Uneven Corona Treatment Across Web Width: Causes, Testing & FixesWhy Uneven Corona Treatment Happens

1. Uneven Electrode-to-Substrate Gap

Corona treaters rely on maintaining a precise, uniform gap between the electrode and the substrate across the full web width. Over time:

• Electrodes wear unevenly due to arcing
• Mechanical stress causes slight distortion
• Mounting hardware loosens
• Gap spacing drifts from original specification

Typical gap values:

• Standard operating gap: 1.5–2.0 mm (0.060–0.080 in)
• OEM target tolerance: ±0.1 mm
• In real production environments, deviations of ±0.15–0.2 mm are common

Even small gap changes matter.
A 0.1 mm increase in gap can reduce effective surface energy by up to 2–3 dynes/cm, depending on power density and line speed.

2. Electrode Wear and Surface Damage

As electrodes age, they may develop:

• Carbon buildup
• Pitting from arc erosion
• Warping from thermal cycling

These defects disrupt the electric field, concentrating discharge in some areas while leaving others under-treated.

3. Non-Uniform Power Distribution

Power may not be delivered evenly along the electrode length due to:

• Aging power supplies
• Faulty connectors
• Poor grounding
• Internal resistance differences across the electrode

The result is localized over-treatment and under-treatment across the same web.

How to Test for Cross-Web Surface Energy Variation

Equipment Needed

• Dyne test pens (32–46 dynes/cm, 2-dyne increments)
• Measuring tape or ruler
• Lab notebook or digital log

Cross-Web Testing Protocol

1. Mark test locations across the web width:
   • Left edge (≈2 inches from edge)
   • Quarter width
   • Center
   • Three-quarter width
   • Right edge
2. Run material through the corona treater at normal production speed and power.
3. Immediately test surface energy at each marked location using dyne test pens.
4. Record results in a standardized format:

Failure Criterion

• Variation greater than 4 dynes/cm across the web width indicates a significant problem requiring correction.

This simple test takes less than 3 minutes and often reveals issues that months of troubleshooting fail to uncover.

How to Fix Uneven Corona Treatment

Immediate Corrective Actions

1. Verify and adjust electrode gap
   • Use calibrated feeler gauges
   • Measure gap at multiple positions across the width
2. Inspect electrodes
   • Look for carbon buildup, pitting, or distortion
   • Clean according to manufacturer recommendations
3. Check power delivery
   • Confirm uniform power output across the electrode
   • Inspect grounding and electrical connections

Long-Term Preventive Solutions

• Implement weekly electrode gap verification
• Document gap measurements across the full width
• Replace electrodes when gap variation exceeds ±0.15 mm
• Consider automatic gap-control systems for high-volume lines
  (Typical ROI: 6–8 months)

Case Study: Cross-Web Testing Eliminates Adhesion Rejects

A label converter experiencing recurring adhesion failures implemented systematic cross-web surface energy testing at the start of each shift.

Results:

• Adhesion reject rate dropped from 8% to 0.3%
• Root cause identified: electrode distortion of 0.2 mm at center width
• Corrective action: electrode replacement and weekly gap checks

The entire problem was solved with minutes of testing, not expensive equipment upgrades.

Why This Problem Is Often Misdiagnosed

Why This Problem Is Often Misdiagnosed

Uneven corona treatment is frequently mistaken for:

• Ink formulation issues
• Substrate quality problems
• Operator error
• Environmental fluctuations

Without cross-web testing, teams often chase symptoms instead of the true cause.

Prevention Checklist

• ☐ Perform cross-web surface energy testing daily on standard material
• ☐ Record dyne values at fixed web positions
• ☐ Verify electrode gap monthly (minimum)
• ☐ Clean electrodes before carbon buildup becomes visible
• ☐ Replace worn electrodes before power compensation becomes excessive

Key Takeaway

Uneven corona treatment across the web width is not random and not unavoidable. It is a mechanical and electrical issue that can be detected early with simple surface energy testing.

A few minutes of verification can prevent thousands of dollars in rejected material and eliminate one of the most frustrating adhesion problems in converting operations.

Related Reading

Corona Treater Arcing: What It Means and How to Stop It

5 Signs Your Corona Treatment Isn’t Working (And How to Test It)

Why Corona Treatment Decays Over Time (And How to Measure It Correctly)