Causes and elimination methods of tone screen printing moiré

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In the printing industry, how to eliminate the moiré in halftone reproduction is an important issue because it has a great influence on the printing quality. In the color stencil screen printing, it is disturbed by the screen, which makes the generation of the moiré more complicated. Improper handling in the screen printing process, two kinds of moiré are easy to appear: overprint moiré and silk screen moiré.

In color reproduction, the predecessor told that the angle between the two strong colors must be greater than 22 degrees and a half, less than 22 degrees and a half, there will be moiré.

First, the production of moiré

In multi-color mesh copying, due to improper technical processing, there will be two kinds of moiré: overprint moiré and silk screen moiré.

1. Overprinting turtle pattern:

The generation of moiré is the result of mutual interference when two or more patterns with different spatial periods overlap. Its size is related to the spatial period difference and the angle of intersection of the two pictures. The larger the difference in the period, the smaller the moiré is, and vice versa.

Semi-tone prints are made up of regular dot arrangements, which is the first reason for the appearance of moiré. Not only the network dots, but also the regularly arranged patterns may interfere with each other if more than two overlaps, and an additional pattern that is not very clear often occurs.

Causes and elimination methods of tone screen printing moiré

It is shown that the parallel line group A having a certain interval coincides with another group of parallel lines B having a certain interval, and a parallel line group C which is not clear than the third pattern appears. In this example, the two sets of parallel lines interfere with each other and should produce a third set of parallel lines. But what if a set of orthogonal lines coincide with another set of orthogonal lines? The pattern produced by mutual interference, that is, the moiré, is still a set of orthogonal lines. In the same set of rules, after a certain interval of orthogonally arranged dots overlap with another set of dots, the resulting regular orthogonal pattern is a moiré.

Causes and elimination methods of tone screen printing moiré

The production of moiré. When the two sets of parallel lines of the interval P1 and the interval P2 coincide with each other at an angle θ, the resulting pattern is the dotted line drawn in the figure. The spacing of the interference pattern, i.e., the moiré, is represented by P, and its direction is represented by α (the angle at which the parallel lines of the interval P1 intersect). Then P and α are calculated by geometric calculation. which is

Causes and elimination methods of tone screen printing moiré

(± is determined by P1cosθ being smaller or larger than P2) or by dividing the line spacing by the number of lines per unit distance. The number of lines of two sets of parallel lines is represented by n1 and n2, respectively. The number of lines of the moiré is represented by N, and the interval between the interval and the number of lines is inversely related to each other.

(1) and (2) or (1') and (2') are the basic formulas for the occurrence of moiré. These formulas are related to the requirement for the generation of moiré in parallel line sets, but also for the case of dot prints. The moiré of the dot prints, especially the multi-color dot prints, also has a dot overlap problem of the same interval (number of lines). At this time, if P1 = P2 (n1 = n2 = n), the formula (1), (2) or (1') (2') can be simplified to:

Causes and elimination methods of tone screen printing moiré

The spacing (number of lines) or angle of the moiré can be easily calculated.

Causes and elimination methods of tone screen printing moiré

In the halftone printing reproduction, a variety of pattern patterns appear, and some patterns such as a snowflake pattern, a star pattern, a circular pattern and the like are beautiful and evenly dense, and are pattern patterns required for printing. Has the ability to reproduce color alignment. Another pattern, the dots are arranged, stacked and evacuated according to a specific law, and have a grid-like, turtle-shell-like pattern, etc., which is to break the dot definition, is an interference pattern in printing, and is commonly called a moiré (Moiré fringe) ).

The appearance of the silk screen moiré is the moiré that appears in the screen printing due to the interference of the mesh cloth and the weft, which is called the silk screen moiré.

The image dot distribution on the screen and the mesh of the screen produce diffraction diffraction of light under certain conditions, which destroys the integrity of the dot, and the defect of the dot blot appears, also known as pitting.

