Analysis of the printing quality caused by offset

2022-08-12
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Analysis of print quality caused by offset press

during the printing process of offset press, it will cause the quality accident of ink bar on the print. The direct cause is the sudden change or mutual displacement of the speed of the mutual contact point of the printing cylinder or the mutual contact point of the ink roller and the contact point with the printing plate cylinder or the junction point during printing. There are many factors that cause sudden changes in speed or mutual displacement. Generally speaking, there are factors in the operation of offset press, as well as factors in the design and manufacture of offset press

this kind of ink bar produced because the gear design does not meet the requirements of the offset press is called the gear ink bar. The gear ink bar is a kind of ink bar that reflects the characteristics of the print. It shows that the entire layout (sometimes sub areas) is filled with ink bars with a spacing equal to the pitch of the drum gear along the transverse direction (drum axis). Because of its special reflection on the printing plate, it is easy for people to recognize it. There is another kind of ink bar. Its reflection on the print is exactly the same as the gear ink bar above, but it is not produced because the gear design does not meet the requirements. It can be called imitation gear ink bar

1. The influence of the diameter difference of the printing cylinder

(1) the given diameter of the printing cylinder

the benchmark for the design and adjustment of the offset press is the embossing cylinder. Both the printing plate cylinder and the rubber cylinder are lined when printing. In order to transfer the pictures and texts on the printing plate to the prints of the impression cylinder with as little deformation as possible, the diameter of the printing plate cylinder after adding the printing plate should be equal to the diameter of the impression cylinder plus the printing sheet. However, due to the different thickness of various printing papers, the diameter of the imprint cylinder should be equal to or slightly larger than the graduation circle diameter of the main gear in order to avoid bare rolling and wear during the operation of the imprint cylinder. Except for the printing plate and backing paper, the bare roll diameter of the printing plate cylinder should be smaller than the graduation circle diameter of the main gear. The bare roll diameter of the rubber drum is smaller, because the rubber blanket is thicker, and the liner must adapt to the changes of soft, hard and neutral lining. Therefore, the diameter of the bare roller of the three printing cylinders is: the diameter of the impression cylinder, the diameter of the printing plate cylinder, and the diameter of the rubber cylinder

(2) diameter change of rubber drum

when pressing printing, the three drum gears mesh with each other on the dividing circle. The diameter of the rubber drum after lining is equal to the diameter of the gear graduation circle plus the rubber compression given by the rubber blanket. This compression can be calculated from the lining thickness specified in the machine manual. At this time, the center distance between the rubber cylinder and the plate cylinder and the impression cylinder is less than the diameter of the rubber cylinder, which is equal to the diameter of the graduation circle. The distance from the contact point to the center of the drum is called the drum printing radius

when there is paper on the impression cylinder, the printing radius of the rubber cylinder becomes smaller, the compression at the contact point of the rubber blanket increases, and the pressure increases. With the change of the thickness of the paper, this radius and pressure also change accordingly. In order to ensure the constant pressure in printing, one method is to automatically shift the force value, clear it and query the data under the normal meshing of the gears. According to the thickness of the printed sheet, the paper with the corresponding thickness is extracted from the rubber lining and padded in the printing plate cylinder lining. At this time, the diameter of the rubber drum and its corresponding printing radius become smaller. The other is the method commonly used by printing plants to ensure the pressure, that is, according to the thickness of the printing sheet, the center distance between the rubber drum and the imprint drum is correspondingly opened, but this method makes the rubber drum gear and the imprint drum gear in an abnormal meshing state

sometimes, due to some negligence or need, the diameter of the rubber cylinder is wrapped too large. In order to ensure the pressure, the center distance between the rubber cylinder and the impression cylinder and the plate cylinder is pulled large, resulting in abnormal meshing of all gears. The printing radius corresponding to the rubber drum also increases

(3) rebound of rubber blanket

rubber blanket is a very flat and dense elastic material composed of fiber and rubber, which is tightly wrapped on the rubber drum. When the blanket is under pressure, the blanket rebounds to the agent. When the rubber drum rolls relative to the acting object, the rubber rebound force will be opposite to the rolling direction of the rubber drum. When the printing radius of the rubber drum is equal to the printing radius of the corresponding embossing drum, the elasticity of the rubber blanket prevents the corresponding drum from rotating during operation; When the printing radius of the rubber drum is smaller than the corresponding printing radius of the embossing drum, the elasticity of the rubber blanket slows down the rotation of the corresponding working embossing drum; When the printing radius of the rubber drum is larger than the printing radius of the corresponding working embossing drum, the force generated by the rebound of the rubber blanket during the work will drive the drum to rotate

