Plastic film gravure printing often has many quality failures, among which the generation of knife lines is one of the most common failures. The production of the knife line makes the product scrapped, which is easy to cause economic loss. As a printing enterprise engaged in flexible packaging, it is urgent to analyze and summarize the causes and find corresponding solutions, so as to reduce the fault of the knife line to the lowest point.
1. The relationship between the generation of the knife line and the viscosity of the ink
The generation of the knife line has a direct relationship with the viscosity of the ink. In actual production, the higher the viscosity of the ink, the easier it is to produce cutting lines during printing. These cutting lines generally appear in two forms on printed products. The first type of knife line is a fine streamline, generally not too long, but appears intermittently on the printed product. The starting point of the fine streamline knife line is mostly close to the printed pattern, and the start of the knife line The starting point is generally thicker than the end. If it is not observed carefully in actual production, this fine streamlined knife line is easily overlooked. This streamlined knife line is often the root cause of quality accidents and must be paid attention to. There are several ways to solve this streamlined knife line, which is also a common method for printing employees, but it should also be determined according to the actual situation.
a. Grind a soft blade with a width of 5cm, a length of 15cm, and a thickness of 140um to be very smooth. Gently pick where the knife line appears. Under normal circumstances, this tiny streamlined knife line will be improved. This kind of operation is generally completed when the machine is running, which requires that you must be familiar with the operator to complete it, so as not to be self-defeating and cause unnecessary losses.
b. The 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 streamline blades are generally eliminated. This is because the viscosity of the ink decreases after the addition of the solvent, and the fluidity increases accordingly. Therefore, the fine streamlined blade line is also related to the deterioration of the fluidity. This should be noted.
The second type of cutting line is the printed pattern as the starting point, but the cutting line is concentrated, and it is relatively rough, and it appears regularly at the same position on the printed product, and often has to continue to the next color. This kind of cutting line is easy to be found . The reason for this knife line is that the viscosity of the ink is too high, the transfer point of the printing plate is poor, and the auxiliary agent in the ink is destroyed with the change of the ambient temperature, especially when the temperature is too high, a thin layer will form on the surface of the ink Dry solid layer, the ink layer accumulated between the printing plate and the squeegee blade will push up the squeegee blade for a long time, thus producing this kind of knife line. The solution to this knife line is to adjust the angle of the doctor blade first. Generally, the angle between the doctor blade and the printing plate is 60 °, but sometimes the angle between the doctor blade and the printing plate can be increased to about 65 ° In this way, the pressure of the doctor blade is increased accordingly. At the same time, add the appropriate amount of corresponding additives or solvents to the ink with high viscosity to reduce the viscosity of the ink and increase the fluidity of the ink. When adding solvent, the ink should be stirred according to the direction of the solvent. Another solution is to put the stirring in the ink tank; the roller, as the plate cylinder rotates, the stirring roller also rotates accordingly. Preventing ink layering on the surface of the ink is also a way to reduce the occurrence of knife lines. The above two solutions are used in conjunction with each other. In the absence of special circumstances, the above-mentioned concentrated thickness of the knife line will be effectively contained. It must be noted that the angle of the squeegee should not be increased too much to prevent damage to the brand roller .
Second, the relationship between the generation of the knife line and the ink particle size
The thickness of the ink particles is closely related to the generation of the knife line. The finer the ink, the better the quality of the print, and the lower the probability of the knife line. There are strict requirements for the particle size of the ink, usually the ink fineness is 15um, the ink fineness does not meet the requirements, the rough particles will push up the scraper, thereby generating a knife line, and at the same time too rough ink particles The damage of the plate is very large, the thickness of the ink should be moderate, too fine is not good.
After a long-term observation of this type of knife line, it is found that the line of the knife line caused by the roughness of the ink particles on the printed product is also different, mainly in three forms.
1. Irregular thickness lines appear on the printed product of the first type of thread, which extend from the printed pattern part to the non-patterned part. The knife line is long, and as the doctor blade swings left and right, the knife line also moves, sometimes one, two, sometimes many. The solution is to adjust the position of the squeegee up and down, and at the same time, observe whether the printed product has color shading. 2. The trend of the second type of knife line is very regular, and it appears in the printed pattern part on the same printing plate, and it is in the same position. With the doctor blade swinging left and right, there is basically no big change. This is because the ink The coarse particles in the jacked up the scraper, and the left and right swing of the scraper cannot drive it at all. If it appears in the middle color, the next color ink will be filled with these knife lines, that is, the next color ink appears at the knife line. 3. The third kind of cutting line is easy to appear on the last color, and usually appears on the white plate when printed in plastic film. Such one or two cutting lines will run through the entire printed product. The knife line is not rough, and it is not easy to notice. From the back of the printed product, you can find the "mark" formed by the knife line throughout. This kind of "mark" will undoubtedly appear when compounding, especially when compounding aluminum film. It can be clearly seen.
