Maintenance of the hottest flexographic press and

The maintenance of flexographic press and the operating skills of flexographic printing in recent years, flexographic printing has been widely used in the packaging industry as a new printing method in China. It has the advantages of fast printing speed, high printing resistance, simple operation, online production of printing and post-processing, short production cycle, and environmental protection, which has attracted more and more attention

flexographic printing has a wide range of materials and high production efficiency. Environmental friendly ink and UV ink are used. Offset printing, gravure printing and silk screen printing units can be integrated in the printing process, and online production has become the focus of attention today

the development of flexo in China is relatively late, the development speed is slow, and the application level is low. There is a big gap compared with European and American countries. However, in recent years, it has been more and more widely used in the packaging of milk bags, self-adhesive trademarks, cigarette labels, wine labels, toothpaste tubes, cartons and so on. Due to the lack of understanding of flexographic printing technology and the limitations of objective conditions, it is not possible to print products comparable to offset printing

talk about our views on the maintenance of different flexo printing machines, how to master flexo printing technology, and how to adjust pressure

I. maintenance of flexographic press

there are certain requirements in the printing process for the synchronization of printing unit power, the balance of ink supply, and the balance of plate rollers

the printing power of the flexographic press is transmitted to the reducer of each unit by the main motor, and the color groups operate synchronously. To make the overprint of printing stable, the reducer should be well maintained. Reducer is one of the key components of the whole printing machine. Pay attention to the oil quantity and oil temperature when starting up (the oil should be replaced if it is muddy). Do not start up at high speed without lubrication to avoid damage. When circumferential alignment is required, start the circumferential version adjustment motor to make the planetary wheel carrier rotate forward and backward, so as to achieve the superposition or superposition of the speed of the printing roller on the synchronous speed, so as to complete the requirements of circumferential alignment

the balance of ink volume is completed by the corrugated roller and scraper. The ink supply of the corrugated roller is accurate and reliable. There is a kind of ceramic line roller. The base roller is coated with ceramic and carved by laser. Its precision, surface roughness, form and position tolerance, hardness and corrosion resistance are all highly required and expensive, and it is produced by professional factories. When using this kind of roller, pay special attention to prevent bumping, keep it clean and prevent the ink from blocking. Clean it immediately after printing. The accuracy of corrugation roller is one of the important factors to ensure the printing quality of flexographic printing machine

the corrugated roller is an accurate ink transfer roller. The amount of ink is determined by the number of lines per inch. The higher the number of lines, the more pits, and the less ink; Fewer lines means less pits and more ink. The reasonable number of roller in the printing center line is determined by the printing color requirements. Screen plus, line printing should choose the number of high lines, low ink volume; Low line count and large ink volume should be selected for field printing

the doctor blade is used to ensure the uniform supply of ink to the printing plate. If you don't use a scraper, use a rubber roller to transfer ink, and the wrinkle roller is pressed tightly. The ink rotation is fast and the speed is slow. There is a great difference in the ink volume of the corrugated roller. The ink volume is more when rotating fast, but less when rotating slowly. For this purpose, an ink scraper is set on the flexographic press. With the doctor blade, the amount of ink transferred to the printing plate by the ink of the roller is limited to the ink storage of the roller hole, so that the balance of the printing plate ink amount can be guaranteed no matter how fast or slow the machine runs. Therefore, the installation of scraper has high requirements. The scraper should also have certain elasticity, toughness and strength, and the blade should be straight without burr and wrinkle. Generally, the steel scraper used is 0.15 mm or 0.2 mm

the scraper can be installed in the forward direction or in the reverse direction. Now the reverse scraper is mostly used, and it is combined into a scraper cavity

II. Problems needing attention in the use of corrugated rollers and scrapers

1 The long-term use of the corrugated roller will cause the bearing wear at both ends, resulting in the incomplete printing ink. At this time, the bearings at both ends should be replaced in time. Before the problem is found in the printed matter, the operator can also check it. The dial indicator can be used to test the circumferential runout of the wrinkle roll, or it is normal to put your hand on the scraper frame and feel stable during operation. If there is vibration, replace the bearing in time. In order to ensure the printing accuracy and the quality of printed products, the runout value of the three rollers of the wrinkle roller, the plate roller and the substrate roller shall not exceed ≤ 0.02 mm

2. After the doctor blade is worn, it should be replaced in time. At ordinary times, pay attention to disassemble the knife rest for cleaning, and clean the ink in the scraper and blade. The ink in the knife rest has not been cleaned for a long time. The installation of the scraper can really make the graphene grow. After the knife is installed, the knife edge will form a curve, uneven and straight. The amount of ink scraped on the roller will form a circumferential ink mark, which is difficult to adjust during printing

the materials of the doctor blade are divided into stainless steel scraper and polyester plastic scraper. Plastic scraper is generally used for field layout printing, which is better. Due to the flexographic press, it does not mean that testing cannot be done. High speed operation and rapid blade wear often lead to polyester plastic scraper debris falling into the ink or sandwiched between the scraper and the embossing roller, and then transmitted to the printing plate, affecting the quality of printing products. Due to small wear, the steel scraper is generally installed in reverse direction for scraping, with an angle of 30 ° ~ 40 °. The angle and pressure of the scraper depend on the adjustment of the scraper spring (cylinder)

the flexographic press runs for a long time, and the wear of the scraper is fast, so the correct installation of the scraper is also particularly important for printing. When replacing the worn doctor blade, pay attention to cleaning the ink in the tool holder and blade. For example, there is dry ink in the knife rest and knife pressing strip, which will cause the knife edge to be bent and uneven after the scraper is installed, and will also cause circumferential ink marks on the wrinkle roller, which is difficult to adjust during printing and affect the print quality

clean the doctor blade after each printing. Be sure to wipe the blade edge with alcohol to evaporate the water on the blade, so as to prevent the blade edge from rusting and affect the uneven scraping during printing

