Six failure analysis of high-speed rotogravure printing
I have been engaged in the quality control of gravure printing for many years, and often visit the site where the fault occurs, and make detailed records of the process of the fault and the solutions. How did these faults occur, how to solve them, the causes, and the results? Or let's go to the scene to take a look.
The first: poor transfer of fine text printing plate ink, with obvious missing strokes
It was a summer evening, the weather was sultry, the temperature and humidity of the workshop were very high, and the relative humidity was close to 100%. In the process of handover, the paper broke suddenly and shut down. The captain of the successor picked up the paper and turned it on. After taking a sample, the black version of the small text was light in color, and the lack of pen was severe. The captain reduced the speed and asked his deputy to wipe the black version carefully. After speeding up, the captain took a few more samples and looked at it, but he was still not satisfied. It turned out that all the small texts were "blocked".
Generally speaking, the small text in the black version has a small area, and the ink consumption is also small, so no ink needs to be added for a long time. As the solubility of the volatile ink in the ink becomes worse, the plate is blocked. In addition, the downtime during the shift is too long, and the ink in the net dries out. When wiping the plate with a solvent, the solvent evaporates a lot of heat when it evaporates rapidly. The moisture in the air condenses on the printing plate, especially when the ambient temperature and humidity are not ideal. The ink is mixed with water and the dissolving ability becomes poor, so the ink has to be completely printed out, and the printing plate is thoroughly cleaned with a decontamination powder, and the plate is cleaned with a dry cloth. Re-adjust the ink and add a small amount of butyl acetate to it. After turning on the machine, the speed will be increased to more than 100 meters. The fine text in the black version is normal.
Second: Poor transfer of fine dots in the gradation printing
Whether the fine dot ink transfer is normal in the gradation printing is the key to the success or failure of printing. This is the second printed product, and the paper used is 230g / m2 white cardboard. After turning on the machine and speeding up the printing, the captain sampled and found that the fine dots of the blue version were seriously lost and there were too many white dots, and the effect of adjusting the viscosity of the shaft ink was not obvious. The captain first compared the paper used, and found that the surface of the printing paper was slightly rough. After the paper roughness reached a certain level, it was difficult for the static ink to transfer the ink in the fine mesh to the paper, so the white spots in the highlight part More. The captain then looked at the printing unit of the green plate and found that the platen roller was not an electrostatic ink leveling roller. Then the whole electrostatic ink leveling system was thoroughly inspected. It was found that the brush was agglomerated, and the brush in some places could not reach the electrostatic ink leveling roller, so he removed the brush to clean it, and checked whether the static generator was normal, replaced the non-static ink leveling roller, and replaced the paper . Everything is ready, adjust the viscosity of the ink, speed up to more than 100 meters after turning on again, the fine dot transfer is normal, and the white dots have disappeared.
Third: Why did the overprinted version suddenly "run out"?
The stability of the printing register is maintained by the tension control system and cursor tracking. The printing status of the cursor has a great influence on the stability of the overprint. In addition, printing pressure, drying temperature, ambient temperature and humidity also have a great influence on the stability of overprinting.
This is a transition period from summer to autumn. It is still very hot during the day, and the weather is cooler at night, and the humidity drops significantly after a north wind. This time the printing on the machine is a wide-format 80g / m2 coated paper, and the product is a beer label. The thinnest part of multi-position typesetting and overprinting is only half a line (that is, about 0.15 mm). After the handover, the captain found that the middle group of printing units was out of version, and hurriedly adjusted the deviation of the overprint, but the result was no response. The captain checked the unwinding tension, the winding tension and the tension between the printing units and found no abnormal tension.
Later, when I checked the cursor of the printed sample, it was found that the cursor of a printing unit in the middle was not embossed, and half of the first half of the cursor was dried, so the cursor signal read out was wrong, and the cursors that followed it appeared in turn. Wrong, this will run out of version. Finally, I checked the position of the floating roller and found that the floating roller came to the top and did not respond to the input overprint deviation. In addition, the temperature and humidity of the workshop environment have changed greatly, the paper stretch rate has changed, and the drying suitability has also changed greatly. It is normal for the overprint to be inaccurate. There are several adjustment methods: first adjust the viscosity of the ink to ensure the complete printing of the cursor, then adjust the drying temperature of each unit in turn, adjust the printing pressure and drying temperature appropriately, and spray some water on the ground to increase the humidity of the environment. After the reading cursor is normal, adjust the overprint deviation and adjust the printing speed appropriately to meet the paper stretchability requirements, so that the overprint is normal.