Regardless of the above-mentioned knife line, the solution is to stop printing and filter the ink. Usually, the ink used in the first and second colors is filtered before printing on the machine. The filter screen is 140 mesh metal screen. It has achieved initial results and is worthy of peer learning and reference. [next]
Third, the relationship between the generation of knife line and the plate cylinder
The generation of the knife line is inseparably related to the surface finish of the roller and the quality of the coating. The printing plate is divided into two types: hand-engraved version and electric engraved version. The hand-engraved intaglio version is not smooth and prone to knife lines; the surface of the electric engraved intaglio plate is smooth, and the probability of generating knife lines is much lower than that of the hand-engraved version. When printing with hand-engraved gravure, 1000 # water-resistant sandpaper is used to polish the layout before the first time on the machine. The specific method is: open the printing speed faster and run it empty. The sandpaper moves back and forth during polishing, but it cannot be overpowered Large, otherwise it will damage the net hole. After polishing, use a soft cloth to wipe the cylinder, and then print on the machine. The knife line phenomenon has been reduced a lot. It should be noted that this work needs to be performed by experienced personnel. If such a knife line appears during printing, the method is the same as before.
The hardness of the coating layer is also related to the generation of the knife line. The hardness of the coating layer is not enough. It is easy to be damaged under the action of the scraper, and the knife line will also be generated. This kind of knife line is only easy to find under the illumination of the reverse side of the fluorescent lamp. For this knife line, there are two solutions: one is to gently grind the coating with 1000 # water-resistant sandpaper after this situation occurs, generally It will be improved, but it often appears after printing for a period of time, so it is best to carry out the deplating process, and if it is more serious, you need to re-make the plate; Hair dryer also received very good results.
Fourth, the relationship between the generation of the knife line and the doctor blade
The generation of the knife line has a lot to do with the quality and installation of the doctor blade. Whether the squeegee is grinded or not has a lot to do with the knife line. Properly grinded, it is not easy to produce a knife line, otherwise it will be easy to produce a knife line. When grinding, usually use oil stone grinding, and then use 1000 # water-resistant sandpaper plus 6 # engine oil to smooth, the back of the knife is grinded into a 25 ° bevel. If the scraper has a serious gap, it should be replaced with a new blade. At the same time, it should also be noted that if the blade is imported with soft elasticity, it is not easy to produce cutting lines, if it is a domestic blade, it is easy to produce cutting lines. The blade must have a certain elasticity. Hard and inelasticity is the main reason for the cutting line. There are other reasons, such as scraping the excess ink on the printing plate, and fogging to affect the quality. Generally, the hardness of copper is between 200HV-300HV, the hardness of chromium is between 800HV-900HV, the hardness of the scraper should be selected between 450HV- (900 + 300) / 2, so that it will not damage the printing plate and leave no knife marks. At the same time, it also reduces the chance of generating a knife line. The tip of the squeegee should be rounded, so that the printing plate can be protected, because the plate is round and the scraper is rounded. Without destroying the printing plate,
It is easy to scrape the ink. Keep the clean and uninhibited ink at the place where there is no pattern when printing. If the blade tip is flat, because the flat edge is very sharp, it will damage the printing plate and affect the printing. During the installation, the scraper should be protected between 0.5cm-1.2cm between the small plate in the lithographic frame under the scraper. The scraper is too protruding, the scraper is easily deformed, and it has a wavy shape, which affects the scraping effect and causes quality accidents. If the installation is too short, the ink will be easily scraped off, but there will be a lot of knife marks on the plate, and as long as the knife prints out, the pattern printed becomes thicker or lighter, and it cannot be used repeatedly. In addition, if the hammer is too important and the pressure of the blade is too high, the cutting line will be generated. The solution is to reduce the weight of the hammer and at the same time to properly blow air to the layout, it will also effectively reduce the generation of the cutting line.
Fifth, the relationship between the generation of knife line and the environment
The generation of the knife line has a great relationship with the quality of the environment. If there is dust in the operating environment, the dust will enter the ink and it will cause a knife line. The workshop should first isolate the printing unit to reduce the flow of personnel. The room is equipped with air conditioning facilities and dust removal equipment. When working, the inks of the first color sequence and the second color are filtered and then printed on the machine, and the dust is removed before work. At the end of the day, there are special sanitation personnel to clean up, and the environment must be kept clean.
The generation of cutting lines is diverse and eclectic, which requires the operator to develop the habit of summing up experience and actively cooperate with the factory management to minimize the cutting line failure.
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