During offset printing, delamination of the paper or paper coating can occur leading to serious print defects and press operation failure. A number of publications have reported tack forces that are measured during the printing of a solid region. However, often parameters are not clear the with regard to the effect of nip loading, speed, and ink rheology on this force.
A device that simulates the press roll is used to characterize the pressure pulse of an ink layer as it travels through a nip. The loading of the nip is controlled by air pressure. The speed of the rolls is well controlled by the computer. The pressure distribution is obtained with a flush mounted piezoelectric sensor. A series of Newtonian fluids are used on the roll surfaces as well as a series of inks that were rated for different ink tacks.
The pressure pulses are similar to past results with a positive pressure as ink goes into the roll, and a sub-ambient pressure as the ink splits. The magnitude of this sub-ambient pressure is called tack. Tack is a function of speed and nip loading, increasing to a point for both. However, at higher speeds, the tack becomes a constant value. This value must be related to the fluid being tested and its ability to withstand tensile force. A magenta ink had tack values that were three times the values of the viscous silicon oil. Inks with different tack ratings did not show the expected trend in terms of tack force in this device.