Correlations between properties of water-based flexographic inks and their print uniformity on PE-coated board

Uniformity of fulltone flexographic prints on PE-coated board was investigated for a large matrix of aqueous inks with varying acrylic-based vehicles and pigment concentrations, and with their applied amounts varied using a wide range of anilox volumes. Print mottle was characterised by coefficient of variation of reflectance, and partitioned into different feature-size classes. For all prints the mottle was caused by the residual pattern of white (poorly-covered) stripes originating from the anilox ruling, and subsequently distorted and redistributed to both finer and coarser length scales during transfer to, and flow on, the print substrate. Accordingly, mottle was largest on the sub-millimetre scales encompassing these stripe features, and generally increased with anilox volume, approximately proportionally so on above-millimetre scales. While greater pigmentation naturally increased both print density and mottle, the vehicle also exerted a strong influence, with mottle decreasing with increased content of soluble (versus emulsion) polymers. The latter was principally due to the associated increase in lowshear viscosity and decreased surface tension, both serving to reduce substrate dewetting. Empirical relations accurately correlating mottle to these two ink properties, and anilox volume and print density, were established