Flexo printing operation skills

In recent years, flexographic printing has become widely used as a new printing method in the packaging industry in China. It has the advantages of fast printing speed, high resistance to printing force, easy operation, on-line production of printing and post-processing, short production cycle, etc., and environmental protection, which has attracted more and more attention.

Flexographic printing materials are widely used in printing, high production efficiency, the use of environmentally friendly ink, UV ink, the printing process can be integrated offset printing, gravure, silk screen printing units, on-line production, has become the focus of people's attention today.

The development of China's flexo edition is relatively late, its development speed is slow, and its application level is low. Compared with European and American countries, there is a big gap. However, in recent years, more and more applications have been made in the packaging of milk packages, adhesive labels, cigarette labels, wine labels, toothpaste tubes, and paper boxes. Due to the lack of understanding of flexographic technology and the limitations of objective conditions, people cannot print products that rival those of offset printing.

For the maintenance and maintenance of different flexo presses, we talked about how to master the flexographic process and regulate the pressure.

First, maintenance and maintenance of flexographic printing machine

The synchronization of the power of the flexo printing unit, the balance of the ink supply, and the balance of the printing roller have certain requirements in the printing process.

The power of the flexo printing machine is transmitted from the main motor to the reduction gear box of each unit, and the color groups are operated synchronously. To make the overprinting of the printed product stable, the gear box should be well maintained. The reducer is one of the key components of the entire printing press. Pay attention to the amount of oil and the temperature of the oil when it is turned on (the oil must be replaced when it is turbid). Do not start the machine at high speed without lubrication to avoid damage. When it is necessary to make a circle to the plate, the circumferential plate-version motor is started so that the planetary out-of-plane frame can produce forward and reverse revolutions, and the speed of the printing roller can be superimposed or overlapped at the synchronous speed, thereby completing the registration requirements in the circumferential direction.

The ink balance is achieved by an anilox roller and a doctor blade. Anilox roller supply is accurate and reliable. There is a ceramic network line roller, the base roller is coated with ceramic, laser engraving. Its precision, surface roughness, shape and position tolerance, hardness and corrosion resistance are all very demanding and expensive, and it is produced by a professional factory. When using this type of anilox roller, special care must be taken to prevent bumping, keep it clean and prevent ink clogging, and clean immediately after printing. The accuracy of the anilox roller is one of the important factors for ensuring the printing quality of the flexographic printing press.

Anilox rollers are precise transfer ink rollers. The amount of ink transferred is determined by the number of network cables per inch. The higher the number of lines, the more the number of network pits and the lower the amount of ink. The smaller the number of network lines, the smaller the number of network pits and the greater the amount of ink. The number of screen lines in the printing process of the anilox roller is reasonable, which is determined by the color requirements of the printing product. Screen screening, screen printing should be selected high number of lines, low ink volume; field printing should be selected low line, large amount of ink.

The squeegee is used to ensure uniform ink supply to the plate. If no scraper is used, the roller is used to transfer the ink, the anilox roller is pressed tightly, and the ink transfer is fast and the speed is slow. Anilox roller with a large difference in the amount of ink, rotating faster and more ink, slow rotation is less. For this purpose, a squeegee is provided on the flexo press. With the squeegee, the amount of ink transferred from the anilox roller to the printing plate is limited to the ink storage volume of the anilox roller. This ensures the balance of the printing plate ink regardless of the speed of the machine. Therefore, there are high requirements for installing scrapers. The scraper should also have a certain degree of elasticity, toughness and strength. The knife edge should be straight and free from burrs and wrinkles. The general choice of steel scraper is 0.15 mm or 0.2 mm.

Squeegee installation, there are positive and reverse mounting, and now more use of reverse squeegee, and combined into a scraping ink chamber.

Second, the use of anilox roller and scraper to pay attention to the problem

1. The use of anilox roller for a long time will cause the bearings at both ends to wear out, which will result in the shortage of printed ink. At this time, the bearings at both ends must be replaced. Before any problems are found in the printed matter, the operator can also check that the dial of the anilox roller can be tested with the dial indicator, or that it is normal to feel steady on the blade holder by hand. If there is jitter, 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 anilox roller, plate roller and printing roller shall not exceed 0.02 mm.

2. After the squeegee is worn out, it must be replaced promptly. Usually pay attention to disassemble the knife holder and clean it, and clean the ink in the blade and the knife bar. The ink in the knife holder has not been cleaned for a long time. After the ink squeegee is installed, the knife edge will be curved and not straight. The amount of ink scraped on the anilox roller forms a circumferential ink mark, which is difficult to adjust during printing.

Squeegee material is divided into stainless steel scraper and polyester plastic scraper. Plastic scrapers are generally used for field layout printing, preferably. Due to the high-speed operation of the flexo printing machine, the knife edge wears quickly, which often causes the scrap of the polyester plastic scraper to fall into the ink or between the scraper blade and the anilox roller, and then passes it on the printing plate, which affects the quality of the printed product. Because of the small wear of the steel scraper, the reverse mounting is generally used for scraping, and the angle is 30° to 40°. The angle and pressure of the blade depends on the adjustment of the blade spring (cylinder).

