Box sections came out of the oven with bare internal walls while the outside read 75 μm; the Faraday cage effect stopped powder at the opening. A 12 percent reject rate followed.
This case guide covers the gun angles, lower voltage inside recesses, air flow control and the cure-recoat sequence that fills box sections without defects.
Common Mistakes and How to Avoid Them
| Mistake | Why It Happens | Practical Fix |
|---|---|---|
| 1. Full voltage inside | Field collapses at the opening | Drop to 40-60 kV |
| 2. Straight-on spray | Shadow at the back wall | Angle the gun 45 degrees |
| 3. Fast passes inside | Powder never reaches depth | Slow sweep into the box |
| 4. High booth air | Powder pulled from walls | Reduce air velocity |
| 5. Recoat before cure | Trapped gas and defects | Cure between coats |
| 6. Corona gun only | Depth limit for deep boxes | Trial a tribo gun |
| 7. No depth check | Shadow hidden at QC | Section the sample box |
| 8. Gun distance fixed | Inside reach differs | Extend distance, hold dwell |
| 9. Powder choice ignored | Particle size matters | Use fine powder for depth |
Best Practices
- Lower voltage to 40-60 kV for internal surfaces
- Angle the gun into corners at 45 degrees
- Sweep slowly and dwell at the deepest point
- Reduce booth air velocity for box sections
- Cure fully before any second coat
- Section sample boxes to verify internal film
Implementation Roadmap
| Step | Frequency |
|---|---|
| Set voltage | 40-60 kV for internals |
| Angle and sweep | 45 degrees, slow dwell |
| Control air | Lower booth velocity |
| First cure | Full cure before recoat |
| Section check | Verify internal walls |
Box section coating flow
- Lower - Voltage to 40-60 kV. (Full voltage shadows the back wall.)
- Angle - Gun at 45 degrees. (Straight-on spray misses corners.)
- Sweep - Slow with end dwell. (Fast passes never reach depth.)
- Verify - Section the sample box. (External readings hide internal shadows.)
The Faraday cage effect is field collapse, not a powder fault. | Fine powder and lower voltage penetrate deeper.
Box section coating data
Reference values for coating recessed box sections.
| Parameter | Reference | Why It Matters |
|---|---|---|
| Internal voltage | 40-60 kV | Prevents field collapse at the opening |
| Gun angle | 45 degrees into corners | Straight-on spray shadows walls |
| Dwell time | 1-2 s at the deepest point | Lets powder land and hold |
| Booth air | Below 0.3 m/s near parts | High air pulls powder out |
| Film target | 60-80 μm internal | Section to verify |
| Recoat | Cure, coat, cure | Second coat needs a cured base |
| Powder type | Fine particle, tribo option | Better internal penetration |
Shadow fault map
| Symptom | Cause | Fix |
|---|---|---|
| Bare back wall | Field collapse | 40-60 kV internal |
| Bare corners | Straight-on spray | 45 degree angle |
| Thin mid-wall | Fast pass | Slow sweep and dwell |
| Powder pulled out | High booth air | Lower velocity |
Voltage and angle by section depth
| Section depth | Voltage | Angle |
|---|---|---|
| Up to 10 cm | 60 kV | 60 degrees |
| 10-20 cm | 50 kV | 45 degrees |
| Over 20 cm | 40-50 kV | 45 degrees, tribo |
Case 1 - 12 percent reject rate on box sections
Scenario. A plant rejected 12 percent of box-section parts for bare internal walls; operators sprayed straight on at full voltage with fast passes.
Action. Internal voltage dropped to 50 kV, the gun angled at 45 degrees with a slow sweep and end dwell, and booth air velocity was reduced.
Result. Internal film reached 60-75 μm, and rejects fell below 1.5 percent in three weeks.
Case 2 - deep enclosures fixed with a tribo gun in Mexico
Scenario. An enclosure maker could not reach the back walls of 25 cm deep housings with corona guns at any voltage or distance setting.
Action. A tribo charging gun was trialed with fine powder, a 45 degree sweep and a low-velocity booth zone.
Result. Back walls covered at 55-70 μm, and the tribo gun became standard for deep housings.
Frequently Asked Questions
What is the Faraday cage effect?
The electrostatic field collapses at the opening of a recess, so powder stops before the walls.
Why lower the voltage inside?
High voltage strengthens the collapse; 40-60 kV lets powder pass the opening.
What gun angle works?
About 45 degrees into the corner, sweeping slowly toward the deepest wall.
Why slow passes?
Powder needs time to reach depth; fast passes never let it land.
Does booth air matter?
Yes - high velocity pulls powder out of the section before it deposits.
When to recoat?
Only after a full cure; an uncured base traps defects between coats.
What about tribo guns?
They charge by friction and penetrate deep recesses better than corona.
How do I verify internal film?
Section a sample part and measure the internal walls with a gauge.
Does powder type matter?
Fine particle powders penetrate deeper; check the supplier's size range.
What Would You Like to Solve?
Describe your box depth, gun type and current internal film readings, and a voltage and angle trial usually opens the shadow.