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Causes and countermeasures of gravure printing knife lines

Causes and countermeasures of gravure printing knife lines

Causes and countermeasures of gravure printing knife lines

Plastic film gravure printing often has many quality problems, among which the cutting line is one of the most common problems. There are many reasons for the knife line, and specific analysis should be made according to the actual situation, and countermeasures should be found. The following is an analysis of the failure phenomenon that occurred.
1. The knife line is related to the viscosity of the ink
The generation of knife lines is directly related to the viscosity of the ink. In actual production, the higher the viscosity of the ink, the easier it is to produce knife lines. This type of knife line roughly presents two shapes on the printed product.
1) The first type of knife line is a fine streamlined thread, which generally does not drag too long, but appears intermittently on the printed product (as shown in Figure 1). The starting point of the fine streamlined knife line is mostly close to the printed pattern, and the knife line is generally thicker at the starting point than the end. If you don't observe carefully in actual production, this kind of fine flow filament knife line is easy to be ignored. The method to eliminate this streamlined knife line is as follows: ① Grind a soft plastic card with a width of 5 cm, a length of 15 cm and a thickness of 140 cm to be very smooth, and lightly pick it where the knife line appears. Under normal circumstances, this kind of fine flow wire will be improved. This kind of operation is generally completed when the machine is running, and it must be done by a skilled operator. ②The most fundamental solution is to add an appropriate amount of solvent to the ink to reduce the viscosity of the ink. After the viscosity of the ink is reduced, the fine streamlined knife lines will generally be eliminated.
2) The second cutting line is the starting point of the printed pattern, but the cutting line is concentrated and rough, and appears regularly in the same position of the printed product, and often continues to the next color. This kind of cutting line is more obvious and easy to be found. The reason for this kind of cutting line is: the ink viscosity is too high, and the printing plate has poor dot transfer, especially when the temperature is too high, a thin dry solid layer will be formed on the surface of the ink. Over time, the ink layer accumulated between the printing plate and the squeegee will lift the squeegee, thereby producing this kind of cutting line. The method to solve this kind of knife line is: ①Adjust the angle of the squeegee. Generally, the angle between the squeegee and the printing plate is 60 degrees. At this time, the angle of the squeegee and the printing plate can be increased to about 65 degrees to increase Squeegee pressure, and at the same time, add an appropriate amount of additives or solvents to the high-viscosity ink to reduce the viscosity of the ink and increase the fluidity of the ink. When adding solvent, the ink should be stirred. ②Put a stirring roller in the ink tank. With the rotation of the plate cylinder, the stirring roller will also rotate accordingly, which can prevent the ink surface from forming a layer. The above solutions are used in conjunction with each other. The above-mentioned concentrated and different thicknesses are usually Will be eliminated. It must be noted that the squeegee angle should not be increased too much, and the pressure should not be too high, otherwise it will easily damage the printing plate cylinder and affect the service life.
2. The relationship between the cutting line and the thickness of ink particles
The finer the ink particles, the better the quality of the print, and the probability of cutting lines is correspondingly reduced. There are strict requirements on the particle size of the ink, and the fineness diameter of the ink is usually required to be 15μm. If the fineness of the ink is not up to the requirement, the rough particles will push up the squeegee, resulting in a knife line. At the same time, the ink particles are too rough, which will damage the squeegee printing plate. The thickness of the ink should be moderate, too thin is not good. After long-term observation of the comparison of the knife line, it is found that the direction of the knife line on the printed product caused by the rough ink particles is not the same, mainly in the following three forms.
1) Irregular cuts of varying thickness appear on the printed product, which extends from the printed pattern part to the place where there is no pattern (as shown in Figure 3). The characteristic is that the cutting line is longer, and as the squeegee swings left and right, the cutting line also moves. Solution: Adjust the position of the squeegee up and down, and at the same time observe whether the printed product has any color shifts. Especially when the printing layout is a shallow screen, the up and down adjustment of the squeegee can easily cause color difference in the printed product, so attention must be paid.
2) The direction of the knife line is very regular, appearing in the same position of the same printed pattern (as shown in Figure 4). It changes with the left and right swing of the squeegee. This is because the coarse particles in the ink push up the squeegee, and the left and right swing of the squeegee cannot drive it away. If it appears in the intermediate color, the ink of the next color will fill these cut lines, that is, the hue of the ink of the next color appears at the cut line.
3) The cutting line is easy to appear on the last color, and usually appears on the white plate when printed in plastic film, and runs through the entire printed product. Observing from the front of the printed product, if the knife line is not thick, it is not easy to be noticed. From the back of the printed product, it can be found that the knife line forms a long "mark" (as shown in Figure 5). This kind of trace will appear during compounding, especially when compounding aluminized film, it will be more clearly distinguishable in a large area of ​​white area.
No matter what kind of cutting lines are produced, printing should be stopped and ink filtration should be carried out. Filters have different choices due to different manufacturers. Generally, it is recommended to filter with a 140 mesh metal screen.
3. The relationship between the knife line and the squeegee
The generation of the knife line has a lot to do with the quality of the squeegee, the installation angle, and the actual adjustment. Excellent printing operator, can adjust the squeegee to the best position. Not only can it reduce the generation of knife lines, but it also has a bearing on the improvement of the quality of the entire printed product. It can be said that the adjustment and installation of the squeegee are the core issues of gravure printing. The quality of the squeegee grinding has a lot to do with the knife line. If it is well ground, it is not easy to produce a knife line, otherwise it is easy to produce a knife line. When grinding, the ink stone is usually used for grinding, then 1000# water-resistant sandpaper and 6# machine oil are used for smoothing. The back of the knife is ground into a 25° bevel. If the scraper produces a serious gap, the blade should be replaced with a new one. At the same time, pay attention to whether the blade is imported or domestic. Imported blades are flexible and not easy to produce knife lines. If it is a domestic blade, the hardness is high, and it is easy to produce cutting lines. The blade must have a certain degree of flexibility. Hard and inelastic is the main cause of the knife line, and at the same time it damages the printing cylinder more severely. If the ink is not cleaned, the transparency will decrease, which will cause "printed product fog" failure. The tip of the squeegee should be rounded to protect the printing plate. When installing, the scraper is divided into main and auxiliary blades, and the auxiliary blade mainly supports the positive blade. When installing, the positive blade is 0.5cm longer than the secondary blade. The positive doctor blade is installed too prominently, and the doctor blade is easily deformed and has a wavy shape, which affects the ink squeegee effect and causes quality failure. If the installation is too short, the ink will be easily scratched, but there will be a lot of knife marks on the printing plate, and as long as the knife is loose, the printed pattern will become thicker or lighter and cannot be used repeatedly. In addition, the pressure of the squeegee is very important. Too much pressure will damage the printing plate, and it will not clean the ink if it is too light.

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