The reason and solution of the dot moiré (1)

According to the editors of the first Asia-Pacific Network Printing Exhibition, the China Screen Printing and Imaging Association has invited the SGIA, FESPA, and Asia Pacific Online Printing and Imaging Association (ASGA). ) Co-organized the Beijing New Century Screen Printing Forum. The forum's statement not only elaborated on the technical points of the screen printing color reproduction process, but also talked about the impact of digital technology on screen printing, and also provided information on China's screen printing industry, such as Lin Guang, with professionalism. The combination of technical and informative features has been welcomed by both domestic and international participants. With the consent of the speakers, these lectures will be published in the “Screen Printing Forum” section of screen printing magazines from this issue to readers. Reproduced without permission. Of all the challenges faced in color-tone screen printing, the moiré is probably the most destructive and difficult to deal with. One of the most prominent problems is that it seems completely unpredictable, with random occurrences and disappearances. Through the practices and observations of the past 15 years, several reasons for this phenomenon have been summarized. Before we discuss the cause and type of moiré, you should have 2 to 3 sets of different detection devices. At a minimum, there should be a 10x small color correction magnifier and a 25-30x handheld microscope. In addition, a 100x microscope should be prepared. The finer the printed dot, the higher the magnification required for the detection. Without these inspection devices, the specific conditions of outlets cannot be observed. For us, moiré is often the most destructive of patterns. It can be ribbon, line, checkerboard, jewel, or cause color shifts. It may be throughout the entire pattern, or it may be concentrated in a specific area. We do need to understand the moiré and learn how to avoid or reduce it. Here, I want to say that it is impossible to completely control the moire. No matter how careful we are, the key is that there are many factors that may cause moiré. We say that the moire phenomenon is somewhat challenging because some of the factors that cause moire are beyond our control. Almost all aspects of screen printing are included. The finer the dots, the more difficult it is to control the electroprinting. Therefore, we must strictly follow some relevant recommendations for the special fine-sleeve nettones (100 lines/inch or 40 lines/cm). Moire can appear at several different stages, including: From the original to the process of reading the film (original to film); 2. Digital film type (film to film and film to screen); 3. Film screening angle (film to film); 4. Mesh and screen (film to screen); 5. Film screening angle and screen angle (film to screen); 6. Image and substrate surface (screen to substrate); 7. Ink viscosity and substrate (ink to substrate); 8. Printing ink layer (ink to ink). There is a possibility of moire at every step of the printing process. In order to reduce the appearance of moire, our operations must be very systematic. Reducing variable factors and standardizing operating methods are critical to our success. If you do not follow this principle, moire may appear and disappear at any time. It can be extremely destructive, even with catastrophic consequences for our production. The easiest to begin with is the original itself. In our environment, moire will appear naturally. There are still some real-life manuscripts that produce moiré patterns once they are photographed. For example, there are regular patterns on the original. Wire mesh textures, wicker chairs, fences, screens, louvers, floor patterns, and any similar material that presents repetitive surface textures. The customer's provision of a printed poster or other similar printed matter to the printing plant is another major cause of the turtle's level due to the original, because the original itself already has the dot pattern generated by the previous printing. To reduce this situation, it is recommended to order a 4 inch x king inch copy film, and then use a transparent film for color separation. In the production process, the dots have been refined to be able to merge with each other to become a point. At this time, there are no dots and dots of moiré. At present, many scanning software also have the function of removing moire to reduce the print texture. When using a laser scanner or imagesetter for film production, special attention should be paid to the dot resolution. The output resolution of the first generation laser scanners (DS, HELL, CROSFIELD) was not particularly high. Electronic Generation Network (EDG) capabilities are still primitive. During the manufacturing process, since the laser produces a series of intermittent and approximate apertures, the finer the dots in the highlight area, the more irregular the edges will be. It can be said that the finer the dots, the rougher the edges, and the resulting dots often have a star-like or other irregular shape. This is really a problem for those print shops that buy large format directly, because the film is made on a very high number of lines (150 to 425), and then scaled up to the last line. All these irregularities are also revealed. The latest scanners and imagesetters have overcome this situation. Before purchasing the film from the color separator, the sample should be taken under a magnifying glass to carefully observe whether the edge of the dot is clear. The second major cause of the turtle's effectiveness is the angle of screening between each other. In order to reduce the point-to-point turmoil effect, the three dark colors (magenta, cyan, black) are 30° from each other because the yellow contrast is not very strong, and it is clamped at 15°. The standard lithographic angles are: yellow 0°, 90°, magenta 15°, cyan 75°, black 45°. The difficulty lies in that 0°, 90°, and 45° are the main moiré generation angles. This is because the network points are in line with the direction of the network cable, which results in the network cable completely blocking the entire network of outlets. At 45°, the intersection of the cable will block the dot. This means that the halftone images produced by these angles almost always show moire. Therefore, if you purchase a color separation sheet that is color-separated by lithographic printing, you can ask for trouble. For the ratio of screen mesh to outlet, the smaller the outlet, the more important the ratio. The higher the ratio of screen mesh to outlets, the less important it is. For any less than 4. O:1 (screen mesh number is greater than the number of screen lines), we have to change the basic angle in the separation. The acceptable angle is to rotate all screen angles by ±4 to 8°. This allows the original to be rotated along the horizontal line during color separation, or it can