In the halftone reproduction of the printing industry, the research on how to eliminate moire is an important issue, because it has a great influence on the printing quality. In the color-adjusted screen printing, it was interfered by the silk screen, which made the generation of moire more complicated. Improper handling in the screen printing process is prone to two types of moire: overprint moire and silk screen moire.
In the color reproduction, the predecessors told that the angle between two strong colors must be greater than 22 degrees and a half, and less than 22 degrees and a half, moire will appear.
1. The generation of moire
In multi-color mesh reproduction, due to improper technical processing, there will be two kinds of moire: overprint moire and silk screen moire.
1. Overprint turtle pattern:
The occurrence of moire is the result of mutual interference when two or more patterns with a small difference in spatial period overlap. Its size is related to the space period difference and intersection angle of the two pictures. The larger the period difference is, the smaller the moire is, and vice versa.
Halftone prints are composed of regular dot arrangements, which is the first reason for moire. It is not limited to network dots, that is, regularly arranged patterns can interfere with each other if more than two overlap, and often another kind of unclear pattern appears. As shown in Figure 1.
(Figure 1 Moire caused by two groups of parallel lines)
Figure 1 shows that a parallel line group A with a certain interval overlaps with another parallel line group B with a certain interval, and a parallel line group C that is not clearer than the third pattern appears. [next]
In this example, two sets of parallel lines interfere with each other, and a third set of parallel lines should have been generated. But what if a set of orthogonal lines coincides with another set of orthogonal lines? The pattern caused by mutual interference, that is, moire, is still a set of orthogonal lines. The same set of regular, regularly arranged dots at a certain interval overlaps with another set of dots, and the resulting regular orthogonal pattern is moire. As shown in Figure 2.
(Figure 2 Moire caused by two groups of dots)
(Figure 3 The relationship between parallel lines and moire)
The generation of moire is shown in Figure 3. When the two sets of parallel lines of the interval Pl and the interval P2 coincide with each other at the angle θ, the resulting graph is the dotted line drawn in the figure. The interval of this interference pattern, that is, the moire, is represented by P, and its direction is represented by α (the angle that intersects the parallel line of the interval Pl). Then P and α are obtained through geometric calculation. which is
(± is determined by P1cosθ being smaller or larger than P2) or the interval of parallel lines is replaced by the number of lines per unit distance. The line numbers of the two parallel lines are denoted by n1 and n2, respectively. The number of lines of the moire is denoted by N, then the relationship between the interval and the number of lines is reciprocal to each other.
(1) and (2) or (1 ') and (2') are the basic formulas for moire. These formulas are related to the requirements for generating moire on parallel lines, but they are also applicable to dot prints. The moire of dot prints, especially multi-color dot prints, also has a problem of dot overlap arranged at the same interval (number of lines). At this time, if P1 = P2 (n1 = n2 = n), then (1), (2) or (1 ') and (2') can be simplified to:
The interval (line number) or angle of the moire can be easily calculated.
A variety of patterns appear in halftone printing and copying. Some patterns such as snowflake pattern, star pattern, round pattern and other patterns are beautiful, uniform and dense, and are the required patterns in printing. It has the performance of reproducing color tone. In another pattern, the dots are arranged, stacked and evacuated according to a specific rule, presenting a grid-like, turtle-like pattern, etc. It is a disruption of the dot definition, a disturbing pattern in printing, commonly known as moir ).
The appearance of silk screen moire is caused by the interference of mesh warp and weft in screen printing, which is called silk screen moire.
The distribution of the image dots on the screen and the mesh of the wire mesh produce light diffraction fringes under certain conditions, destroying the integrity of the dots, and showing the defect of dot imprinting, also known as pitting.
