Flexible printing plate is one of the common production processes currently used for printing packaging products. It is widely used in packaging and printing industry due to its convenient plate making, easy plate mounting operation, wide printing adaptability, high production efficiency, less equipment investment and low production cost The wider the application. As with any printing process, some flexographic printing problems inevitably occur during the flexographic printing process, which affects the printing quality of the product. Therefore, understanding and mastering the flexographic printing-prone problems can be compared It can reduce or avoid the chance of printing quality failure, realize normal production and improve the printing quality of products.
The ink color of the printed product is uneven and there is a flower or paste phenomenon
In the flexographic printing process, sometimes the ink color of the printed surface of the printed product will be uneven, flowering or scumming. The main reasons and corresponding solutions are as follows:
1. The thickness of the printing plate is inconsistent or the hardness is too low. When making a plate, it is required to select a plate material with the same thickness for the production; for the layout where the field and the small text coexist, a thin layer of adhesive tape can be stuck on the back of the field plate; if it is a problem caused by the poor quality of the printing plate, consider using a high-quality plate Make a plate.
2. The ink is dried on the printing plate. Dry ink solidified on the printing plate will produce poor ink transfer phenomenon, so, in the printing process, those who need to stop for a long time should clean the machine in time. If the ink drying speed is too fast, you can use an appropriate amount of slow drying agent to adjust the dryness of the ink.
3. It is caused by poor precision of the anilox roller or too much or too little contact pressure with the printing plate. If the shaft head of the anilox roller is worn or loose, corresponding repair measures should be taken to eliminate it; if the structure of the traction roller is separated and the pressure is unstable, the cause should be identified and dealt with to ensure that the anilox roller is pressed. No bouncing phenomenon; if the roller pressure on both sides is inconsistent and the roller pressure is too large or too small, it should be accurately adjusted and adjusted.
4. Caused by unstable working of the pressure roller. For two-color flexographic printing presses with direct printing on corrugated cardboard, if the pressure roller shaft head is bent, deformed, worn or loose, corresponding measures should be taken to repair; if the position of the pressure roller is too high or too low, it should be passed Adjust the proper position of the pressure roller so that the gap between the pressure roller and the plate cylinder matches the thickness of the cardboard to ensure that the ink color on the printing plate surface remains stable.
5. Caused by bad rubber roller. If the rubber roller ages and deforms, the roller shaft head may loosen or jump, which may cause unstable ink supply and affect ink color quality. In this regard, a new rubber roller should be replaced, and corresponding repair measures should be taken to eliminate the loosening and bounce of the rubber roller shaft head.
6. Caused by bad doctor blade. At present, the flexographic printing machine ink delivery system does not use a rubber ink roller, but the anilox roller is directly immersed in the ink tank. When rotating, the ink on the roller surface is scraped off by the doctor blade. In machines with this structure, if there is wear and deformation or the doctor blade is loose and bounces, it will also cause ink supply instability and affect the quality of the ink color. Straight and straight lines; take repair measures to eliminate loose or bounced parts.
7. The viscosity of the ink is not suitable. If the viscosity of the ink is too large or too small, it is easy to make the ink color of the printing plate surface appear to bloom or paste. Generally, the viscosity of ink for printing anilox plates and form plates should be a little lower, while for printing solid plates and graphic plates, the viscosity of the ink should be a little higher. The viscosity of water-based ink is generally controlled between 40-50S. However, because the temperature when using ink varies from place to place, which will affect the viscosity and printing effect of the ink, it is necessary to scientifically grasp and adjust the actual situation according to the printing environment.
8. Caused by poor ink fineness. If the dots are printed with ink with poor fineness, then because of the coarse pigment particles in the ink, the printing plate surface is prone to paste and dot defects, and the printing durability of the printing plate will also decrease. Therefore, for printing dot products, ink with good fineness should be used for printing. The fineness of the ink has a large concentration, which can make the dots of the printed product clear and full.
9. Improper selection of adhesive tape. Improper selection of paste-on tape can easily cause the ink color of printed products to bloom. Therefore, the tape should be selected reasonably according to the characteristics of the printing plate. The low-density tape should be used for the thin line and screen layout; the medium-density tape can be used for the text line and the solid version; High-density tape can be used.
Phantom appears on the print
When the substrate is paper, there is a cloud-like phantom in the ink layer of the printing plate or another part of the shadow of the large area on the ground, and there is a blank embossed shape on the ground, and the print of the cellophane is cloudy. Phantom phenomenon. The reasons are: uneven application of ink; too small amount of ink distribution and poor ink fluidity and unclean anilox roller.
The solution is: change the diameter of the plate cylinder and the different speed of the anilox roller; thoroughly clean the anilox roller and the printing plate with a solvent; replace the ink and use the correct solvent formula; use slow drying agent as appropriate ; Appropriately increase the amount of ink supply to ensure a certain amount of ink on the anilox roller, increase the anilox roller immersion area; reduce the solvent when printing cellophane; control the drying of ink.
Small dots in the ink layer of the printed product
Small dot-shaped eyelets appear on the ink layer of flexo products, and the printing industry is habitually calling it a "pinhole". There are two manifestations of "pinholes" in flexo products. One is that there are many many dot-like marks on the printed products, and the distribution is not regular, but the size of the "pinholes" are also different, usually called " "Chemical pinhole", which usually appears on cellophane and overprinted products. Another kind of "pinhole" is a "pinhole" trace that appears on a large area of ​​the ink layer of the printed product, and its size is relatively uniform, usually called "mechanical pinhole". There are many reasons for the occurrence of "pinholes" in the flexographic printing ink layer. Corresponding solutions should be taken according to the problems in order to better eliminate the fault and ensure the printing quality of the product. If there are "pinholes" in the ink color of the printed matter in the production process, there are several situations as follows:
If the surface tension of the printing is uncomfortable, add about 1.5% of the surface tension regulator of water-based ink to adjust and control.
