Precautions for using gravure printing units on flexo presses
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With the continuous development of printing technology, the flexographic printing line is no longer limited to a single printing method, and has begun to transform into a diversified processing method that combines various printing processes and post-press processing. There are many successful cases in which a variety of processing methods such as gravure printing, screen printing, cold stamping, embossing, die cutting, etc. are added to a flexo printing machine.
Because gravure printing has high printing durability and long service life, the ink loading is larger than flexo printing, the ink layer is softer and thicker, and the color is softer than the soft printing. It is favored by packaging and printing enterprises. Adding a gravure printing unit to the flexo printing machine can be used to realize special printing processes such as gold ink, silver ink, matte ink, snowflake ink and the like, which have coarser particles and thicker ink layers. Combining years of work experience, the author summarizes the precautions for using gravure printing machines on flexo printing machines, and would like to share them with peers.
Printing tension control
Since the printing pressure of the gravure printing unit is much larger than that of the flexographic printing unit, when the two printing methods are used on the same equipment, there will be a problem that the printing tension of the flexographic printing unit does not match the printing tension of the gravure printing unit, resulting in gravure printing. Color registration and flexo color set overprinting or overprint fluctuations occur frequently. If the circumferential registration of the gravure printing unit is adjusted, the registration deviation of the flexo printing unit will be more serious. Therefore, how to control the printing pressure to maintain the stability of the printing process overprinting, and reduce the impact of the gravure circumferential registration adjustment on the flexographic printing unit is an important problem to be solved. After continuous exploration, research and practice, the author believes that it can be controlled from the following four aspects.
(1) Scientific arrangement of printing color sequence
Scientific and reasonable arrangement of the printing color sequence. Production practice found that the closer the color group is to the gravure printing unit, the greater the influence of the gravure printing unit on its registration. Therefore, when arranging the printing color sequence, try to arrange the color with low registration requirement before and after approaching the gravure printing unit. Location, the registration requires high color away from the gravure printing unit.
(2) Reasonable arrangement of idle units
If there are idle units, they should be arranged as close to the front and rear of the gravure printing unit.
(3) Reduce the printing pressure of the gravure printing unit
The printing pressure of the gravure printing unit should be minimized to reduce the influence on the printing tension of the flexo printing unit.
(4) Installation tension control system
A tension control system is installed at the end of the flexo printing unit to create a closed loop of tension control throughout the printing process to maintain print tension stability.
Practice has proved that the above four methods can significantly improve the problem of mismatch between the printing tension of the gravure printing unit and the flexographic printing unit, and effectively solve the registration problem.
Use of scraper
As the saying goes, "Open a gravure press, relying on a knife." The counterparts who have operated the gravure press know that the correct use of the scraper determines the quality of the gravure and the service life of the gravure. Under normal circumstances, the scraper mechanism of the gravure printing machine is mainly provided with: a detachable scraper splint mechanism, a scraper angle adjusting mechanism, a scraper left and right tilting mechanism, a scraper clutch pressing mechanism and the like. The specific considerations are as follows.
1. Removable scraper plate mechanism
The function of the detachable blade clamp mechanism is to secure the blade to the tool holder and to facilitate the operator to replace the blade. The upper and lower splints must be thoroughly cleaned before installing the scraper. Care must be taken during installation. The scraper blade must not be damaged. The scraping blade must be kept straight on the clamping plate. When tightening the screws on the clamping plate, the two sides should be tightened one by one to prevent the blade from being displaced, and the cutting edge of the blade is parallel to the axis of the intaglio.
2. Scraper angle adjustment mechanism
The blade angle adjustment mechanism is usually adjusted by a worm gear, and the size of the blade directly determines the thickness of the gravure ink layer. Generally, the smaller the angle between the doctor blade and the intaglio plate, the greater the thickness of the ink layer, but the ink in the gravure blank is not easy to be scraped, and the printed matter is prone to the phenomenon of knife wire and ink drawing; the larger the angle between the blade and the intaglio, the ink The smaller the layer thickness, the easier the ink in the blank of the gravure is, and the problems such as the knife wire and the ink are not easy to occur.
3. Scraper left and right tilting mechanism
The left and right flapping mechanism of the scraper is driven independently by the low speed motor, and the left and right tilting of the scraper is realized by the eccentric shaft on the low speed motor. The left and right turbulence of the scraper can change the trajectory of the scraper at the intaglio, and carry away the ink collected at the scraper to avoid the problem of the knife and the ink. When the printing equipment is running, it is necessary to pay attention to whether the scraper mechanism is running normally.
4. Scraper clutch pressure mechanism
The blade clutch pressing mechanism realizes the clutch pressure by the cylinders installed at both ends of the blade holder, and adjusts the blade pressure by increasing or decreasing the cylinder air pressure. The blade pressure should be as small as possible so that the ink in the gravure blank can be wiped clean. The smaller the blade pressure, the less wear on the intaglio pockets and the doctor blade. If the blade pressure is too large, the blade edge is prone to slight bending deformation, and even when it is severe, it may even form a wave-shaped curve, causing problems of flying ink and ink. Under normal circumstances, the blade pressure should be controlled within 0.2MPa. The minimum blade pressure used by our company is 0.04MPa, which still meets the printing requirements.
It should be noted that the blade angle, the blade pressure, the type of the blade, and the mounting method of the blade have a great influence on the quality of the gravure. In actual use, the depth of the gravure, the thickness of the ink layer, the hue requirement, and the ink performance are determined. Such factors are reasonably adjusted to make the printing quality meet the needs of customers.
Ink viscosity control
The viscosity of the ink is closely related to the ink transfer effect, the thickness of the ink layer, and the color.
When the viscosity of the ink is low, the solvent content of the ink is relatively high, and the content of the resin, pigment and the like is small, which may cause the overall hue of the printed product to be shallow, whitish and faint, and in serious cases, water quality and the like may occur. Especially when printing gold and silver ink, it is necessary to pay attention to the viscosity of the ink to avoid the situation that the content of gold and silver powder is low and the solvent content is too much.
When the viscosity of the ink is high, the fluidity of the ink will be deteriorated, and the ink cannot be smoothly transferred out of the cell. It is easy to cause problems such as loss of dots, paste, white spots, etc. In severe cases, the ink viscoelasticity is deteriorated. The ink is not easily separated from the blade during printing, and the ink is accumulated on the blade mouth to cause a serious ink squeezing phenomenon.
In addition to the ratio of solvent, resin, pigment, etc., temperature is also an important factor affecting the viscosity of the ink. When the temperature of the printing shop is low, especially below 10 ° C, the fluidity of the ink becomes poor and the viscosity is correspondingly high.