Design of Coating Structure for Flexographic Printing, 2007 Coating & Graphic Arts Conference

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In flexo printing, advances in plate making technology and pre-press have improved the quality greatly and allow flexo printing to compete effectively with rotogravure and offset quality. Paper substrate requirements include the correct surface topography and porosity to enable good transfer of the ink to the surface, without any reticulation of the ink. The surface energy of the substrate may need to be altered to ensure good wetting of the ink.

This study investigated the influence of different pigment types, binder levels and calendering conditions on the structure of board coatings. The board samples were then printed using a laboratory flexo printer and the print characteristics studied. Selected samples were submitted to the University of Bristol for sectioning using a Focussed Ion Beam FIB). This showed penetration of the ink into the coating layers.

This work has outlined some of the most important coating structural properties required for flexo printing of coating linerboard. The choice of coating pigment has a major impact on the properties of the final board. 100% PCC gave a high gloss surface and a high brightness and whiteness. However on its own, the surface was rather porous leading to higher ink penetration and lower print density. As lightly higher print gloss and density are achieved on substrates where the ink is better held on the surface, i.e. a broad particle size distribution carbonate, and one with a higher latex level. In this case, the dot spread further giving a higher total printed area, in the half tone region, and a larger dot perimeter. For the same printed area fraction a higher print density was also achieved.

However addition of a platey partner clay to the porous PCC coating can fill up some of the surface pores. This demonstrates that a multipigment approach can add value to the papermaker by engineering a suitable surface for printing whilst still maintaining optically efficient coatings.