Flexographic printing skills
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In recent years, flexographic printing has been widely used in the packaging industry as an emerging printing method in China. It has the advantages of fast printing speed, high printing durability, easy operation, printing and post-processing can be produced in line, short production cycle, etc., and environmental protection is attracting more and more attention.
Flexographic printing has a wide range of materials and high production efficiency. It uses environmentally-friendly ink and UV inks. It can be integrated with offset printing, gravure printing, silk screen printing units and in-line production. It has become the focus of attention today.
China's flexographic development is relatively late, the development speed is slow, and the application level is low, which is quite different from that of European and American countries. However, in recent years, the application of milk packets, self-adhesive labels, cigarette labels, wine labels, toothpaste tubes, cartons and the like has become more and more widespread. Due to the lack of understanding of flexographic printing technology and the limitations of objective conditions, it is impossible to print products that can compete with offset printing.
For the maintenance and maintenance of different flexo presses, talk about our views on how to master the flexo printing process and adjust the pressure.
First, the maintenance and maintenance of the flexo printing machine
The synchronization of the printing unit of the flexo printing machine, the balance of the ink supply, and the balance of the plate roller have certain requirements in the printing process.
The power of the flexo printing machine is transmitted from the main motor to the gearbox of each unit, and the color groups are synchronized. To make the printing overprint stable, the gearbox should be well maintained. The reducer is one of the key components of the entire press. Pay attention to the oil quantity and oil temperature (the oil is turbid to be replaced) when starting the machine. Do not start the machine at high speed without lubrication to avoid damage. When the circumferential alignment is required, the circumferential adjustment motor is activated to make the planetary output frame rotate forward and reverse, so that the speed of the printing roller can be superimposed or superimposed at the synchronous speed, thereby completing the alignment requirement in the circumferential direction.
The balance of the amount of ink is achieved by the anilox roller and the doctor blade. The anilox roller provides accurate and reliable ink supply. There is a ceramic wire roller, which is coated with ceramic on the base roller and is laser engraved. Its precision, surface roughness, shape tolerance, hardness and corrosion resistance are all required to be high and expensive, and are produced by professional factories. In the use of such anilox roller, special care should be taken to prevent bumping, to keep it clean and to prevent ink clogging, and to clean it immediately after printing. The accuracy of the anilox roll is one of the important factors to ensure the printing quality of the flexographic printing press.
The anilox roll is a precise transfer roller. The amount of ink transferred is determined by the number of screens per inch. The higher the number of lines, the more net pits, the less the amount of ink; the fewer the number of screen lines, the less the number of net pits, the greater the amount of ink. The number of screen lines in the anilox roll is reasonable, which is determined by the color requirements of the printed product. Screen screening, network cable printing should select high line number, low ink volume; solid printing should choose low line number, large ink volume.
The squeegee is used to ensure the amount of ink evenly supplied to the plate. If the scraper is not used, the ink is transferred by the rubber roller, the anilox roller is pressed, the ink is transferred quickly, and the speed is slow. The amount of ink that the anilox roller has is very different, the amount of fast ink is much higher, and the rotation is slower. To this end, a doctor blade is placed on the flexo press. With the squeegee, the amount of ink delivered to the plate by the anilox roller is limited to the amount of ink stored in the anilox roll cell, so that the amount of ink in the plate can be balanced regardless of the speed of the machine. Therefore, the installation of the scraper has high requirements. The scraper must also have a certain degree of elasticity, toughness and strength. The knife edge should be straight and free of burrs and wrinkles. The steel scraper is generally 0.15 mm or 0.2 mm.
The blade is installed in the forward direction and reversed. Now the reverse squeegee is used and combined into the squeegee.
Second, the use of anilox roller and scraper
Problems to be aware of
1. The long-term use of the anilox roller will cause the bearing at both ends to wear, resulting in the defective ink of the printing product. At this time, the bearing at both ends should be replaced in time. Before the print has found no problems, the operator can also check that the dial gauge can be used to test the circumferential beat of the anilox roller, or to feel smooth on the doctor blade by running it. If there is jitter, replace the bearing in time. In order to ensure the printing accuracy and the quality of the printed product, the three-roller jump value of the anilox roller, plate cylinder and printing roller shall not exceed ≤ 0.02 mm.
2. After the squeegee is worn, it should be replaced in time. Usually pay attention to disassemble and clean the knife holder, and clean the ink in the scraper and the pressure strip. The ink in the tool holder is not cleaned for a long time. After the squeegee is installed, the knife edge will form a curved shape, which is not straight. The amount of ink scraped on the anilox roller forms a circumferential ink mark, which is difficult to adjust during printing.
