Factors affecting the accuracy of gravure printing

As with other printing methods (such as offset printing, screen printing, etc.), gravure printing products also require that each single version must have a precise and accurate overprint position. If the position of the overprint position deviates, deviations from the rules and overprint errors will occur, resulting in printed images. The changes in the hue and the level make the image unclear, the overprint of the text appears trailing, missing white, etc., and it is more difficult to process the subsequent processes (such as hot stamping, embossing, die cutting, indentation, etc.). Affect the quality of printed products, causing undue cost increases. Below, the author will discuss the factors that affect the accuracy of web gravure printing and talk about his own opinions.

The tightness of a roll

1. The effect of the tightness of the paper roll on the tension of the paper

The degree of tightness of the web of paper rolls directly affects the change of the paper tension, which tends to cause instability in the printing tension, which causes the back printing to be more advanced and lagging than that of the reference, thus affecting the registration of printed graphics. Although almost all gravure printers now have a pre-printing paper tension adjusting device, if the paper roll is rolled too loosely and beyond the range of device adjustment, the tension roller will fluctuate up and down during the printing process, resulting in overprinting. Not allowed.

Once we used Yajin cardboard to print the “Amber King” cigarette label of the Jinan Cigarette Factory, we found that overprinting of printed products was both good and bad, with large deviations. Because the product is six-color printing, it occupies all the printing units, so initially we thought that this situation was caused by the inconsistent printing pressure of the printing units, but after careful adjustment of printing pressure, the effect was not obvious. . Later, when a printing worker was transporting ink and solvent, he found that the web on the airbag shaft at the unwinder was loosened and the inner paper core was deviated, causing the tension roller to move up and down too quickly, sometimes hitting the floor, and sometimes reaching. The highest position has caused overprinting. After replacing the new roll, the overprint problem was solved.

2. How to judge the tightness of the paper roll

The author believes that first remove the overwrapping kraft paper on the roll paper, reveal the internal printing paper, and then carefully observe the two sides of the paper roll roll uneven, if the paper edges are relatively homogeneous, indicating that the paper roll tightness is uniform, Can be printed on the machine; if the paper edge out of the abduction, uneven, it indicates that some part of the paper roll is too loose or too tight, not suitable for printing. According to our many years of experience, we have imported webs such as American "Visvik", Finnish "Double A", Canadian "Butterfly" and other white cardboard, Japan's "Sanyo National Policy" coated paper and domestic Ningbo. The tightness of the paper roll of the white paper of China Paper and the paper of the copper paper of Jindong Paper is relatively uniform, and there is generally no problem of overprinting during the printing process.

Second change roll, take over paper factor

1. The effect of the paper on the overprint

At present, the gravure printing production line unwinding machine adopts the static paper-feeding and dynamic-feeding. Regardless of whether it is a static splicer or a dynamic splicer, the paper at the unwinder is at a standstill under the action of the tension roller at the instant of change of the splicing paper. At this point, the tension roller moves up and down quickly, causing the paper tension to change slightly, and the printing of text and graphics may occur. In fact, under normal circumstances, this overprinting phenomenon can be automatically restored to the normal overprint position by overprinting the master console. However, if the new paper roll exchanged is not produced by the same manufacturer as the paper roll being printed, there may be problems with overprinting. On one occasion, when we printed Harbin cigarette labels for Harbin Cigarette Factory, due to suppliers, we needed to use another brand of paper, the “Red Tower” brand web. After we printed the "Vickerswick" paper and connected the new paper roll, we discovered that the overprint problem could not be recovered by the overprinting master console. Therefore, we could only reprint it by adjusting the printing pressure. Afterwards, through analysis, we believe that due to the different quantification and thickness of the two papers, the printing pressure has changed, which has affected the registration.

2. The effect of paper joints on overprinting in paper rolls

The number of paper joints in the roll can also affect the overprint of the product. This is because the two papers at the joints overlap each other. The front and back sides are sticky and become relatively thick. When the joint paper reaches the pressure roller of the printing unit, the printing pressure changes, resulting in misprints. The more paper roll joints, the more times the paper tension changes, the more times the print pressure changes, and the more unstable the overprinting, resulting in wasted paper. Each connector in a roll of paper is equivalent to changing the paper and rewinding the paper once, causing the tension to change and affecting overprinting. Before printing the roll, carefully inspect the unmarked paper joints for outsourcing or open the packaging kraft paper to check whether there are paper joints, and make preparations in time. At the same time, it is recommended that gravure printing manufacturers do not use webs with paper joints in the roll to avoid unnecessary waste.

