Causes and countermeasures of knife lines in gravure printing
Release Time:
Nov 07,2013
The gravure printing of plastic film often encounters many quality issues, among which knife lines are one of the most common problems. The causes of knife lines are varied and should be analyzed based on actual conditions to find countermeasures. Below is an analysis of the fault phenomena that occur.
1. Knife lines are related to the viscosity of the ink.
The occurrence of knife lines is directly related to the viscosity of the ink. In actual production, the higher the viscosity of the ink, the more likely knife lines will occur. These knife lines generally present two shapes on the printed product.
1) The first type of knife line appears as fine streaks, usually not extending too long, but intermittently appearing on the printed product (as shown in Figure 1). The starting point of fine streaks is mostly close to the printed pattern, and the knife line is generally thicker at the starting point than at the end. In actual production, if not observed carefully, these fine streaks can easily be overlooked. The methods to eliminate these streaks are as follows: ① Smoothly grind a soft plastic card that is 5cm wide, 15cm long, and 140cm thick, and lightly lift it at the location of the knife line. Under normal circumstances, these fine streaks will improve. This operation is generally completed while the machine is running and must be performed by a skilled operator. ② The fundamental solution is to add an appropriate amount of solvent to the ink to reduce its viscosity. Once the viscosity of the ink is reduced, the fine streaks are generally eliminated.
2) The second type of knife line starts from the printed pattern, but the knife lines are concentrated and relatively rough, and they appear regularly at the same position on the printed product, often continuing to the next color. This type of knife line is more obvious and easy to detect. The cause of this type of knife line is: the viscosity of the ink is too high, and the dot transfer on the printing plate is poor, especially when the temperature is too high, a thin solidified layer forms on the surface of the ink. Over time, the accumulated ink layer between the printing plate and the doctor blade will lift the doctor blade, thus producing this type of knife line. The methods to solve this type of knife line are: ① Adjust the angle of the doctor blade. Generally, the angle between the doctor blade and the printing plate is 60 degrees. At this time, the angle can be increased to about 65 degrees to increase the pressure of the doctor blade, while adding an appropriate amount of additives or solvents to the high-viscosity ink to reduce the viscosity and increase the fluidity of the ink. When adding solvent, the ink should be stirred. ② Place a stirring roller in the ink tank. As the printing cylinder rotates, the stirring roller will also rotate accordingly, preventing the surface of the ink from forming a layer. The above several solutions should be used in combination, and the concentrated, uneven knife lines mentioned above will usually be eliminated. It must be noted that the angle of the doctor blade should not be increased too much, and the pressure should not be too high, otherwise it may damage the printing plate cylinder and affect its service life.
2. The relationship between knife lines and the particle size of the ink.
The finer the ink particles, the better the quality of the printed product, and the lower the probability of knife lines occurring. There are strict requirements for the fineness of the ink, usually requiring the ink fineness diameter to be 15μm. If the ink fineness does not meet the requirements, rough particles will lift the doctor blade, thus producing knife lines. At the same time, overly rough ink particles can cause significant damage to the doctor blade and printing plate. The fineness of the ink should be moderate; being too fine is also not good. Through long-term observation of knife lines, it has been found that knife lines caused by rough ink particles have different orientations on the printed product, mainly presenting the following three forms.
1) Irregular knife lines of varying thickness appear on the printed product, extending from the printed pattern to the non-pattern area (as shown in Figure 3). The characteristic is that the knife lines are relatively long, and as the doctor blade swings left and right, the knife lines also move. The solution: Adjust the position of the doctor blade up and down, while observing whether there are any color depth variations on the printed product. Especially when the printed surface is a light screen, adjusting the doctor blade up and down can easily cause color differences on the printed product, which must be noted.
2) The direction of the knife lines is very regular, appearing at the same position of the same printed pattern (as shown in Figure 4). They change with the left and right swing of the doctor blade. This is because the coarse particles in the ink lift the doctor blade, and the left and right swing of the doctor blade cannot eliminate them. If they appear in the middle color, the next color of ink will fill in these knife lines, resulting in the next color's hue appearing at the knife line.
3) Knife lines are likely to appear on the last color, usually appearing on the white plate when printing on plastic film, and they run through the entire printed product. From the front of the printed product, if the knife lines are not thick, they are not easy to detect; however, from the back of the printed product, the knife lines can be seen forming a long 'mark' (as shown in Figure 5). This mark will become apparent during lamination, especially when laminating aluminum-coated film, where it is even clearer in large areas of white.
Regardless of the type of knife line that occurs, printing should be stopped, and the ink should be filtered. The filtering mesh varies by manufacturer. Generally, it is recommended to use a 140 mesh metal screen for filtering.
3. The relationship between knife lines and the doctor blade.
The occurrence of knife lines is greatly related to the quality, installation angle, and actual adjustment of the doctor blade. An excellent printing operator can adjust the doctor blade to the best position. This not only reduces the occurrence of knife lines but also relates to the overall quality improvement of the printed product. It can be said that the adjustment and installation of the doctor blade are core issues in gravure printing. The quality of the doctor blade grinding is closely related to knife lines; if ground well, knife lines are not easily produced; otherwise, knife lines are likely to occur. During grinding, it is usually selected to use ink stone for grinding, and then polished with 1000# waterproof sandpaper and 6# machine oil for smoothness. The back of the blade should be ground to a 25° bevel. If the doctor blade has severe notches, it should be replaced with a new blade. It is also important to note whether the blade is imported or domestically produced. Imported blades have elasticity and are not easily prone to knife lines. If it is a domestic blade, the hardness is higher, making it easier to produce knife lines. The blade must have a certain degree of elasticity; being hard and inelastic is a major cause of knife lines and can also cause significant damage to the printing cylinder. Failing to scrape the ink cleanly leads to a decrease in transparency, resulting in 'printed product haze' faults. The tip of the doctor blade should be chosen to be arc-shaped to protect the printing plate. During installation, the doctor blade is divided into primary and secondary blades, with the secondary blade mainly supporting the primary blade. It is advisable for the primary blade to be 0.5cm longer than the secondary blade during installation. If the primary doctor blade is installed too prominently, it is easy for the blade to deform and take on a wavy shape, affecting the ink scraping effect, thus causing quality faults. If installed too short, the ink can be easily scraped off, but there will be many knife marks on the printing plate, and as soon as the blade is loosened, the printed pattern will become darker or lighter, making it impossible to reuse. Additionally, the pressure of the doctor blade is crucial; too much pressure can damage the printing plate, while too little pressure will not scrape the ink cleanly.
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