The author of this article graduated from the Beijing Printing Institute and has eight years of working experience in the packaging and printing industry. He has in-depth research on color quality management of packaging and printing and ink color matching.

For four-color printing, solid density is an important factor affecting color reproduction. The solid density determines the thickness of the ink layer to a certain extent, but also determines the expansion of the dots of the printed matter and the reproduction of the tone of the printed matter. Under normal circumstances, the level of the original is relatively rich, such as photos, and after copying into a print, the level will have a certain loss and compression. If the field density is too small, the reproducibility of the gradation is low, so increase the field density value and minimize the compression of the gradation to improve the tone reproduction ability; but if the field density value is too high, because the ink layer is too thick, The influence of printing pressure causes the dots to expand too much, which will also cause the loss of the layer. Especially, the dark tone part of the layer is very easy to lose. Therefore, controlling the density of the field is very important for the reproduction of the entire tone.

However, different brands and different specifications of inks are printed on different substrates, and the achievable field density is different, and the network expansion is also different. Therefore, in actual production, it is necessary to determine the best solid density and suitable dot expansion value for different substrates and different inks.

Discussed here is the method of using printing contrast to determine the best field density and dot expansion.

Printing contrast or K value refers to the contrast between solid density and dark tone density (mainly refers to 75% or 80% density). The larger the printed contrast value, the more layers it can reproduce, and the richer the details of the dark tone; conversely, the smaller the printed contrast value, the fewer layers it can reproduce. The calculation formula is:

K = (Ds -Dt) / Ds = 1-Dt / Ds

In the formula, Ds is the solid density value (which can include paper or subtract paper density), and Dt? R75% or 80% density value (which can include paper or subtract paper density).

It can be seen from the formula that if the Ds value of a certain printed product is as large as possible and the Dt value is as small as possible, the printing contrast can be maximized.

Of course, if you directly measure Ds and Dt and then calculate, it is more cumbersome, so the general spectrodensitometer has the function of directly measuring the printing contrast. The following example analyzes the relationship between printing contrast and solid density.

Print an ink on a bond paper, gradually increase the ink volume, measure the printing contrast of each ink volume, and measure Ds and Dt at the same time, the results are as follows:

Solid density
(Ds)

75% density
(Dt)

Printing contrast
(K value)

description

0.00

0.00

0%

Not printed yet, blank paper, contrast is 0

0.50

0.4

20%

Less ink, not enough to show the entire tone level

1.00

0.65

35%

Increased ink volume, increased field density, increased contrast

1.50

0.82

43%

The ink volume is further increased, the contrast value is further increased, and the level is fully displayed

2.00

1.25

38%

The ink volume is too large, the dot expansion is dominant, the contrast is reduced, and the level is partially lost

2.5

2.5

0%

The ink volume is too large, Dt is close to Ds, so that the dark tone level is completely lost, the contrast is 0

Plot the data in this table into a graph of "Relationship between printing contrast (K value) and solid density (Ds)".

It can be seen from the figure that when the solid density Ds = 1.60, the printing contrast value is the largest (K = 44%), so the best solid density should be 1.60. Sub-division? The measurement dots increase, and the increments of the dots at 25%, 50%, and 75% are divided into 8%, 18%, and 10%. When the solid density Ds <1.60, the reproduction ability of tone gradation is low; on the contrary, when the solid density Ds> 1.60, the expansion of dark tone dots causes the printing to decrease, and the gradation is lost.

Therefore, for this ink and this paper, the solid density should be set to 1.60, and the dot increase at 25%, 50%, and 75% is divided into 8%, 18%, and 10%. At this time, the amount of gradation compression is the smallest, and the tone reproduction performance is the highest.

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