Flexographic printing is a flexible flexible letterpress printing method. It has the advantages of low equipment investment, high process yield, favorable for reprint printing, wide printing adaptability, easy operation of equipment, high printing efficiency, and relatively high quality of printing ink color. Stable and a series of significant advantages, at the same time as the process uses environmentally friendly water-based ink printing, can better avoid the ink on the printed matter or interior products, so it is more suitable for food, medicine and other packaging products, printing, deep Affected by the printing manufacturers and customers, as China's low-carbon economic trends continue to approach, the flexographic printing process will be more widely promoted and applied.

1. Analysis of the characteristics of the flexographic printing process

The flexographic printing plates are produced using rubber plates, and solid or liquid resin plates are used for plate making. The printing performance and printing quality of different materials are also quite different. Due to the flexible nature of the plate, the printing pressure is light and the printing adaptability is strong. It can not only print thick corrugated cardboard, but also print thin and soft plastic film; it can print strong absorbing paper towels. Can print non-absorbable aluminum foil and other materials. Due to the use of liquid ink with strong fluidity, and the plate material is soft and smooth, it can overcome the rough printing defects of the surface of the material and obtain a thick, colorful print quality effect of the ink layer. The flexographic printing machine can also be combined on-line with offset printing, gravure printing, silk screen printing and other post-finishing finishing processes. The flexibility of splicing is strong, and the printing production efficiency is high, which can shorten the production cycle of products and meet the urgent needs of users. In addition, due to the flexo printing process using non-toxic, odorless water-based ink printing, the production process can effectively prevent the pollution of the product, the printing product has the advantages of clean and hygienic, safe and environment-friendly, and is an ideal printing process to achieve green packaging. Suitable for printing of medicines and food packaging products. However, the flexo printing process also has its disadvantages, that is, due to the softness of the plate material, the dot enlargement value is relatively large, and the product of printing fine mesh plate layout is obviously inferior to the offset printing product. Due to the inherent characteristics of the flexo printing process, the printing plates are prone to deformation defects. Therefore, the plate layout design and printing must pay attention to properly adjust the structure of the printing plate surface, such as the size and level of the dots, the thickness and direction of the lines, and the clearance of the layout. Wait.

2. The factors that plate design should consider

Since the flexographic plate material is flexible, has a certain elasticity, and has a relatively large thickness, when it is pasted on a cylindrical plate cylinder, the longitudinal direction of the plate corresponding to the circumferential surface of the cylinder will produce a certain bending deformation, especially The greater the thickness of the printing plate, the greater the coefficient of deformation of the surface of the printing plate after affixing, which in turn causes errors in the size of the printed image and original. If the printing layout of the dot structure, dot distortion is the main problem causing the printing color deviation. Therefore, whether the technology of the flexographic plate designing process is in place has a great influence on the printing quality. Therefore, in the plate design, according to the different thickness of the plate elongation, according to the characteristics of the plate at the time of plate-making properly reduce the layout properly, so as not to cause the printing plate surface size is greater than the original layout specifications. For the printing of the dot layout product, the dot gain should be compensated by adjusting the color separation characteristic curve of the flexible plate before the film is printed out. For a product with a bar code layout, the bar code line should be aligned with the circumferential direction of the roller as far as possible when imposing the design, so as not to affect the normal use due to the deformation of the bar code line gap. Due to the fine dots of the flexographic plate, it is difficult to obtain a good reproduction in the process of plate making and printing. In particular, high-light spots are prone to loss. And under the effect of pressure, the soft printing plate is prone to enlarge, paste, and become like the dark spot. The expansion of outlets will result in a lack of layered layout of printed products, and the quality of print copying will be significantly reduced. Therefore, for the design of halftone and line number, it is necessary to reasonably determine the hardness of the substrate and the gloss of the surface. General printing materials are relatively soft, the paper surface is not very smooth, high gloss layout parts should be considered appropriate to increase the dot, usually the layout highlights of the network design is not less than 4%, while the dark tone network to control within 85%. Usually, the number of screen printing screens depends on the quality of the printed materials. The number of screens in the corrugated box is obviously smaller than that of the preprinted box paper. If high-grade coated whiteboard paper is used, high-performance flexibility is now used. The printing equipment is printed, and some of the plate-making screen lines can reach 175 lines/inch or even higher. In addition, the flexible version should not be designed too thin lines and small text, so as not to cause the layout of the paste dirty and affect the print quality.

