Formation of printing dots in printing manuscripts and its production mechanism III

Third, the dot size

1. basic concept:

The change of the printed image's shades is reflected by the amount of reflected light. The light shines on the paper, the inked area absorbs light, and the blank area reflects light to the human eye. Large dots of printing ink on a large area, absorbing more light, less reflected light, low optical density, making people feel dark, that is, dark tone or low-key areas; small dots, large area, absorb light, reflect light, optical The high density makes people feel bright, that is, a bright tone area or a high profile area. The medium-sized dot area is the middle tone area or the middle tone area. Various dots of different sizes show different levels of tone based on the different proportions of absorbed light and reflected light (Figure 9).

Figure 9

In general, people use dot coverage to indicate the size of the dot. The coverage rate of outlets is the percentage of the area occupied by outlets within a unit area. It is also called the percentage of outlets. In our country, it is customary to express the size of the outlets as "comprised" (Figure 10). Under normal circumstances:

Figure 10

The site coverage in the field is 100%, that is, 10% of outlets; 95% to 99% of outlets are called white dots; 1%—5% of outlets are called black dots, also called extremely high light areas; 5%—10 % of the network is called the highlight area; 10% - 30% of the network is called the highlight area; 30% - 60% of the network is called the intermediate tone area; 70% - 95% of the network is called the dark tone area.

During printing, if the dark tone area can display 95% or more of the outlets, it is a fine product; if 90% of the outlets are printed in the field, but 85% of the outlets are still legible, it is a better product; if 80% and The networks of Ibu are printed on the ground and can only be called general products; if 70% of the outlets are also in common with the field, the products are unqualified. The above is the requirement for the dark tone portion of printed products.

Based on the fact that the human eye is more sensitive to a highlight portion than to a dark tone portion, the print reproduction requirement of the highlight portion is higher. In general, ultra-fine products require printing of 0.5% of dots; fine products require printing of 1%-4% of dots; just the ability to print 5% of dots is a better product; if only 10% of dots can be identified as General product. The finer the area of ​​the highlight area is, the better.

2. Network distortion:

Ideally, when the original is screened to obtain a print version of the mesh, the light and shade levels on the original are accurately converted to the dot coverage of the corresponding size; when the printing plate is inked to obtain prints, the dot size can be reproduced perfectly. The tone level of the image, in the entire process flow, is not expanded or reduced as the dot of the image carrier.

However, in fact, when the dot-swimming network undergoes a series of processes such as screening, printing, copying, developing, printing, printing, etc., it must be accompanied by an increase or decrease in the percentage of area, which affects the reproduction and color of the image tone level. The composition and clarity of the guarantee (Figure 11).

Percentage of Original Sites in Figure 11 (%)

Figure 11 percentage of print outlets (%)

The dot distortion is divided into visual deformation, optical distortion and mechanical deformation.

The so-called visual distortion refers to only 10% of the white light is reflected to the human eye when the light is irradiated to the edge of the halftone dot on the white paper, and the remaining 90% of the white light is absorbed. We feel a circle of gray, and thus Visually feel outlets expanded (Figure 12). The expansion of visual outlets is objective and exists for all prints and all outlets.

Figure 12

The so-called optical distortion is that when copying and printing, the light is scattered through the film base layer of the film, and the dots are deformed when they are transferred. When a negative image is copied into a positive image, the dot is enlarged and the white point is lost. When the positive figure is used to print a plate, the dot is reduced and the black dot is lost. This is also difficult to avoid in traditional platemaking.
The so-called mechanical deformation refers to the printing process, due to the printing pressure caused by ink droplets to the blank part of the extrusion; or due to the compression deformation of the blanket caused by the network in the transfer of the slip; or due to the use of fountain solution, causing ink droplets to the surrounding Diffusion; or due to paper deformation, causing radial, axial extension of the network. Mesh deformation caused by mechanical deformation is mainly based on dot expansion, and machine printing cannot avoid a certain amount of dot gain.

In the actual printing process, the non-standard operation will intensify the deformation of the dot. Such as the exposure time and the development time is insufficient or excessive; the developer concentration is too large or too small; the printing shop temperature and humidity is too high or too low; the printing pressure is too large or too small; the ink layer in the field is too thick; More; fountain solution with ink and PH value does not meet the requirements and so on.

Statistics show that under the same circumstances, the amount of deformation of the outlets is proportional to the perimeter of the outlets. Therefore, (1) Among the percentage of outlets, the number of outlets with 50% of outlets is the largest, ie, the expansion rate of 50% outlets ranks first among all outlets. (2) The perimeter of square outlets is larger than that of circular outlets and other shapes. That is, the expansion ratio of square outlets is larger than that of circular outlets and other outlets. Therefore, when printing quality inspection, 50% of square dots are commonly used to determine the expansion of the dot. (3) Combination of ink and all papers, dot gain values ​​increase with the surface roughness of the paper, the newsprint dot gain value is the most people, and the dot enlargement value of coated paper is smaller than that of offset paper. (4) The expansion value of the Cyan version is 3% to 4% higher than that of the other three-color version.

In actual operation, on the one hand, it is necessary to strictly control the specification and control of various numerical values. On the other hand, it is also possible to appropriately compensate for the deformation of the dot by using the adjustment of the tone level curve.
(to be continued)

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