DIRECT MEASUREMENT OF TENSILE-STRESS (TACK) IN THIN INK FILMS

A printing press tackmeter was built by fitting a pressure sensor into the printing cylinder of a laboratory printing press. This tackmeter has been used to measure pressure profiles of thin (3-14-mu-m) ink films under printing conditions. We define the maximum tensile stress (or negative pressure) that the ink can withstand in the nip exit before splitting to be the tack of the ink. The tack of inks without dissolved polymer appears to be independent of film thickness. The tack of inks with dissolved polymer resin increases strongly with increased amount of ink in the nip, while pigment and dispersant content have a minor effect on ink tack. This suggests that at high speed, ink tack is mainly characterized by the response of polymer molecules toward rapidly applied stress. No universal correlation was found between the tack and viscous flow properties of the ink.