Abstract
Fountain solution induced mottling in offset printing manifests as non-homogenous printing area with optical variation has several influencing factors. One mechanism is that the fountain solution from the previous printing unit has no time to absorb into the coating before ink transfer or the ink cannot emulsify the water and stays on top of the ink surface as an additional hydrophilic layer. In paper coating development, the paper chemistry influences the surface chemistry features of the ink-paper coating interaction and can cause water repellency. Porosity and the absorbing capacity of paper and its coating is of great importance to avoid press related problems. In this paper, we have examined paper substrates which were reported to cause wet repellence mottling and tested their porosity ratio of micro and macro pores and their water/ink absorption properties. The micro macro porosity was determined by using easy and low-cost proprietary technique for the porosity ratio determination. We have measured ink stabilization values, penetration dynamics, wet repellence mottling and micro and macro porosity on paper samples printed with laboratory sheet-fed offset printing. We have found that the lower number of macropores and non-optimal micro and macropore distribution influenced the occurrence of water induced wet repellence and lowered the optical homogeneity of the samples.
Highlights
The paper substrate during printing in offset technology is exposed to two different liquids
To evaluate the micro and macro porosity ratio to wet repellence we have compared the macro pores which are mainly influencing the water/dampening solution absorption with the optical density values measured on the test samples
Our results indicate that micropores do not influence the water repellency as considered to absorb oils like the vehicle from offset printing inks, while the calculated macropores and their ratio exponentially induce water repellence in offset printing
Summary
The paper substrate during printing in offset technology is exposed to two different liquids (printing inks and fountain solution). The diameter of the capillary structure of the paper determines the separation effects which drains off the ink vehicle part and leaves the pigment on or near the surface. Bauer & Hirch (2010) researched the effect of calendaring on the pore structure change and its influence on the ink setting behaviour Their findings indicate that fast ink setting can be realised by the use of fine pigments as well pigments with steep particle size distribution. Their results demonstrate that the pore size of coated papers on average gets compressed permanently and reduced by using calendering This trend towards smaller pores in their opinion should be beneficial but due to denser coating structure which reduces the total numbers of voids (absorptivity) can lead to slower ink settings and problems in printing. The ratio of the two can be a very valuable tool in paper production to reduce all printability problems
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