2. Imitation gear ink bar

when the printing cylinder gear engages on the dividing circle, the paper feeding of the imprinting cylinder rotates, and the printing radius of the rubber cylinder is less than the thickness radius of the imprinting cylinder. Chen quanxun, counselor of the State Council, President of China Nonferrous Metals Industry Association and Secretary of the Party committee, inspected the innovation group. In gear meshing, the speed of the rubber drum and the embossing drum is the same, and the linear speed of the paper surface is greater than the linear speed of the rubber drum. The deformation resilience of the rubber blanket at the contact point makes the paper cling to the surface of the embossing drum. When the paper is thicker, the resilience is greater

what happens if the distance between the rubber lining and the center of the drum is adjusted so that the printing radius of the rubber drum is greater than the radius of the embossing drum plus the thickness of the paper? Since the embossing cylinder is the main force, the rotation speed of the two cylinders is the same, and the linear speed of the rubber cylinder printing point is greater than that of the paper. At this time, the deformation resilience of the rubber blanket pushes the imprinting cylinder forward, and the force acting on the tooth surface of the imprinting cylinder increases. The resilience of drinking water and water heating pipes on the impression cylinder is actually on the paper. Because the friction force between the blanket and the paper is much greater than that between the impression cylinder surface and the paper, if the resilience on the contact point is large enough, the paper will loosen and slide on the impression cylinder surface, the resilience will disappear temporarily, and an ink bar will be generated on the paper. As the rebound force disappears, the additional force on the tooth surface suddenly disappears. As the machine continues to rotate, the additional force gradually accumulates, and the resilience of the blanket at the contact point slowly increases, causing the paper to slide again, forming another ink bar. Because this rebound force accumulation corresponds to the accumulation of additional force on the tooth surface, that is, to the gear pitch, an ink bar is generated on each pitch, and a full page ink bar with the same spacing as the gear pitch is generated on the whole paper, thus forming a gear like ink bar

the formation of gear like ink bar is not that the rubber blanket at the contact point slides on the paper surface, but that the paper at the contact point slides on the embossing cylinder. To generate this sliding force, the speed of the rubber drum contact point must be greater than the paper speed, which corresponds to the gear pitch. The tighter the rubber blanket is, the more obvious it is

when the thickness radius of the embossing cylinder plus the paper is greater than the printing radius of the rubber cylinder, the main force of the embossing cylinder will accelerate the rubber cylinder. Due to the existence of the side clearance of the gear, the reverse side of the gear tooth of the rubber cylinder contacts the reverse side of the gear tooth of the embossing cylinder, which will cause the rubber blanket to slip on the paper. This slip should also correspond to the gear pitch. However, in the actual production, this kind of gear like ink bar does not appear. Although the thicker the paper, the worse the printing, and the invisible ink bar appears, it does not correspond to the gear pitch. Then analyze the situation of rubber cylinder and plate cylinder. It is reasonable that different printing radii will also cause contact points to slip and form ink bars. Similarly, this kind of situation did not appear in the test

how to judge whether the full-scale equal pitch ink bar on the printing sheet is a gear like ink bar can be understood from its production principle: the elasticity of the blanket and the sliding of the paper on the surface of the imprinting cylinder. The most direct way is to turn over the printing sheet with a full version of equal pitch ink bar and look at it carefully at a certain angle. If you see a scratch on the corresponding front ink bar, it must be an imitation gear ink bar, which is particularly obvious on coated paper. Observe the surface of the imprint cylinder. Unless the surface of the imprint cylinder was wiped particularly clean, you will see scratches corresponding to the ink bar, which is a record of paper slipping

another way to judge is to select a half full (line) graphic and half blank printing plate (divided vertically) and install it on the machine, wipe the blanket, so that there are obvious scratches on the blanket. Turn on the machine for printing, stop the machine when there is an equal pitch ink bar on the printing sheet, and watch the blanket. There are ink bars on the image and text on the blanket, and there are dirty marks connected with the ink bars on the image and text in the blank

3. Conclusion

most of the gear like ink bars appear on offset presses that have been overhauled with the diameter of the embossing cylinder. The diameter of the embossing cylinder of this machine has been grinded too much and has not been supplemented. In order to ensure the printing pressure, it is bound to increase the printing radius of the rubber cylinder, which provides conditions for the generation of gear like ink bars. Therefore, overhaul the imprint roller of offset press, such as manufacturer, price, quality, etc. to make a preliminary judgment. After grinding, the diameter of the cylinder must be restored

in the production and manufacturing of offset press, it is rare to have gear like ink bars, but improper debugging may also occur. Offset printing machines have high requirements for the accuracy of drum gears, but they are not generally taken for granted. The research on the possible faults of offset press is to avoid printing quality accidents and other accidents in the use of offset press, such as imitation gear ink bar, which will not appear in normal operation

source: Chinese ink technology

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