In the gravure printing process, the print knife line is a relatively common fault. Many of them appeared in the blank part of the printed matter because the squeegee failed to scrape off the ink on the blank space of the layout, and was easy to check and find. Some knife lines appeared in the graphic part of the printed matter. This situation is relatively rare because the squeegee excessively scrapes the ink somewhere in the picture and text, causing the amount of ink at the place to be less than that of other places, resulting in the ink trace becoming imaginary. The ink color is slightly lighter than the printed image (hereinafter referred to as the dark line). When the large solid color patch is printed, this kind of knife line appears relatively more and is more obvious. The knife line can be generally divided into two types: one is a fixed, regular knife line; the other is an intermittent, irregular knife line.
Paper factor
In the papermaking process, additives such as fillers are usually added to improve the smoothness of the paper surface. If the filling effect of the filler is poor, the filler on the surface of the paper will fall off when the paper is moved or subjected to stress. Secondly, most of the papermaking uses recycled fiber raw materials, which have short fibers and brittleness. In addition, the sizing process of papermaking is insufficient, and the paper is subject to the effects of embossing force and ink stickiness, and the paper will appear fluffing and powder dropping. The detached fillers, paper and paper dust adhere to the surface of the printing plate. As the printing plate rotates, it may accumulate on the squeegee or mix in the ink. When these substances adhere to the squeegee, the squeegee is affected. The effect of squeegeeing results in an unclean squeegee and a knife line.
This knife line is usually thick and is not fixed in position and moves back and forth along the platen as the squeegee moves.
When this happens, soft and hard materials, such as bamboo chips, can be used to carefully scrape the foreign matter attached to the knife edge. The knife line can be eliminated and the plate will not be damaged. If it fails to eliminate the knife line, it can only stop, wipe the knife blade clean and then start printing. If the knife line still appears more frequently, you need to filter the ink, or replace the paper with better quality.
Ink factor
1. Effect of ink viscosity
The ink viscosity should be matched with the printing speed. In the high-speed printing, the viscosity of the ink is controlled between 12-15 seconds (3# Zahn Cup); when printing at medium speed, the viscosity of the ink should be controlled in 16-18 seconds (3#) Zahn Cup) between. Ink viscosity is too high, the adhesion of the plate cylinder is too strong, is not conducive to the squeegee to scrape the excess ink on the layout, the probability of the occurrence of a knife line on the print is high. During the printing process, the viscosity of the ink should be measured and controlled at regular intervals, and it is usually measured every 20-30 minutes.
2. Effect of ink dryness
The ink dries too fast and is prone to crusting. If the crust is not dissolved during the ink circulation, it will adhere to the knife edge or blade as soon as it contacts the squeegee, resulting in a printed knife line. If the ink circulation system uses an open ink tank, ink skinning and agglomeration are more likely to occur. In order to reduce the chance of occurrence of such a knife line failure, it is necessary to constantly mix the ink in the ink tank; add an appropriate amount of slow-drying agent as required; if necessary, install a sealing cover, which can effectively control and reduce the failure of the print knife line.
3. Effect of ink fineness
Gravure ink is mainly composed of pigments, binders, auxiliaries and other components. If the pigment is not sufficiently ground, the particles are coarse, and they are insoluble in the binder and the solvent, and they remain on the squeegee blade during printing, resulting in fine and irregular cutting lines. The fineness of the pigment particles has a direct influence on the frequency of occurrence of the knife line. For example, the pigment particles used in the blue ink have a high hardness, and even if they are sufficiently ground, it is difficult to achieve a very fine particle size and the ink is easily damaged during the printing process. Knife or affect the squeegee effect, knife line failure often occurs when printing blue ink.
4. Effect of ink activity
In the ink production process, a certain amount of active agent is generally added. The plate cylinder has a certain activity after chrome plating. If the activity of the ink and the plate cylinder is too high, the active material will adhere firmly to the cylinder during printing, and the squeegee is also difficult to scrape clean, thus forming a knife. line. At this time, the plate cylinder is first cleaned, the active material on its surface is removed, and an active slowing agent is added to the ink to eliminate the knife line.
5. Effect of ink quality
Ink storage time is too long, the solvent balance changes, resulting in reduced ink solvency; or ink deployment, improper use, cause chemical reactions, will make the ink produce precipitation. During the printing process, the deposits in the ink are liable to accumulate on the squeegee blade to cause a knife line and the ink needs to be replaced.