3. Reasonable selection of flexographic printing plate

The flexo plate is different from the plate of the offset printing process, and the plate is relatively soft and easily deformed under pressure, and the printing adaptability of the plate with different thickness and hardness is also quite different. The thinner and harder the plate is, the smaller the deformation coefficient is, and the better it is to improve the printing quality. Therefore, when printing high-line carton boxes, color boxes, and other more elaborate color products, the use of 2.84 mm thick flexographic printing is better, and printing corrugated cardboard or other uneven material should be used in the thickness of the larger Plate material, otherwise, due to the shallow relief of the printing plate, the printed blank layout is prone to dereliction. When printing with a thicker plate, you can overcome the difference in thickness or thinness of the substrate by compressive deformation, so that the surface of the substrate with uneven surface can also obtain a relatively uniform ink color, which will help improve the printing quality of the product. . Resin type flexo editions are divided into solid and liquid resin editions. Due to differences in material properties and manufacturing processes, there are certain differences in the prices of plates and printing effects. When using them, they should be reasonably selected according to actual production conditions. The solid resin plate is a semi-finished plate material using blanks, directly exposed to the printing plate, washed (developed), dried or subjected to secondary exposure, and becomes a finished plate, which has the characteristics of convenient and rapid plate making. The thickness of the solid resin plate is relatively uniform, and its expansion rate is much smaller than that of the rubber plate and the liquid resin plate. Because the plate has the characteristics of good surface ink resistance, high printing rate, etc., and the plate material has a good latitude, it can reproduce finer high-gloss dots and small texts and lines, but the plate material is more expensive. The general solid version is more suitable for printing high-grade, fine preprinted carton paper, color boxes and other packaging products. Also from the cost point of view, the number of products to print a larger number to be cost-effective. The liquid photosensitive resin plate is based on liquid photosensitive resin as the material, to go through the shop flow, the back of the mask exposure, front exposure, back full exposure, recycling uncured resin, development, drying and then exposure process, plate making time is relatively longer It usually takes about 1 hour, but its raw material cost is about half that of the solid resin plate. In addition, because the accuracy of the liquid plate thickness is not as high as that of the solid plate, and the coefficient of deformation is relatively large, the printing 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 ink

Ink is one of the important raw materials for printing, and it is also a major factor affecting the quality of printed products. The correct use of ink is actually to control the quality of the ink. Fine water-based inks have relatively fine pigments and filler particles, good color rendering effect, and relatively small amount of ink when printing, so that satisfactory color effects can be obtained. The fineness of water-based inks for flexo printing can be measured with a fineness squeegee. Generally, the fineness of water-based flexo inks is within 20 μm. The lower the value is, the stronger the ink's color concentration is, and the amount of ink used can be reduced. , you can get the ideal color, printing is not prone to paste and other ills. Viscosity is also one of the main indicators of water-based inks, and has a great influence on the quality of printed products. If the viscosity is too high, the leveling properties of the ink are not good, affecting the uniform application of the ink, and the disadvantages such as dirty plates and paste plates are prone to occur. On the contrary, if the viscosity is too low, the printing color quality is not good. Generally high-grade water-based ink viscosity should generally be controlled in about 20 seconds, its strong coloring, bright colors. Adjust the viscosity of the water-based ink, by controlling the temperature of the ink and the amount of diluent, so that the viscosity of the ink to meet the printing requirements. The viscosity of water-based inks is generally measured by the use of viscosity coating No. 4 cup filled with ink, and the time from the time when the discharge hole is released to the ink in the cup is the test result. In addition to fineness and viscosity, the PH value of water-based inks is also an important control factor that cannot be ignored, and changes in pH will change the viscosity of the ink. Generally, the pH value of the water-based 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 also relatively stable. When the pH value is higher than 9.5, the viscosity of the ink will decrease due to the strong alkalinity of the ink, the drying speed becomes slow, and the water resistance performance also deteriorates. When the PH value is lower than 8.5, the viscosity of the ink becomes high due to the weak alkalinity of the ink, the ink is prone to dryness, and the printing plate or the anilox roller is clogged, thereby causing the layout to be dirty. Therefore, the printing process should pay attention to the control of the PH value. Generally, the PH value stabilizer is used to adjust the PH value of the ink. When appropriate, the pH stabilizer is added to the ink and mixed evenly or directly into the circulating ink pump.

5. Control of flexographic printing process technology

Flexible plate is soft due to the plate material, and it is full of strong elasticity. It is sensitive to the reaction of printing pressure. If the printing pressure is too high, it will easily cause the dot, text or lines to expand and deform, resulting in rough printing, double image or stencil printing. Other ills affect the reproduction quality of the product. Therefore, the flexographic printing process can achieve a uniform and thick printing ink quality effect as long as a relatively small pressure is used. In addition to the reasonable adjustment of 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 pressure on the ink layer, so that the printed version of the plate layout defects. Conversely, if the pressure between the anilox roller and the printing plate is too small, uniform and sufficient coating of the ink can be affected. Therefore, each installation of a version should pay attention to adjust the appropriate pressure. In addition, the printing process should also pay attention to the characteristics of the ink, substrate and printing plate, to grasp the appropriate, constant printing speed, in order to maintain the consistency of the entire batch of printing ink. The printing area is relatively small, the use of good performance inks and substrate materials are also relatively good products, can be properly used to print at higher speeds. On the contrary, the speed of printing should be appropriately reduced to obtain a saturated printing ink quality effect.

In summary, the technical control of the flexo printing process involves all aspects of the production process, as long as the emphasis is placed on the rational selection of the plate material and the design of the plate making process technology; from the selection and rational adjustment of the ink to the technological process of the printing process, comprehensive Serious technical control and control are carried out so that the production process can not only effectively avoid the occurrence of such or such quality problems, but also can better improve the printing quality of products and meet the requirements of users.

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