Impact of coating surface properties and print impression on the non-uniformity of sheet-fed offset prints, 2010 TAPPI Advanced Coating Fundamentals Symposium

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The results from a pilot coating and full scale sheet-fed offset printing trial are reported. The aim was to study the impact of latex and clay content in a calcium carbonate coating on surface properties and print quality. Special emphasis was addressed to non-uniformity in print. The effect of calendering and print pressure was also considered.

It was found that uniformity in surface porosity decreased with increasing latex content but increased with clay content. Surface energy, wettability and water absorption rate also increased with higher clay content. Ink setting became slower when the latex content increased.

Non-uniformity in print was analyzed in terms of back trap mottle (BTM), water interference mottle (WIM) and print gloss variation (PGV). Mottle was evaluated for a 50% raster, while PGV was analyzed on heavily inked (400%) surfaces. BTM was low and showed a slight decrease with increasing setting time (slower setting). WIM was severe in the fifth unit and was found to be influenced by a number of factors e.g. print impression and water absorption rate. Calendering and uniformity in coating porosity had a significant impact on PGV. The observed non-uniformity in print is discussed in terms of non-uniformities in coating porosity and coating micro-roughness.