Assessment of the causes of a print defect in a film-coated LWC-paper

A print defect in a film-coated wood-containing LWC paper was investigated. The printed surface gave an uneven impression, with alternating areas of seemingly high and low gloss and surface roughness. The aim of this work was primarily to demonstrate which experimental methods that could be used to quantify and characterize the types of feature responsible for these print defects. The same paper grade was used throughout the study. Samples from important steps in the paper manufacturing process were gathered: pre-calendered base paper, uncalendered and calendered film-coated paper, as well as heat-set offset-printed coated paper. It was found that a high standard deviation in the reflectance of burnout-treated uncalendered coated paper correlated with a poorer print result. A high coefficient of variation for the macro-gloss of coated calendered paper was also found to correlate with these print defects. The findings indicate that the type of printing problem encountered here is associated with an uneven coating mass distribution. These two measurement techniques could be used to estimate prior to printing the risk that such print defects might occur. An evaluation of the base paper properties indicated that these were not the main cause of the print defects, and that the origin of this type of print defect should be sought in the coating process.