How to control the flexo printing technology
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Flexographic printing is a method of printing with a flexible flexible letterpress. It has low equipment investment, high process profit, good for reprinting, wide printing adaptability, easy equipment operation, high printing efficiency, and relatively good printing ink quality. A series of significant advantages such as stability, and because the process uses environmentally-friendly water-based inks for printing, it can better avoid the contamination of printed matter or interior products by inks. Therefore, it is more suitable for printing of food, medicine and other packaging products. Appreciated by printing manufacturers and customers, as China's low carbon economy trend is approaching, the flexographic printing process will be more widely promoted and applied.
1. Analysis of process characteristics of flexographic printing
The printing plate of the flexo printing process is made of a rubber plate, and it is also made of a solid or liquid resin plate. The printing performance and printing quality of different materials are also very different. Due to the flexible nature of the plate, the printing pressure is lighter and the printing adaptability is strong. It can print thick corrugated cardboard and print a thin and soft plastic film. It can also print absorbent paper towels. Can print non-absorbent aluminum foil and other materials. Due to the use of liquid ink printing with strong fluidity, and the plate is soft and smooth, it can better overcome the printing defects of the surface roughness of the material, thereby obtaining a thick and colorful printing quality effect of the ink layer. The printing machine of the flexo printing process can also combine offset printing, gravure printing, silk screen printing and other finishing processes, with strong stitching flexibility and high printing production efficiency, which can shorten the production cycle of the product and meet the urgent needs of users. In addition, since the flexographic printing process uses non-toxic and odorless water-based ink for printing, the production process can effectively prevent contamination of the product. The printed product has the advantages of being clean, hygienic, safe and environmentally friendly, and is an ideal printing process for realizing green packaging. Suitable for printing of medicine and food packaging products. However, the flexo printing process also has shortcomings, that is, due to the softness of the plate material, the dot gain value is relatively large, and the product of the fine mesh surface printing is obviously inferior to the offset printing product. Due to the inherent characteristics of the flexographic printing process, the printing plate is prone to deformation defects. Therefore, the plate design and printing must pay attention to the proper adjustment of the structure of the printing plate, such as the size and level of the dots; the thickness and direction of the lines, and the gap between the plates. Wait.
2. Factors that should be considered in plate design
Since the flexographic plate is soft, has certain elasticity, and has a relatively large thickness, when it is pasted on the cylindrical plate cylinder, the longitudinal direction of the plate corresponding to the circumferential surface of the roller causes a certain bending deformation, especially The plate with the thicker thickness has a larger coefficient of deformation of the surface of the plate after pasting, which in turn causes errors in the size of the printed image and the original. If it is the printing layout of the dot structure, the expansion of the dot is the main problem causing the printing color deviation. Therefore, the technical control of the flexographic plate design process is in place and has a great impact on the printing quality. Therefore, in the plate making design, according to the plate elongation of different thicknesses, the layout should be appropriately reduced according to the characteristics of the plate material at the time of plate making, so as to avoid the printing plate size being larger than the original plate size. For the printing of the dot layout product, the dot gain should be compensated by adjusting the flexographic separation characteristic curve before the film is produced. For barcode layouts on the product, the orientation of the barcode lines should be as close as possible to the circumferential direction of the drum during the imposition design, so as to avoid the normal use due to the deformation of the barcode line gap. Due to the difficulty in obtaining a good reproduction of the fine dots of the flexographic plate during the plate making and printing process, especially the high-gloss dots tend to be lost. And the soft printing plate under the action of pressure, the dark-adjusted dot portion is prone to enlargement, paste and become solid. The expansion of the dot will result in a lack of layering in the printed layout, and the quality of print reproduction will be significantly reduced. Therefore, for the design of the dot tone and the number of lines, it is determined according to the hardness of the substrate and the gloss of the surface. Generally, the substrate is relatively soft, and the paper surface is not very smooth. The high-gloss layout should be considered to increase the dot properly. Generally, the dot of the high-gloss portion of the layout is designed to be no less than 4%, and the dark-tone dot is controlled to be within 85%. Usually, the number of screens for printing plates is mainly determined by the quality of the printed materials. The number of screened lines of corrugated boxes is obviously smaller than that of preprinted carton paper. If high-grade coated whiteboard is used, high-performance flexibility is now used. The printing equipment is printed, and the number of plate-making cables can reach 175 lines/inch or even higher. In addition, the flexographic version should not be designed with thin lines and too small text, so as to avoid the layout of the paste and affect the printing quality.
