An empirical laboratory one step and one colour impression ink transferring at sheet-fed printing configuration of an intensive black low viscosity/tack ink was made directly from a hard roll, and in a second set of experiments through a blanket using similar pressure, to a matt and a glossy paper, i.e. comparing letterpress versus dry offset principle of ink transfer. Ink level was increased in twelve steps at the relatively low speed of sheet-fed press configuration used to determine gravimetrically how ink was to be transferred, both within the letterpress and dry offset configuration with a similar nip line pressure, and how matt and glossy coated papers affected the transfer and the followed laboratory print quality. Conventional AM-halftone screening at 133 lines/inch ( ˜ 5.2 lines/mm) was also included for the laboratory offset configuration at printing plate tone values of 20 %, 50 %, 70 %, 90 % and fulltone coverage.
The letterpress configuration, or hard roll direct to paper transfer configuration, showed a maximum for ink transfer % for the coated papers. Glossy coated paper shoved a narrover ink transfer % maxima at a lower plate inking level. Offset ink transfer was primarily depending on the plate to blanket ink transfer, while blanket to substrate transfer gave less differences between the coated papers.
Print density readings determined at different inking levels and time-points of evaluation showed less variations for the glossy coated paper printed in offset configuration. The colour space, i.e. CIE-L*a*b* values, underlined that a smooth layer of printing ink was transferred in the offset configuration at all inking levels, where the matt coated paper showed larger variations, especially seen for a* and b* values. Print gloss declined slightly after the determined maximum level in ink transfer % obtained in letterpress configuration for the glossy coated paper. Offset transferred print gloss showed a similar trend, but with more variation. Matt coated paper print gloss was found to increase in both ink transfer configurations. Halftone dry offset ink transfer gave a maximum in the normal colour intensity (NCI) level for the matt coated paper, while glossy paper showed a linear behaviour. The NCI levels for 70 % and 90 % plate halftone areas declined drastically for both paper types at a similar fulltone print density due to toning of the waterless printing plate. Physical and optical dot gain, i.e. Murray Davis dot gain, was seen as larger for the glossy coated paper, i.e. dot gain enlarging at increasing inking level.
This fast laboratory procedure contributes valuable information of an ink, or film, transfer process giving a possibility to study in laboratory the properties of all involved materials, i.e. plates, inks, blankets, and substrates to be printed. Laboratory waterless offset print determinations can give the today needed trend information for full-scale printing trials, e.g. where in the press configuration the compensation needs arise.
Keywords: Coated Paper, Dryography, Ink gloss, Ink transfer, Laboratory Printing, Waterless Offset, Prüfbau, Dry Offset
Laboratory Waterless Ink Transfer Evaluation for Coated Papers Using the Prufbau Printability Tester