Electrostatic Spraying: Key Parameters for Transfer Efficiency and Film Quality in Powder Coatings
Category: Powder Coatings
The same powder can give a smooth, even film in one shop and orange peel, pinholes and bare edges in another. The difference is often not the powder but three gun settings: voltage, gun distance and powder feed. This article explains how they interact — and which one to adjust first when something goes wrong.
1. How Electrostatic Spraying Holds Powder on the Part
In a corona gun the electrode is charged to a negative high voltage while the workpiece is grounded, creating a strong electrostatic field between gun and part. Powder passing the corona zone picks up a negative charge and is driven onto the part by the field and the airflow, then melts and flows into a film in the oven. The field also wraps powder around to the back of the part — the "wrap-around" effect that sets electrostatic spraying apart from conventional air spraying.
Grounding must be sound: with poor grounding, transfer efficiency drops first, and no voltage adjustment will recover it.
2. Corona vs Tribo Guns: Two Ways to Charge the Powder
The mainstream choice is the corona gun: the power supply charges the powder, voltage is adjustable, and most powders work with it. A tribo gun uses no high-voltage supply — powder is charged by friction against the gun barrel — and it penetrates Faraday-cage areas (deep recesses, cavities) better, but it needs tribo-specific powders and generally coats more slowly. Choose by part shape: flat panels and structural parts suit corona; parts with deep cavities, mesh or complex internal corners are candidates for tribo evaluation.
3. Voltage: Higher Is Not Always Better
Typical working voltage is 60–90 kV (professional guns reach about 100 kV). Voltage sets field strength: higher voltage means faster deposition and better edge coverage — but past the breakdown point of the deposited layer, back ionization occurs, showing up as pinholes, orange peel and powder fall-off.
| Workpiece | Starting voltage | Gun distance |
|---|---|---|
| Thin sheet (<1.5 mm) | 60–70 kV | 200–250 mm |
| Medium structural parts | 70–85 kV | 250–300 mm |
| Thick parts and castings | 80–100 kV | 250–300 mm |
| Deep cavities and recesses | 80–100 kV | Shorten the distance and pre-spray — voltage alone will not coat a recess |
Change one parameter at a time: test-spray in 4–5 steps from low to high, and judge on test panels — film thickness, edge coverage and absence of defects. All figures here are starting values; confirm them on test panels for your own powder and part.
4. Gun Distance and Gun Movement: Start at 150–300 mm
Too close (<150 mm) deposits too much powder too fast, giving a thick film before it can flow out — orange peel and sagging follow. Too far (>350 mm) and airflow disturbs the powder cloud, cutting transfer efficiency and leaving uneven film and poor edge coverage. Start at 150–300 mm — manual guns toward the near end, automatic lines toward the far end — then fine-tune by part shape.
Move the gun steadily, keep it parallel to the surface, and spray in one pass without stopping mid-part.
5. Film Thickness: Powder Feed and Air Pressure
A typical powder film is about 50–100 µm (2–4 mil). Common working ranges are 0.30–0.55 MPa conveying air and 0.30–0.45 MPa atomizing air: higher pressure deposits faster but wastes powder and wears gun consumables; lower pressure gives poor atomization and uneven film. If thickness fluctuates, first check that powder feed is stable and fluidizing air is correct. Measure thickness at several points — a single reading misleads.
6. Quick Troubleshooting
- Low transfer efficiency, bare edges: shorten gun distance first, then consider raising voltage; check workpiece grounding.
- Orange peel: reduce gun distance or film build first; a levelling agent such as WD88 helps flow after melting — it cannot fix wrong spray settings.
- Pinholes, powder fall-off: suspect back ionization first — lower voltage and current; for thick films (>80 µm) the degassing role of benzoin also matters (see article #2, TGIC vs HAA, on thick-film pinholing).
- Recesses not coating (Faraday cage effect): lower voltage, shorten gun distance, change gun angle and pre-spray — or evaluate a tribo gun. Do not just raise voltage.
- Uneven film, uneven matting: uneven film makes the matting effect of agents such as MB68 look uneven — stabilize spray settings before judging gloss.
7. Recovery and Safety
Overspray can be collected, sieved and blended with virgin powder at the ratio the powder supplier recommends; never reuse unsieved reclaim directly. Keep the booth well grounded and ventilated: powder dust is combustible, so no open flames in the spray area and clean up accumulated powder regularly. Follow the equipment manual and local regulations for the details.
8. Frequently Asked Questions
Q1: Does 100 kV give the fastest coating?
Not necessarily. Past the breakdown point, back ionization causes pinholes and fall-off; pick the voltage that gives the best thickness and coverage with no defects.
Q2: What if deep recesses never coat?
Lower voltage, shorten gun distance, change angle and pre-spray; if that fails, evaluate a tribo gun — do not rely on voltage alone.
Q3: Can reclaimed powder be reused directly?
Sieve it and blend with virgin powder as the supplier directs; do not use reclaim that is damp, lumpy or contaminated.
There is no universal spray setting — only "starting values plus test-panel verification." Record voltage, gun distance and powder feed on the process card, and re-check them on test panels whenever the powder or the part changes. That alone stabilizes most of film quality.