Quality control in gravure printing production site notes
high speed rotary gravure printing is fast and the printing paper path is long. Once there is a fault, whether it is shutdown adjustment or startup adjustment, the waste caused is amazing. There are many reasons for the failure. First, it is affected by the physical and chemical changes of the ink; Secondly, it is affected by the wetting condition of the printing plate and the doctor blade and the change of the depth of the printing plate; Third, it is affected by the physical and chemical properties of the paper surface and the electrostatic inking system; In addition, it is also affected by the operation of machine personnel and the changes of printing environmental factors
I have been engaged in gravure printing quality control for many years, often visiting the scene of the failure, and making a detailed record of the process of the failure and the solution. How do these faults occur, how to solve them, what are the causes and results? Let's go to the scene and have a look
first: the ink transfer of small text printing plate is poor, and there are obvious missing pens and broken strokes
it was a summer night, the weather was muggy, the temperature and humidity of the workshop were very high, and the relative humidity was close to 100%. During the shift handover, the paper suddenly broke and the machine stopped. The captain on duty connected the paper and then started the machine. After sampling, it was found that the black version of the small text was light in ink, and the pen was missing and broken seriously. The captain reduced the speed of the aircraft and asked his deputy to carefully wipe the black version. After speeding up, the captain took a few samples and looked at them, but he was still not satisfied. It turned out that all the small words were "blocked"
generally speaking, the black version is small, the text area is small, the ink consumption is small, and there is no need to add ink for a long time. As the solvent in the ink evaporates, the solubility of the ink becomes poor, and the version is blocked slowly. In addition, the downtime during shift handover was too long, and the ink in the hole dried up. When wiping a plate with a solvent, the sharp evaporation of the solvent takes away huge heat, and the moisture in the air condenses onto the plate, especially when the ambient temperature and humidity are not ideal. The ink is mixed with water and the solubility becomes poor, so you have to print out all the ink, thoroughly clean the printing plate with detergent powder, and clean the plate with a dry cloth. Redeploy the ink and add a small amount of butyl acetate into it. As soon as you start the machine, the speed will be increased to more than 100 meters, and the small text of the black version will be normal
second: poor transfer of fine dots in hierarchical printing
whether the transfer of fine dots in hierarchical printing is normal is the key to the success of printing. This is the second printing product, and the paper used is 230g/m2 white cardboard. After starting up the plate and speeding up the printing, the captain found that the fine dots of the green plate were seriously lost and there were too many white dots, so 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 degree, it was difficult for the electrostatic ink to transfer the ink in the small holes to the paper, so there were more white spots in the highlights. Then the captain looked at the cyan printing unit and found that the embossing roller was not an electrostatic ink leveling roller. Then the whole electrostatic inking system was comprehensively checked. It was found that the brush had caked, and the brush in some positions could not contact the electrostatic inking roller, so he removed the brush for cleaning, checked whether the electrostatic generator was normal, replaced the non-static inking roller, and replaced the paper. Everything is ready. Adjust the viscosity of the ink. After starting up again, speed up to more than 100 meters. The transfer of fine dots is normal, and the white dots also disappear
third: how can a good overprint version suddenly "run out of print"?
the stability of printing registration depends on the tension control system and cursor tracking. The printing condition of the cursor has a great influence on the stability of overprint. In addition, printing pressure, drying temperature, ambient temperature and humidity also have a great impact on the stability of overprint
this is a transitional period from summer to autumn. It is still hot during the day, and the weather is cooler at night. After a north wind, the humidity decreases significantly. This time, 80g/m2 wide coated paper was printed on the machine, and the product was beer label. There is only half a line (about 0.15 mm) at the thinnest part of multi position typesetting overprint. After the handover, the captain found that the printing unit in the middle ran out of version, and hurriedly adjusted the overprint deviation, but there was no response. The captain checked the unwinding tension, rewinding tension and the tension between the printing units, and found no abnormal tension
later, when checking the cursor of the printing sample, it was found that the cursor of a printing unit in the middle was not accurately embossed, and half of the first half of the cursor had been dried. In this way, the cursor signal read out was wrong, and the cursor following it was wrong in turn, so the version ran out. Finally, I checked the position of the floating roller and found that the floating roller ran to the top and did not respond to the input overprint deviation. In addition, the ambient temperature and humidity of the workshop have changed greatly, the paper expansion rate has changed, and the drying suitability has also changed greatly. It is normal for overprint to be inaccurate. There are several adjustment methods: first, adjust the ink viscosity to ensure the integrity of cursor printing, then adjust the drying temperature of each unit in turn, appropriately adjust the printing pressure and drying temperature, and sprinkle some water on the ground to increase the humidity of the environment. After the reading cursor is