If the wettability between the ink and the surface of the printed product is too poor, causing the ink layer on the printed product to partially shrink and produce "pinhole" marks, consider adding some help to improve the wettability and leveling in the ink Agent.
If the temperature in the printing workshop is low or the humidity is too high, and the viscosity of the ink is too small or the drying is too slow, so that the ink cannot completely wet the surface of the printed product, you can adjust the ink viscosity by increasing the temperature of the printing workshop and reducing the humidity. Or add some additives to improve the suitability of the ink. If the ink adjustment is invalid, you can consider replacing the ink type to solve.
If the ink is too thin or the printing ink layer is too thin, the viscosity of the ink should be adjusted, and the ink supply for printing should be appropriately increased.
If the ink is condensed, air bubbles are generated because of air entry, and no uniform ink layer is formed on the surface of the printed product, some ink additives can be added to improve the fluidity of the ink.
If the printing ink dries fast on the anilox roller and causes uneven ink transfer and uneven ink application, slow drying agent can be used for adjustment. If it takes a long time to stop in the middle of printing, the machine should be cleaned in time.
If the anilox is clogged or the anilox roller is badly worn to reduce the amount of ink, the anilox roller should be cleaned or replaced with a new anilox roller to ensure the ink supply and maintain a moderate thickness of the printing ink layer.
If the surface of the printed product is rough and the ink layer is difficult to maintain close contact with the depressed points on the surface of the printed sheet, the printing pressure, the amount of ink coating, or the solid plate should be overprinted twice.
If the surface of the printing plate is worn to form rough marks, or the printing plate is caused by slight corrosion due to poor acid resistance on the surface, a new printing plate should be replaced.
If there is dirt on the impression cylinder, the impression cylinder should be cleaned.
If the printing pressure is too light to cause poor transfer of the ink layer on the printing plate, the printing pressure should be increased appropriately.
If impurities adhere to the plate or printed sheet, measures should be taken to prevent dust, dust and paper dust from remaining on the plate and printed surface.
Loss or deformation of flexographic dot printing
In the printing production process, 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 high-gloss part of the layout are easy to be lost, and the dark-tone dots are prone to expand and paste and become like the ground. Therefore, the flexographic printing process is difficult to express the dots below 5%, and the dots above 95% are easily pasted together, and the middle and dark tone parts are easily lost. At the same time, due to the ductility of the flexible plate, it is difficult to improve the accuracy of plate overprinting. 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. It can be seen that the printing plate material is soft and elastic, which is the main reason why the flexographic printing dots are prone to quality problems. The process analysis shows that during the flexographic printing process, under the action of pressure, the graphic size of the printing plate becomes larger, so that the dot area becomes larger, and the small dots are easy to be lost due to wear or uneven pressure. On the other hand, because flexographic printing uses low-viscosity ink, when the contact pressure between the surface of the printing plate and the printed product is slightly larger, the ink on the printing plate will spread outward, making the middle ink layer of the expanded dot thin and the edge The thickness of the ink layer is thick, forming a hollow-like dot. When the magnifying glass is used to observe, the edge of the dot is smeared, which 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 ink particles used, the larger 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, the rubber roller, the liner or the flexographic plate, the greater the dot expansion; the worse the accuracy caused by the aging of the machine, 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. Therefore, to prevent the occurrence of printing quality problems of flexographic dots, a better solution is to pay attention to the maintenance of equipment, choose high-quality, high-precision plates and high-quality inks, and pay attention to good operation techniques. It is related to the printing process technology. When the printing unit has excess, when printing the screen version of the screen version and the field version, you should consider separating the screen version and the field version and printing separately, which is conducive to the accuracy of the printing pressure. Control is also conducive to the uniform coating of ink and improve the printing quality of flexo products.
Ink marks appear on the ink layer of the printed product
When the thickness of the printing plate is uneven or the double-sided tape at the bottom has wrinkles, the old version should be replaced. Use high-precision high-quality plates or re-stick the double-sided tape on the back of the plate to eliminate wrinkles.
When the key parts such as the roller shaft head or gears are worn and loose, it is easy to vibrate or slide during the printing process, so that the printed plate has horizontal bar marks, and the screen dot area expands, so you should pay attention to equipment Lubrication work to prevent the failure problem.
When the squeegee is improperly installed or the fastening device is loose, the squeegee should be reinstalled and the squeegee should be re-fastened to prevent the squeegee from moving.
When the ink fountain roller and the anilox roller are partially worn out, new ink fountain roller and anilox roller should be replaced. In normal operation, care should be taken to prevent dry friction of the roller body, and the amount of ink must be sufficient. In the absence of ink, never allow the squeegee or ink fountain roller to make close contact with the anilox roller.
When the anilox roller has ink drying on some of the cells, the anilox roller should be thoroughly cleaned to effectively remove the accumulated ink in the cells.
When the engraved anilox roller mesh holes are arranged incorrectly or the anilox roller mesh holes have linear scratch marks, the anilox roller should be replaced and reprocessed.
When the particle impurities in the ink are stuck in the doctor blade or the anilox roller cavity, the ink impurities in the doctor blade should be removed or the fine ink should be replaced for printing. In addition, when using old ink, you should pay attention to filtering before using.
In summary, there are many reasons for the quality failure of flexographic printing, including production environment, printing materials, equipment, process and operation technology, etc. However, as long as we focus on the source link, take some necessary prevention and Corrective measures, strict control in production process technology, operation technology and quality management, not only can effectively prevent or reduce the occurrence of one or other printing quality failures, but also can improve production efficiency and product printing quality, The flexographic printing process is better applied in the field of modern packaging.
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