Mesh screen-printed moire

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 depth, looks uncomfortable, destroying the performance of the dot reproduction color, is a taboo printed, as shown in Figure II. Unusual patterns are called moire patterns.
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 produces moiré patterns when photographing objects with regularly changing objects, 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.
Second, the film itself of the moire As a result of the use of electronic color separation machine or desktop prepress system Photographics machine for laser screening, the edge of the network is not smooth, the finer the network, the higher the roughness of the green side, so in the image The probability of producing moiré in the high light area is greater. 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 “off”. 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.
Third, the silk screen caused by the standard pattern of offset printing screen angle is: yellow 0 ° or 90 °, magenta 15 °, blue 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.
Fourth, due to inappropriate ratio of mesh/dot caused by moiré This kind of moiré even if the angle is correct. Frequently there are stripes, pine planks, and white lines.
Please keep in mind that the occurrence of moiré is caused by the mutual cover of repeated pictures. The larger the ratio of mesh/dot, the smaller the chance of deformation of the dot, as shown in Figure VII. If mesh/dot=3:1, the dots can easily be printed in the shape of incomplete. If there are enough distortions in the dots, we will see the moire.
In order to reduce the occurrence of these moirés, two things need to be done. First, print with a large mesh/dot ratio as much as possible. As shown in Table 1, if mesh/dot is greater than 5:1, printing will produce good results. Second, when using a ratio less than 5.0, To avoid ratios where the first decimal place is an even number (in other words, to avoid 0, 2, 4, 6, 8 in the first decimal place), if you can guarantee that both the deciles and the percentiles are odd, that will be more Ok, like 3.55. The use of an odd number relationship can cause the repeated pattern generated to be scattered to the point where the eyes are not aware, that is, when the screen is affected by the ink in the screen, the warp and weft lines affect the ink penetration of the dots. The dot distortion generated by each line will not be the same, and the dots generated by the line will not be the same. The same, it will not interfere, our eyes will not see the moire.
Another issue to note about the relationship between mesh/dot is that the actual number of nets given by the vendor may not be accurate, especially when using line/cm units but converting to line/Inch.
When discussing the number of meshes, it should also be taken into account that the mesh itself is also inaccurate. When the wire mesh is produced in the factory, there is an error of ±5%, and it is difficult to ensure that the number of meshes in all places is the same. Occasionally, the occurrence of moiré in certain individual areas is related to the non-uniformity of the number of meshes.
Finally, it should be considered that the ratio determination and calculation are based on 50% of the outlets. With the change of the percentage of outlets, the ratio of mesh/dot will also change. The smaller the number of outlets is, the smaller the ratio will be. For example, 10% of the dot area is 1/5 of the 50% dot. If you use 65 line halftone printing, the original mesh/dot ratio is 50% dots is 5.5:1, and 10% dot/dot mesh/dot The ratio is 1.1:1, which means 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 the special dot area. When printing an ash scale, you may see patterns on certain 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 also why it is very difficult to print a large area of ​​monochromatic color patches.
V. Mesh tension Tension changes in the mesh will affect the ease of ink penetration and cause the dot shape to change. If the screen tension is not uniform, the ink will produce different amounts of ink during printing. Dots will appear differently in different areas and appear moire. In addition, it should be noted that the actual number of meshes will decrease with the increase of tension. If the tension is 30N/cm inch, it is also possible that the 355-mesh mesh becomes 305-310 meshes. In fact, the mesh/dot calculation should be accurate after the network is opened. 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 network, and small dots cannot be printed, which affects the reproduction of the layers.
Sixth, the ink layer of the print is also related to the production of moiré We know that the thicker the screen, the more ink through, the thicker printing ink. Under normal circumstances, the ink should have a certain leveling to eliminate screen printing marks. However, when using UV inks to print fine meshes, due to the solid components of the ink, which are dry and fast, the ink has only a small amount of leveling. It is not easy to eliminate the screen marks and the mesh surface will also have a moiré pattern. This type of moiré often appears in the midtone region of the tone printing above 85 lines. The common approach to reduce this problem is to use silk screens that are as small as possible, especially for UV ink printing.
In addition, there are two methods that can be combined to solve this problem. Its purpose is to reduce the thickness of the ink layer. The first is the use of gray component replacement technology for color separation, the purpose of which is to use black ink instead of the gray component formed by the yellow, the product, and the green in the multicolor or neutral color region. The maximum ink volume can be controlled to be about 210% during color separation, so that the total thickness of the ink layer can be reduced to prevent such moiré. The second method is to use a plain weave screen.
7. Another reason for the occurrence of moiré in the printing process is that the probability of occurrence of moire is high when the substrate is caused to print on irregular and regularly changing substrates. Since the pressure of the blade is small, the ink only Can be deposited to the convex part, and will not fall to the concave part, the dot structure will be deformed, then prone to moire. This can easily happen when printing on fabrics. There are two ways to solve this problem. One is to reduce the number of screens as much as possible, and print with larger dots to reduce the pressure on the squeegee. The second is to find ways to make the surface of the substrate smoother.
Eight, scraper pressure in the larger network distance, due to the scraper pressure is too large, it will cause the mesh stretch, so that the network point deformation or loss. Under the high-power microscope, it can be observed that the dots in the front part of the printing stroke are clear and the edges are good, while the edges of the dots in the back part are not clear and the dots are deformed. (Liu Wuhui) ("Screen Printing")