Three printing pressure

In all printing methods, gravure printing exerts the greatest printing pressure. The uniformity and size of the printing pressure directly affect the change of the paper tension, and thus affect overprinting.

1. Uniformity of printing pressure

According to many years of practice, the author believes that for products with less color number (within 4 colors), the uniformity of the printing pressure will not affect the overprint as long as the printing pressure reaches the requirements of the printed product; but for printing When the number of colors is 4-6 colors or even more, the pressure roller of each printing unit must be adjusted so that the pressure roller of each unit remains flat and consistent, and the pressures of the front and rear units are the same, otherwise the unit There may be a “crepe” phenomenon between the unit and the paper, causing the paper to become slack, or the paper wrapping around the gravure plate cylinder and damaging the squeegee blade. In addition, in the daily work, it is necessary to keep the surface of the imprint rubber roller clean, because some abnormal shutdown (such as paper break, sudden stop, etc.) will make the ink tank and small ink tank overflow the ink, and the imprinting roller will inevitably stick. On the other hand, the ink forms a solid ink on the rubber roller after drying, which increases the local circumference of the rubber roller, thus changing the uniformity of the entire rubber roller and accelerating the aging of the rubber roller.

2. The size of the printing pressure

The size of the printing pressure is also more critical. Under normal circumstances, the print pressure can be verified by normal shutdown. If the machine is normally stopped, the pressure roller of each unit can just lift automatically, which means that the printing pressure is right. Of course, at this time, for some color blocks (such as gold, big red bottom, etc.), false printing may occur, that is, the phenomenon of “defectiveness” may occur. Therefore, according to actual printing conditions, a certain unit can be appropriately added on the basis of maintaining pressure uniformity. The printing pressure.

The thickness of the four-print version shaft head

The gravure plate cylinder is divided into a shaft plate cylinder and a shaftless plate cylinder. For a shaftless version, a plate shaft is required to support each plate change. After the plate shaft is used many times, the shaft head will wear more and more thinly, and the outer metal sleeve will move up and down, so that the eccentric motion will occur when the plate cylinder rotates, and overprinting will be affected. For shafted cylinders, due to the higher platemaking costs, the printing manufacturer will use the plate cylinder for multiple plate-out operations. During this process (more than 8 million rotations), the shaft head will become finer and thinner. The above result. If you want to continue to use this version of the drum, it is recommended to relatively reduce the inner diameter size of the shaft head metal sleeve, because the production cost of a gravure plate roller with a shaft is higher, and the processing price of the shaft head metal sleeve is relatively cheaper.

Five plate tight plug

In the change of the plate, if the gravure tight plug is loose, it will cause the plate cylinder to move axially under the action of the embossing rubber roller, and there will be failures in the horizontal and horizontal overprinting. Therefore, at the end of the plate change work, before the product overprinting, it is necessary to carefully check whether the solid plugs of all gravure printing units are loose.
The wear of the internal bearings of the fastening plugs will also have an effect on overprinting. If the wear is severe, there will be a failure of the shaft head metal sleeve buckling and gravure jumping. In order to prevent the wear of the bearing, it is necessary to apply a certain amount of butter to the inner bearing of the fastening plug at each change of plate.

Six photoelectric scanning head

The photoelectric scanning head is the key part of detecting gravure overprinting, and the scanned signal is directly transmitted to the main console, so whether the signal is stable or not plays an important role in overprinting. The following is a brief description of the use and maintenance of the photoelectric scanning head.

The detection angle of the photoelectric scanning head is closely related to the printed color. In general, darker colors such as red, black, gold, and green, because the signal is strong, the scanning head can be directly detected; for yellow (transparent yellow, medium yellow, light yellow), light brown, gold and silver For white printed on cardboard, etc., because the signal is weak, the scanning head must be rotated at a certain angle for detection, and the fine adjustment knob on the scanning head must be gently adjusted until the signal displayed on the main console is stable. When arranging a gravure process and making a gravure plate, a darker color is generally used as the reference color for overprinting. The advantage of this is that it is convenient for overprinting.

In order to prevent the scanning head from providing the wrong signal, the photoelectric scanning head should be kept clean during the daily work. The main cover personnel should regularly check the scanning head so that the detection head is transparent and clear, preventing dust, paper hair, paper dust and ink particles. Other debris enters the photocell and interferes with the scanning signal.

Source: Ke Yin Media - "Printing Technology"