Many faults are easy to occur in gravure printing of flexible packaging, such as the printed products are blooming and dull; smearing occurs, the shades are different; there are knife lines and so on. These failures may come from the film itself, but also from mechanical rotation, operator technology, room temperature and humidity. Most of these failures are closely related to ink scrapers. Therefore, correct selection, installation and use of doctor blades will greatly reduce the failure rate.
Squeegee blades play an important role in gravure printing. Proper use of squeegee blades can not only saturate the ink color of printed products, but also minimize printing quality accidents.
When the gravure press runs at high speed driven by the motor, the doctor blade also swings accordingly. The structure of the squeegee 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 Industry Machinery Factory used by our company, the scraper swing is driven by a special micro motor, and the 800 type gravure printing machine produced by Zhejiang Haining People's Machinery Factory, the scraper swing is driven by the transmission Driven by gears. Regardless of the type of swing structure, we must achieve one goal: scrape off excess ink, while reducing the friction of the blade of the doctor blade and the printing plate to extend the service life of the doctor blade and the printing plate.
1. The choice of doctor blade
If the squeegee is not elastic, it cannot be quickly restored to its original position, and the knife wire, squeegee powder, and copper plate powder generated when the pendulum is left on the printing plate will have a great influence on the color of the printed product.
A doctor blade containing a certain proportion of C, Cr, Mn, and Si should be selected. The blade thickness is usually 0.1 to 0.18 mm. The squeegee is too thick and has no elasticity, and the ink is not cleaned. When it is reflected on the printed product, it often appears foggy.
If the squeegee is too thin, the squeegee will easily leave a mark on the printing plate. Under the action of friction, the squeegee will gradually wear out, which will inevitably affect the quality of the print. We noticed that when printing a large area full-page white background, the use of too thin scrapers often shows longitudinal streaks.
The hardness of the doctor blade also affects the print quality. The squeegee is iron, and the gravure is chrome-plated on the surface of the copper cylinder. Because the hardness of copper is 200-300HV and the hardness of chromium is 700-900HV, it is best to choose a squeegee with a hardness of 450-600HV, so that it will not damage the printing plate , And not easy to leave a knife mark, you can also scrape off the excess ink on the layout.
The blade of the scraper should be rounded, which is conducive to protecting the printing plate. Because the surface of the printing plate is arc-shaped, if the blade edge is also arc-shaped, it is not easy to damage the printing plate, and it is also easy to scrape the ink, to ensure that the non-graphic area 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 scraping effect, which affects the printing quality.
2. Installation of doctor blade
When installing a doctor blade, it is often supported by a sub-blade. The secondary blade is generally made of hard steel sheet. The thickness of the secondary blade selected by our company is 120 ～ 150μm. The secondary blade is generally clamped in the center of the squeegee with an 8 # nut, about 8mm from the back of the squeegee blade, and about 7mm from the blade edge of the squeegee (the specific location should be determined by testing).
The squeegee is installed too prominently, and the squeegee is easily deformed and wavy. In high-speed printing, some places are cleaned and some are not cleaned, which not only affects the printing quality, but also the scrapers are easy to wear. The squeegee extends too short and lacks the necessary elasticity. It is necessary to increase the pressure to scrape off the non-graphical (no mesh) ink and damage the printing plate. Therefore, the proper installation of the scraper is critical, and it should be adjusted according to the actual situation.
The installation position of the doctor blade (the distance between the contact point of the doctor blade and the plate cylinder to the impression point of the impression cylinder and the plate cylinder) has a great influence on the printing quality. Most gravure presses can adjust the blade up and down and back and forth by adjusting the adjustment nut of the auxiliary blade. If the adjustment is not good, the printed product will be dull and unclear, and dry printing, blocking, coloring, virtual edges, and poor adhesion of the ink layer will occur during printing. The installation position of the doctor blade is related to the printing speed and the 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 15 cm is used. When the machine speed is about 60 m / min, the distance between the doctor blade and the printing point is 6 cm. The printing speed is fast, the distance is better, the printing speed is slow, the distance should be closer. [next]
3. Use of doctor blade
Before installing the squeegee, whether it is a new blade or an old blade, the cutting edge should be ground first to ensure that there are no gaps and burrs, and the angle of the cutting edge is preferably 25 °. In our factory, we usually add a few drops of engine oil to the oil stone for rough grinding, and then use a 1000 # water-resistant abrasive cloth to add a few drops of engine oil for fine grinding. Adding a few drops of engine oil can lubricate, reduce friction, and the grinding effect is generally good.
Most printing factories often encounter the phenomenon that the ink color of the printed product is light or dark. When printing the mesh to adjust the printed product, in addition to the transfer of ink, the angle between the doctor blade and the plate cylinder cannot be ignored. Most gravure printing machines can choose 45 ° ～ 65 ° angle. The angle is small, the printing ink layer is dark in color, and the gloss is good, but the effect of scraping ink is poor, and it is easy to appear unsharp and blurry. If the included angle is too large, the shallow mesh part may not be able to achieve ink transfer, such as the phenomenon that the main pattern with the shadow pattern is printed out, but the shadow is not printed. The solution is to adjust the position of the scraper and increase the position of the knife holder, so that all the ink is transferred to the printing base film.
To adjust to the best angle, factors such as cell depth, scraper position and printing speed must also be considered. If the net hole is too deep, the angle should be larger.
The pressure of the doctor blade will also have a very important effect on the quality of the printing. The pressure can be adjusted according to the depth of the hole and the level of the graphic. If the plate cylinder is too long and it is corroded by gravure, the applied pressure will be greater. Because the corrosion gravure mesh is deep, but the edges of the pattern are prone to burrs and other phenomena. But too much pressure can easily scrape off the ink in the shallow mesh cavity, causing the shallow mesh part to not be printed; and too much pressure will also make the contact angle smaller, it is difficult to scrape the excess ink on the layout, resulting in the accumulation of ink under the blade Knife line. Under normal circumstances, the scraper system in good working condition, the contact angle should be about 60 °.
If the pressure of the scraper is too light, it will not work, and it will cause problems such as hair dragging. The solution is to properly blow air to the blade edge, that is, the interface between the blade and the printing plate. Special blowing facilities can be used for blowing air.
After the doctor blade is used for a period of time, there will be burrs, cracks and other phenomena. The doctor blade cannot work effectively. At this time, it is necessary to sharpen the knife or replace it with a new doctor blade. In actual work, sometimes the squeegee breaks, and the service life can be prolonged by grinding or padding. First, the cost of the squeegee can be saved, and the corresponding quality failure can also be solved.
The squeegee should be kept clean and lubricated after use. The usual practice is:
(1) Clean the used squeegee with a cloth dampened with solvent.
(2) After drying, apply motor oil, put it in a plastic bag, and store it in a ventilated place. Usually polyester blade is coated with 6 # motor oil.
(3) Classified storage for future use.
In summary, the correct selection and use of doctor blades is essential for improving the quality of gravure products.
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