Common Challenges in UV Flexographic Label Printing

Date: May 19 2025 From: Star Color Views:

When UV flexographic ink is used for label printing, various issues that affect printing quality and production efficiency may arise due to the interaction of ink performance, printing equipment accuracy, substrate characteristics, and environmental factors. For example, ink curing, adhesion issues, color deviation, etc. can all occur. Let's take a look at how to solve these problems

1. Incomplete Curing (Residual Rate >5%)

Root Causes & Solutions Matrix

Factor	Test Standard	Optimization	Validation
Low UV intensity	<800mJ/cm²@395nm	Upgrade to LED-UV/Mercury lamp	Energy ≥1200mJ/cm²
Insufficient PI	TPO<2% / ITX<0.5%	3% TPO + 1% ITX blend	FTIR: NCO conversion>98%
Excessive speed	>80m/min	Reduce to 50-60m/min	Cure depth ≥15μm
Oxygen inhibition	O₂>1000ppm	Nitrogen purge (<200ppm)	Surface hardness ≥2H

2. Poor Adhesion (>5% peel-off)

3-Step Enhancement

Substrate prep
- Corona treatment: ≥42mN/m (BOPP)
- Primer coating: 0.5-1g/m² PU
Ink reformulation
- Add 2% adhesion promoter (e.g., BYK-4510)
- Low-shrinkage resins (<5%)
Curing optimization
- Pre-cure 300mJ/cm² + Main 800mJ/cm²
- Wavelength ratio: 30% 365nm + 70% 395nm

3. Foaming (>0.2mm bubbles)

Key Controls

Type	Trigger	Solution	Metric
Mechanical	Viscosity <30s (cup #4)	Increase to 40-45s	≤5 bubbles/m²
Chemical	Solvent >200ppm	Switch to 100% solids	Residuals <50ppm
Thermal	ΔT >8℃	Chiller (25±1℃)	Leveling: Grade 4

Equipment:

Viscometer (Brookfield DV2T)
Thermal camera (FLIR T540)

4. Color Variation (ΔE >1.2)

End-to-End Control

Ink management
- Batch ΔE <0.5
- Viscosity tolerance ±0.5s
Press settings
- Anilox: 200-250LPI (solid) / 300-350LPI (halftone)
- Pressure: 1.5-2.5Bar (closed-loop)
Environment
- 23±1℃ / 55±3%RH
- Class 100,000 cleanroom

5. Skin Formation/Clogging

Prevention Protocol

Stage	Action	Standard
Printing	Add 0.3% inhibitor (HQ)	Open time >4h
Short pause	Oxygen barrier film (50μm)	O₂ permeability <5cc
Extended idle	Low-speed circulation	Viscosity Δ <±2%

Cleaning:

UV ink cleaner
Ultrasonic (40kHz/15min)

6. Poor Weather Resistance (ΔE>3@300h)

Durability Solutions

Formulation
- 1.5% UV absorber
- Epoxy-acrylate resins (+50% weatherability)
Process
- Dual-cure: UV + thermal (80℃/30min)
- Nano-SiO₂ topcoat (2-3μm)
Testing
- Xenon aging: ISO 4892-2 Cycle 1
- Thermal cycling: -20℃↔60℃/20x