Infiltration of ink during printing (4)

The coated paper consists of a coating layer and a paper substrate, and the coating layer and the paper substrate have different capillary structures. The penetration of the ink in the coated paper is not only related to the respective capillary structures of the coating layer and the paper substrate layer, but also to the relative position of their capillaries, and therefore is more complicated than the penetration of ink in non-coated paper. In the following, in the absence of printing pressure, the ink is subjected to so-called "free penetration" in a free state for some discussion.

Figure 7-11 shows the structure of the coated paper. The upper part is the paint layer. If there are n1 capillaries of radius r1 per square centimeter, the thickness of the lesser layer is  when the ink completely wets the coating (cosθ1=1). When the total capillary force per unit area is

(a)

The total amount of ink penetrated is

(b)

Press (7-3) to fill all the coating layer capillaries for

(c)

From this

(d)

Therefore, the total amount of ink that has penetrated can be written as

(e)

The lower part of the coated paper structure is a paper substrate. If there are n2 capillaries with a radius of r2 per square centimeter and the paper substrate is parallel to the coating capillary, then when the ink completely wets the paper substrate (cosθ2=1), the unit The total capillary force in the area is

(f)

According to equation (7-3), the penetration depth and penetration time of the ink to the paper substrate are

(g)

(h)

The above discussion considers the coating layer and the paper substrate layer as separate systems. In fact, when the ink enters the paper substrate through the coating layer, all the n1 capillaries used in the coating layer are filled with ink, and F1 should be equal to F2, ie, So the pressure difference of each capillary in the coating layer should be

(i)

The total amount of ink in the coating layer can be written as

(j)

T'l is the penetration time of the ink in the paint layer with a pressure difference of ΔP.

In order to find t′l, it is assumed that the paint layer and the paper base layer have the same n and r, ie n1=n2, r1=r2, but have different thicknesses 1=2, and the capillary distribution is shown in Figure 7-12. .

When cosθ=1, the capillary pressure is

(k)

Therefore, the total amount of ink in the coating layer is

(l)

Obtained by (j)(l)

(m)

(n)

If the total penetration time of the ink to the coated paper is t, the time required for the ink to pass through the coating layer capillary is t'1 when the coating layer capillary is completely filled with ink, then the time for the ink to penetrate the paper substrate is t· T′1, which can be calculated according to formula (7-3). The depth of penetration of the ink in the paper substrate is the total penetration depth of the coated paper in the time t.

(7-5)

(7-5) shows that the penetration depth of free penetration of ink in the coating is related to the surface tension and viscosity of the ink; it is related to the thickness of the coating layer of the paper; and the size and distribution of the capillary of the paper coating layer and the paper substrate layer. related. It should also be noted that the formula (7-5) is derived under the assumption that the surface tension of the ink is large, the viscosity is low, and the coating layer of the paper can be completely infiltrated.