Mesh screen printed moiré
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Screen printing and printing, as well as printing, use dots to reproduce the tone of the original. The Y, M, C, and K color screen dots interfere with each other when printing, and various interference patterns and patterns change with the change of the angle difference of the dot points. All pattern patterns appear in the 45° angle and repeat in the 90° range, as shown in Figure 1. The normal pattern is even and dense, looks comfortable, and has normal gradation reproduction performance. Unusual pattern, the dots are piled up according to specific rules, scattered, coarse patterns, uneven color, seem uncomfortable, destroying the performance of dot reproduction color, is a taboo for printing, as shown in Figure 2. Unusual patterns are called moiré.
The dot moiré is caused by the uneven distribution of the density caused by the uneven distribution of the two colors. This uneven distribution is between the full weight of the dots and the complete juxtaposition, as shown in Figure 3. Obviously, the density of the heavy dots is less than the density of the parallel dots. If the density difference is too large and alternately appears regularly, a moiré appears.
In the practice of Photo, in order to avoid the appearance of moiré, the angle between the color plates of offset and embossed four-color printing is greater than 22.5°, and the angles of yellow, fine, blue and black are generally arranged at 90° and 15° respectively. 75 °, 45 °, so that only the angle between the yellow plate and other color plates is less than 22.5 °, but the yellow is lighter, the formation of the turtle is not very obvious.
Since the screen-woven structure of the silk screen is also arranged regularly like the dots, there is a possibility that a moiré is generated between the screen and the film. In addition, the printing control of silk screen printing is more variable than offset printing, so the moiré control of tone screen printing is more complicated. This article will analyze the reasons for the generation of moiré in the screen printing, and explore the method of controlling the moiré.
1. The moiré of the original itself
When photographing an original of a regularly changing object, it may cause moiré on the original itself, such as a fabric pattern, wicker furniture, a fence, a blind, and the like. In addition, the printed original will have a moiré after scanning.
For the original moiré, it is important to know that after the screen printing, the moiré will be more obvious. It is necessary to explain clearly with the customer, try to replace the originals as much as possible, or properly blur these places during the printing separation process; for the printed originals, be sure to do the de-net processing during scanning to remove all the moiré. Of course, this is at the expense of the clarity of the original.
2. The film of the film itself
Because the laser is screened by the electronic color separation machine or the phototyper of the desktop prepress system, the edge of the dot is not smooth, and the finer the dot, the higher the green roughness on the side, so that the moiré is generated in the highlight area of the image. The chance is greater. As shown in Figure 4, the edge of the traditional dot is very smooth, and the edge of the electronic screen of the electric extension and the PostScript dot of the imagesetter are not smooth. They are related to the resolution of the imagesetter, because their screening methods are exposed by the laser beam "on" or "off". When a 16×16 dot laser is used to generate a dot, the dot is The situation is as shown, it is obvious that they are not as smooth as traditional photo spots anyway.
3. Moiré caused by the wire mesh
The standard offset screen angle is: yellow 0° or 90°, magenta 15°, blue 75°, black 45°. The difficulty faced by screen printing is that 0°, 90°, and 45° are the basic angles at which the moiré is generated. Since the network cables are fixedly arranged in one direction, the screen mesh cable directly blocks the mesh dots from passing through. As shown in Fig. 5, the 45° image is blocked by the intersecting fibers at a basic intersection to prevent the formation of dots. The halftone image generated by the corner is most prone to moiré. Figure 6 is the common shape of the moiré produced by the screen. Because the moiré generated by the screen is related to the size of the dot and the mesh number of the mesh, the finer the dot, the more important the angle of the image of the color separation image, the ratio of the mesh to the mesh (screen mesh line / film plus network = net The ratio of the mesh to the cable, such as 355 mesh / 100 Lpi = 3.55.) The larger the cable angle is, the less important it is. If the ratio is less than 4.0, the angle of the color separation used will change. The method of variation is to produce a variation of ±4-8° based on the angle of the offset screen.
The second way to solve this problem is to use a diagonal mesh, while the color separation film still uses the same angle as the offset. The method of oblique network can be carried out by the method of diagonally placing the screen, or by 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; the second is that there is a lot of trouble in tension balance due to the inconsistency between the direction of the inkjet and the tension of the screen during printing.
The third way to solve this problem is to obliquely print the plate, and rotate the positive piece 4-8° on the screen with a mesh angle of 0°. With this method, the printing rules and the printing table should be rotated during printing, which is troublesome for overprinting, feeding and receiving, and the corresponding frame and substrate area should be increased.
Moiré generated by angles often appear in a certain area for high mesh/dot ratios. The moiré will first appear in the high-profile area, and as the mesh/dot ratio decreases, the moiré will fill the entire tone.
4. Moiré caused by inappropriate mesh/dot ratio
This type of moiré can be produced even if the angle is correct. Frequently appear are stripes, loose patterns, off-white lines, and the like.
