Solving Ink Filling in Flexo Printing: Causes & Solutions

The Problem: Ink Filling (Bridging)
Ink filling occurs when ink fails to transfer completely from the plate to the substrate, accumulating on dot shoulders. This causes:

1. Highlight Dot Gain: Small dots (<30%, especially <5%) print larger due to excess ink.
2. Midtone Bridging (Ink Fill-In): Ink fills recesses between dots (45-55% area), causing them to connect irregularly. Visible as severe ink buildup on fine screens after washing.

Key Causes:

1. Incompatible Anilox/Plate Screen:
   • Core Issue: Plate’s smallest dot diameter < Anilox cell opening diameter. Causes ink “spill-over” into dot recesses. Avoid rigid 4-7x anilox:plate ratio rules; prioritize this quantitative match.
2. Excessive/Uneven Pressure:
   • Mechanical: Roller runout (>20µm tolerance).
   • Operational: Excessive ink metering pressure (anilox to plate) distorts dots, floods ink. Over-compensating with impression pressure worsens filling.
   • Plate Mounting: Uneven plate height.
3. Ink/Plate Surface Tension Mismatch:
   • Poor ink release from plate surface allows ink to pool in recesses. Wet plates (after washing) significantly worsen this.

Effective Solutions:

1. Anilox Selection:
   • Mandatory: Smallest plate dot diameter ≥ Anilox cell opening. Obtain precise cell data from suppliers.
2. Precision Pressure Control:
   • Fix roller runout (≤20µm target).
   • Set ink pressure LIGHTLY first. Gradually increase only if needed. Reduce ink pressure slightly after setting impression – improves filling even if solids slightly hollow.
   • Shim plates (NEVER increase pressure): Use tape (~50µm) under solids; shellac (~4µm/layer) under screens.
   • Use low-density mounting tape for screens (better resilience).
3. Plate Layout & Mounting:
   • Gradual pressure transition: Arrange image from low to high screen density.
   • Minimize bounce: Use bearer bars or staggered plate layout around the cylinder.
4. Optimize Ink & Plate Surface:
   • Select ink with surface tension close to the plate resin.
   • Consider harder plates or adjust UVC/UVA exposure during platemaking.
   • THOROUGHLY DRY plates with compressed air after washing before restarting.
   • (Use plate pre-treatment solvents cautiously per supplier guidance).

Conclusion: Prevent ink filling by quantitatively matching anilox to plate dots, minimizing pressure, ensuring mechanical precision, and optimizing ink/plate compatibility. Focused process control is key.