Analysis of Interaction between Ink and Fountain Solution for Offset Printing and Ink Balance (I)

At present, the application of offset printing in the printing industry is very extensive, and the balance of ink and ink in offset printing has always been the focus of attention in the industry. In the long-term production practice, operators have accumulated rich experience. At the same time, the printing press further created conditions for the ink balance in terms of hardware.

In the offset printing process, if any color ink reaches a certain density value on the coated paper, the thickness of the ink layer is about 1 μm. This paper uses this value as a basic basis to discuss the control of the ink layer thickness of the blanket, plate, and ink roller and the effect of the thickness of the dampened liquid film layer to illustrate the basic ink layer and water layer thickness in the ink balance. Problems and their impact on the balance of water and ink.

Ink system

1. Structure

The ink system of the printing press is composed of ink fountain roller, ink transfer roller, ink fountain roller, ink roller, etc., and the rubber ink roller and the metal ink roller are staggered, and the ink in the ink fountain is finally delivered to the printing plate.

2. Ink separation

During the transfer of ink, the thickness of the ink layer on the ink roller gradually decreases. As shown in Figure 1, when the ink between two adjacent ink rollers (the ink roller on the upper layer is thick and the ink roller on the lower layer is thin) enters the nip, its pressure begins to rise, not far from the center of the nip. The maximum is reached at one point and then drops to a level lower than the atmospheric pressure at the exit of the nip. Since at least one of the two ink rollers is a rubber ink roller, deformation after contact between the two ink rollers will increase the pressure between the nips, and the greater the elastic deformation, the greater the pressure applied to the ink. The ink layer begins to split at the exit of the nip. After separation, the thickness of the ink layer on each roller is equal and equal to half the total thickness of the ink layer before passing through the nip, so-called semi-partial separation theory.

In addition to the ink transfer roller and the ink fountain roller, the surface speeds of all the ink rollers are basically the same, so the ink is transferred between the nips mainly because of the average separation of the ink. This ink transfer process passes through two adjacent ink rollers. Ink layer contact and separation to complete. From this, the ink layer thickness of the entire inking system can be deduced.

3. Ink layer thickness

Assuming that solid ink is printed, the amount of ink discharged by the ink fountain roller is the same (approximate ideal state). If the thickness of the ink layer reaching the coated paper is 1 μm, according to the semi-partial separation theory, the thickness of the ink layer on the blanket before entering the nip (with the impression cylinder) is 2 μm, after the nip is 1 μm; the same printing plate is on The ink layer thickness is 3 μm before passing through the nip (with the blanket cylinder) and 2 μm after passing through the nip. The ink application roller (2 or 4) has to make up the thickness of the printing plate by 1 μm at a time, so the ink layer thickness of the ink roller is Between 2.5 and 3.5 μm, the ink layer thickness of the adjacent ink roller is 4-6 μm, and these deduced data have been confirmed by the thickness of the ink layer actually measured by the infrared detector.

The ink layer thickness increase and decrease gradient that occurs in order by the ink roller only exists when the ink is transferred. If the ink roller of this series is not in contact with the ink fountain roller and the plate (non-printing state), it adheres to the ink roller. The ink will redistribute and each ink roller will have the same ink layer thickness (on average about 4-6 [mu]m), so when the ink is reprinted, the thickness of the ink layer on the ink roller is slightly thicker than desired.

When the ink roller of each printing unit is covered with a uniform and uniform ink layer, a better impression of the printing plate will be obtained, that is, the amount of ink transferred to each printing unit during the entire printing process is consistent, but actually The number of ink rollers required to meet such requirements is much greater. Because the transfer roller does intermittent swing, ink is transferred from the slow-rotating ink fountain roller to the fast-rotating ink roller, resulting in inconsistent transmission speed, so multiple ink rollers are required to be evenly distributed; 30% of the space on the plate cylinder will also make ink Transmission discontinuities; different printing units print layout distribution of different locations, not to the printing plate continuous transmission of ink, so by the ink fountain screws in accordance with the layout of the printing plate layout to adjust the amount of ink into the lateral; increase the additional ink roller can reduce the ink bars ( The density of the ink printed on the paper on the circumference of the drum changes abruptly.) When the ink roller is low, the fountain solution will be transferred back to the ink fountain of the printer.