Formation of Patterns on Paper Coated with a Metering Size Press, 2000 Coating Conference Proceedings

M. Wikstrom, J. Gron--The metering size press (MSP) technique is rapidly gaining acceptance for coating lightweight-coated paper (LWC). The coating technique is interesting not only for its potential to improve the process efficiency but also for quality reasons. At low coat weights, below 8 g/m 2, the MSP technique provides excellent covering ability in comparison with conventional blade coating. However, severe colour penetration into the surface voids of the base sheet may lead to irregularities (patterns) in the coating layer, and these may be visible in the final printed product. Patterns related to film splitting (i.e. orange-peel) may also occur on MSP-coated paper, especially at higher coat weights. The character and the distinctness of coating patterns for LWC paper coated at various machine speeds and with different filler contents were evaluated with some new techniques. The patterns governed by the filling-in of surface voids of the base paper clearly dominated in these paper samples, but it is possible that patterns related to film splitting also occurred. Patterns related to the filling-in of surface voids (i.e. openness or roughness of the sheet) were much less noticeable after coating at the highest machine speeds (i.e. 1800 and 1950 m/min), and this was interpreted as being an effect of a more ideal thickness of the non-immobilised film under those conditions. The patterns had in most cases a significant MD-orientation, probably governed by the base paper structure rather than by the film splitting. It was also demonstrated that the patterns influenced offset print results.