With the continuous development and growth of China's packaging and printing industry, the application of flexo printing technology is becoming more and more extensive, and the technology is becoming more and more mature. In packaging and printing, some advantages of flexo printing are gradually recognized and valued by people. However, as with other printing processes, if you neglect the control of process technology, printing quality failures are likely to occur during the printing process, which will cause certain obstacles to normal production.
Several technical control points for correct use of water-based ink
Prevent the ink from becoming color impure. Because the water-based ink is liquid in nature, compared with offset and relief printing inks, it is most prone to delamination or precipitation, so when using or sampling, the ink should be fully stirred before being put into use to ensure the stability of the printing color .
The ink viscosity is determined according to the printing speed. Due to the "thixotropic" characteristics of the ink, the viscosity of the ink should be determined according to the area of ​​the printing plate and the speed at the time of printing.
Correctly adjust the ink suitability. When the viscosity is too high, use water or special solvents for dilution adjustment. If the viscosity is too low, it is best to use the original ink to adjust, and try to use less or no thickener. When the color density is high, the diluting agent can be used for proper diluting treatment.
Do ink testing. During the printing process, you should always pay attention to tracking the changes of ink viscosity, and do a good job of ink viscosity detection and adjustment to ensure the stability of product printing quality.
Prevent excessive foaming of ink. If more foam appears during the printing process, it is easy to affect the quality. A small amount of defoamer can be added to eliminate the foam. The amount of defoamer should be controlled within 0.2% of the amount of ink.
Control the printing process. In order to prevent the post-printing ink from making the dried background ink turbid, when overprinting on the background color, a printing process of gradually increasing the viscosity of the ink and gradually increasing the PH value of the ink should be used, which is also beneficial to the drying of the ink.
Perform a small sample test on the ink. Printing non-absorbent materials, such as plastic film, vacuum aluminized paper and other substrates, it is best to conduct a small sample test to check the abrasion resistance, adhesion and drying effect of the ink on the printed sample in order to select the appropriate The inks required for product quality are printed in batches to prevent quality accidents.
Do a good job of cleaning the machine. After printing, the rubber roller, anilox roller and printing plate on the machine should be cleaned as soon as possible.
Pay attention to store ink. The ink should never be placed in an open air environment. The temperature for storing ink should be around 20 ℃. The unused ink should be put back in the barrel and kept in a sealed and cool place.
Pay attention to filtering the old ink. When using old ink, it is best to use a filter mesh of more than 80 mesh to filter out impurities before use, to prevent the occurrence of printing quality failures.
Pay attention to the ink shelf life. The shelf life of general ink is about 12 months, while the shelf life of gold ink is half a year, or even less than half a year. Therefore, according to the production situation, we must purchase the ink brand and quantity suitable for the characteristics of the product to ensure the printing quality of the product.
Use good old ink. When printing, the ink fountain only maintains a certain amount of ink to achieve normal inking printing, so there must be a certain amount of remaining ink. If Yu Mo does not continue to use it, it will cause waste and increase production costs. Therefore, when distributing ink, try to control the amount of ink adjustment. It is better to store the remaining ink separately and seal it well, so as not to mix the ink skin or debris. If impurities appear, they should be filtered and put into use.
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Operation technology control of using doctor blade correctly
The squeegee is an important part of the flexographic printing machine's ink delivery, and it is also a control condition that can not be ignored. The squeegee and ceramic anilox roller are matched to form an ink transfer system, which generally uses a plug-in reverse squeegee structure, which has the advantages of convenient operation and uniform ink scraping. Correct adjustment of the pressure of the doctor blade should be based on the scraping of excess ink on the anilox roller surface. If the pressure of the scraper is too high, it is easy to scratch or wear the ceramic anilox roller. Due to the contact of the doctor blade with the surface of the anilox roller when scraping ink, it is easy to wear during use, which affects the uniformity of the doctor blade and the stability of the ink transfer quality. Therefore, the new doctor blade should be replaced in time to ensure the printing ink color quality. It is worth mentioning here: in the absence of ink, the ceramic anilox roller must not let the squeegee contact with it, so as not to damage the surface of the anilox roller due to dry friction. When using the scraper, also pay attention to whether the installation angle is appropriate, which is a very important technical control link. Generally, the angle of the scraper should take into account the number of lines and the pressure of the scraper, so as to maintain a good scraping effect. Generally, the installation angle of the reverse scraper is 30º ～ 40º of the tangent line. If the number of the anilox is higher, the angle of the scraper can be slightly larger, and the pressure between the scraper and the anilox roller should be increased accordingly. In the process of scraping the friction of the scraper, it is inevitable that there will be wear and tear and affect the uniformity of the scraping. In production, care should be taken to replace the scraping scraper in time. In addition, we should pay attention to the reasonable selection of the material of the doctor blade, which has an important effect on better reducing the wear of the doctor blade and the anilox roller. The production process practice shows that the steel squeegee is easy to wear, and it is easy to cause damage to the anilox roller, and the squeegee of polypropylene material is relatively wear-resistant. Choosing a polypropylene squeegee can better guarantee the ink scraping The stable quality is conducive to reducing the printing defects in production and improving the printing efficiency and quality.
