The Relationship Between Ink Setting Rates, Backtrap Piling And Micro-Picking, 2004 Coating and Graphic Arts Conference

Backtrap mottle refers to disruption of an otherwise uniform “early down” ink layer in a later printing unit. Its onset and severity are believed to depend, in part, on the rate of ink setting. Recent results show that the pigment type has some influence on backtrap mottle. However, clear mechanisms that can be used to predict mottle are not given in the literature.

Under many circumstances, the disruption affects only the ink. In some cases, a variation in the ink film thickness is produced that shows up as an optical density mottle. Differences in the ink set rate, from pointto- point, may cause variations in the ink film split and the ink film thickness. However, in other cases, the backtrap cause a disruption that may extend through the ink layer and affect the coating surface. In this case, backtrap related picking and piling in the later printing unit will cause the optical density mottle, but this issue has not been well studied. One goal of this paper is to determine whether greater uniformity explains the suppression of backtrap related mottle by certain types of pigments.

A series of coatings with a range of pigment types and size distributions are applied onto paper with a pilot coater at the same latex level, coat weight and finishing conditions. The ink setting rates of these samples were measured. These samples were printed on a six unit press with black ink under conditions meant to cause severe backtrap disruption. Comparing the gloss of the first down ink and the last down ink linked the amount of print disruption to the ink and coating layer. The same ink was used in the first and last unit to compare the back-trapping effect. The samples were also tested with a nano-indenter, the micro-tack tester, and the liquid bridge-porosity probe, to measure the variability of the coating structure. Electron microscope images of the printed samples were collected as well as visual results.

Electron microscope images showed that the back-trapped samples with a mottle appearance had regions of “micro-picking”. These regions correlate to a low print density region of the print. Small parts of the coating are pulled from the surface leaving small points unprinted. Two different trends are seen when comparing this event with ink setting rates: kaolin and ground calcium carbonate samples follow one curve, while the precipitated calcium carbonate (PCC) coatings follow quite a different curve. The samples that had the micro-picking effect were uniform in terms of compressional mechanical tests. The micro-tack tester did show some relationship between ink setting rate uniformity and backtrap mottle appearance, but the relationship is not strong. One possible reason for these results is that certain coatings may wick away fountain solution from the top coating layer surface keeping that layer strong enough to withstand the tack forces in the last nip.