When the ink doctor blade is worn and causes streaks during printing, it should be replaced promptly. Regularly disassemble and clean the blade holder, ensuring that the ink is thoroughly cleaned from the blade and pressure bar. If the ink in the blade holder is not cleaned for a long time, the blade edge will become curved and uneven after the doctor blade is installed. This will cause circumferential ink marks on the anilox roller, making it difficult to adjust during printing. Plastic doctor blades are generally used for solid area printing and perform well. Due to the high-speed operation of flexographic printing presses, the blade edge wears quickly, often leading to fragments of the polyester plastic doctor blade falling into the ink or becoming trapped between the doctor blade and the anilox roller, and then transferred to the printing plate, affecting the print quality. Steel doctor blades, due to their lower wear rate, are generally installed in reverse for scraping, at an angle of 30° to 40°. The angle and pressure of the doctor blade significantly affect its wear rate, so correct installation of the doctor blade is crucial for printing. After each printing run, the ink doctor blade must be cleaned. Always wipe the blade edge with alcohol to evaporate any moisture, preventing rust and ensuring even ink scraping during printing.
Precautions for using ink doctor blades:
1. Blade Sharpness
The sharpness of the blade depends on whether it is a new blade, the blade angle during sharpening, and the type of sandpaper (whetstone) used. The ideal sharpness should effectively scrape off the ink without producing blade streaks. Overly sharp blades are prone to causing streaks and also wear down the printing plate.
2. Pressure
Factors affecting the ink doctor blade pressure include the cylinder pressure, the type of blade (hard or soft), and the angle between the ink doctor blade and the printing plate. The greater the angle, the greater the pressure. Excessive pressure reduces the ink transfer rate and causes greater wear on the printing plate and blade; insufficient pressure can easily lead to dirty plates or doctor blade lines.
3. Distance between the Ink Doctor Blade and the Printing Plate
Reducing the distance between the doctor blade and the printing plate helps to improve the ink transfer rate, but it can easily lead to dirty plates; conversely, increasing the distance can effectively solve the problem of dirty plates and prevent some fine lines, but it will reduce the ink transfer rate.
4. Movement of the Ink Doctor Blade
The left-to-right movement of the ink doctor blade plays a crucial role in reducing doctor blade lines, improving blade utilization, and minimizing wear on the printing plate. If the ink doctor blade is not moving properly, improvements should be made.
One cause is damage to the printing plate, and another is a dull blade. However, another reason is that foreign matter is stuck to the blade edge. The solution is to move the ink doctor blade back and forth and up and down to dislodge the adhering material. If it cannot be dislodged, the machine must be stopped to clean the ink doctor blade.
5. Flatness of the Blade
The flatness and warping of the blade mainly depend on the installation method, although it may also be related to foreign matter adhering to the blade holder groove or the blade and liner.
Poor blade flatness can cause some droplet-shaped marks on the edges of the printed surface, or intermittent line marks. Some believe that this phenomenon only occurs when the ink is coarse or has poor fluidity; however, poor blade flatness can also cause this problem.
