Screen printing uses the dots to reproduce the tonality of originals, as well as plain printing and lead printing. Y, M, C, and K versions of each dot print interfere with each other, and various types of interference patterns and patterns appear to vary with the size of the dot difference. All pattern patterns will appear within an angle of 45° and repeat within 90°, as shown in Figure 1. The normal pattern is dense and dense, looks comfortable, and has a normal gradation reproduction performance. Unusual patterns, network points according to specific rules of accumulation, evacuation, pattern coarse, uneven color, look uncomfortable, destroy the dot reproduction of color performance, is the taboo of printing. .
The dot moiré is caused by the uneven distribution of density caused by the uneven arrangement of two colors in dot printing on the Internet. This uneven distribution is between the full weight of the dots and the complete juxtaposition, as shown in Figure 3. Obviously, the density of heavy dots is less than the density of parallel dots. If the density difference is too large and appears regularly alternately, moire appears.
In practice, in order to avoid the occurrence of moiré, the angle between the four-color printing of offset printing and embossing is greater than 22.5°. Generally, the dot angles of yellow, blue, green, and black are arranged as 90°, 15°, and 75, respectively. °, 45 °, so that only the yellow version and other color version of the included angle is less than 22.5 °, but the yellow is shallow, the formation of moire is not very obvious.
Because the silk screen weaving structure is also arranged in the same way as the dots, there is a possibility that the silk screen and the film will have a moire. In addition, the printing control of silk screens has more variable factors than offset printing, so the control of the moiré of the screen printing is more complex. This article will analyze the reason for the occurrence of moiré on silk screen printing and discuss the methods for controlling moiré.
1. The moiré of the manuscript itself
When you take a manuscript copy of a regularly changing object, it can cause moire patterns, such as fabric patterns, wicker furniture, fences, and shutters. In addition, the printed original will have a moiré after scanning.
For the original tortoise shell, the first thing to do is to clearly understand the moiré that is generated after screen printing. It is necessary to clarify with clients first, try to replace the originals as much as possible, or apply appropriate blurring to these places during the printing separation process; for print originals, it is necessary to do a net removal process during scanning to remove all the moiré patterns. Of course, this is at the expense of the clarity of the original.
2. The film's own moire
Due to the use of an electronic color separation machine or a photofinishing machine for desktop prepress systems for laser screening, the edges of the dots are not smooth, and the finer the dots are, the higher the green roughness of the edges is, and thus the probability of occurrence of moire in the high-light areas of the image is high. The bigger it is. As shown in Figure 4, the edge of the traditional network is very smooth, while the electronic extension of the extension and the edges of Postscript nodes of the imagesetter are not smooth. They are related to the resolution of the imagesetter because their screening methods are performed using the laser beam “on†or “offâ€. When a 16x16 matrix laser is used to generate a dot, the dot pattern is As shown in the figure, it is obvious that they cannot be as smooth as traditional camera dots in any case.
3. Moire caused by wire mesh
Standard offset printing screen angles are: yellow 0° or 90°, magenta 15°, cyan 75°, black 45°. The difficulties faced by screen printing are: 0°, 90°, and 45° are the basic angles that produce moire. Since the network cables are fixedly arranged in one direction, the screen network lines will directly block the penetration of the lined network dots. As shown in Fig. 5, the images of the screening network at 45° prevent the formation of network points due to crossover fibers at a basic intersection. The halftone image generated by the corner is the most prone to moire. Figure 6 is a common moire shape created by the screen. The moire produced by the screen is related to the size of the dot and the number of screen meshes. The finer the dot, the more important the angle of the dichroic picture cable is, and the ratio of the mesh to the cable (mesh/mesh/film/line) For example, 355 meshes/100Lpi=3.55. The larger the mesh size, the smaller the importance of the mesh line angle. If the ratio is less than 4.0, the angle of the dichroic plate used will change, and the variation will be a ±4-8° change based on the offset screen angle.
The second solution to this problem is the use of diagonal meshes, and the color separation sheet still uses the same angle as offset printing. The oblique network method can be implemented by obliquely laying out the screen on the screen, or using a dedicated oblique network device. However, there are two problems with this method: the orientation of a four-color screen is difficult to be consistent; second, there is a lot of trouble in tension balance due to the inconsistency between the orientation of the ink knife and the tension of the screen during printing.
The third method to solve this problem is oblique printing. The screen is rotated 4-8° on a screen with a screen angle of 0°. With this method, overprinting rules and printing tables have to be rotated during printing, which is troublesome for overprinting, feeding, and receiving, and the corresponding frame and substrate area must be increased.
The moiré generated by the angle effect often appears in a certain area for high mesh/dot ratios. The tortoise will appear first in the high-profile area, and as the ratio of mesh/dot drops, the tortoise will cover the entire tone.