III. printing pressure of flexographic printing

flexographic printing is a direct printing method with less pressure, which is called "kissing" printing. Because the pressure of flexographic printing is very small, and the magnitude of the pressure will significantly change the print quality, the control of the pressure is very important

different models have different adjustment methods. For example, the adjustment of "Heiniu" flexographic press is basically the same as that of yajiafa flexographic press

since pressure is the most sensitive problem in flexographic printing machine (unlike gravure printing, the printing quality is not affected by pressure), the operator of flexographic printing machine should pay attention to the adjustment of pressure during machine operation. Generally, when the flexible printing plate is finished, the thickness error of the whole printing plate should be controlled within 0.015 mm ~ 0.02 mm. When printing, the pressure is higher, and the printing plate transfers more ink to the substrate. Therefore, in order to ensure that the whole pattern part can be inked evenly without hairiness, plate pasting and other phenomena, the printing pressure, that is, the compression of the printing plate, should be strictly limited within 0.1 mm (plate 0.02 mm). If the pressure is too high, the text and image will collapse, point deformation and interconnection, which is prone to problems such as inaccuracy or pattern edge printing. When the flexo is compressed by 0.01 mm, the embossing force will double (1 ~ 3 kg/cm2), and the increase value of line plate points is serious, which will make the dark part of the whole plate points become solid, and the points appear hollow points, and the points are not bright and clean. Due to the change of point level, the color will be seriously biased, so that the whole picture looks unclear and unclear, the subtle level cannot come out, and the stereoscopic sense is not strong; If there are too many intermediate layers, the picture will be darker, the color restoration will be poor, and the color will be less bright, which will seriously affect the product quality. The edge effect and hard mouth appear in the field, lines and words, the impression looks uneven, the ink color is not thick, and the words are pasted, so that the characteristics of letterpress printing with rich color and clear writing peaks are lost

determines the tonal reproducibility of the printed matter

according to the principle of geometry, point increase means that the edge part expands evenly outward. For different percentage points, the edge increase of each proportion is consistent. For points with the same area and different shapes, under certain pressure, the larger the circumference, the greater the point expansion rate

so 6. Graphics: appropriate graphics and tables have user-defined graphics, labels and automatic adjustment proportion. The quality of flexographic printing is closely related to the operation adjustment pressure. The size of the printing force should be adjusted to the condition that the point does not expand or the expansion rate is the smallest, and the edge effect and hard mouth cannot be seen or obvious, which is very important. When adjusting the pressure, the pressure should be zero and the pressure on both sides should be consistent (during plate making, balance lines and test strips should be made on both sides of the plate to ensure that the pressure on both sides of the plate is consistent). The actual printing depends on the specific situation of the layout. For example, the pressure is relatively small when printing a fine line version, while the pressure is relatively large when printing a field version. In addition, the pressure adjustment also includes the adjustment of the contact pressure between the surface of the embossing roller and the surface of the printing plate. When the pressure between the two is too high, the ink will be squeezed from the surface of the plate to the surrounding of the image and text part, and stacked on the shoulder of the word to form a paste plate. At this time, even if the printing pressure is appropriate, there will inevitably be point overlap on the print; On the contrary, if the pressure between the two is too small, it will affect the transmission of ink in the traditional aluminum alloy modification. Before using a set of plate rollers every time, be sure to use "plate gauge" (feeler gauge) to carefully adjust. Under the condition of ensuring the printing pressure, the pressure between the printing plate and the embossing roller should be the thickness of the printing plate + the thickness of the double-sided adhesive -0.01 mm, and the pressure between the printing plate and the substrate should be the thickness of the printing plate + the thickness of the double-sided adhesive + the thickness of the substrate - about 0.01 mm, so as to determine the above two pressures. When printing on the spot, it is necessary to avoid excessive printing pressure. Generally, the ghosting on the edge cannot be recognized by the naked eye shall prevail

without measuring the plate gauge (feeler gauge), you can clamp the paper of the substrate between the printing plate and the embossing cylinder, and then gently pull it. When you can pull it out with a little force, the pressure is basically appropriate

the printing pressure is also related to the printing speed. The printing speed is fast, and the contact time between the printing plate and the substrate is short. The printing pressure should be appropriately increased, otherwise the printing pressure should be reduced

the pressure of the plate roller and the plate roller is constant (0.02 mm ~ 0.03 mm), and the pressure of the plate roller and the substrate roller is adjustable according to the performance of the substrate

source: Huicong printing industry channel Author: Yang Junjian, song xiongzhuang