Q&A on flexographic printing technology (3)
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In the first two issues, we discussed the general issues of flexographic printing and the plate making of flexographic plates. In this issue, we will take the 300 series and 500 series unit flexo printing machines produced by Allied Gear & Machine Company of the United States as an example to talk about the characteristics of the unit type flexographic printing machine and the problems that should be paid attention to during operation.
What are the characteristics of the unit type flexographic printing machine?
The unit type flexo printing machine is one of the most popular types in the world today. The structural feature is that each printing unit is independent of each other and arranged in a straight line in a horizontal direction. The main motor is located at the rear of the die-cutting unit with the largest load, and then transmits power through a common driving axial unit at the rear of the machine. Therefore, this model can determine the number of printing units according to the specific requirements of the user, up to 10 units, that is, 10 colors can be printed. This feature of the unit-type flexo press provides many advantages such as good accessibility and short preparation time before printing. In the United States and Canada, such narrow-format unit flexo presses have dominated most markets in the areas of self-adhesives, small folding cartons, commercial form paper, bills, tags and soft drink packaging. In recent years, the packaging printing market has shown a new trend. Manufacturers are constantly introducing new packaging designs, and the number of printing per product is relatively reduced, which gives the flexible unit-type flexo printing machine a better opportunity.
The foreign packaging and printing industry has traditionally referred to flexographic printing machines as "Converting Machines" because it is not only easy to convert, but also flexible. Compared with the wide-format flexo printing machine, the unit type flexo printing machine can complete multiple printing and die-cutting processes in one pass. If equipped with an electric discharge processing system, the narrow-width flexo printing machine can also print plastic film; On-line self-adhesive compound stripping--printing--recombining device can print the required information on the inside of the self-adhesive label; if the automatic rewinding device is not stopped, it can also solve the problem that the flexo press is replaced by the machine. A problem that affects the efficiency of use. A flip frame is mounted between the printing units to enable double-sided printing of the substrate. The flip frame can be placed between any two printing units according to the specific process requirements; the printer can also use the numbering machine to print the number. On the substrate; if the last unit of the machine is equipped with an ultraviolet curing system, the printed product can be UV-glazed; if all units are equipped with an ultraviolet curing system, the machine becomes a flexographic version that can be printed with UV ink. Printing press. In the post-press section, the narrow-format flexo press can use a variety of processes and associated equipment at the die-cutting station based on the printer's processing order. For example, if there is a die-cutting station, there can be various combinations such as bronzing/indentation/die cutting, dragon/indentation/die cutting, bronzing/indentation/crosscutting, embossing/indentation/crosscutting, etc. the way. Finished or cross-cut finished products can also be collected using a finished collector equipped with a counter. In addition, in the post-press part, it can also be equipped with yin and yang punching machines, fan folding machines, etc., for the production of commercial computer paper. It is also possible to carry out on-line filming and slitting. In the appropriate position at the rear of the machine, a stroboscope or even a computer-colored monitoring system can be installed to facilitate operator quality control. When purchasing a flexo printing machine, the user should select the corresponding supporting equipment according to the specific source of the printing processing order, to avoid the need to entrust the processing due to the incomplete function of the machine, or to cause the waste, or because the supporting equipment is idle for a long time.
What are the components of the unit type flexographic printing press?
The unit type flexo printing machine generally consists of the following components: unwinding device, rectifying device, tension and register control device, printing unit, drying system, die cutting station, waste disposal device, laminating and glazing unit, finished product collection device Wait.
What role does the correcting device play?
In order to ensure the lateral position of the printing material before entering the printing unit, the unit type flexo printing machine often adopts an automatic correcting device, which can detect the deviation of the lateral position of a certain edge of the printing material and correct it in time, with an accuracy of ±0.5 mm. . Generally, photoelectric sensors are often used for opaque materials such as paper and aluminum foil, and ultrasonic sensors are used for transparent materials such as films. Once the printing material enters the printing area and the die-cut rewinding area, the horizontal position is stabilized, which is ensured by the processing and mounting accuracy of the machine itself and the parallelism between the rolls.
What are the characteristics of the unwinding device?
In order to reduce the scrap rate, the printing time of each roll of printing material should be extended, but considering the handling convenience of the operating staff and the power limitation of the unwinding control motor, the diameter of the paper roll is generally about 1 meter. The open reel uses a pneumatic brake device to form an unwinding tension zone, and the magnitude of the braking force varies with the diameter of the paper roll to obtain a stable unwinding tension. The unwinding device is generally equipped with a paper receiving table, and if necessary, a dust and static device can be installed after the unwinding device to ensure the surface quality of the printing material entering the printing unit.
How to adjust the tension and register control
At present, the tension control system of the unit type flexo printing machine is to set the appropriate tension value according to the condition of the printing material. The system adopts closed-loop automatic control to keep the tension of the printing material constant from beginning to end. The register control is divided into two directions, with the paper feed direction as a reference, and the horizontal registration is to laterally move the plate cylinder, and the handle on the plate roller shaft can be rotated clockwise or counterclockwise. Generally, in the initial stage of trial printing, firstly ensure the lateral registration, and then adjust the vertical registration. First, the last unit of the first color pressure printing is stopped, and the coarse adjustment is performed. By rotating the plate cylinder, the registration error is within one tooth, and then the power is turned on, and the fine adjustment is manually performed by using the registration control button. The longitudinal registration system uses a high-speed stepping motor that drives the plate cylinder to rotate through the impression cylinder gear, which actually changes the longitudinal relative position between the plate cylinder and the substrate. Unlike wide-format flexo presses, unit-type flexo presses rarely use fully automatic longitudinal registration systems, because if each unit is equipped with such a system, the investment will account for a significant portion of the total investment in the machine. It has lost the characteristics of less investment and quick effect of the unit type flexo printing machine.
How to choose a printing unit
The number of printing units is determined by the user at the time of purchase of the machine, depending on the particular print order. The inking system of the printing unit mainly has the following forms: an ink-jet roller/anilox roller, an ink-feeding roller/anilox roller/squeegee blade or an anilox roller/squeegee blade. The ink roller is also called the metering roller. The hardness of the surface rubber and the pressure between the anilox roller and the number of the mesh of the anilox roller are the main factors affecting the amount of ink. Generally, in the case of printing a solid printed solid color and the uniformity requirement is not very strict, an ink-receiving roller/anilox roller type ink transfer system can be used, but the printing quality of such a system is inferior to that of the doctor blade. Inking unit. Therefore, although the use of a doctor blade increases the preparation time before printing and increases investment, more and more users are now choosing a printing unit with a doctor blade or an inking unit.
Since the flexographic material is soft and compressible, the pressure between the printing plate and the anilox roller on the printing unit and between the printing plate and the surface of the printing material needs to be finely adjusted. The adjustment knobs are usually made with very thin threads and require four directions of adjustment independent of each other. After fine adjustment, the best condition achieved should be maintained throughout the printing process.
What is the function of the drying system?
The unit type flexo printing machine is equipped with a dryer at the back of each unit, which uses infrared resistance wire to heat the air, and uses a blower to blow hot air at high speed to the printed substrate to form an impact gas to achieve the best. Drying effect. When printing in the field with a thicker ink layer, the speed of the flexo press is sometimes affected by drying problems. When this happens, try to increase the impact velocity of the airflow and try to make the lateral airflow uniform, because for water-based ink, the impact velocity of the dry airflow is more effective than the heating, and the heating of the printing material. Excessive use of the assembly caused the material to overextend, resulting in inaccurate overprinting.