normal, adjust the overprint deviation and appropriately adjust the printing speed to meet the paper scalability requirements, so that 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 cannot be eliminated after cleaning, it will sometimes crush the plate when entering the printing unit, causing the printing plate to burst. This time, it happened that a Fuchsia base plate had been printed with 500000 large sheets. After changing to the new version, the printing color of the Fuchsia base plate was quite different from the original. The ink layer was thick and the hue was obviously darker, 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 concentration. Generally, add diluent to adjust the color concentration, and fine tune the toner ink to adjust the yellow phase, magenta phase and cyan phase in purplish red ink. The toner ink can be golden red, dark red and medium yellow. In this way, the technologist needs to readjust the ink formula, and all the previously adjusted ink is readjusted, which makes the captain's head painful. In addition to the impact on the production of the shift, the consumption of re edition and color matching will increase significantly
fifth: the cooling roller is dirty
the printing adaptability of gold ink is relatively excellent, but sometimes unexpected failures will occur if you don't pay attention. This fault occurred after the gold bar was overprinted with a yellow base. Due to the poor adhesion fastness of the gold ink, it was easy to stick to the cooling roller. On the night shift, the captain thought that this kind of work was better for printing, so he raised the printing speed higher. The yellow base sub plate consumes a lot of ink, the ink is updated quickly, and the ink printing adaptability is good. Gold ink has strong hiding power, and there is generally no problem of overprint failure in normal overprint. The ink layer is too thick after gold ink superposition, and the peeling increases. If the drying is not complete, or the temperature of the cooling water is not appropriate (too high), it will affect the adhesion fastness of the gold ink
from the perspective of paper gravure printing process, after printing the color ink in turn, cover it with varnish at last. The overprint ink of the intermediate color group is too thick, which is easy to cause insufficient adhesion fastness. In addition, the poor cooling effect of the gold ink directly affects its adhesion fastness. It's not easy to find out during night shift sampling. In fact, it's clear when you look at the sample with the light on its side. It turned out that the cooling roller was hot and full of gold ink. The drying temperature of the oven was too low, and the gold ink was not fully dried. The captain stopped the machine, completely removed the ink stuck on the cooling roll, adjusted the drying temperature, and adjusted the machine speed to the appropriate speed, and the problem was solved. Some are careless, and the captain injured the cooling roll with a hard ink shovel, which makes it easier to get dirtyIn fact,
next, I want to talk about the improvement of plate making and printing on the issue of "the obvious difference between the axial and circumferential thick lines of light color frame printing"
almost all border prints have differences in axial and circumferential performance, except for light ink printing. It is more obvious that this system adopts pci2.1 bus standard. There is a 2m or so border with a gold base around it. Regardless of the way of typesetting, its circumferential and axial expressiveness is different. Along the axial direction, the lighter the yellow ink is printed, and the lines are thinner; The light yellow lines in the circumferential direction have 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. Moreover, the area of the printing plate supporting the doctor blade in the axial and circumferential directions is also different, which leads to an inevitable difference
the preventive measures are as follows: in terms of design, many cigarette packs prefer to use some thin thick line borders, which is exactly the difficult side of gravure printing. In fact, this kind of light colored and small lines can be avoided from the design. In terms of plate making process, we can take advantage of the strong hiding power of gold ink to make the embossing edge more than 0.5m, which is actually equivalent to widening the thickness of lines. In this way, even if the axial impression is not solid, the gold bottom can also be covered. In terms of ink, the method of increasing the color concentration and reducing the viscosity of ink can be adopted. Improving the wettability of ink and scraping on the printing plate is an ideal electrode material for mobile batteries and power supplies. In this way, the difference in the amount of ink transfer in the axial and circumferential directions is reduced. In addition, there is another way is to appropriately increase the depth of the printing plate, especially for thicker paper is an effective way
finally, let's introduce the electrostatic fault. Generally, static electricity occurs in winter and spring. The weather is relatively dry. Static electricity is accumulated when the high-speed paper rubs against the metal roller. When the static electricity is accumulated, it will discharge to the surrounding conductors when the exports of super developed countries are mainly at the middle and low ends, and a certain capacity is exceeded. From the printing sample, especially the filaments formed around some thin lines, at first it seems that the doctor blade is dirty, and there is no good effect after replacing the doctor blade. What is more serious is that after the paper accumulates electric charge, it is easy to cause difficulties in cutting, separating and receiving paper, and even cause traffic jams. The most serious problem is that the charge on the paper will accumulate to a certain extent and cause sparks. If the solvent concentration reaches a certain degree, it will even cause an explosion. To prevent such faults, we can spray water on the ground, add a humidifier between units, or use direct grounding discharge to eliminate electrostatic faults
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