The generation of silk screen moire depends on the period of pitting difference. The smaller the period is, the less obvious the moire is, and no mesh moire will be generated when there is no regularity. Whether it is multi-color overprinting or monochrome printing may produce moire. The screen mesh with good reproducibility is mostly yellow polyester screen mesh. The matching of the number of screen wires and the mesh number of mesh should be appropriate to prevent the occurrence of silk screen moire. [next]
2. The method of eliminating moire:
1. Match the number of screen lines of the original manuscript correctly with the number of screen meshes: the smaller the mesh / line ratio, the more serious the moire will be, and the general requirement is 3.5: l or more. Internationally, the ratio of mesh / line is more than 4.5: 1 times. Within this range, moire appears and is slightly mottled, which is within the scope of printing permission. Color? Adjustable screen printing is generally used above 5: 1. This ratio is generally not stretched net or positive version will not appear moire.
Michelle Kay, an expert in France: A lot of experiments have been done on the generation and elimination of moire in halftone screen printing. In the case of mesh / line mismatch, he used the method of diagonally stretching the net Eliminate the occurrence of moire. He gave two examples:
The above method works well, but the disadvantage is that it consumes mesh.
2. Master the plate-making technology of the template correctly:
In the stretched net, pay attention to the surface tension of the screen, if the stretched net is not good, the screen will deform, appear wavy, or the internal square is not good, will cause local moire. The screen must be stretched according to the specifications, and two sets of screens (four sets per set) must be stretched at a time, and they must also meet the print size requirements.
The positive stretch net (zero degree stretch net) mesh wire diameter is parallel to the net frame, which can save the net fabric and make small positioning correction. Oblique stretch net, the mesh warp and weft wire stretches the net at an angle of ± 4 ° -8 ° with the edge of the net frame. The mesh number and tension of the four screens must be consistent. The disadvantage is the cost of the mesh cloth, and the advantage is that it can ensure that the screen wires are completely parallel and square. In order to save the net cloth, a positive stretch net can be used, and the oblique exposure plate is ± 4 degrees to 8 degrees. During exposure, the negative film can be placed obliquely on the film plate to fix the exposure plate, or the platen positioning should be rotated by an angle during printing.
To master the correct exposure time, the current process is using photo-crosslinking type diazo photosensitive glue light source matching ultraviolet light, the wavelength is 365 to 420um. The exposure is equal to the illuminance times the time (H = Ext). The mesh number and color have an effect on the exposure time. The higher the mesh number, the thinner the film and the shorter the exposure time; the lower the mesh thickness, the longer the exposure time; the white silk screen exposure time is shorter, and the color substitution silk screen exposure time is longer.
3. Choose ink and substrate:
The ink should be selected to be compatible with the substrate material. The pigment particle size should be fine ink. Generally, the pigment particle size of the ink should be one third of the inner diameter of the mesh of the mesh, otherwise it will be blocked. The printing material should choose the printing material with smooth surface and ink affinity. The texture should use double-sided coating board paper.
4. Correctly select the screen angle:
It is very important to choose the angle of screening, the purpose is to prevent the generation of moire, to compose the beautiful pattern of graphic dots and to reproduce the powerful color of the graphic. The screen angles that are normally set are 0, 15, 45, 75, and four angles. Another method is insufficient mesh / number matching, and the angle of oblique stretch net or oblique printing plate is adopted. It is ensured that no silk screen moire will be generated between the negative film and the silk screen, and at the same time, no overprint moire will be generated after overprinting of the various color plates. The single-intersection angle of 45 degrees is generally best set as the main color angle, 75 degrees and 15 degrees as the strong color angle, and 0 degrees (90 degrees) as the weak color yellow angle.
5. Selection of the shape of the dot:
According to the tone of the middle and dark tone of the original image, choose different dot-shaped dots. The images with rich midtone levels mostly use diamond dots and round dots. The square tones are mostly used in the dark tone rich layers. Round dots can be used for low-level tone, and diamond dots are generally used for popular ones.
6. Correct operation of printing process:
(1) Commissioning on the machine: the color-separated version should be positioned accurately, and the mesh distance should be consistent, not too large or too small. The graphic area should match the length of the scraper. Generally, the scraper grows 3 cm at each end. The distance between the frame and the edge of the picture and text is 10 to 15cm.
(2) Master the relationship between the five elements and the five variables in printing.
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