This article is part of the FLEXO FLAW FIXING series
Flexo printing adds vibrancy to our daily encounters with packaging, but when fill-in or bridging defects affect the flexo printing process, the urgency for correction becomes paramount.
Fill-in, the opposite of skip-out, is the unwanted spread of ink over small features, and bridging is the blending of individual print dots. Identifying these defects often requires an experienced eye, looking for blurred graphics or bridged dot patterns on substrates. Identifying and correcting these problems is critical for those seeking flawless flexo printing.
Causes and Remedies of Fill-In and Bridging in Flexo Printing
Press Mechanics
Over Pressure
- Cause: Uneven ink distribution and image distortion typically indicate excessive pressure during printing.
- Remedy: Adjusting the downward pressure on the impression cylinders can prevent over-application of ink and maintain image integrity.
Low Pressure
- Cause: Insufficient pressure can result in incomplete ink transfer, leaving telltale signs such as skipping or faint printing.
- Remedy: Increasing the printing pressure or changing the anilox roller configuration may provide more uniform ink application.
Ink Formulation
Improper Viscosity
- Cause: Thicker ink may not transfer adequately, risking both filling and bridging defects.
- Remedy: Regular viscosity checks and timely adjustments with additives or temperature controls can keep the ink at optimum flow.
Solvent Composition
- Cause: Inappropriate solvent ratios can result in premature ink drying or poor deposition on the substrate.
- Remedy: Adjusting the ink formulation to balance the solvent ratios will help change the drying speed, resulting in better print quality.
Inconsistent pH-Value
- Cause: Print inconsistencies result from fluctuating pH levels in water-based inks.
- Remedy: Maintaining a stable pH through constant monitoring ensures that ink properties remain conducive to high-quality printing.
Anilox Roller Configuration
Unbalanced Ink Transfer
- Cause: Over inking is often the result of an inappropriate anilox roller configuration for the job.
- Remedy: Selecting an anilox roller with the correct cell volume and number for the job will ensure the proper amount of ink is deposited.
Cell Volume Too Coarse
- Cause: Fine details can disappear if the anilox roller doesn’t precisely control ink in smaller areas.
- Remedy: Use anilox rollers with higher cell counts and finer engravings for precise ink release on intricate graphics and text.
Debris and Particle Buildup
- Cause: Particle contamination of the ink system can inadvertently block cells and affect ink flow.
- Remedy: Implement a rigorous ink system cleaning regimen and use filtered inks to minimise the risk of clogging.
Unbalanced Transfer Between Inking Units
- Cause: Differences in drying rates between units can cause inconsistencies in ink transfer.
- Remedy: Synchronising solvent evaporation rates and heat settings across all units stabilises ink condition at each transfer point.
Conclusion
Fill and bridge defects can seriously degrade a flexo job. Identifying the root cause of such defects – printing inconsistencies, ink viscosity, screen configurations, or other mechanics – is a critical first step. Armed with this knowledge, printers can make precise adjustments.
Through regular maintenance, close monitoring of ink properties, and precise equipment settings, printers can promote resilience against these print quality adversaries and achieve a consistent, defect-free result that stands out on any shelf.
