Infiltration of ink during printing (1)

Paper is a porous sheet that is interwoven with fibers and has numerous pores of different sizes and shades. During the printing process, when the paper enters the embossing zone (ie, the area where the plate cylinder and the impression cylinder contact each other through the paper), the ink is pressed into the larger pores of the paper under the effect of the printing pressure; when the paper passes through After the embossing zone, the ink penetration is clearly dependent on the capillary action of the paper for free penetration. Therefore, the penetration of the ink is significantly dependent on the size (aperture and length) and distribution of the paper capillary. In the process of ink penetration, there are sometimes phenomena such as “moisture”, “printing out”, and “powdering,” and the quality of printed matter will be reduced. Ink penetration is different for non-coated and coated papers, discussed separately below.

First, non-coated paper ink penetration

Infiltration of ink mainly refers to the penetration of the binder in the ink to the paper. The ink binder during infiltration can generally be considered as Newtonian fluid. In order to determine the penetration depth of the ink in the pores of the paper, it is necessary to first understand the flow of the Newtonian fluid in the straight tube.

Assume that the fluid makes a steady flow in the round pipe, that is, the flow parameters, such as speed, flow, pressure, etc., at various points in the flow field do not change with time, and the flow assumes a laminar flow state, and the flow (flow through the pipe V to the time The rate of change of t (dV/dt) has the following relationship:

Q=dV/dt=(πr4·ΔP)/δηl (7-2)

(7-2) is the so-called Poiseuille' formula. In the formula, ι is the length of the tube, r is the radius of the tube, ΔP is the pressure difference between the two ends of the tube, and η is the viscosity of the liquid. (7-2) shows that the flow through the circular pipe is proportional to the fourth power of the pipe diameter and the pressure difference at both ends of the pipe, and inversely proportional to the pipe length and the viscosity of the fluid.

Assume that the paper capillary is a straight tube with a radius r. It is assumed that the binder in the ink is freely permeated to the paper, that is, assuming that there is no printing pressure, and the gravity of the binder itself is ignored, the binder is only adsorbed by the wall, that is, the capillary force F, and the tube penetrates a certain depth. , as shown in Figure 7-7. If γ indicates the surface tension of the ink binder and θ indicates the contact of the ink binder to the capillary wall of the paper, then

The formula (7-3) shows that if there is no printing pressure or the printing pressure is small, if the paper is constant and completely wetted by the ink, the ink penetrates only by the capillary action of the paper, the depth of penetration and the penetration time. The square root is proportional to the square root of the viscosity of the ink.