Plate factors
1. Effect of layout finish
Most gravure printing plates are currently produced using electronic engraving process. In the process of electric engraving, burrs occur at the edge of the cell site. If the burr is not polished during polishing, the burr area on the plate will not be easily scratched during printing. In severe cases, the squeegee may be damaged, thus forming a knife line. For the printed knife line caused by the rough surface of the printing plate, the printing plate can be lightly polished with 1200-grit fine sandpaper, but it is generally better not to polish the printing plate so as not to damage the printing plate.
2. Chromium hardness effect
In plate making, if the chrome plating time is too long, the hardness of the chrome layer on the surface of the printing plate is too high, and the squeegee may be easily damaged, and an indefinite cutting line may appear on the printed matter. On the contrary, if the chrome plating time is too short, the chrome layer on the surface of the printing plate is too soft, and the squeegee will easily damage the printing plate, and a fixed cutting line will appear on the printed matter.
3. Effect of layout cleanliness
The debris generated during the plate-making grinding process is not completely removed and adhered to the plate. During the printing process, the plate and the squeegee are damaged, thereby generating a cutting line. Before the new version is printed on the machine, it is best to use a solvent to thoroughly clean it once again to remove oil stains and impurities, so as to reduce the occurrence of print cutter failures.
4. Influence of machining accuracy
The difference in machining accuracy of the plate cylinder can also lead to the failure of the print line, and it is difficult to detect or difficult to check. Therefore, it is easily overlooked. In actual operation, it can be checked on the machine. In addition, the installation accuracy of the printing plate should also be strictly controlled so as to avoid excessive occurrence of errors due to concentricity errors and excessive impact on the squeegee during high-speed printing.
other factors
1. The effect of the squeegee
The hardness of the squeegee is lower than the hardness of the chromium layer on the surface of the printing plate. If the hardness of the squeegee is too high, it will accelerate the wear of the plate and cause scratches: If the quality of the squeegee is poor, it will wear out in a short period of time, resulting in the ink of the layout can not be scraped clean, there is a knife line At this time, frequent adjustments and tool changes are required.
When the gap of the squeegee blade is damaged, there will be regular left and right moving blade lines, which are easier to identify. If the gap on the squeegee is fine, it can be polished with whetstone or fine sandpaper until it is smooth and smooth. If the gap is large, a new squeegee needs to be replaced. When replacing a new knife, the accumulated ink of the pressing blade and the knife groove must be thoroughly cleaned to avoid affecting the smoothness of the squeegee blade installation, and the length of the squeegee blade that exposes the pressure blade part must be the same, about 5 mm. During the printing process, some squeegee blades need to run in conjunction with the plate for a long time to achieve a good squeegee effect. At this time, the squeegee blade can be smoothed with metallographic sandpaper so that the squeegee can reach the squeegee. Better squeegee effect, and help improve production efficiency.
Second, the poor flatness of the squeegee installation can also result in a printed knife line. Therefore, when installing the squeegee, it is necessary to ensure smoothness and evenness of the squeegee. In addition, the pressure and angle of the squeegee knife also have a great influence on the print knife line. When printing, according to the degree of squeegee on the layout and the inking condition of the printed matter, the angle and pressure of the squeegee should be properly adjusted so as to achieve a good squeegee effect, thereby reducing the occurrence of print knife line failure.
It is suitable to use self-sharpening squeegee with close material and moderate hardness. The operation should pay attention to grasp the angle and pressure of the squeegee, so that not only can the ink be scraped clean, but also can reduce the wear of the squeegee on the plate and the wear of the squeegee itself, thereby increasing the plate and squeegee The life span reduces the production of knife lines.
2. Effect of solvents
In the printing process, solvents are often used to adjust the viscosity and dryness of the ink. In actual operation, the solubility should be as strong as possible. High volatilization solvents such as ethyl acetate and isopropanol. Different types of inks should use different kinds of solvents. If the solvent is used improperly, the ink will not dissolve sufficiently and will stick together. Once it adheres to the squeegee, it will form a knife line.
This knife line is usually dark, sometimes not. The use of gold ink is more common when printing solid blocks. Since gold ink is made of gold powder, gold transfer oil and solvents, the metal particles are heavy and easy to precipitate. If the solvent used has poor solubility, stirring is not necessary. Full, metal particles can accumulate due to uneven distribution. If it stays at the edge of the squeegee, it will form a thick dark line. The characteristic of this dark line is that it disappears after a period of time and then it forms again, which seriously affects the quality of the printed matter. This type of knife line is difficult to eliminate in the printing, need to start from the roots of preparation work before printing and down time, deployment of ink using the appropriate solvent, fully stir to fully dissolve, keep the ink during printing a good cycle.
Source: Longjiang Printing Network