Five factors in the gravure printing that cause the fault of the printed cutter line
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In the gravure printing process, the print cut line is a relatively common fault. It appears in the blank part of the printed matter because the squeegee fails to scrape the ink in the blank space of the printing plate, and it is easy to check and find. Some knives appear in the graphic part of the printed matter. This situation is rare because the squeegee scrapes off the ink somewhere in the graphic, so that the amount of ink in the place is less than other positions, so that the ink is faint. The ink color is slightly lighter than the printed graphic (hereinafter referred to as the dark line). When the large solid color block is printed, the cut line appears relatively more, and is more obvious. Knife lines can generally be divided into two types: one is a fixed, regular cut line; the other is an intermittent, irregular cut line.
First, the paper factor
In the papermaking process, in order to improve the smoothness of the surface of the paper, it is usually necessary to add an auxiliary agent such as a filler. If the filling effect of the filler is poor, the filler on the surface will fall off when the paper is moving or stressed. Secondly, most of the papermaking uses recycled fiber raw materials, the fibers are short and brittle, and the papermaking sizing process is insufficient. When printing, the printing force and the viscosity of the ink will cause the paper to fluff and fall off. The detached filler, paper wool, and paper powder adhere to the surface of the printing plate, and may accumulate on the squeegee or be mixed into the ink as the printing plate rotates. When these substances adhere to the squeegee blade, the squeegee is affected. The effect of scraping the ink causes the ink to be wiped out to form a cut line.
Such cut lines are generally thick and have a non-fixed position and move back and forth along the lateral direction of the layout as the doctor blade moves.
In this case, the soft and hard material, such as bamboo, can be used to carefully scrape the foreign matter attached to the knife edge, and the knife line can be eliminated without damaging the printing plate. If the knife line is not removed, it can only be stopped. After wiping the knife blade clean, it will start printing. If the knife line still appears frequently, you need to filter the ink or replace the paper with better quality.
Second, the ink factor
1. Effect of ink viscosity
The viscosity of the ink should match the printing speed. When printing at high speed, the viscosity of the ink is controlled between 12 and 15 seconds (3#Cahn cup). For medium speed printing, the viscosity of the ink should be controlled at 16 to 18 seconds (3#) Between the Chaen Cup). The viscosity of the ink is too high, and the adhesion to the plate cylinder is too strong, which is not conducive to scraping the excess ink of the plate by the squeegee, and the probability of the stencil being printed is high. During the printing process, the viscosity of the ink should be measured and controlled regularly, usually measured every 20-30 minutes.
2. Effect of ink drying
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 edge of the knife or the bottom of the knife once it comes into contact with the squeegee to produce a print line. If the ink circulation system uses an open ink tank, ink crusts and agglomeration are more likely to occur. In order to reduce the occurrence of such cutter line failure, 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 failure of the print line.
3. Ink fineness
Gravure inks are mainly composed of pigments, binders, additives and other components. If the pigment is not sufficiently ground, the particles are coarse, and it is insoluble in the binder and solvent, and stays at the edge of the doctor blade during printing, resulting in a fine, irregular cut of the print. The fineness of the pigment particles has a direct influence on the frequency of occurrence of the cut line. For example, the pigment particles used in the blue ink have high hardness, and it is difficult to achieve very fine particle size even after sufficient grinding, and it is easy to damage the scratching during the printing process. The knife or the effect of the ink scraping often causes a knife line failure 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 roller during printing, and the squeegee is difficult to scrape it to form a knife. line. At this time, the plate cylinder is first cleaned to remove the active material on the surface, and an activity slowing agent is added to the ink to eliminate the cut line.
5. Effect of ink quality
The ink storage time is too long, the solvent balance changes, causing the ink to dissolve, or the ink is mismatched and used improperly, causing a chemical reaction, which will cause the ink to precipitate. During the printing process, the precipitate in the ink tends to accumulate at the doctor blade to cause the knife line, and the ink needs to be replaced.
Third, the printing factor
1. Impact of layout finish
Gravure printing plates are mostly made by electronic engraving and plate making process. During the electric engraving process, the edges of the cells will produce burrs. If the burr is not cleaned during polishing, during the printing process, the ink on the surface of the burr is not easily scraped, and in severe cases, the squeegee is damaged, thereby forming a cut line. For printed stencils caused by rough surface of the printing plate, the printing plate can be lightly polished with 1200 mesh fine sandpaper, but it is generally best not to polish the printing plate to avoid damage to the printing plate.
