Reasons for inaccurate overprint of the hottest si

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Reasons for inaccurate overprinting of single sheet paper and two-color offset press

when the single sheet paper and two-color offset press are overprinting in multi-color, there is inaccurate overprinting in vertical and horizontal directions, which is mostly caused by the following aspects:

1 The printing plate is not calibrated

2. The gap between the tooth pad of the swing arm and the iron plate of the paper feeding table is improperly adjusted

3. The large cam of swing arm, connecting rod pin shaft, roller, etc. are worn or broken

4. The collision between the swing positioning block and the two sides of the limit screw is different, which makes the metal material of the large tension spring fatigue

5. The handover time of big and small teeth is too short

6. Improper adjustment of paper trailing tip positioning brush wheel

7. Serious longitudinal deformation of paper

8 is different from jjg762 (2) 007 "extensometer" The thickness of drum lining changes

9. There is a circumferential displacement of a drum

for the inaccurate overprint in the vertical and horizontal directions caused by the above reasons, the operator can eliminate the fault and print the products that meet the quality requirements as long as he carefully observes the change law of the crosshair and checks the relevant mechanical parts. However, we once encountered a special case of inaccurate overprint in the vertical and horizontal directions of the two-color machine, which took 5 working days to eliminate the fault. Now the details are described as follows for the reference of colleagues, so as to avoid detours when you encounter similar faults

fault phenomenon

print a batch of four color jobs on j2204a two-color machine, using 300g/m2 high-grade coated white board paper. According to the normal printing color sequence, put the color group on red and put down the color group on yellow. After all the preparations are in sequence, start proofreading. At this time, he said: "What excites people is the current problem. No matter how you pull the version, the two colors in the vertical and horizontal directions cannot be overprinted together. Later, you pull the yellow version and try to register it with the red version. After repeated calibration for several times, you can't register it. After observing the overprint of the crosshair, you find that when the red version doesn't move, the Yellow crosshair changes irregularly, and the change distance on the side of the body is large, about 2mm up and down, and the computer animation changes on the outside The imitation of 50000 audience is small, less than 1mm. According to this change, the circumferential movement of the plate cylinder and the rubber cylinder can be preliminarily ruled out, because the movement distances on both sides of the imprint are different, and there is no ghosting

check the tightness of the blanket and the printing pressure, and no abnormality is found. Everything is in line with the technical indicators. Therefore, we changed a version correction method, that is, monochrome version correction. First print a few red copies, and then use red to overprint yellow. As a result, the version was corrected soon. After that, just print a few yellow samples, and then overprint them with yellow and the upper group of red, which can also be registered. We overprint red and red, yellow and yellow separately, which are very accurate, but when the two colors are pressed together, the red and yellow overprint is not correct


first, according to the normal operation state of the machine, we ruled out the possibility of failure of each drum, and focused on the large and small sets of teeth. Therefore, we first check the teeth of the swing arm, move the machine to the position where the teeth are about to take paper on the table, make the tooth pad of the swing arm at the lowest point, observe whether the tooth pad is in the same plane, and adjust individual uneven tooth pads. Next, check the teeth of the impression cylinder again, so that the opening angle of each tooth is basically the same, and the holding force remains the same after closing the teeth. After patient and meticulous work, and then startup inspection, the printing quality has been improved, and the crosshairs and corners are basically registered, but the text and text are still not overprinted accurately, especially the downward arc of the next color group image in its mouth, resulting in the inaccurate overprint of the image in the lower area of the middle of the two colors. We have repeatedly adjusted the two sets of teeth, and the fault has not been eliminated

after more than two days of analysis and research, we decided to focus on the inspection of the teeth of the imprint cylinder. First, replace the severely worn tooth pads, remove all the small compression springs in the teeth, and replace them with new springs of high quality. Then, adjust the teeth of the impression cylinder according to the following steps

1. Under the condition of closed teeth, pad a 0.25mm thick paper between the dental shaft plug and the adjusting screw, and adjust the positioning screw to maintain a distance of 0.25mm from the positioning block

2. Loosen all tooth adjusting screws and tooth clamp screws so that the tooth and clamp can move freely on the tooth shaft

3. Place a piece of kraft paper with the same width as the tooth between the tooth piece and the tooth pad, rotate the tooth body and the clamp by hand to make the tooth piece press the kraft paper, tighten the tooth body adjusting screw, and take out the kraft paper; Pull the clamp by hand to keep the tooth body parallel to the clamp, and tighten the clamp screw

4. By adjusting the screw of the tooth body, the paper holding force of each tooth is consistent

5. Adjust the adjusting screw of the dental slice to make the closing and opening of the dental slice consistent

6. Remove the paper pad between the toothing positioning block and the positioning screw, and the toothing adjustment of the imprint cylinder is completed

after the above adjustment of the teeth of the embossing cylinder, the machine is turned on for inspection, and a large number of industrial platforms of the next color group figure 1 are also established: the radian of the image disappeared on the 25th of last month, the two colors are completely overprinted accurately, the fault is eliminated, and the production returns to normal

failure cause analysis

the causes of this failure may be the following:

1 Due to the long-term use of the machine, the tooth pad of the imprinting cylinder is worn, and the surface is uneven, so that the paper is not in the same plane. Especially when the machine is used for printing paper smaller than the open width for a long time, the tooth pieces on both sides are in direct contact with the tooth pad for a long time, which intensifies the wear of the tooth pad, forms grooves, weakens the paper holding force, and the holding force of the middle tooth is relatively strong. In this way, when printing paper with a large format, when it is first imprinted, The two sides of the paper are slightly separated from their teeth, forming a natural arc; When entering the second color printing, make the mouth of the two-color imprint in a downward circular arc, and increase the arc

2. When we adjusted the "stopper" of the teeth, we found that the gap between the positioning block and the positioning screw was too large. Even in the closed state, the gap was 2 ~ 3mm, which was far from the specified gap of 0.25 ~ 0.3mm. In this state, the tooth holding spring compression deformation is also large, and the tooth holding will produce secondary tooth closing and tooth opening actions, which is very detrimental to overprint. Because it is a very short process from taking the paper out of the drum to completing the printing, the higher the machine speed, the shorter the time required, and the secondary tooth closure of the teeth will damage the positioned paper. When printing again, due to the stripping force of the blanket and the adhesion of the ink, the paper is more prone to circumferential displacement, making the two-color printing in the same process produce irregular overprint in the vertical and horizontal directions

(MA Tingyu, Feitian color printing factory, Zhangye, Gansu)

reprinted from: printed in China

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