Density meter principle and application of printing quality control method (3)

Three parameters that affect printing quality

The main parameters affecting the print quality in printing are the density of the field, the dot gain, and the relative contrast. These quality parameters are related to the thickness of the ink film on the printed sheet. From another perspective, it is related to the density. Therefore, determining the optimal ink layer thickness has important significance for controlling the printing quality.

1, field density

The field density refers to the coverage area of ​​the printed products on the Internet. The coverage area of ​​the printed product is completely covered with the ink layer. The density is measured with a reflection densitometer. The definition is based on the definition of the reflection density:

If R represents the reflectivity of the ink layer to the light, then the thicker the ink layer, the more light energy absorbed, the larger the reflectivity R, the smaller the density Dv, and the thicker and denser the ink layer.

The density in the field does not increase indefinitely with the increase of the ink layer. When the thickness of the ink layer increases to a certain value, the thickness of the ink layer continues to increase. The density in the field has reached its maximum value and no longer increases, as shown in Figure 9-8. Show
Figure 9-8 Relationship between density in the field and ink layer thickness

China's printing industry standard recommended offset printing product dark-density range

Table 9-2 Planographic Printed Shadow Density Ranges Colors Fine Prints General Prints

2, network increase value

The density in the field can only reflect the thickness of the ink and cannot reflect the change in the dot size during printing. During the proofing or printing process, the proper increase of the dot is a normal phenomenon, but it must be controlled within the allowable range, otherwise it will affect the printing. The tone reproducibility and color reproducibility of the product.

Table 9-3 50% dot gain value Fine print (%) Normal print (%)

3, relative contrast

Relative contrast, also known as print contrast, is an important parameter for controlling image tone. Measure out-of-field density Dv and dot density Den on the printed matter or on the measuring strip. Substitute the following formula to calculate the K value.

K=1-

The value of K varies from 0 to 1. The larger the value of K, the smaller the ratio of dot density to field density, and the smaller the dot gain. There are many factors affecting the K value, such as the thickness of the paper, the ink layer, etc. In general, the K value of the coated paper is larger than the K value of the offset paper. Under a stable printing pressure and a good print job condition, the K value is the largest, the dot Increase the minimum value.


Table 9-4 Relative Contrast K Value Color Fine Print General Print

Application of four high magnification magnifiers

The so-called high-magnification magnifying glass refers to a magnifying glass with a magnifying power of 25 times or more. In the detection of print quality, this kind of magnifying glass is used not only to observe the fitting of prints, but more importantly, to observe changes in the dot so that the printing process can be made accordingly. Adjustments. Mainly in the following areas:

(1) Observe whether the outlets are elongated, ghosting halo, and regular expansion.
(2) Pay attention to whether the outlets are hollow, hairy, and regularly shrinking.
(3) The observed area mainly refers to: high-light areas, such as the cheekbones and other parts of the figures, around the bright spots of the eyes. Middle tone, that is, the situation of a half-dozen outlets. Dark tone area, 7 to 9 percent of the network is not not paste.
(4) Detect the depth of printing plate drying.