Do a good job in controlling the quality of flexographic dot printing
With flexible printing dots, if the production process is not properly controlled, quality problems such as dot loss and deformation expansion are prone to occur. For example, small dots in the highlight part of the layout are easy to be lost, and the dark tone dots are easy to expand and paste and become like the ground. Due to the expansion and deformation of the dots, the print layout lacks a sense of hierarchy, and the quality of the printing copy is significantly reduced. The printing plate material is soft and elastic, which is the main reason why the flexographic printing dots are prone to quality problems. Process analysis shows that in the flexographic printing process, under the action of pressure, the size of the printing plate becomes larger, so that the dot area becomes larger, and small dots are easy to be lost due to wear or uneven pressure. On the other hand, due to the use of low-viscosity inks for flexographic printing, when the contact pressure between the surface of the printing plate and the printed product is slightly greater, the ink on the printing plate will spread outward, making the middle ink layer of the expanded dot thin. The ink layer on the edge is thick, forming a hollow-like dot. When the magnifying glass is used to observe, the dot edge can be seen to be pasted. This is the basic feature of the increase of the dot of the flexographic printing. The production process also shows that the higher the dot line number of the flexible version, the greater the dot increase value; the thicker the particles of the ink used, the greater the dot expansion; the larger the amount of ink in the layout, or the ink is too thin, the dot expansion The greater the printing pressure and the anilox roller pressure, the greater the dot expansion; the softer the substrate, rubber roller, liner or flexographic plate, the greater the dot expansion; the aging of the machine so that the accuracy is worse, the dot expansion The bigger. In addition, the thickness of the printing plate is inconsistent, and it is easy to make the printing dots expand or lose.
Control well the technological process of plate making
Because the flexographic plate material is soft, the plate surface is prone to deformation. This is because the flexographic printing machine is a high-speed rotary machine, and the radius of the plate cylinder is small, so the bending deformation of the plate is large. When the plate is pasted to the cylindrical cylinder After the surface, the printing plate is curved and deformed along the circumferential surface of the cylinder, which makes the size of the printed graphics more obvious. In addition, if the web printing material has soft characteristics, it is easy to change in elongation under the action of tension and printing pressure during printing, and the substrate in the axial direction of the cylinder shrinks. In addition, the printed graphic part of the flexographic version is convex, but the non-graphical area is concave, so that if the printing speed is adjusted lower; the printing pressure is adjusted more; the machine accuracy is lower; the blank layout area is larger ; The greater the depth of the graphic relief, the more severe the deformation of the printing plate. The variation of the graphic specifications of the printing layout makes the printed semi-finished products fail to meet the requirements of process design, which has a greater impact on the prints with higher quality requirements, such as overprinting; the semi-finished products are die-cut, cut or trimmed into After the single-plate structure of the finished product, there is a deviation error in the printed graphic layout, especially the multi-imposition structure of the printed layout. This performance is particularly powerful, often making the finished product fail to meet the quality standards or process design requirements. In view of the above-mentioned print quality defects, it is necessary to take measures to overcome changes in print size. First, the printing plate should be pasted flat, the plate center should be in close contact with the surface of the cylinder, and the plate should be aligned at one time to prevent deformation of the plate; second, the moderate printing pressure should be controlled, and the pressure should not be too large; three It is to control the tension of the roll material transportation, and there must not be too loose or too tight; Fourth, the printing speed should not be too fast or too slow, and maintain a constant printing speed; Fifth, pay attention to the maintenance of the machine and all the components Lubrication work to prevent the wear of key components and affect the printing accuracy; Sixth, according to the usual accumulated production experience, properly adjust the layout specifications in the plate making process, so that the deformed layout after printing meets the quality requirements of overprinting, die cutting and trimming.
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Several technological essentials for quickly changing the color of car wash
Master the car wash method. When washing the car, do not circulate and return the car wash water in the same bucket. Instead, put the inlet port in a bucket filled with fresh water, and put the outlet port in another empty bucket to prevent dirty water during washing. Back into the clean water bucket. When the cleaning is almost clean, you can let the inlet nozzle suck a little ink (color) to be printed, and then clean it again with clean water, so that the cleaning can achieve both water saving and fast cleaning of the ink circulation system, which can greatly improve car washing Efficiency and quality.
Disassemble and wash some parts of the ink circulation system at the right time. The pump head, ink storage tank, ink collection tank and filter screen of the ink circulation device are often not easy to clean, and some accumulated ink will remain for a long time. Therefore, these "dead corner" parts should be disassembled and cleaned in time. In this way, the car is usually washed Changing colors is easier to clean.
Correctly select the ink delivery tube. If the ink inlet and outlet (retention) pipes in the circuit of the ink circulation system are aging or damaged, please pay attention to the selection of tubes with good flexibility, good elasticity, not easy to aging and deformation, smooth inner wall, ink delivery pipe when replacing It should be as short as possible to facilitate the smooth flow of ink and rapid cleaning.
Pay attention to prevent the ink from drying on the machine during the printing process. If the shutdown time is longer for some reason during the production process, the ink circulation system should be cleaned to prevent the ink from drying and skinning, affecting the smoothness of the circulation system, and thus affecting the rapid car wash color change.
Reasonably arrange the printing color sequence to facilitate fast car wash color change. If you want to print multi-color carton products, it takes 2 or more times to complete the printing, you should pay attention to the reasonable arrangement of printing color sequence, in order to better improve the efficiency and quality of car wash color change. If a two-color flexo printing machine is used to print a red, blue, black, and green (without overprinting) 4-color carton product, the color sequence can be arranged as follows: for example, the first group is printed in red before the group is printed in blue; The second front group should be printed in black, and the rear group should be printed in green, so that car wash color change is relatively fast and easy to clean. Various color conversions are easier to have the following conditions: transfer dark colors on light colors; transfer red on yellow; transfer green on yellow; transfer brown on red; transfer red or deep red on gold-red, etc., Can be flexibly mastered according to the actual situation.
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