Ink Transfer Mechanisms in Flexographic Printing

The work presented here describes the wetting and transfer characteristics of ceramic anilox rolls, used to meter ink flow on a flexographic printing press. The effect of the type of laser used to make engraved cells on the anilox was examined. Cells engraved by a conventional carbon dioxide (C02) laser, a modified C02 laser and a neodymium doped yttrium aluminium garnet (nd:YAG) laser were studied. Printing was carried out on a commercial OMA T wide web flexographic printing press and a laboratory scale RK press. The ink transfer from the cells was measured, along with the wettability, surface energy, geometry and interior surface roughness of the cells.

The wetting characteristics of the anilox rolls were extensively studied using contact angles measurements. Two instruments were used to make the measurements and both equilibrium and advancing contact angles were measured. The contact angle data was used in conjunction with the Wenzel, Shuttleworth and Bailey, and Cassie and Baxter theories to understand the effect of the highly structured roughness produced by the engraving process, on the wettability of the anilox rolls.

The findings of this work were that the laser type used to make the engraving changed the wettability of the anilox surface. Surface energy calculations revealed that the change was due to the polarity of the roll surface, which made the roll less receptive to wetting by polar liquids. The nd:YAG laser was found to produce cells that were less receptive to polar liquids than the C02 lasers.

Analysis of the contact angle data showed that the magnitude of the observed advancing contact angles was in agreement with the theory of Shuttleworth and Bailey, that the gradient of the roughness, which in this case was the slope of the cell walls, dictated the resistance to wetting by polar and non polar liquids. When the liquids were allowed to reach thermodynamic equilibrium, wetting by polar liquids was found to be consistent with the theory of Cassie and Baxter, that pockets of liquid/air interfaces were formed in the bottoms of the cells, producing a composite surface under the test liquid of solid/liquid and air/liquid areas. This produced the increase in observed contact angles with polar liquids on the engraved areas. The well-known Wenzel equation was found not to be applicable to the anilox surface wetting with any test liquid.

Printing was carried out with a standard UV curable flexographic ink. Ink transfer was observed to be affected by ceJl geometry. Equilibrium contact angle measurements were not directly related to ink transfer. However, it was the finding of this investigation that the relationship between advancing contact angles and ink transfer was likely and warrants further study.