower voltage near corners and recesses.
What gun distance?
15-25 cm; closer causes back-ionization that thins the film.
Why check grounding?
A weak ground drops the powder before it sticks, wasting the first pass.
What film thickness?
60-80 μm cured; below 50 μm coverage and protection fail.
Why cover corners first?
The first pass carries the powder; corners starved early stay thin.
How fast should I pass?
Slow enough for one even pass; two fast passes build uneven film.
How to measure corners?
Use a film gauge on the corner radius, not only on the flat face.
Can I recoat thin spots?
Yes, but cure first; uncured recoat traps defects between layers.
What Would You Like to Solve?
Share your part shapes, gun settings and film readings, and a grounding and gun-distance check usually explains thin corners.