The hottest tone screen printed moire

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silk printing, like printing and lead printing, uses dots to reproduce the tone of the original. Y. The m, C and K color plates interfere with each other during dot printing, and various interference patterns will appear, and the patterns will change with the size of the dot angle difference. All patterns will appear within the included angle of 45 ° and repeat within the range of 90 °, as shown in Figure 1

the normal pattern is uniform and dense, looks comfortable, and has a normal step tone to reproduce the performance expressed by DuPont high performance materials division. Abnormal patterns are piled up and scattered according to specific rules. The patterns are coarse, the color depth is uneven, and it looks uncomfortable, which destroys the performance of dot reproduction color. It is a big taboo of printing, as shown in Figure 2. Abnormal patterns are called tortoise patterns

dot moire is caused by uneven density distribution caused by uneven arrangement of dot printing of more than two colors. This uneven distribution is between the complete weight and complete juxtaposition of points, as shown in Figure 3. Obviously, the density of focus is less than that of juxtaposition. If the density difference is too large and appears alternately in a regular manner, moire will appear

in practice, in order to avoid the appearance of moire, the included angle between the four-color plates of offset printing and embossing printing should be greater than 22.5 °. Generally, the dot angles of yellow, magenta, cyan and black are arranged as 90 °, 15 °, 75 ° and 45 ° respectively, so only the included angle between the yellow plate and other color plates is less than 22.5 °, but the yellow is lighter, and the formation of moire is not very obvious

since the silk braiding structure of silk screen printing is also arranged in a regular manner like dots, there is the possibility of Moire between silk and film. In addition, the printing control of silk screen printing is more variable than offset printing, so the moire control of step tone silk printing is more complex. This paper will analyze the causes of moire in tone screen printing and discuss the methods of controlling moire

I. moire of the original

when shooting the original of some regularly changing objects, it will lead to moire of the original itself, such as fabric patterns, wicker furniture, fences, shutters, etc. In addition, the printed manuscript will also have moire after scanning

for the moire of the original, first of all, be aware that the moire will be more obvious after warp printing. Make it clear to the customer first, try to find a way to replace the original, or blur these places appropriately during printing and color separation; For the printed manuscript, it must be processed during scanning to remove all moire. Of course, this is at the expense of the clarity of the manuscript

second, the moire of the film itself

due to the use of electronic color separation machine or desktop prepress system Phototypesetter for laser processing, the edge of the point is not smooth, the thinner the point, the higher the green roughness of the edge, so the greater the probability of moire in the highlight area of the image. As shown in Figure 4, the edge of the traditional point is very smooth, while the postscript point edge of the electronic summing imagesetter of the electric extension is not smooth. They are related to the resolution of the Imagesetter, because their addition methods are exposed by using the laser beam "on" or "off". When a 16x16 dot matrix laser is used to generate a dot, the dot situation is as shown in the figure. It is obvious that the refining and chemical base development strategy is planned to be implemented, and they can't be as smooth as traditional photo dots anyway

III. moire caused by silk

the standard offset plus angle is: Yellow 0 ° or 90 °, magenta 15 °, green 75 °, black 45 °. The difficulties faced by silk printing are: 0 °, 90 ° and 45 ° are the basic angles to produce moire. Because the lines are fixed in one direction, the silk line will directly block the penetration of the points forming the line, as shown in Figure 5. The image with 45 ° plus prevents the formation of points on each basic intersection point because of the crossed fibers, and the halftone image generated by this corner is most likely to produce moire. Figure 6 shows the common moire shape caused by silk. The moire produced by silk is related to the size of dots and the number of silk meshes. The finer the dots are, the more important the angle of the color separation image line is. The ratio of mesh to line (silk mesh line/film plus line = mesh to line ratio, such as 355 mesh/100lpi=3.55.) The larger the line angle, the less important it is. If the ratio is less than 4.0, the angle of the color splitter used will change. The method of change is to produce a ± ° change on the basis of the offset angle

the second way to solve this problem is to use oblique, while the color separations still use the same angle as offset printing. The oblique method can be carried out by tilting the version, or by using special oblique equipment. However, this method has two problems: it is difficult to achieve the same placement direction of a four color plate silk; Second, there will be a lot of trouble in tension balance due to the inconsistency between the direction of ink knife and the direction of wire tension during printing

the third way to solve this problem is to print the plate obliquely. On the plate with an angle of 0 °, turn the positive plate by °. Using this method, the overprint rules and the printing table should also rotate during printing, which will cause trouble for overprint, feeding and receiving, and the corresponding frame and substrate area should be increased

moire caused by angle often appears in a certain area for high mesh/dot ratio. Moire will first appear in the high-profile area. As the mesh/dot ratio decreases, moire will cover the whole tone

