Brushing is one of the problems often encountered in gravure printing, which involves many factors such as gravure printing machine, plate cylinder, doctor blade, printing environment, ink, etc. 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. The drawing phenomenon of black, cyan, and magenta is more obvious, and it usually 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.
First, the drawing of the shallow net position
1. Eliminate the phenomenon of drawing from the source of design and plate making
Fully consider printability in manuscript design and plate making. Be sure to consider the printing direction and try to avoid printing from light to dark; black and magenta versions are most prone to wire drawing on the shallow net, especially the black version. Due to the coarse carbon particles in the ink, when the black net is very shallow, a dot It can hardly fit a carbon particle, so it is more likely to be brushed. 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. , Affecting the quality of printing.
(2) The flatness of the doctor blade. When installing the squeegee, the blade should be flattened, and the screws should be tightened from the middle to the two sides in sequence, not in one step. It needs to be repeated two or three times to ensure that the blade is evenly stressed.
(3) The 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 have to 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 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 blending inks, on the premise of not affecting other patterns on the same plate, consider using slow-drying solvents, and do not add too much auxiliary solvents 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.
Second, the drawing of the solid color block from the knife position
The drawing of the knife position of the solid color block is represented by the lifting of the ink blade at the edge of the solid color block, resulting in a dotted line at the edge of the printed color block, usually a few millimeters of short wire. The main reason is that the ink viscosity is too large and the ink There are too large pigment particles caused by crystallization. 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 in the printing blank is characterized by irregular ink channels that are sometimes absent and long or short, usually thicker. The causes and solutions are as follows.
1. Foreign objects stick to the blade edge or under the blade. The squeegee can be moved back and forth, up and down during the printing process, and the sticky foreign objects can be removed or directly removed on the squeegee blade with a bamboo stick (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, due to the lack of good circulation, the resin precipitates, forming crystals and agglomerates of different sizes, or excessive drying causes the formation of ink skin on the surface (in summer, ink skin is easy to produce, 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 slightly 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 new ink should be filtered. Generally, a 140 mesh / inch filter screen is 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.
3. Other forms of drawing
1. Drawing phenomenon caused by printing plate
(1) Surface treatment of printing plate. Now most of the gravure uses electronic engraving. During the engraving process, many small burrs will form on the edge of the engraved mesh. These small burrs will also damage the squeegee during the printing process and cause drawing. But this brushing is limited to the part of the printing plate with dots, and will not extend to the blank space. Generally, the new version is prone to this situation. Within a certain range, as the printing volume increases, the drawing will gradually decrease. This requires that in the process of making the plate cylinder, on the one hand, it is necessary to ensure that the hardness of the engraved copper layer is moderate, to reduce the formation of small burrs during the engraving process, and the copper layer must be treated after engraving; The polishing process includes the type of abrasive belt used, the speed of the plate cylinder, the swing frequency and movement speed of the polishing head, and the polishing time. If the drawing phenomenon occurs during the printing process, you can also use 2 # metallographic sandpaper or 600 # water sandpaper or a special nylon polishing net to polish and polish the surface of the printing plate, which can generally solve the drawing problem.
(2) Installation of printing plate. If the printing plate beats greatly in the circumferential direction during printing, it may also cause drawing. This requires checking the concentricity of the printing plate and the plate shaft (the center of the shaftless version is to be checked), whether the bearings and transmission parts are deformed, worn, or even the machine base is level.
2. Drawing phenomenon caused by the environment
The environment of the printing site must be kept clean. If the operating environment is dusty, it may be mixed into the ink, resulting in drawing. Therefore, as much as possible to reduce personnel movement at the printing operation site, do a good job of sanitation and cleaning work, and qualified companies can isolate the printing unit, install air conditioning facilities and dust removal equipment.
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