be rotated while scanning. The first method is less used because the scanner section now provides several sets of scan angles. The second method is to require the scanner to make some settings and adjustments. In practice, some scanners cannot complete this task and need to use the first method. Before determining the rotation angle, be sure to consult your service provider. Also, the rotation of the angle allows the operators to reset the raster image processor (RIP). Usually they are not willing to do so. Care must be taken to check the received film to ensure that it is the angle you requested. There is a third way to solve this problem. The screen is stretched to the frame in the offset printing angle. I do not agree with this method. There are several reasons. First, it is difficult to accurately position the screen and the frame at 4°. Second, when you are doing diagonal stretching and sticking, the tension of the mesh does not match the force of the blade. In this case, I can't believe how stable the tension of the screen is. But intuition tells me that when the thread is in line with the direction of the force, we can hardly guarantee the stability of the tension. If you use an adjustable frame, it becomes more important when the screen tension deviates from the direction of the thread. Together with the role of the blade, it becomes more difficult and more deformed. The moiré caused by the angle interference usually presents a regular moire band. When the ratio of screen mesh to dot is high, the moiré band first appears in the highlight area. With the decrease in the number of screen meshes and dots, moire will spread to various shade areas. If the ratio is higher than 5.0:1, the angular ridges are basically absent. The reason for the next generation of turtle effects is the relationship between the number of screen meshes and outlets. Simply put, it is the ratio of the number of screen meshes to the outlets, that is: the number of screen meshes/number of screens added = the ratio of screen meshes to outlets. In this respect, there have been many reference ratios in the past. The most common of these is the minimum ratio of not less than 3.5:1. Although I also agree with this view, there are still many issues that deserve our attention. Note that the moiré is formed by the overlapping textures. When the moiré appears, what we see with the naked eye is uneven printing. Some are round, some are half, and some are even lost. The higher the ratio of the number of screen meshes to the outlets, the less chance of the outlets being distorted during printing. For example, if the ratio is 3:1, the dot can easily change from the original dot to a square dot. Once the rows of grids are more deformed, we will feel the moiré on the naked eye. In order to reduce duplication, two things are necessary. First, use higher rates as much as possible. As long as the ratio is higher than 5.0, you can certainly get good results. Second, if the ratio is lower than 5.0, it should be avoided that the first digit after the decimal point is even, and it is better to ensure that the single digit and the hundred digits after the decimal point are odd (for example, 3.55). It should be realized that when the ratio is odd, we can disperse the texture so that the change in texture is not noticeable to the naked eye. We must ensure that when the filaments interfere with the dots and block the holes, the interference at each dot is different. As a result, a row of different but unreplicated dots is obtained. In this way, even if the spots are different, we don't look like moire with our eyes. In addition, we should also pay attention to the relationship between the number of screens and outlets. The number of screens to be marked is not necessarily the actual number of screens. All screens are woven according to metrics, such as how many heads per centimeter. In an imperial country, we usually convert the number of centimeters by 2.54 to inches. In this way, in most cases, the obtained screen day value is a divisor, which is approximately 5 orders. One of the best examples is that the conversion from 150 mesh/cm to English system is 390 mesh/inch. In fact, the actual value after conversion should be 381. At the same time, it should be understood that the number of meshes is not the number of meshes under the stretched state. As the tension increases, the number of meshes will decrease. If you don't use a screen size meter, you cannot know its effective mesh number. We must not ignore this point. As the tension continues to increase, the effective mesh of the screen will continue to change. When we are discussing screens, we should see the reality that the number of screens is inaccurate. When the screen is woven, the warp threads are first fixed to the knitting machine, so the number of warp threads is fixed. In general, the warp measurement of the screen is more accurate. The latitude line is not very definitive, and fluctuations of ±5 on the basis of the number of headings are common. The error range of the number of screen meshes I see is even more severe than ±5. This is one of the causes of local moiré, which we cannot control. The number of wefts is not static in the printing process. Therefore, the ratio of our screen mesh to outlets is constantly changing within a certain range. There is one final point about the relationship between the number of screen meshes and outlets. The ratio is calculated based on the 50% of the selected line number. Since the dot size varies with the percentage of hue, the screen mesh number and the dot ratio change are caused. The smaller the outlets, the lower the ratio. This is why it is so important that the first digit after the decimal point is odd. The diameter of 10% of the dots is 1/5 of the diameter of the 50% dot. If our number of screens is 65 lines/inch, the first ratio is 5.5:1. At 10% of the screens, the ratio of screen mesh to screen points becomes 1.1:1. This shows that the dot of 1 case is almost impossible to print. As a screen printer, you should understand that the first ratio is only a reference value, but it is relative to the 50% of the screen lines you have calculated. Because the ratio is constantly changing, the moiré that is caused often appears in certain hue regions, ie, moiré. If you print a gray ruler, you will find that the moiré appears only in a certain section of the gray ladder. Most importantly, the texture of the moire changes with this mathematical relationship, which is important when exploring the causes of the moiré. The moiré will be concentrated in the highlight area, and as the density of the dots slowly increases, the moire becomes a checkerboard or diamond. This is why it is so difficult to print large areas of a single hue (such as the sky, large skin color). A particularly uniform ratio relationship (150 dots on any screen) will cause the moiré of each hue area to change continuously. Wire diameter and dot diameter