Experience of using ink scraper in gravure printin

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Experience of using ink scraper in flexible packaging gravure printing

many faults are easy to occur in flexible packaging gravure printing, such as the printed matter is scratchy and Matt; Drag shadow occurs, with different depths; There are knife lines, etc. These faults may come from the film itself, or from mechanical rotation, operator technology, indoor temperature and humidity. Most of them are closely related to the ink scraper. Therefore, the correct selection, installation and use of doctor blade will greatly reduce the failure rate

the doctor blade plays a very important role in gravure printing. The correct use of the doctor blade can not only make the printing ink bright and saturated, but also reduce the printing quality accident to the lowest point

when the gravure machine runs at high speed driven by the motor, the doctor blade also swings accordingly. The structure of the doctor blade swing mechanism of gravure printing machines produced by different manufacturers is different. For example, the ys620 rotary gravure printing machine produced by Guangdong Shanzhang light industrial machinery factory used by our company, the blade swing is driven by a special micro motor, while the 800 gravure printing machine produced by Zhejiang Haining people's machinery factory, the blade swing is driven by the transmission gear. No matter what form of swing structure, we should achieve a goal: scrape off the excess ink, and reduce the friction between the blade of the doctor blade and the printing plate, so as to prolong the service life of the doctor blade and the printing plate

first, the choice of the doctor blade

if the doctor blade is not elastic, it cannot be quickly restored to its original position, and it will also leave the knife wire, scraper powder and copper plate powder generated during the swing on the printing plate, which will have a great impact on the color of the print

the doctor blade containing a certain proportion of C, Cr, Mn and Si should be selected, and the thickness of the doctor blade is usually 0.1 ~ 0.18mm. The scraper is too thick and inelastic, and it can't scrape the ink clean. When it is reflected on the print, it often appears foggy, which not only affects the next color overprint, but also the quality of the same batch of prints will be very different before and after

if the scraper is too thin, it is easy for the scraper to leave knife marks on the printing plate. Under the action of friction, the scraper will gradually wear out, which will inevitably affect the quality of printing. We noticed that when printing a large area of full page white background, longitudinal streaks often appear when using a too thin scraper

the hardness of the doctor blade also affects the quality of the printed matter. The scraper is iron, while the gravure is chrome plated on the surface of the copper drum. Because the hardness of copper is 200 ~ 300hv and the hardness of chromium is 700 ~ 900HV, it is best to choose an ink scraper with a hardness of 450 ~ 600hv, so that it will not hurt the printing plate, it is not easy to leave knife marks, and it can also scrape off the excess ink of the layout

the blade of the scraper should be circular arc, which is conducive to protecting the printing plate. Because the surface of the printing plate is arc-shaped, if the blade edge of the scraper is also arc-shaped, it is not easy to damage the printing plate, and it is easy to scrape the ink away, so as to ensure that the layout of non graphic parts is clean and does not stagnate ink. If the blade tip is flat, because the flat edge is very sharp, it is easy to damage the printing plate and affect the ink scraping effect, thus affecting the printing quality

II. Installation of doctor blade

when installing the doctor blade, it is often supported by the auxiliary blade. The auxiliary blade is generally made of hard steel. The thickness of the auxiliary blade selected by our company is 120 ~ 150 m. Generally, the auxiliary blade can also be fixed in the center of the doctor blade with 8 # nut clamps. It is about 8mm away from the back of the doctor blade and about 7mm away from the blade edge of the doctor blade (the specific position should be determined through tests)

the scraper is installed too prominently, and the scraper is easy to deform and appear wavy. When printing at high speed, some places are scraped clean, and some places are not scraped clean, which not only affects the printing quality, but also the scraper is easy to wear. The scraper extends too short and lacks the necessary elasticity. You must increase the pressure to scrape the ink in the non graphic part (without holes), which will also damage the printing plate. Therefore, it is very important to install the scraper properly, which should be adjusted according to the actual situation

the installation position of the doctor blade (the distance between the contact point between the doctor blade and the plate cylinder and the impression point between the impression cylinder and the plate cylinder) has a great impact on the print quality. Most gravure printing machines can adjust the scraper up and down and back and forth by adjusting the adjusting nut of the auxiliary scraper. If the adjustment is not good, the printed matter will be dull and unclear, and there will be dry plate, blocking plate, no coloring, false edge, poor ink layer adhesion fastness and other phenomena in printing. The installation position of the doctor blade is related to the printing speed and ink drying speed. The drying speed of the ink can be adjusted according to the printing speed, so the installation position of the doctor blade should be adjusted according to the printing speed. Generally, a plate cylinder with a diameter of about 15cm is used. When the machine speed is about 60m/min, the distance between the doctor blade and the impression point is 6cm. Printing speed is fast, distance is better, printing speed is slow, distance should be closer

III. use of doctor blade

before installing the doctor blade, whether it is a new blade or an old blade, the cutting edge should be ground first, so as to ensure that there is no notch and burr, and the cutting edge angle should be 25. Our factory usually adds a few drops of engine oil to the oilstone for rough grinding, and then uses 1000 × water-resistant emery cloth to add a few drops of engine oil for fine grinding. Adding a few drops of engine oil can play a lubricating role and reduce friction. The grinding effect is generally good. Peers may wish to have a try

most printing plants often encounter the phenomenon of light or thick ink color of printing products. Especially when printing eye adjusted printing products, this is not only related to the ink transfer, but also the angle between the doctor blade and the plate cylinder can not be ignored. Most gravure machines can choose 45 ~ 65 angles. The included angle is small, the color of the printing ink layer is deep, and the gloss is good, but the ink scraping effect is poor, and it is easy to scrape unclean and blurred graphics and texts. If the included angle is too large, the ink transfer may not be realized in the shallow part, such as the main pattern with shadow pattern is printed, but its shadow cannot be printed. The solution is to adjust the position of the scraper and raise the position of the knife rest to try to transfer all the ink to the substrate film

to adjust to the best angle, factors such as hole depth, scraper position and printing speed should also be considered. If the acupoint is too deep, the angle should be larger

the pressure of the doctor blade will also have a very important impact on the quality of prints. The pressure can have great development potential in the efficient drilling and processing of composite materials. It can be adjusted according to the depth of holes and graphic levels. If the plate cylinder is too long and corrodes the gravure, the pressure will be greater. Due to corrosion, intaglio holes are deep, but burrs are easy to appear on the edge of the pattern. But too much pressure is easy to scrape off the ink in the shallow hole, causing the shallow part to not print out; Moreover, too much pressure will also reduce the contact angle, making it difficult to scrape the excess ink on the layout, resulting in ink accumulation under the scraper and knife lines. Generally, the contact angle of the doctor blade system in good working condition is about 60

too light pressure of the scraper will not work, which will cause faults such as galling. The solution is to blow properly to the blade of the scraper, that is, the interface between the scraper and the printing plate. Special blowing facilities can be used for blowing

after using the doctor blade for a period of time, there will be burrs, cracks and other phenomena, and the doctor blade cannot work effectively. At this time, it is necessary to sharpen the blade or replace it with a new one. In practical work, sometimes the scraper breaks, and its service life can be extended by grinding or padding, which can not only save the cost of the scraper, but also solve the corresponding quality problems

the doctor blade should be kept clean and lubricated after use. The usual practice is:

(1) clean the used doctor blade with a cloth soaked in solvent. (2) After drying, apply engine oil, put it into plastic bags and store it in a ventilated place. Polyester blades are usually coated with 6# engine oil. (3) Keep it by category for future use

to sum up, the correct selection and use of doctor blade is very important to improve the quality of gravure printing products

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