In the process of gravure printing, the knife line is a common fault. Most of them appear in the blank part of the printed matter, because the squeegee fails to scrape the ink off the blank of the layout, and it is easy to dry check and find. Some of the knife lines appear in the graphic part of the printed matter. This situation is relatively rare. It is because the squeegee scrapes off the ink in the image and text too much, so that the amount of ink in this place is less than that in other places, so that the ink is faint The ink color is slightly lighter than the printed graphics (hereinafter referred to as the dark line). When printing large solid color patches, this kind of knife line appears relatively more and is more obvious. The knife line can be generally divided into two types: one is fixed and regular knife line; the other is intermittent and irregular knife line.
Paper factor
In the papermaking process, in order to improve the smoothness of the paper surface, additives such as fillers are usually added. If the filling effect of the filler is poor, the filler on the surface will fall off when the paper is in motion or under stress. Secondly, most of the papermaking uses recycled fiber raw materials. The fibers are short and brittle. In addition, the papermaking sizing process is insufficient. During printing, the paper will be fluffed and powdered due to the embossing force and the viscosity of the ink. The falling filler, paper wool, and paper powder adhere to the surface of the printing plate. As the printing plate rotates, it may accumulate on the squeegee or be mixed into the ink. When these substances adhere to the squeegee, it affects the squeegee. Ink scraping effect, resulting in dirty ink scraping, forming a knife line.
This kind of blade line is usually thick, and the position is not fixed. With the squeegee of the doctor blade, it moves back and forth along the lateral direction of the layout.
In this case, use soft and hard materials, such as bamboo, to carefully scrape off the foreign objects attached to the edge of the knife. The knife line can generally be eliminated without damaging the printing plate. If you can not eliminate the knife line, you can only stop the machine, wipe the blade of the squeegee clean, and then start printing. If the knife line still occurs frequently, you need to filter the ink or replace the paper with better quality.
Ink factor
1. Influence of ink viscosity
The viscosity of the ink should match the printing speed. When printing at high speed, the viscosity of the ink should be controlled between 12 and 15 seconds (3 # Zahn Cup); when printing at medium speed, the viscosity of the ink should be controlled between 16 and 18 seconds (3 # Zahn Cup). The viscosity of the ink is too high, and the adhesion to the plate cylinder is too strong, which is not conducive to the scraping knife to scrape off the excess ink on the plate, and the probability of a knife line on the printed product is high. During the printing process, the ink viscosity should be measured and controlled at regular intervals, generally every 20-30 minutes.
2. Influence of ink drying
The ink dries too quickly and is prone to skinning. If the skinning is not dissolved during the ink circulation process, it will stick to the blade or the bottom of the blade once it contacts the squeegee, resulting in a print line. If the ink circulation system uses an open ink tank, ink caking and caking are more likely to occur. In order to reduce the occurrence of such knife line faults, the ink in the ink tank should be stirred frequently; an appropriate amount of slow-drying agent should be added as needed; if necessary, a sealing cover can be added to effectively control and reduce the printing knife line faults.
3. Influence of ink fineness
Gravure ink is mainly composed of pigments, binders, additives and other components. If the pigment is not sufficiently ground, the particles are coarse, and they are insoluble in the binder and solvent, and stay at the squeegee blade during printing, it will cause small, irregular knife lines in the print. The fineness of the pigment particles has a direct impact on the appearance of the knife line. For example, the pigment particles used in blue ink have high hardness, and even after sufficient grinding, it is difficult to achieve a very fine particle size, and it is easy to damage the ink during the printing process The knife may affect the effect of scraping ink, and the knife line often fails when printing blue ink.
4. Influence of ink activity
In the production process of ink, a certain amount of active agent is generally added. The plate cylinder also has a certain activity after chrome plating. If the activity of the ink and the plate cylinder is too high, the active substance will firmly adhere to the cylinder during printing, and it is difficult for the doctor blade to scrape it off, thus forming a knife line. At this time, first clean the plate cylinder to remove the active substances on the surface, and add an active retarder to the ink to eliminate the knife line.
5. Influence of ink quality
If the ink storage time is too long, the solvent balance will change, resulting in a decrease in the solubility of the ink; or the ink is improperly formulated and used, causing a chemical reaction, which will cause the ink to precipitate. In the printing process, the deposits in the ink are easy to accumulate in the blade of the doctor blade and cause the knife line, so the ink needs to be replaced.
