Factor analysis of gravure knife line failure on t

Analysis on the factors of gravure printing knife line fault on paper

in the process of gravure printing, the knife line of printed matter is a relatively common fault. It mostly appears in the blank part of the printed matter, which is caused by the failure of the doctor blade to scrape the ink in the blank of the page, and it is easy to dry, check and find. Some knife lines appear in the graphic part of the printed matter, which is relatively rare. It is because the ink scraper scrapes away the ink at one place of the graphic too much, so that the ink volume at that place is less than that at other places, so that the ink is weak, and the ink color is slightly lighter than the printed graphic (hereinafter referred to as dark lines). When printing large field color blocks, this knife line appears relatively more, and it is more obvious. Generally, the knife line can be divided into two kinds: one is fixed and regular knife line; The other is intermittent and irregular knife line

paper factor

in the process of papermaking, fillers and other additives are usually added at the bell ringing ceremony in order to improve the smoothness of the paper surface. If the filling effect of the filler is so poor, the filler on the surface of the paper will fall off when the paper is moving or under force. Secondly, recycled fiber raw materials are mostly used in papermaking, and their fibers are short and brittle. In addition, there are deficiencies in papermaking sizing process. Due to the effect of pressure printing force and ink viscosity during printing, the paper will appear fuzzing and powder dropping. The fallen fillers, paper wool and paper powder adhere to the surface of the printing plate. With the rotation of the printing plate, they may accumulate on the doctor blade or mix into the ink. When these substances adhere to the edge of the doctor blade, they affect the scraping effect of the doctor blade, resulting in dirty scraping and the formation of knife lines

this kind of knife line can comprehensively evaluate whether it conforms to national standards or relevant regulations of the industry. It is often thick and the position is not fixed. It moves back and forth horizontally along the layout with the serial movement of the doctor blade

in this case, materials with moderate hardness, such as bamboo chips, can be used to carefully scrape away the foreign matters attached to the knife edge. The knife line can generally be eliminated without damaging the printing plate. If the knife line cannot be eliminated, you can only stop the machine and wipe the ink scraping blade clean before starting the printing. If the knife line still appears frequently, you need to filter the ink or replace the paper with a better quality

ink factor

1. The influence of ink viscosity

the ink viscosity should match the printing speed. When printing at high speed, the ink viscosity should be controlled between 12 ~ 15 seconds (3 # chaen cup); When printing at medium speed, the viscosity of the ink should be controlled between 16 and 18 seconds (3# chaen cup). The ink viscosity is too high, and the adhesion to the plate cylinder is too strong, which is not conducive to the doctor blade to scrape the excess ink on the plate, and the probability of knife lines on the printed matter is high. During the printing process, the ink viscosity should be measured and controlled regularly, usually every 20-30 minutes

2. Influence of ink dryness

if the ink dries too fast, it is prone to scaling. If the scaling is not dissolved in the ink circulation process, once it comes into contact with the doctor blade, it will adhere to the edge or bottom of the blade, resulting in printed matter knife lines. If the ink circulation system adopts an open ink tank, ink scaling and caking are more likely to occur. In order to reduce the probability of such knife line failure, it is necessary to often stir the ink in the ink tank; Add an appropriate amount of slow drying agent as needed; If possible, a sealing cover can also be added, which can effectively control and reduce the knife line failure of printed matter

3. Influence of ink fineness

gravure ink is mainly composed of pigments, binders, additives and other components. If the pigment is inadequately ground, the particles are coarse, and insoluble in binders and solvents, and stays at the edge of the doctor blade during printing, small and irregular knife lines will appear on the printed matter. The fineness of pigment particles has a direct impact on the occurrence frequency of knife line. For example, the pigment particles used in blue ink have high hardness, and it is difficult to achieve a very fine particle size even after full grinding. In the printing process, it is easy to damage the doctor blade or affect the ink scraping effect. Knife line failures often occur when printing blue ink

4. Influence of ink activity

in the process of ink production, a certain amount of activator is generally added. The printing plate cylinder also has certain activity after chrome plating. If the activity of ink and printing plate cylinder is too high, the active substances will firmly adhere to the cylinder during printing, and it is difficult for the doctor blade to scrape it clean, thus forming a knife line. At this time, first clean the plate cylinder to remove the active substances on its surface, and add an active moderator to the ink to eliminate the knife line