Photo Please remember that the generation of the moiré is caused by the mutual covering of the repeated images. The larger the mesh/dot ratio, the smaller the chance of mesh deformation, as shown in Figure 7. If mesh/dot=3:1, the dots are easily printed into a broken shape. If the dots in the row are deformed enough, we will see the moiré.
In order to reduce the generation of such moiré, two things are to be done. First, try to print with a large mesh/dot ratio. As shown in Table 1, if the mesh/dot is greater than 5:1, printing will produce good results. Second, when using a ratio less than 5.0. To avoid the ratio of the first decimal place of the odds (in other words, to avoid the first digit of the decimal, 0, 2, 4, 6, 8), if you can guarantee that the tenth and the percentile are odd, then it will be even more Ok, like 3.55. The application of the odd relationship can make the generated repetitive pattern disperse to the point that the eye can not perceive, that is, when the latitude and longitude of the screen affects the ink transmission of the dots, the deformation of the dots generated by each line will be different, and thus the generated dots are not The same, there will be no interference, our eyes will not see the turtle.
Another issue to be aware of regarding the mesh/dot relationship is that the actual number of meshes given by the vendor may be inaccurate, especially when using line/cm production, but converting to line/Inch.
When discussing the number of meshes, it should also be considered that the mesh itself is also inaccurate. When the screen is produced in the factory, there is a ±5% error, and it is difficult to ensure that the number of meshes in all places is the same. Sometimes, the presence of moiré in some individual areas is related to the unevenness of the number of meshes.
The final consideration of Photo is that the ratio determination and calculation are based on 50% of the network. As the percentage of the dot changes, the ratio of mesh/dot will also change. The smaller the number of dots, the smaller the ratio. . For example, 10% of the dot area is 1/5 of the 50% dot. If you print with 65 lines of halftone, the initial mesh/dot ratio is 50% of the dot is 5.5:1, and the mesh/dot of 10% of the dots. The ratio is 1.1:1, which indicates that 10% of the dots cannot be printed at all, as shown in Figure 8.
Therefore, the ratio of mesh/dot is changed, and the resulting moiré will often change in a special dot area. When printing a gray scale, you may see the pattern on some steps. The most important thing is that the turtle pattern will change with the mathematical relationship between the dots, which will be very important in determining the cause of the moiré. This is why it is very difficult to print large-area color patches of a single color.
5. Net tension
Photo mesh tension changes can affect the ease of ink penetration and cause changes in the shape of the dots. If the screen tension is not uniform, the ink will produce different amounts of transmission during printing. The dots will have different deformations in different areas and the moiré will appear. In addition, it should be noted that the actual number of meshes will be reduced with the increase of tension. For example, the tension is 30N/cm, and it is possible to change the mesh of 355 mesh to 305~310 mesh. In fact, the calculation of mesh/dot should be after the exact net. The mesh tension has a certain effect on the performance of the ink on the substrate, especially in the high light area. Under low tension, the high viscosity ink tends to adhere to the net, and small dots cannot be printed, which affects the layer reproduction.
6. The imprint of the ink layer is also related to the production of moiré.
We know that the thicker the screen, the more the ink is transmitted, the thicker the ink layer is printed. Under normal circumstances, the ink should have a certain leveling to eliminate the screen printing marks. However, when the fine mesh is printed by the UV ink, since the solid content of the ink is large, the drying is fast, the ink is only leveled, and it is not easy to eliminate the trace of the screen, and the mesh surface also generates the moiré. Such moiré often occurs in the midtone region of tone printing above 85 lines. A common way to reduce this problem is to use a wire with a small wire diameter to print as much as possible, especially for UV ink printing.
In addition, there are two methods that can be combined to solve this problem, the purpose of which is to reduce the thickness of the ink layer. The first is to use the gray component replacement technology for color separation, the purpose of which is to replace the gray component formed by yellow, yellow and blue in the complex color or neutral color region with black ink. The maximum ink can be controlled at the time of color separation, and the printing amount is about 210%, which can reduce the total ink layer thickness and prevent such moiré. The second method is to use a plain weave.
7. Another reason for the production of moiré during the printing process is caused by the substrate.
When printing on a substrate that is uneven and has regular changes, the probability of occurrence of moiré is large. When the blade pressure is small, the ink can only be deposited on the convex portion without falling to the concave portion, and the dot structure will When it is deformed, it is prone to moiré. This is apt to occur when printing on fabrics. There are two ways to solve this problem. One is to minimize the number of screen lines, to print with larger dots, so that the pressure on the squeegee can be reduced. The second is to find a way to make the surface of the substrate smoother.
8. Scraper pressure
When the mesh distance is large, the mesh pressure is too large, causing the mesh to elongate and deform or lose the mesh. Observed under a high power microscope, it can be observed that the dots in the front portion of the printing stroke are clear and the edges are good, while the edges of the dots at the back portion are unclear and the dots are deformed.