IV. moire caused by improper mesh/dot ratio

this moire will occur even if the included angle is correct. Stripes, loose board lines, off white lines, etc. often appear

please remember that moire is caused by the mutual covering of repeated pictures and texts. The larger the mesh/dot ratio, the smaller the chance of point deformation, as shown in Figure 7. For example, when mesh/dot = 3:1, the dots are easy to be printed into incomplete shapes. If the rows of dots have enough deformation, we will see moire

in order to reduce the occurrence of this kind of moire, two things should be done. First, try to print with a large mesh/dot ratio, 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, avoid the ratio that the first decimal place of the ratio is an even number (in other words, avoid the occurrence of 0, 2, 4, 6, and 8 in the first decimal place). It would be better to ensure that the tenths and percentiles are odd, such as 3.55. The application of odd number relationship can make the generated repeated patterns disperse to the point that the eyes cannot detect, that is, when the ink of the points affected by the silk longitude and latitude passes through, the point changes generated in each line will be different according to the sample shape of different diameters, and the resulting lines of points are different, so there will be no interference, and our eyes will not see the moire

another problem that should be noticed about the relationship between mesh/dot is that the actual mesh number given by the seller may be inaccurate, especially when the wire/cm is used for production. 5. The horizontal tensile testing machine system of steel wire rope is controlled by a full microcomputer, but it is converted to wire/inch

when discussing the mesh number, we should also consider that the mesh itself is also inaccurate. When the silk is produced in the factory, there is an error of ± 5%, and it is difficult to ensure that the mesh number in all places is consistent. Sometimes, the appearance of moire in some individual areas is related to the uneven mesh number

finally, it should be considered that the determination and calculation of the ratio are based on 50% points. With the change of the percentage of points, the mesh/dot ratio will also change. The smaller the number of points, the smaller the ratio. For example, the 10% dot area is 1/5 of the 50% dot. If you print with 65 lines of halftone, the initial mesh/dot ratio is 5.5:1 for 50% dots, and 1.1:1 for 10% dots, which indicates that 10% dots cannot be printed at all, as shown in Figure 8

therefore, the mesh/dot ratio changes, and the resulting moire will often change in special point areas. When printing a gray ladder ruler, you may see patterns on some steps. The most important thing is that the turtle pattern will change with the change of the mathematical relationship between points, which will be very important in determining the cause of the turtle pattern. This is why it is very difficult to print large areas of monochrome color blocks

v. eye tension

changes in eye tension will affect the ease of ink penetration and cause changes in point shape. If the silk tension is uneven, the ink will produce different transmission during printing. Points will produce different deformation in different areas and appear moire. In addition, it should be noted that the actual mesh number will decrease with the increase of tension. For example, if the tension is 30n/cm inch, it is also possible to change 355 mesh into 305 ~ 310 mesh. In fact, the calculation of mesh/dot should be after the exact tensioning. The eye tension has a certain impact on the performance of the ink transferred to the substrate, especially in the high light area. Under low tension, the high viscosity ink is easy to adhere to the substrate and cannot print small dots, which affects the reproduction of layers

VI. the imprinting of the ink layer is also related to the production of moire pattern.

we know that the thicker the silk is, the more the ink passes through, the thicker the printing ink layer is. Generally, the ink should have a certain leveling property to eliminate silk screen marks. However, when printing fine mesh with UV ink, due to the large solid components of the ink and fast drying, the ink has only a small amount of leveling, which is not easy to eliminate the silk trace, and the surface will also produce moire. This kind of Moire often appears in the middle tone area of gradient printing above 85 lines. The common way to reduce this problem is to print with small wire diameter as much as possible, especially for UV ink printing, this method is the most effective

in addition, there are two methods that can be combined to solve this problem. The purpose is to reduce the thickness of the ink layer. One is to use gray component substitution technology for color separation, which aims to use black ink to replace the gray components formed by yellow, magenta and cyan in the multicolor or neutral color area. In addition, the maximum ink printing volume can be controlled to be about 210% during color separation, which can reduce the total ink layer thickness and prevent this kind of moire. The second method is plain weave

VII. Another reason for moire in the printing process is that it is caused by the substrate

when printing on an uneven and regularly changing substrate, the probability of moire is very high. Because the pressure of the scraper is small, the ink can only be deposited to the convex part, but will not fall to the concave part, and the point structure will be deformed, so moire is easy to appear at this time. This is easy to happen when printing on fabric. There are two ways to solve this problem: first, try to reduce the number of extra lines and use larger dot printing, so as to reduce the pressure of the scraper; The second is to find a way to make the surface of the substrate smoother

VIII. Scraper pressure

when the distance is large, too much scraper pressure will cause eye elongation and point deformation or loss. Under a high-power microscope, it can be observed that the points in the front part of the printing stroke are clear and have good edges, while the points in the back part are not clear and have deformation. (liuwuhui) (silk printing)

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