The long-term operation of the flexographic printer makes the doctor blade wear faster, so the correct installation of the doctor blade is especially important for printing. When the squeegee is worn out, pay attention to cleaning the ink in the knife holder and the knife bar. For example, there are dry inks in the knife holder and the press blade, which will cause the blade to be bent on the back of the knife, which is not straight. It will also cause the anilox roller to produce circumferential ink marks, which are difficult to adjust during printing and affect the quality of printed products.

To clean the squeegee each time printing is finished, be sure to wipe the blade with alcohol to evaporate the moisture on the blade so as to prevent the blade from rusting and affect the unevenness of the squeegee during printing.

Third, flexographic printing pressure <br> <br> flexographic printing is a pressure-less direct printing method is called "kiss" printing. Since the flexographic printing pressure is very small, the pressure will have a significant change in the quality of the print, so the pressure control is very important.

Different models have different adjustment methods, such as "Black Bull" brand flexo printing machine and Akai hair flexo printing machine adjustment is basically the same.

Since pressure is the most sensitive issue in flexographic printing presses (unlike gravure, the print quality is not affected by pressure), the operator of the flexographic printing press must pay attention to the pressure adjustment during the operation of the machine. When the general flexo plate is finished, the entire plate thickness error should be within 0.015 mm to 0.02 mm. Larger pressure during printing, printing plate transfer to the substrate more ink. Therefore, in order to ensure that the entire pattern can not only get ink evenly, but also produce no hairiness, stencil, etc., the printing pressure, that is, the compression of the printing plate should be strictly limited within 0.1 mm (screen 0.02 mm). If the pressure is too great, the text image will appear crumbling and the dots will be deformed and connected to each other. This may cause problems such as false appearances or inaccurate printing on the edge of the pattern. The flexographic plate is compressed by 0.01 mm, the embossing force is doubled (1 to 3 kg/cm2), and the dot gain value of the cable screen edition is severe. This will cause the dark tone of the entire screen dot to be ground and the dots appear hollow. The outlets are not clean. Due to changes in the network level, serious color cast will appear, causing the entire screen to look unclear and unclear. The subtle levels will not come out and the three-dimensional effect will not be strong; the middle level will cause the screen to be dark and the color reproduction will be poor. Brightness and other issues seriously affect product quality. In the field, lines, and texts, there are edge effects, hard mouths, and the imprints look unfair, the ink is not thick, and the text is stenciled, resulting in the loss of the color of the letterpress printing and the sharpness of the text.

The size of the outlet determines the reproducibility of the print.

According to the principle of geometry, the dot gain is a uniform outward expansion of the edge portion. With different percentages of outlets, the increase in the margin of each ratio is the same. For the same area and different shapes of the outlets, under certain conditions of pressure, the larger the perimeter, the larger the outlet expansion rate.

Therefore, the quality of flexographic printing is closely related to the adjustment pressure. The size of the print embossing force should be adjusted to the point where the dot is not enlarged or the expansion rate is the minimum, and the edge effect and the hard-edged state are invisible or inconspicuous, which is crucial. When adjusting the pressure, the pressure should be zero, and the pressure on both sides should be consistent (balance lines and test strips must be made on both sides of the plate at the time of plate making to ensure that the pressure on both sides of the plate is consistent). The actual printing should be based on the specific circumstances of the layout, such as printing a fine screen version of the pressure to be relatively small, and the pressure to print the field version to be relatively large. In addition, the adjustment of the pressure also includes the adjustment of the contact pressure between the surface of the anilox roller and the surface of the printing plate. When the pressure between the two is too great, the ink will be squeezed from the surface of the printing plate to the periphery of the graphic portion and accumulated. On the shoulder of the word, a stencil is formed. Even if the printing pressure is appropriate, the splicing phenomenon of the stencil will inevitably occur on the printed matter. On the other hand, if the pressure between the two is too small, the transfer of ink will be affected. Always use "plate gauges" (feel gauges) carefully adjusted before using each plate roller. Under the condition of ensuring the combined pressure of printing, the pressure between the printing plate and the anilox roller should be the thickness of the printing plate + the thickness of the double-sided tape - 0.01 mm. The pressure between the printing plate and the substrate should be the thickness of the printing plate + the double-sided tape. Thickness + substrate thickness - about 0.01 mm to determine the above two pressures. When printing in the field, it is necessary to avoid excessive pressure on the printing, and it is generally impossible to distinguish the ghosts on the edges by the naked eye.

Without measuring the gauge, the paper of the substrate can be sandwiched between the plate and the impression cylinder, and then gently pulled to pull it out after a little force, basically Stress is more 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, and the printing pressure should be appropriately increased. Otherwise, the printing pressure should be reduced.

The pressure of the anilox roller and the plate roller is constant (0.02 mm to 0.03 mm). The pressure of the plate roller and the printing roller can be adjusted according to the performance of the substrate.

Source: Longjiang Printing Network