Gravure Printing Common Problems

Drawing is one of the most frequently encountered problems in gravure printing. It involves many factors such as gravure printing machine, plate cylinder, doctor blade, printing environment, ink, etc. Improper handling will affect production and quality. Drawing, also known as drawing, starting line, when the ink performance and ink color is different, its performance is also different, the general black, blue, magenta drawing phenomenon is more obvious, and generally appear in a large area of ​​the shallow network. The common wire drawing is mainly divided into three types: drawing in the shallow position, drawing in the squeegee in the solid color, and drawing in the blank in the printing blank.

First, shallow wire position drawing

1. From the origin of the design plate to prevent wire drawing

Take full account of printability in manuscript design and plate making. Must take into account the printing direction, try to avoid printing from light to dark; black, magenta version of the most likely to appear shallow online drawing, especially the black version, due to the coarse carbon particles in the ink, when the black network is very shallow, a network of points Almost no carbon particles can be loaded, so drawing is more likely to occur. Therefore, a large area of ​​black shallow net should be set as a special gray version, reducing the difficulty of printing and reject rate.

2. Correct use of squeegee to avoid drawing

(1) The selection of the squeegee. The surface hardness of the chrome plated roller is HV850～1100. The hardness of the squeegee should be lower than the hardness of the plate. About HV600, the hardness is too low. The ink scraping is not clean. If the hardness is too high, the plate wear will be accelerated and a tool mark will be generated. Affect the quality of printed products.

(2) The flatness of the squeegee. When the squeegee is installed, the blade should be flattened first, and then the screws should be tightened from the middle to the two sides in order. It cannot be reached in one step. It needs to be repeated two or three times to ensure that the blade is evenly loaded.

(3) The sharpness of the squeegee. The 600# water sandpaper or 2# metallographic sandpaper can be used to grind the squeegee, or use a special grinder to grind the back of the squeegee into a bevel of about 25°, even if a new knife is used.

(4) Squeegee angle. The squeegee angle is smaller than normal when printing, and at the same time, the distance between the squeegee and the plate contact point and the imprinting point is shortened.

3. Reasonable matching ink to prevent wire drawing

(1) When the screen has a large area of ​​shallow net or level transition, good quality ink should be used to prevent the ink pigment particles from being too thick to cause wire drawing. Usually the fineness of the ink is 15μm, too coarse ink particles to the squeegee The printing plate has a lot of damage.

(2) When dispensing ink, under the premise of no influence on other patterns on the same plate, slow-drying solvents should be considered. Do not add too much auxiliary solvents such as ethyl acetate to prevent the ink from changing. print.

(3) Ensure that the ink viscosity is moderate and stable. The generation of drawing has a direct relationship with the viscosity of the ink. In general, the higher the viscosity of the ink, the easier it is to produce a knife line during printing. In addition, due to the different performance of each brand of ink, the amount of solvent added is not the same, the ink can be mixed with reference to the specific data provided by the manufacturer, and combined with their own printing conditions and the printed product for specific operations.

Second, the field color block knife drawing

The drawing performance of the real-color color block is represented by the fact that the edge of the solid color block is raised by the squeegee and the edge of the printing color block is dotted. Generally, the short line is several millimeters. The main reason is that the ink viscosity is too high and the ink There is excessive crystallization of pigment particles. The solution is as follows.

1. Filter the ink and add a certain amount of new ink during use.

2. Adjust the squeegee angle slightly, and shorten the distance between the squeegee blade's point of contact with the plate and the embossed point.

Irregular drawing of blank spaces in printed blanks Random drawing of blank spaces is sometimes irregular, often short, and often irregular. The causes and solutions are as follows.

1. There is foreign matter on the edge of the squeegee blade or under the knife. It is possible to move the squeegee forward and backward and up and down during the printing process to remove the sticking foreign matter or remove it with a bamboo stick directly on the squeegee blade (must be completed by a skilled operator to avoid unnecessary loss due to improper operation) If necessary, stop the squeegee thoroughly.

2. Insufficient ink circulation. Part of the ink in the ink tank dead space, due to not get a good circulation caused by resin precipitation, the formation of different sizes of crystals and agglomeration, or excessive drying caused the formation of the surface of the ink (summer easy to produce ink, spring and autumn because the temperature There are many cases of ink crystallization and agglomeration. As the ink is circulated, a part of the ink is crystallized or the plate roller is rotated to the blade edge, and the squeegee breaks it to form a random wire drawing. Therefore, improvements must be made in both the circulation and filtration of the ink.

(1) Cycle improvement. The first method is to tilt the ink tank to the left or 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. In the second method, the ink tank is laid flat, but the ink must be injected from both ends simultaneously. In the middle of the reflow, it can also form a very good cycle, can not be injected from the single side of the ink.

(2) Filter improvements. Before the ink is used, it must be filtered for the first time. Even the new ink must be filtered. A filter of 140 mesh/inch is generally used. The filtration work of the ink circulating in the ink tank is more important. It can well filter out the resin crystals and foreign impurities formed during the use of the ink. The mesh selected should be finer. The method is to set a stocking under the ink fountain. , Form a filter bucket, after filtering for a period of time, if the filter is blocked, ink flow is not smooth, you can insert some holes in the top of the bucket with a bamboo stick to increase the ink-conducting effect. The lower part of the bucket can also be used. The role of good deposition of impurities is shown in Figure 3. Some large printing plants in Shanghai and Guangdong have used this method to control intermittent ink drawing problems.

3. Ink fatigue. This situation occurs mostly 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 will not be replenished, and the proportion of each component of the ink will change, thus causing ink fatigue and affecting normal use. Appropriately adding new ink will solve the problem.

Third, other forms of drawing

1. The phenomenon caused by the printing plate

(1) Surface treatment of printing plates. Gravure is now mostly electronically engraved. During the carving process, many small burrs are formed on the edge of the engraved mesh cavity. These small burrs also damage the squeegee during the printing process, causing the wire drawing. However, this type of drawing is limited to the dots on the printing plate and will not extend into the blank space. The new version is prone to this situation. Within a certain range, with the increase in the number of prints, drawing will gradually decrease. This requires that in the production process of the plate cylinder, on the one hand, the hardness of the engraved copper layer should be ensured, the formation of small burrs in the engraving process should be reduced, and the copper layer should be processed after the engraving; on the other hand, a standardized chromium layer should be established. The polishing process includes the type of belt used, the rotation speed of the platen, the frequency and speed of the polishing head, the polishing time, and the like. If the wire drawing phenomenon occurs in the printing process, you can also use 2# metallographic sandpaper or 600# water sandpaper or a dedicated nylon polishing net to grind and polish the surface of the printing plate, which generally solves the drawing problem.

(2) Installation of printing plates. If the printing plate runs large during the printing process, it may also cause wire drawing. This requires checking the concentricity of the printing plate and the printing plate shaft (no shaft mounting plate to check the top), checking whether the bearing and transmission parts are deformed, worn, or even the machine base is horizontal.

2. Wire drawing caused by the environment

The printing site environment must be kept clean. If there is dust in the operating environment, it may be mixed into the ink, resulting in drawing. Therefore, in the printing operation site, the number of people should be reduced as much as possible, and commuting should be performed in a clean and hygienic manner. Conditional companies can isolate the printing units, install air-conditioning facilities and dust removal equipment.