Plate factors
1. Effect of layout finish
Most of the gravure printing plates are made by electronic engraving and plate-making technology. During the process of electric engraving, burrs will be generated on the edges of the net holes. If the burrs are not polished during the polishing process, the ink with burrs on the plate is not easy to be cleaned during the printing process. In serious cases, the squeegee will be damaged, thereby forming a knife line. For the cutting line of the printed product due to the rough surface of the printing plate, you can use 1200 mesh fine sandpaper to gently polish the printing plate, but in general, it is best not to polish the printing plate, so as not to damage the printing plate.
2. Effect of chromium layer hardness
When making a plate, if the chrome plating time is too long, the hardness of the chromium layer on the surface of the printing plate is too high, which is easy to damage the doctor blade, and the indefinite knife line will appear on the printed product. On the contrary, if the chrome plating time is too short, the chromium layer on the surface of the printing plate is too soft, the squeegee is easy to damage the printing plate, and a fixed knife line will appear on the printed product.
3. The impact of the cleanliness of the layout
The debris generated in the grinding process of the plate-making process is not completely removed and adheres to the printing plate. During the printing process, the printing plate and the doctor blade will be damaged, resulting in a knife line. Before the new version is printed on the machine, it is best to clean it thoroughly with solvent to remove oil and impurities to reduce the occurrence of faults in the printed product.
4. Influence of machining accuracy
The poor machining accuracy of the printing plate cylinder will also lead to the failure of the printed product knife line, and it is difficult to detect or difficult to detect, so it is easy to be ignored, and it can be checked on the machine during actual operation. In addition, the installation accuracy of the printing plate should be strictly controlled to avoid excessive impact on the squeegee during high-speed printing due to concentricity error exceeding the standard, resulting in the occurrence of faults in the cutting line of the printed product.
other factors
1. Impact of squeegee
The hardness of the doctor blade is lower than the hardness of the chrome layer on the surface of the printing plate. If the hardness of the squeegee is too high, it will accelerate the wear of the printing plate and produce scratches: if the squeegee is of poor quality, it will wear out in a short period of time, resulting in the ink on the plate surface not being cleaned, and a knife line appears At this time, frequent tool adjustments and tool changes are required.
When the squeegee blade is damaged and there is a gap, there will be a regular line of left and right movement on the printed matter, which is easier to identify. If the gap on the doctor blade is relatively fine, it can be polished with oilstone or fine sandpaper until smooth and smooth. If the gap is large, a new doctor blade needs to be replaced. When replacing a new knife, the accumulated ink on the pressure blade and the knife groove must be completely cleaned to avoid affecting the flatness of the installation of the doctor blade, and the length of the blade exposed to the pressure blade should be the same, about 5mm. In the printing process, some squeegee blades need to run in with the printing plate for a long time to achieve a good squeegee effect. At this time, metallographic sandpaper can be used to smooth the edge of the squeegee blade, so that the squeegee blade can reach Better ink scraping effect, and is conducive to the improvement of production efficiency.
Secondly, the poor flatness of the installation of the doctor blade will also cause the knife line of the printed matter. Therefore, when installing the doctor blade, make sure that the flatness is good, so that the doctor blade is evenly stressed. In addition, the pressure and angle of the squeegee have a great influence on the print line. During printing, the angle and pressure of the squeegee blade should be adjusted according to the degree of squeegee on the layout and the inking condition of the printed matter, so as to achieve a good squeegee effect, thereby reducing the occurrence of faults in the printed article.
It is more suitable to choose self-grinding squeegee with tight material and moderate hardness. Pay attention to the angle and pressure of the doctor blade during operation, so that it can not only wipe the ink clean, but also reduce the wear of the doctor blade on the printing plate and the wear of the doctor blade itself, thereby improving the printing plate and doctor blade The life of the tool reduces the generation of cutting lines.
2. Effect of solvent
In the printing process, solvents are often used to adjust the viscosity and dryness of the ink. In actual operation, it should be as soluble as possible. Solvents that evaporate quickly, such as ethyl acetate and isopropyl alcohol. Different types of inks should use different types of solvents. If the solvent is not used properly, the ink will not dissolve sufficiently and stick together. Once it sticks to the doctor blade, it will form a knife line.
This kind of knife line is usually a dark line, sometimes there is no. It is more common to print solid color blocks with gold ink. Since gold ink is formulated with gold powder, gold oil and solvent, the metal particles are relatively heavy and easy to precipitate. Fully, metal particles will accumulate due to uneven distribution. If it stays at the blade of the squeegee, a thick dark line will form. The characteristic of this dark line is that it disappears automatically after a certain period of time, and then forms again, which seriously affects the quality of printing. This type of knife line is difficult to eliminate in printing. It is necessary to start from the roots and work from the pre-printing preparation work. Use appropriate solvents when mixing the ink, stir it to fully dissolve, and maintain a good ink circulation during printing.
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