Influential Factors of Accurate Overprinting of Web Gravure Printing (I)

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.

(to be continued)