5. Influence of ink quality

the storage time of ink is too long, and the solvent balance changes, resulting in the decrease of ink solubility; Or ink deployment, improper use, causing chemical reactions, will make ink precipitation. In the printing process, the sediment in the ink is easy to accumulate on the blade of the doctor blade, causing the knife line to be 24% higher than that of all glass fiber products, and the ink needs to be replaced

printing plate factors

1. Influence of layout smoothness

gravure printing plates are mostly made by electronic engraving plate making process at present, and burr will be generated at the edge of the hole in the process of electric engraving. If the burr is not polished clean during polishing, the ink with burr on the layout is not easy to scrape clean during the printing process, and in serious cases, the ink scraper will be damaged, thus forming a knife line. For the knife line of printed matter caused by the rough surface of the printing plate, the printing plate can be gently polished with 1200 mesh fine sandpaper, but generally, it is best not to polish the printing plate to avoid damaging the printing plate

2. Influence of chromium layer hardness

during plate making, if the chromium 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 there will be variable knife lines on the printed matter. On the contrary, if the chromium plating time is too short, the chromium layer on the surface of the printing plate is too soft, and the doctor blade is easy to damage the printing plate, there will be fixed knife lines on the printed matter

3. The impact of layout cleanliness

the debris generated in the grinding process of plate making cannot be completely removed and attached to the plate, which will damage the plate and the doctor blade during the printing process, resulting in knife lines. Before the new version is printed on the machine, it is best to thoroughly clean it with solvent to remove the oil and impurities, so as to reduce the occurrence of knife line failure of printed matter

4. Influence of machining accuracy

the poor machining accuracy of the printing plate cylinder will also lead to the failure of the printing knife line, which is difficult to detect or check, so it is easy to be ignored. In actual operation, it can be checked on the machine. In addition, the installation accuracy of the printing plate should also be strictly controlled to avoid excessive impact on the doctor blade during high-speed printing due to the concentricity error, resulting in the failure of the knife line of the printed product

other factors

1. The influence of the doctor blade

the hardness of the doctor blade is lower than that of the chromium layer on the surface of the printing plate. If the hardness of the doctor blade is too high, it will accelerate the wear of the printing plate and produce scratches: if the quality of the doctor blade is poor, it will wear out in a short time, resulting in the ink of the layout not being scraped clean and the knife line appearing. At this time, it is necessary to adjust and change the knife frequently

when the doctor blade is damaged and there is a gap, there will be regular knife lines moving left and right on the printed matter, which is easier to identify. If the notch on the doctor blade is fine, you can polish it with oilstone or fine sandpaper until it is smooth and smooth. If the gap is large, replace the doctor blade with a new one. When replacing a new knife, the ink accumulation on the pressing blade and the knife groove must be completely removed first, so as not to affect the flatness of the installation of the doctor blade, and the length of the exposed part of the doctor blade must be consistent, about 5mm. In the printing process, some doctor blades need to be run in with the printing plate for a long time before they can achieve a good ink scraping effect. At this time, the edge of the doctor blade can be polished smooth with metallographic sandpaper, so that the doctor blade can achieve a 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 print knife line. Therefore, when installing the doctor blade, ensure good flatness and uniform force on the doctor blade. In addition, the pressure and angle of the doctor blade also have a great impact on the print knife line. During printing, the angle and pressure of the doctor blade should be adjusted appropriately according to the ink scraping degree of the layout and the inking condition of the printed matter, so as to achieve a good ink scraping effect, so as to reduce the occurrence of knife line failure of the printed matter

a self sharpening doctor blade with compact material and moderate hardness is more suitable. Pay attention to the angle and pressure of the doctor blade during operation, so that the ink can be scraped clean, and the wear of the doctor blade on the printing plate and the wear of the doctor blade itself can be reduced, so as to improve the service life of the printing plate and the doctor blade and reduce the generation of knife lines

2. Influence of solvent

in the printing process, solvents are often used to adjust the viscosity and dryness of inks. During actual operation, solvents with strong solubility and fast volatilization speed should be used as much as possible, such as ethyl acetate, isopropanol, etc. Different kinds of solvents should be used for different kinds of inks. If the solvent is not used properly, the ink will not dissolve enough and stick together. Once it sticks to the doctor blade, it will form a knife line

this kind of knife line is generally dark, sometimes not. It is common to use gold ink to print field color blocks. Because the gold ink is mixed with gold powder, gold blending oil and solvent, the metal particles are relatively heavy and easy to precipitate. If the solvent used has poor solubility and insufficient stirring, the metal particles will accumulate because of uneven distribution. If it stays at the blade of the doctor blade, a thick dark line will be formed. The characteristic of this kind of dark line is that it disappears automatically after appearing for a period of time, and then forms again, which seriously affects the quality of printed matter. This kind of knife line is difficult to eliminate in printing, so it is necessary to start from the root, and work hard from the preparation before printing. When mixing ink, use appropriate solvent, fully stir to make it fully dissolved, and maintain a good circulation of ink during printing

source: Longjiang printing