The generation of the mesh moiré depends on the period of the pitting difference. The smaller the cycle, the less obvious the moiré is, and no mesh moiré will occur when there are no rules. Moiré can be produced in either multi-color overprint or monochrome printing. A reproducible screen is made of yellow polyester mesh. The matching of the number of screen lines and the number of mesh meshes should be appropriate to prevent the generation of silkworm moiré. Second, the method of eliminating the turtle pattern:

1. Correctly select the matching of the number of screen lines of the original and the mesh number: the smaller the mesh/line ratio, the more serious the moiré is, and the general requirement is 3.5:1 or more. At present, the ratio of the target/line is more than 4.5:1 times. Within this range, the moiré is also a slight moiré, which is within the printing permission. Color screen printing is generally selected to be 5:1 or more. This ratio is generally stretched or positively printed without the appearance of moiré.

Michelle Kay's expert in France: I have done a lot of experiments on the generation and elimination of moiré in halftone screen printing. In the case of mismatched eye/line, he used the method of oblique netting. Eliminates the generation of moiré. He gave two examples:

Causes and elimination methods of tone screen printing moiré

The above method works well, and the disadvantage is that it consumes mesh.

2. Correctly master the plate making technology of the template:

Pay attention to the surface tension of the screen in the stretch net. If the stretch net is not good, the screen will be deformed, showing a wavy shape or a bad internal view, which will cause local moiré. The mesh must be stretched according to the specifications, and two sets of screens (four sets each) must be stretched at the same time, which must meet the printing size requirements.

The use of the positive stretch net (zero degree stretch net) mesh wire diameter parallel to the net frame, can save the mesh and can make a small positioning correction. The oblique stretch net, the warp and weft of the net and the edge of the net frame form an angle of ±4 degrees -8 degrees, and the mesh number and tension of the four nets are consistent. The disadvantage is the cost netting, which has the advantage of ensuring that the wire lines are completely parallel and square. In order to save the mesh, a positive stretch net can be used, and the oblique printing plate can be ±4 degrees -8 degrees. When the exposure is performed, the negative film can be obliquely placed on the film plate to fix the printing plate, or the platen positioning should be rotated at an angle during printing.

Mastering the correct exposure time, the current process is using a photo-crosslinking type of diazo photosensitive light source matched to ultraviolet light with a wavelength of 365 to 420 um. The exposure is equal to the illumination multiplied by time (H=Ext). The mesh number and color have an influence on the exposure time. The higher the net mesh number, the thinner the film and the shorter the exposure time; the film with a low mesh length has a long exposure time; the exposure time of the white screen is short, and the exposure time of the color filter is long.

3. Select inks and substrates:

The ink should be selected from the binder ink of the substrate material. The pigment particle size should be fine. In general, the pigment particle size of the ink is one-third of the inner diameter of the net mesh. Otherwise, the net is plugged. The substrate should be chosen to have a smooth surface and an ink-compatible substrate. The texture should use double-sided paint board paper.

4, the correct choice of screening angle:

It is important to choose the screening angle. The purpose is to prevent the generation of moiré, the beautiful patterns of the graphic points and the color reproduction of the powerful graphics. The conventionally set screening angle is 0, 15, 45, 75, and four angles. In another method, the eye/number matching is not enough, and the angle of the oblique mesh or oblique printing plate is adopted. It is guaranteed that there will be no mesh moiré between the film and the screen, and at the same time, it will ensure that the imprinted moiré will not be produced after the color plates are overprinted. The single-crossing angle of 45 degrees is preferably set to a dominant color angle, 75 degrees and 15 degrees for a strong color angle, and 0 degrees (90 degrees) for a weak yellow angle.

5, the choice of dot shape:

Different dot-shaped dots are selected according to the middletone of the original and the shading of the image layer. Rich and medium-sized images use diamond-shaped dots and circular dots. The dark layer is rich in square dots. Circular dots can be used for the dark level, and diamond dots are generally used.

6, the correct operation of the printing process:

(1) Commissioning on the machine: The color separation version should be positioned accurately, and the mesh distance should be consistently not too large or too small. The area of the figure should match the length of the blade. Generally, the blade will grow 3cm on each end. The distance from the frame to the edge of the graphic is 10 to 15 cm.

(2) Master the relationship between the five elements and the five variables in printing.