3. Reasonable selection of flexographic printing plates
The flexographic plate is different from the plate of the offset printing process. The printing plate is relatively soft, and it is prone to elastic deformation when pressed, and the printing suitability of plates of different thickness and hardness is also completely different. The thinner the plate, the greater the hardness, the smaller the coefficient of deformation, which is more conducive to improving the printing quality. Therefore, when printing high-speed carton boxes, color boxes and other fine color products, the flexographic printing with a thickness of 2.84 mm is better, and the printed corrugated cardboard or other uneven materials should be thicker. Plate, otherwise, due to the shallow height of the plate embossing, the printed blank layout is prone to dirty defects. When printing with a plate having a large thickness, it is possible to overcome the defects of thickness difference or poor gloss of the substrate by compressive deformation, so that the surface of the substrate having uneven surface can obtain a relatively uniform ink color, which is beneficial to improving the printing quality of the product. . Resin-based flexographic plates are available in solid and liquid resin versions. Due to differences in material properties and manufacturing processes, plate prices and printing effects are also different. When used, they should be reasonably selected according to actual production conditions. The solid resin plate is a semi-finished plate made of blank, which is directly exposed to the exposure, rinsing (developing), drying or double exposure, and becomes a finished version, which has the characteristics of convenient and quick plate making. The solid resin plate has a relatively uniform thickness and its expansion ratio is much smaller than that of the rubber plate and the liquid resin plate. Because the plate has the characteristics of good surface ink and high printing durability, and the plate has a good tolerance, it can reproduce finer high-gloss dots and small characters and lines, but the plate price is more expensive. The general solid version is more suitable for printing high-grade, fine pre-printed carton paper, color boxes and other packaging products. In addition, from the perspective of cost, the number of products printed is larger to be more cost-effective. The liquid photosensitive resin plate is made of liquid photosensitive resin, which is subjected to the steps of spreading, back mask exposure, front exposure, full back exposure, recovery of unhardened resin, development, drying and exposure, etc., and the plate making time is relatively longer. It usually takes about 1 hour, but the raw material cost is about half that of the solid resin plate. In addition, since the accuracy of the thickness of the liquid plate is not as high as that of the solid plate, and the coefficient of deformation is relatively large, the print quality effect is not as good as that of the solid plate. Therefore, it is generally suitable for products with a small number of prints and a relatively simple layout.
4. Technical control of proper use of water-based inks
Ink is one of the important raw materials for printing and a major factor affecting the quality of printed products. The correct use of ink is actually to control the quality of the ink. The water-based ink with good fineness has finer pigment and filler particles, good color rendering effect, and less relative coating amount of ink when printing, so that a satisfactory color effect can be obtained. The fineness of the water-based ink can be controlled by a fineness scraper. Generally, the fineness of the water-based ink is preferably within 20 μm. The lower the value, the stronger the color density of the ink, and the amount of ink can be relatively reduced. , you can get the ideal color, and it is not easy to appear paste and other defects during printing. Viscosity is also one of the main indicators of water-based inks. It has a great influence on the quality of printed products. If the viscosity is too high, the leveling of the ink is not good, which affects the uniform coating of the ink, and is prone to dirty plates and pastes. On the other hand, if the viscosity is too low, the printing color quality is not good. Generally, the viscosity of high-grade water-based inks should be controlled to about 20 seconds, and the coloring power is strong and the color is bright. Adjusting the viscosity of the water-based ink can control the viscosity of the ink to meet the printing requirements by controlling the temperature of the ink and the amount of the diluent. The viscosity of the water-based ink is generally detected by the viscosity paint No. 4 cup filled with ink, and the time from the release of the discharge hole to the flow of the ink in the cup is the test result. In addition to fineness and viscosity, the pH of aqueous inks is an important control indicator that cannot be ignored, and changes in pH change the viscosity of the ink. Usually, the pH of the aqueous ink is controlled within the range of 8.5-9.5. At this time, the printing performance of the ink is relatively good, and the printing quality is relatively stable. When the pH is higher than 9.5, the viscosity of the ink is lowered due to the strong alkalinity of the ink, the drying speed is slow, and the water resistance is also deteriorated. When the pH value is lower than 8.5, the viscosity of the ink becomes high due to the weak alkalinity of the ink, and the ink tends to be dry, causing the plate or the anilox roll to clog, thereby causing the surface to be dirty. Therefore, the printing process should pay attention to the control of pH. Generally, the pH stabilizer is used to adjust the pH value of the ink. When used, the pH stabilizer is added to the ink to be evenly stirred or directly added to the circulating ink pump.
5. Control of flexographic printing technology
Due to the softness of the plate and the strong elasticity, the flexographic plate is sensitive to the reaction pressure. If the printing pressure is too large, it will easily cause the deformation of the dots, characters or lines to appear, resulting in rough, double-shadow or paste on the printing plate. Other ills affect the quality of the product's reproduction. Therefore, the flexographic printing process can achieve uniform and thick printing ink quality effects with a small pressure. In addition to the reasonable adjustment of the printing pressure, the correct adjustment of the contact pressure between the surface of the anilox roller and the surface of the printing plate is also an important technical control link. If the contact pressure between the anilox roller and the printing plate is too large, it will be due to Excessive squeezing of the ink layer causes paste paste defects in the printing plate. Conversely, if the pressure between the anilox roller and the printing plate is too small, it will affect the uniform and sufficient coating of the ink. Therefore, every time you install a set of plates, you should pay attention to adjusting the appropriate pressure. In addition, in the printing process, we must pay attention to the appropriate, constant printing speed according to the characteristics of the ink, substrate and printing layout, in order to maintain the consistency of the entire batch of printing ink. For products with relatively small printing area, good performance inks and substrate materials, it is suitable to print at a higher speed. On the contrary, the printing speed should be appropriately reduced to obtain a saturated printing ink quality effect.
In summary, the technical control of the flexographic process involves all aspects of the production process, as long as the focus is on the rational selection of the plate to the design of the plate making process; from the selection and rational adjustment of the ink to the process technology of the printing process, comprehensive Carry out serious technical control and control, so that the production process can not only effectively avoid the occurrence of such quality problems, but also improve the printing quality of the products and meet the requirements of users.