Fourth: Difficulties caused by changing the new version halfway
There are some hard particles on the surface of the paper. If the paper is not cleaned after being cleaned, it will sometimes be crushed when entering the printing unit, causing the plate to burst. This time it happened to be a fuchsia background version that had printed 500,000 sheets. After the new version was printed, the fuchsia background version had a large difference in printing color from the original. The ink layer was thick and the hue was obviously dark, and adjusting the ink viscosity did not work. In this way, we have to ask the technologist to adjust the ink composition, and adjust the printing hue by changing the ink color density. Generally, diluent is added to adjust the color density, and fine-tuning is added to adjust the yellow phase, magenta phase, and cyan phase in the magenta ink. The color ink can be gold red, dark red, and medium yellow. In this way, the craftsman needs to readjust the ink formula, and the original adjusted ink is all readjusted, which makes the captain's head hurt. In addition to the impact on duty output, the consumption of color readjustment will greatly increase the consumption of reversion.
Fifth: the cooling roller is dirty
The printing adaptability of gold ink is relatively good, but sometimes unexpected failures may occur if you are not careful. This failure occurs after the gold line is overprinted with yellow background. Due to the poor adhesion of the gold ink, it is easy to stick to the cooling roller. During the night shift, the captain thought that this kind of work was better for printing, and increased the printing speed. The yellow version has large ink consumption, fast ink renewal, and good ink printing adaptability. The golden ink has strong hiding power, and there is no problem of overprinting in normal overprinting. After the gold ink is superimposed, the ink layer is too thick and peeling increases. If the drying is not thorough, or the temperature of the cooling water is not suitable (higher), it will affect the adhesion of the gold ink.
From the point of view of paper gravure printing process, after printing color inks in turn, cover them with varnish. Too thick overprinting ink of the intermediate color group is likely to cause insufficient adhesion fastness, and the poor cooling effect of the gold ink directly affects its adhesion fastness. It is not easy to find when sampling the night shift. In fact, you can understand the sample by looking at the light sideways. It turns out that the cooling roller has become hot, and it is full of gold ink. The oven blowing temperature is too low, and the gold ink is not fully dried. The captain stopped and the ink on the cooling roller was completely removed, the drying temperature was adjusted, and the machine speed was adjusted to a suitable speed. The problem was solved. Some careless captains used a hard ink shovel to hurt the cooling rollers, making them more likely to get dirty.
Next, I would like to talk about the improvement of plate making and printing on the issue of "obvious difference between the axial and circumferential lines of light-colored border printing".
Almost all the frame prints have different performance in the axial and circumferential directions, but the lighter inks are more obvious when printed. There is a gold base around the border of about 2m. Regardless of the typesetting method, its circumferential and axial expressions are different. The light yellow ink along the axial direction is printed lighter and the lines appear thinner; the light yellow lines in the circumferential direction have a good wetting effect along the direction of the doctor blade, the ink transfer is normal, and the lines are obviously thicker than the axial direction. In addition, the area of ​​the printing plate supporting the doctor blade in the axial direction and the circumferential direction is also different, which causes an inevitable difference.
The preventive measures are as follows: From the design point of view, there are many cigarette pack design styles like to use some thin isometric borders, which is exactly the more difficult side of gravure expression. In fact, you can avoid such light and thin lines from the design. In terms of plate-making process, you can take advantage of the strong hiding power of gold ink to make the embossed edge more than 0.5m, which is actually equivalent to widening the thickness of the line. In this way, even if the axial imprint is not true, the gold bottom can also be covered. In terms of ink, the method of increasing the color density and reducing the viscosity of the ink can be used to improve the wettability of the ink on the printing plate and the doctor blade, so that the difference in the amount of ink transfer in the axial direction and the circumferential direction is reduced. In addition, there is another way to increase the depth of the printing plate properly, especially for thicker paper.
Finally, introduce the electrostatic fault
Generally, static electricity occurs in winter and spring. The weather is relatively dry. Static electricity accumulates when the paper running at high speed rubs against the metal roller. When the static electricity accumulates beyond a certain capacity, it discharges to the surrounding conductors. From the printed proofs, especially some thin lines produce filaments. At first glance, it seems that the squeegee is dirty. After replacing the squeegee, no good results have been seen. What is more serious is that the accumulation of electric charge on the paper is likely to make it difficult to cut and separate the sheets, and even cause traffic jams. The most serious is that the accumulation of electric charge on the paper to a certain degree will cause sparks, and if the solvent concentration reaches a certain level, it may even cause an explosion. To prevent such faults, static electricity faults can be eliminated by spraying water to the ground, adding a humidifier between the units, or using direct ground discharge.
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