Modern printing mainly uses digital screening. The main task of this technology is to reproduce the conventional screening angle combination. The main obstacle in accomplishing this task is the moire pattern generated by the interaction between the four color dot patterns. Turtle refers to an unpleasant web pattern that appears in the proof or printing due to the improperly matched angle of the positive plate. Because the amplitude modulation screen printing is to use the dot size of the regular arrangement to represent the color depth of the image, and these regularly arranged dots will generate moire, so that in the two-color or multi-color mesh printing because the mesh image is overprinted All kinds of moiré are produced, so the moire phenomenon usually appears in the mode of the amplitude modulation screening. If proper measures are taken, moire can also be avoided. Usually there are 3 methods: (1) The yellow plate is set at 90°, and the screen angle difference between the strong-color plates of magenta, cyan, and black is not less than 30°; (2) When a light-colored version is available, the light-colored ink must not be darkened; (3) If a printed matter is used as a manuscript, the screen angle selected for each color version must be offset from the corresponding screen angle on the original document, or be descreened during scanning. First, the moiré produced by monochrome printing Most people think that moiré can be produced only in the process of color reproduction. In fact, this idea is a bit one-sided, and sometimes it may produce moiré in monochromatic printing. For example, in a screen printing, a monochrome image is printed. On a silk fabric with a certain regular distribution, if the angle of the screen angle of the image and the angle of the silk fabric just coincide with the conditions for producing the moiré pattern, after the printing is completed, the moire interference will also be generated in the human visual sense. effect. This kind of situation is often encountered in daily life. Some people wear clothes that look very uncomfortable to others. The striped feeling on the top is mixed, and it may feel dazzling. The reason is that the manufacturer is printing on clothes. The image does not take into account the moiré problem. There are two methods to avoid: if the silk fabric is passed from the screen printing machine in the direction of the grain, the angle with the direction of the printing machine cannot be smaller than the minimum angle that can generate the moiré; if the screen does not take into account the generation of the moiré Or the direction of the silk fabric can also be appropriately adjusted when printing silk fabric. Second, the main factors and control technology The production of moire almost includes all aspects of screen printing. Inaccurate operation of any one of them may produce moire. The appearance of the moire roughly includes the following aspects: the process from the manuscript to the screening film; the type of the digital film; the screening angle of the film; the screen mesh and the screen; the screening and screen angle of the film; the image and the printing Surface; ink viscosity and substrate, etc. The following will start from the main factors that affect the moiré, analyze the causes and provide suggestions for controlling the moiré. 1. The moiré produced by the manuscript The moiré produced by the characteristics of the manuscript itself mainly includes the following three conditions: 1 Some real manuscripts themselves have a strong periodicity, and when they are photographed, moire occurs. Such as: the manuscript has regular graphics, fabric patterns, shutters and so on. 2 The manuscript itself contains a dot pattern, and when electronic color separation is performed, if the screen angle and the number of screen lines are not proper, moire will be generated on the negative film. 3 After the printed original has been scanned, moire also appears. When scanning a screened image (such as a printed matter), moire is generated due to the interaction of the scan line with regularly arranged dot patterns in the original. When scanning such images, high-resolution scanning can be used to reduce the distortion of the dots during scanning, or to scan using the scanning force that maximizes the spatial frequency of the moiré. 4 Scanning the device itself may also cause moire. At present, many scanning software have the function of removing moire. To avoid moire, proper descreening should be performed during scanning. 2. Color separation film moire In color screen printing, the pixels on the C, M, K, and Y color positive images are composed of regularly arranged meshes of different sizes. Therefore, when making screens, if the angle difference changes, different interferences will occur between them. Some of these interferences are due to poor collocation of the screen angle, regular aggregation and discrepancies, dark colors in the aggregation part, and light colors in the discrete parts, resulting in the occurrence of dark and thin lines on the entire screen, affecting the color uniformity of the screen. In this case, it is necessary to re-set the angles of each color plate to ensure that the angle difference between the plates is greater than 15°. Generally, the angle difference between two plates is set to 30°. If the smoothness of the edge of each color patch itself is low, it will also affect the quality of the screen and the color reproduction during printing, resulting in a color shade pattern. At this time, the method of making the various colors of film should be changed and re-produced. In the process of plate-making, moiré can be generated during vacuuming, exposure and development. If the vacuum is not high, it will produce moiré during the printing process. In this case, first check the air-tightness of the air-exhausting device and consider whether it is necessary to use a high-powered vacuum pump or a new blanket to ensure that the film and the printing plate and the glass plate are in close contact with each other to ensure good vacuum conditions. During the exposure process, light hits the screen and moves along the wire to the negative. During the movement of light, a small amount of UV energy is released, causing halation in some areas of the pattern, resulting in excess exposure, resulting in moire. To eliminate halation, we must first maintain a good vacuum environment; followed by the use of orange screen; Third, use high-intensity UV lamps and exposure time as short as possible. During the development process, fine dots of 80μm or less, under the obstruction of the screen, some fine dots will not be completely washed, resulting in inconsistent moiré in the highlight region. In this case, high pressure, low flow nozzles can be used to flush the screen.
3. Moire produced during plate making