Gravure printing machine is mainly divided into three parts: uncoiling part, printing part, slitting and die cutting part. The printing part is the core part of the gravure printing machine, which completes the printing, overprinting and drying of various colors.
1. Select the number of color groups
The selection of the number of color groups of the gravure printing machine is very critical, which is mainly related to the leading products of cigarette packaging printing enterprises and the environment of domestic cigarette decoration. The pattern and process of domestic cigarette pack design are relatively complicated. In the past few years, the domestic cigarette pack printing equipment was mainly 6 colors and 7 colors. In the past two years, the domestic cigarette pack printing equipment was mainly 8 colors. The basic configuration mode includes a full color, four general colors, two spot colors and a glazation unit. The more color groups, the better. The more color groups, the more difficult it will be to control the deformation of paper (encyclopedia) and overprinting of various colors, and the capital investment will also increase. However, if the number of color groups is too small, it is difficult to reflect the advantages of high speed and high efficiency of gravure printing machine and one-time molding, so users must choose according to their own needs and combined with future development plans. Although the unit type gravure machine manufacturers have promised to increase the unit in the future, but then increase the unit not only the investment is larger, but also due to the technical upgrade of the gravure machine and the installation location and space of the gravure machine and other factors, it will be subject to many restrictions.
2. Selection of printing format
The requirements of various manufacturers for printing format are very different. The wider the format, the higher the requirements for paper, and the greater the impact on overprinting. Cigarette bag printing equipment is more suitable for the format of 650mm and 820mm two. Before the printing factory basically choose the former, in typesetting, plate making, printing plate installation, die-cutting cylinder, paper specifications and other aspects are more economical. During typesetting, soft-packed small boxes can be arranged in 6 rows and 4 columns, hard-packed small boxes in 6 rows and 2 columns, strip boxes in 1 row and 2 columns, die-cutting cylinder in 6 rows and 2 columns or 8 rows and 2 columns, with a paper width of 650mm. Due to the small width, the paper deformation during printing is small, which is very beneficial to overprinting of products. However, there is a key problem, that is, when printing box products, the paper fiber direction is perpendicular to the printing direction. For the current high-speed cigarette packaging machine, its packaging adaptability is poor, the appearance of poor molding, resulting in more quality problems. For this reason, many printing plants have to adopt the practice of printing only one piece at a time, which seriously affects the efficiency of the gravure printing machine.
In the past two years, the industry has basically reached a consensus that if printing boxes, a gravure printing machine with 820mm width is selected, so that boxes can be arranged in 1 row and 2 columns, 6 rows and 3 columns of hard-packed boxes and 6 rows and 5 columns of soft-packed boxes during typesetting. For high overprint accuracy requirements and the need to use a variety of printing process products. It is recommended to arrange as few as possible, generally 12~16 for the ideal number of typesetting, which can reduce the waste caused by paper deformation and multi-channel processing technology. If you use online die-cutting, typesetting can be free from this restriction.
3. Installation method of plate cylinder
The installation method of the printing plate cylinder has a great influence on the printing accuracy, printing flatness and the beating of the printing plate cylinder, and plays a key role in the high-speed operation of the printing equipment. In the past, it was mostly mechanical installation, but there are two problems: the installation tightness is inconsistent and easy to loosen, in the case of high-speed operation, it is easy to cause the overprint accuracy to drop. Now the installation method of the printing plate cylinder is basically changed to pneumatic locking, the locking force is consistent, and it is not easy to loosen, suitable for high-speed printing.
4. overprint control
The overprint accuracy is the core index of the gravure printing machine, which plays a key role in product quality and loss control. In the past, the gravure printing machine can only carry out longitudinal automatic registration control. The overprint mark is generally a square or rectangle of 4mm * 4mm or 4mm * 6mm, and the adjustment of horizontal registration is mainly completed manually. At present, both horizontal and vertical registration adopt automatic adjustment, which requires that there must be horizontal overprint marks. There are mainly two kinds of control. One is to add an overprint mark to control horizontal overprint on the original basis, so that each color group must use two scanning positions to control. Another is to change the overprint mark to a triangle, using the hypotenuse of the triangle as the control point, and the length of the right-angle side of the triangle is 4~8mm, which is currently used more control mode. In the adjustment of overprint deviation, the alignment of color overprint was realized mainly by adjusting the position of the printing plate cylinder relative to the paper before. At present, the position of the paper relative to the printing plate cylinder is mainly adjusted. This method does not need to change the running speed of the printing plate cylinder, which is conducive to overprint adjustment under high-speed operation, and the adjustment accuracy is easy to control and the overprint accuracy is high. In order to ensure the high quality of the printed matter, the precision of the gravure printing machine should be controlled within ± 0.10mm.