2. Effect of chromium layer hardness
When 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 it is easy to damage the squeegee, and the stencil will be changed 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 is likely to damage the printing plate, and a fixed cutting line appears on the printed matter.
3. Influence of layout cleanliness
The chips generated by the plate grinding process are not completely removed and adhere to the printing plate, which may damage the printing plate and the doctor blade during the printing process, thereby generating a cutting line. Before printing on the new version, it is best to clean it thoroughly with solvent to remove oil and impurities to reduce the occurrence of print line faults.
4. Influence of machining accuracy
Poor machining accuracy of the plate cylinder can also cause the print cutter line to be faulty, and it is difficult to detect or difficult to 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 be strictly controlled to avoid the excessive impact on the squeegee during high-speed printing due to the concentricity error exceeding the standard, which may cause the failure of the printed stencil.
Fourth, the impact of the squeegee
The hardness of the squeegee is lower than the hardness of the chrome layer on the surface of the plate. If the hardness of the squeegee is too high, it will accelerate the wear of the printing plate and cause scratches: if the squeegee is of poor quality, it will wear out in a short period of time, resulting in the ink of the printing plate not being cleaned and the stencil line appearing. At this time, it is necessary to frequently adjust the knife and change the knife.
When the squeegee blade is damaged and there is a gap, a regular knife-and-left movement line appears on the printed matter, and the knife line is relatively easy to recognize. If the gap on the squeegee is relatively small, it can be polished to smooth and smooth with oil stone or fine sandpaper. If the gap is large, a new squeegee needs to be replaced. When replacing a new knife, the ink accumulated in the pressure blade and the sipe must be completely removed, so as not to affect the flatness of the squeegee installation, and the length of the squeegee exposed to the pressure blade portion should be consistent, about 5 mm. In the printing process, some squeegees need to be lapped with the plate for a long time to achieve a good squeegee effect. At this time, the edge of the squeegee can be smoothed with metallographic sandpaper to make the squeegee reach Better ink squeegee effect and increase production efficiency.
Secondly, the flatness of the squeegee installation will also cause the print line. Therefore, when installing the squeegee, make sure that the flatness is good and the squeegee is evenly stressed. In addition, the pressure and angle of the doctor blade also have a large effect on the print line. At the time of printing, the angle and pressure of the squeegee should be appropriately adjusted according to the degree of squeegee of the layout and the inking of the printed matter, so as to achieve a good squeegee effect, thereby reducing the occurrence of print stencil failure.
It is suitable to use a self-shaping squeegee with tight material and moderate hardness. In operation, pay attention to mastering the angle and pressure of the squeegee, so that the ink can be scraped cleanly, and the wear of the squeegee on the printing plate and the wear of the squeegee itself can be alleviated, thereby improving the printing plate and the squeegee. The life span reduces the generation of cut lines.
Fifth, the impact of solvents
Solvents are often used to adjust the viscosity and dryness of the ink during the printing process. In practice, the solubility should be as strong as possible. A solvent that evaporates quickly, such as ethyl acetate, isopropanol, and the like. Different types of inks should use different types of solvents. If the solvent is used improperly, the ink is not dissolved sufficiently and sticks together. Once adhered to the doctor blade, a cut line is formed.
This cut line is generally a dark line, sometimes not. It is more common to use gold ink to print solid color blocks. Since gold ink is prepared by using gold powder, gold oil and solvent, the metal particles are relatively heavy and easy to precipitate. If the solvent used is poor in solubility, plus stirring. Sufficient, metal particles will accumulate due to uneven distribution. If it is stuck at the edge of the doctor blade, a thicker dark line is formed. The characteristic of this dark line is that it disappears automatically after a period of time, and then forms again, which seriously affects the quality of the printed matter. Such cut lines are difficult to eliminate in printing, and it is necessary to start from the root cause to prepare for the work before printing. When the ink is prepared, a suitable solvent is used, and the mixture is fully stirred to fully dissolve, and the ink is kept well in circulation.