THE INFLUENCE OF INK-COATING INTERACTION ON FINAL PRINT DENSITY IN MULTICOLOR OFFSET PRINTING

摘要

The relationship between non-uniform coating properties and variation in final print density is not well understood. The effect of ink-coating interaction on final print density in multicolor offset printing is investigated on a series of well-characterized model coatings. A novel test procedure is proposed to screen samples for backtrap mottle problems and to characterize ink retransfer in multiple nips. All samples were printed on a laboratory press using the printing conditions similar to commercial presses. Two coated papers with different backtrap mottle problems on a commercial press give quite different results in the procedure. Print density is reported for single and multiple nip contacts. After a single nip, little difference is found between the samples with different degrees of backtrap mottle problems and as well as model coatings with different structure. But after three nips, a significant difference in print density between these samples is seen. This result implies that print tests with a single unit cannot predict the final print density. The extent of print density change depends on the ink-coating interaction. The results demonstrate that the final print density after backtrap increases with increasing ink setting rate. Print gloss does not change after multiple nips. These results further suggest that controlling the uniformity of the coating layer is key to give uniform ink setting rates that lead to reduce backtrap mottle.