5. Drying method
In the past, gravure printing machines mainly used hot air drying methods, mainly for solvent-based inks, but now gravure printing uses many types of inks and varnish, such as alcohol-soluble inks, water-based inks and varnish, UV inks and varnish, etc. Only hot air drying methods can no longer solve the problem. At present, the drying method of infrared heating air is mainly used, especially after the extensive use of water-based ink, the hot air drying device with large air volume has become the basic configuration. Hot air drying devices have electric heating and steam heating. The electric heating method has a great demand for electric capacity, about 150kVA. If the electric power needs to be increased and there is a ready-made steam pipeline, the steam heating method can be directly adopted. At the same time, the number of equipment using UV curing is also increasing, but there are still some problems in ink and process. I believe that in the near future, UV curing will also become the standard configuration.
6. Ink circulation system
Gravure printing ink, especially the deployment of spot color ink, if the compatibility of two or more binder resin is poor, it is easy to precipitate. In addition, large particles of gold, silver ink is also prone to precipitation. Therefore, the circulation of ink is crucial. The ink circulation system must have the following functions: stirring the ink to avoid precipitation or ratio change; Normal delivery of ink, it is not easy to produce bubbles, precipitation and splash during the delivery process; Filter ink, filter mesh should be about 120 mesh; Solvent supplement function to ensure proper ink viscosity.
7. Electrostatic ink transfer device
The disadvantage of gravure printing process is that screen mesh adjustment printing is greatly limited, especially when printing full-plate transition screen or local level effect, it is very difficult to reach 175 lines/inch, and small screen dots are easy to lose, and the printing effect is very rough. The electrostatic ink transfer device can obviously improve this situation, it forms a high-voltage electrostatic zone between the conductive surface of the electrostatic pressure roller in contact with the paper, the paper and the printing plate cylinder, and under the action of static electricity, the ink is sucked out of the screen cavity of the printing plate cylinder and transferred evenly to the paper surface, and the ink transfer rate reaches or approaches 100%. It is not necessary to configure each color group with an electrostatic ink transfer device. Generally, a production line with 4 color groups is sufficient. The installation position should be reserved on each color group. When the position of the printing color group changes, the electrostatic ink transfer device can switch between color groups. Now, the role of domestic electrostatic ink transfer device has been very ideal, but also can be exchanged with foreign devices, the price is only about 1/5 of the imported device.
8. Shaftless transmission mode
Gravure printing machines generally use universal shaft drive, but the linear speed of the Chinese and Indian plate cylinders varies greatly, and even affects the accuracy of overprinting. In the past two years, the non-shaft transmission technology has been rapid development, high-grade gravure printing machine basically adopts non-shaft transmission, which greatly improves the precision of multi-color gravure printing machine transmission and overprint, and can fundamentally realize the transmission control of single unit, which is the main way of multi-color unit transmission in the future. At present, the more shaft-less transmission mode is that each unit is directly driven by a DC motor, and then centralized control is realized on the basis of sub-unit control. For example, the speed change of a single unit can be realized, and the unused monochrome unit can be controlled to stop operation, so as to reduce wear and energy consumption, and it is also very important to improve the overprinting accuracy under high-speed operation.
9. Tension control mode
In the past, the entire gravure printing machine production line only used a set of tension control system, placed behind the latter printing unit, and adjusted the tension of the entire production line by controlling the relative speed difference between the paper feed speed and the paper output speed. Tension change range is larger, generally more than 50 Newton, plus its by adjusting the position of the plate cylinder to achieve overprint, the tension between the color group change is greater, so the stability of the color overprint is also very poor. Now it is basically changed to the tension control of the color separation group, and the tension control of the whole production line is distributed to the color groups, which greatly reduces the variation range of the tension. Generally, the tension changes of the color groups are about 5 Newton, which lays a good foundation for accurate overprinting. In the whole production line, the tension value of each color group should be as consistent as possible. Generally, according to the difference of the paper, a fixed value between 200 and 300 Newton should be selected to make the tension of the paper uniform during the printing process and reduce the paper deformation to a certain extent.
10, Online Quality Monitoring
Online quality monitoring can be divided into three levels: online local overprint monitoring, online product pattern monitoring and alarm, online product pattern and ink color monitoring. On-line local overprint monitoring is to print the local overprint cross hairs or key fine patterns on the format, through a digital camera, and then zoom in, generally 15 to 20 times, and synchronous tracking. The operator can monitor the overprint effect of the entire printing format and adjust the problem in time. Online local overprint monitoring is a basic and necessary online quality monitoring mode. Generally, one or two parts are installed, and the price of a system is about 50000~80000 yuan. The pattern monitoring and alarm of online products is to input the sample pattern into the computer, scan the online printed matter, and compare the change of the pattern position of the printed matter with the sample pattern position. If there is any change, it will alarm, remind the operator to adjust, and mark or reject the online product. This method is now widely used, and the price of a system is about 800000 yuan. The pattern and ink color monitoring of online products is based on the second online quality monitoring method, which increases the automatic adjustment of the equipment and the control of the ink color. This method is used very little and is mainly used in the banknote printing industry because the cost of the monitoring equipment is relatively high.