Aspects of water-induced mottle when printing on coated paper in sheet-fed lithographic offset

Uneven print density, print mottle or mottle, call cause severe print quality problems and is a frequent problem for printers. Print mottle can occur ill sheet-fed offset due to several reasons related to both the paper and its coating and the printing press. The aim of the project was therefore to improve our knowledge of the influence on mottle related to the fountain solution which dampens the paper surface, water induced mottle. Pilot coated papers were produced and printed in a full-scale 4-colour sheet-fed offset printing press. During printing a specially designed test form was used. This must be used together with a blanket which has cut-out areas corresponding to the areas oil the test form. In this way it is possible to create a number of different printing situations oil the same printed paper sheet. The results from the print trials show that fountain solution from the previous printing unit can cause a decreased print density and dot gain. Water induced print mottle is Caused by the fountain solution from the previous printing unit. Back-trap from the following print units Will have a smoothing effect and will decrease the mottle. The results are based oil a hypothesis test at the 5% level. Water induced mottle can be related to both the printing process and the paper. The structure of the paper coating, Uneven porosity distribution, has a high influence oil water induced mottle while the effect of the Surface chemistry is low. The paper with high SB latex content showed the most uneven porosity distribution.