Drawing is one of the problems often encountered in gravure printing, involving gravure printing machine, plate cylinder, doctor blade, printing environment. Many factors such as ink, such as improper handling, will affect production and quality.
Drawing is also called drawing and drawing. When the performance of the ink and the degree of color development of the ink are different, the performance is also different. Generally, the drawing phenomenon of black, cyan and magenta is more obvious, and it generally appears on a large area of ​​shallow net.
Common drawing is mainly divided into three types: drawing at the shallow mesh position, drawing at the starting position of the squeegee of the solid color block, and irregular drawing at the printing blank.
Drawing in shallow mesh position
1. Eliminate the phenomenon of drawing from the source of design and plate making
Fully consider printability in manuscript design and plate making. As shown in Figure 1a, the printing direction must be considered, and printing from light to dark should be avoided as far as possible; Figures 1b and 1c should be considered as a factor when designing because the printing operation is relatively difficult. Black and magenta versions are the most prone to light wire drawing, especially the black version. Due to the coarse carbon particles in the ink, when the black network is very light, almost no carbon particles can be installed in one dot, so wire drawing is more likely to occur. Therefore, a large area of ​​black and light mesh should be set as a special gray version as much as possible to reduce printing difficulty and scrap rate.
2. Use the doctor blade correctly to avoid drawing
(1) Selection of doctor blade. The surface hardness of the chrome plated cylinder is HV850 ～ 1100. The hardness of the squeegee should be lower than the hardness of the printing plate, about HV600. If the hardness is too low, the ink will not be cleaned. If the hardness is too high, it will accelerate the wear of the printing plate. , Affecting the quality of printing.
When the knife is used, the blade should be flattened, and the screws should be tightened from the middle to the two sides in sequence. It cannot be done in one step. It needs to be repeated two or three times to ensure that the blade is evenly stressed.
(3) Sharpness of the doctor blade. You can use 600 # water sandpaper or 2 # metallographic sandpaper to sharpen the squeegee blade, or use a special sharpener to grind the back of the squeegee blade to an angle of about 25 °, even if you use a new knife.
(4) Squeegee angle. The angle of the doctor blade is smaller than that of normal printing, and at the same time, the distance between the contact point of the doctor blade and the printing plate and the imprint point are shortened.
3. Reasonably select ink to prevent drawing
(1) When there is a large area of ​​shallow screen or gradation transition on the layout, better quality ink should be used to prevent the ink pigment particles from being too thick and causing wire drawing. The fineness of the ink is usually 15 μm. The damage to the printing plate is very large.
(2) When preparing ink, on the premise of not affecting other patterns on the same printing plate, consider using slow-drying solvents, and do not add too much auxiliary solvent such as ethyl acetate to prevent the ink properties from changing and preventing normal operation. print.
(3) Ensure that the viscosity of the ink is moderate and stable. The production of brushing has a direct relationship with the viscosity of the ink. Generally speaking, the higher the viscosity of the ink, the easier it is to produce knife lines during printing. In addition, because the performance of each brand of ink is different, the amount of solvent added is also different, when preparing the ink, you can refer to the specific data provided by the ink manufacturer, and combine the printing conditions and the printed product for specific operations.
Brushing the solid color block from the knife position
The drawing of the solid color block from the knife position is shown in Figure 2. It appears that the edge of the solid color block is lifted by the squeegee, resulting in a dotted line on the edge of the printed color block, usually a few millimeters of short wire. The main reason is that the viscosity of the ink is too large and the pigment particles in the ink are too large. Caused by. The solution is as follows.
1. Filter the ink and add a certain amount of new ink during use.
2. Adjust the angle of the doctor blade to be smaller, and shorten the distance between the contact point of the doctor blade and the printing plate and the stamping point.
Irregular drawing in the printing blank
The irregular drawing of the printing blank is characterized by irregular ink channels that are sometimes not available and long or short. Usually thick. The causes and solutions are as follows.
1. Foreign objects stick to the blade edge or under the blade. You can move the squeegee back and forth, up and down during the printing process, remove the adhering foreign objects or use a bamboo stick to directly remove the squeegee blade (must be completed by a skilled operator to avoid unnecessary losses due to improper operation) , If necessary, stop the machine to thoroughly clean the doctor blade.
2. Ink circulation is not enough. Part of the ink is in the dead corner of the ink tank. The resin is not precipitated due to the lack of good circulation, forming crystals and agglomerates of different sizes, or excessive drying causes ink skin to form on the surface (in summer, it is easy to produce ink skin, spring and autumn due to temperature There are many cases of ink crystallization and agglomeration). With the circulation of ink, a part of the ink crystallization or ink skin is brought to the blade by the rotating plate cylinder, and the ink blade breaks it to form irregular drawing. Therefore, improvements must be made from the aspects of ink circulation and filtration.
(1) Cycle improvement. Method one, tilt the ink tank left and right to ensure that the ink can be injected from the higher end and flow out from the lower end to form a good circulation: method two, the ink tank should be placed flat, but the ink must be injected from both ends at the same time. Reflow in the middle, so that a good circulation can also be formed, and ink cannot be injected from one side.
(2) Improved filtering. The ink must be filtered for the first time before use, even for new ink. 140 mesh / inch filter screen is generally used. The filtering work of the ink circulating in the ink tank is more important. It can filter out the resin crystals and foreign impurities formed during the use of the ink. The mesh should be finer. The method is to put a stocking under the ink outlet To form a filter sleeve. After filtering for a period of time, if the filter is blocked and the ink flow is not smooth, you can insert some small holes with a bamboo stick above the sleeve to increase the ink guiding effect. The effect of depositing impurities is very good, as shown in Figure 3. Some large printing plants in Shanghai and Guangdong use this method to control the intermittent ink drawing problem.
3. Ink fatigue. This situation often occurs when the printing area of ​​the printing plate is relatively small. During the printing process, the resin in the ink is continuously consumed. If the solvent is continuously added without adding new ink, the resin cannot be replenished, which changes the proportion of each component of the ink, causing ink fatigue and affecting normal use. The problem can be solved by adding new ink appropriately.
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