The squeegee material is divided into a stainless steel scraper and a polyester plastic scraper. Plastic scrapers are generally used for solid layout printing, preferably. Due to the high speed operation of the flexo printing machine, the knife edge wears faster, which often causes the polyester plastic scraper debris to fall into the ink or sandwich between the scraper and the anilox roller, and then is transmitted to the printing plate, which affects the quality of the printed product. Due to the small wear and tear, the steel scraper is generally installed in reverse for scraping at an angle of 30° to 40°. The angle and pressure of the blade depends on the blade spring (cylinder) adjustment.
The long-term operation of the flexo press makes the scraper wear faster, so the correct installation of the scraper is especially important for printing. Pay attention to cleaning the ink in the knife holder and the pressure bar when replacing the squeegee. If there is dry ink in the knife holder and the pressure bar, the knife edge will be bent and not straight, which will cause the anilox roller to produce circumferential ink marks, which is difficult to adjust during printing and affect the quality of the printed product.
Every time you need to clean the squeegee, you must use alcohol to wipe the knife edge and evaporate the water on the squeegee to avoid rusting the knife and affecting the unevenness of the ink during printing.
Third, the flexographic printing pressure
Flexographic printing is a lighter, direct printing method known as "kissing" printing. Since the printing pressure of the flexographic printing is very small, the pressure will have a significant change in the quality of the printed product, so the control of the pressure is very important.
Different models have different adjustment methods, such as the "Black Bull" brand flexo press and the Ajia hair flexo press adjustment is basically the same.
Since pressure is the most sensitive issue in flexographic printing presses (unlike gravure, the quality of the print is not affected by stress), the operator of the flexographic press pays attention to the adjustment of the pressure during machine operation. When the general flexographic plate is finished, the entire plate thickness error should be within 0.015 mm to 0.02 mm. The pressure during printing is greater, and the ink transferred to the substrate is more. Therefore, in order to ensure that the entire pattern portion can be uniformly inked without causing hairiness, paste, etc., the printing pressure, that is, the compression amount of the printing plate should be strictly limited to 0.1 mm (screen size 0.02 mm). If the pressure is too large, the text image will be crushed, the dots will be deformed and connected to each other, and it may be unreal or the edges of the pattern may not be printed. The flexo is compressed by 0.01 mm, and the embossing force is doubled (1 to 3 kg/cm2). The increase in the dot size of the network cable version causes the dark portion of the full-page dot to become solid, and the dot appears hollow. The outlets are not clean. Due to changes in the level of the outlets, the color will be severely biased, so that the whole picture looks unclear and unclear, and the subtle levels cannot be produced. The stereoscopic effect is not strong. The middle level is too large, which will make the picture dark, and the color reproduction is not good. Bright and other issues seriously affect product quality. Fields, lines, and texts have edge effects and hard mouths. The prints look uneven, the ink color is not thick, and the text pastes the plate, which results in the loss of the color of the letterpress printing and the sharpness of the writing pen.
The size of the dots determines the tone reproducibility of the print.
According to the principle of geometry, the dot gain is a uniform outward expansion of the edge portion. For different percentages of dots, the edge increase of each ratio is consistent. The outlets of the same area and different shapes have a larger expansion rate of the dots with larger circumferences under certain pressure conditions.
Therefore, the quality of flexographic printing is closely related to the operational adjustment pressure. The size of the printing embossing force should be adjusted to the point where the dot is not enlarged or the smallest, the edge effect and the hard mouth are not visible or inconspicuous, which is crucial. When adjusting the pressure, the pressure should be zero, and the pressure on both sides should be consistent (balance line and test strip should be made on both sides of the plate during plate making to ensure the pressure on both sides of the plate is consistent). The actual printing should be determined by the specific conditions of the layout. For example, the pressure should be relatively small when printing a fine screen version, and the pressure should be relatively large when printing a solid version. 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 large, the ink is squeezed from the surface of the plate to the periphery of the image portion. On the shoulder of the word, a paste is formed. At this time, even if the printing pressure is appropriate, the dot overlap phenomenon will inevitably occur on the printed product; if the pressure between the two is too small, the ink transfer will be affected. Be sure to use the “Plan” (small gauge) to carefully adjust each time you use a set of rollers. Under the condition of ensuring the printing pressure, the pressure between the printing plate and the anilox 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 + double-sided adhesive Thickness + substrate thickness - about 0.01 mm to determine the above two pressures. When printing on the ground, to avoid excessive printing pressure, the gross image on the edge is generally not recognized by the naked eye.
In the case where the gauge (the feeler gauge) is not measured, the sheet of the substrate can be sandwiched between the printing plate and the impression cylinder, and then gently pulled, and then pulled out after a little force, basically It is more appropriate to stress.
The printing pressure is also related to the printing speed. The printing speed is fast, 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 roll and the plate roll is constant (0.02 mm to 0.03 mm), and the pressure of the plate roll and the roll is adjustable depending on the properties of the substrate.