On the most popular flexographic printing process

2022-08-05
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A brief discussion on flexographic printing process the method of printing by using rubber relief plate and photosensitive resin plate with flexible characteristics to transfer the ink roller through the ink roller is called flexographic printing process. Flexible plate material is soft and elastic, which can adapt to the hard and soft, gloss and roughness of the substrate material. The printing plate has high hardness resistance. Flexographic printing has the advantages of simple process, high production efficiency and stable quality. It is widely used in the printing of packaging and decoration products. Due to the gradual recognition of the process advantages of flexographic printing, the flexographic printing process has developed rapidly in recent years, and the proportion of use has been rising. It has become one of the main processes of packaging printing

I. advantages of flexographic printing and reasons for its rapid development

flexographic printing process has the commonness of relief printing, lithography printing and gravure printing, and is applicable to the printing of various packaging products. Its printing plate structure is relief plate feature. When imprinting, the soft plate is in direct contact with the substrate, and the printing plate feature is similar to the blanket of offset printing process. In terms of mechanical structure, the machine with rotary web printing can not only print various papers or plates, but also print other substrates that can only be printed by gravure printing process. The inking system of flexographic printing process is relatively simple. The printing plate is inked by a corrugated roller, unlike offset and embossing equipment, which has multiple or even more than a dozen inking rollers, simplifying the components of the equipment, making the inking control response faster and the operation more convenient. Due to the use of volatile dry ink with alcohol as the main solvent, the printing product can be dried quickly, which can meet the requirements of high-speed and multi-color printing. Flexographic printing has light pressure, stable operation, good performance and low raw material consumption. Flexographic printing process also has the advantages of convenient and easy plate making, short production cycle, less equipment investment and quick results. Therefore, with the continuous progress of printing technology and the rapid development of social economy, flexographic printing process will get rapid development in the future

II. Types of flexographic plates and plate making process methods

flexographic printing plates mainly include natural rubber, synthetic rubber and photosensitive resin. The flexo made of rubber can be divided into two types: Hand Carved rubber version and vulcanized rubber version. The printing plate made of photosensitive resin includes liquid photosensitive resin plate and solid photosensitive resin plate

1. Manufacturing method of engraving rubber plate

to make a rubber plate by hand engraving, first draw the original picture and text on the flat rubber plate to form a picture and text outline trace on the surface of the plate, then carefully carve the picture and text outline with a carving knife to remove the rubber of the blank plate, so that the image civilization protrudes from the blank part and becomes a flexible relief plate. This kind of plate making is manual from drawing graphics and texts to the surface of the rubber plate to the whole engraving process. The whole plate making process is labor-consuming and time-consuming, which is very troublesome. This kind of plate making method has poor accuracy and low efficiency, and can only be applied to products with thick graphics and texts and simple layout. For example, this plate making method is usually used in the corrugated plate printing machine with manual paper continuation

2. Manufacturing method of vulcanized rubber plate

this plate making method is as follows: first, the graphics and text on the original are made into a copper zinc plate, the graphics and text are convex, then the thermosetting phenolic resin is used as the material, the copper zinc plate is used as the model, and the phenolic resin plate with the graphics and text concave is made by hot pressing with a profiler, then a layer of raw rubber is coated on the motherboard with the graphics and text concave, and the rubber convex plate is made by vulcanizing it under high temperature and high pressure. The manufacturing process of vulcanized rubber plate is easy to cause pollution problems, and the production process is complex. There are many defects such as large plate size shrinkage deformation and poor accuracy. This plate making process has great limitations

3. Manufacturing method of liquid photosensitive resin plate

a printing plate made of liquid photosensitive resin is called a liquid photosensitive resin plate. Its plate making process is: flow spreading, back mask exposure, front exposure, back full exposure, recovery of unhardened resin, development, drying and post exposure. Flow spreading is to inject the configured photosensitive resin solution into the hopper of the exposure molding machine, then lay the negative film on the lower glass plate, and cover the transparent plastic protective film to prevent the negative film from adhering to the resin. The hopper and the film base roll move along the negative film, and the photosensitive resin in the hopper flows out. The photosensitive resin is scraped to a certain thickness by the scraper at the top, and the film base is hydraulically applied to the resin by the film base roll, and then a piece of glass is pressed. Back mask exposure is to use a pre-made mask for back exposure to increase the depth of the printing plate. Front exposure is to turn off the upper ultraviolet light source and turn on the lower ultraviolet light source for exposure after the mask is exposed. Full back exposure is to remove the mask on the back after the front exposure, and fully expose the back with the upper UV light source to form a printing plate foundation. After the last 3 exposures, the unhardened resin is recovered. Since there is no light at the blank part of the printing plate, the printing plate should be placed vertically on the recovery table to make the unhardened resin flow down for recycling. Development is to form a clear boundary between the text and the blank part by washing, and remove the resin at the edge of the text outline. After the plate is developed, it is dried by electric heating wire or infrared device, and then exposed to light to improve the printing resistance of the plate

4. Plate making method of solid resin plate

the solid resin plate uses the UV irradiation of the resin to make the photosensitive resin cross-linked, polymerized and insoluble, while the resin without UV irradiation remains soluble. In this way, after development and washing, the non hardened resin will fall off, while the exposure hardened resin will form a relief relief structure. Solid resin plate making process includes exposure, washing, drying and post exposure. The exposure is completed by the plate printer, and the exposure is realized by the close contact between the negative and the front of the resin plate through the vacuum adsorption device in the printer. Generally, the printing plate is gently washed with warm water of 40 to 50 degrees and soft sponge to peel off the resin of the non graphic plate and realize the development of the printing plate. The people reported that the developed plates were dried and then exposed in the plate printer to enhance the strength of the plate and improve the printing resistance

III. control of flexographic printing process

pressure is the medium for imprint transfer. It is very important to properly adjust and control the pressure, such as how good the ink quality of the printed product is, whether the layout text is clear or not. Ink volume pressure, inking pressure and printing pressure are the specific manifestations of the working pressure in the process of flexo printing. The so-called ink volume pressure is the contact pressure between the ink bucket rubber roller and the grain transfer ink roller. The magnitude of this pressure directly determines the ink output. The ink volume pressure is inversely proportional to the ink output. Therefore, when the printing layout is large, the ink volume pressure should be appropriately small; On the contrary, when printing a small area of layout and line printing, the ink volume pressure should be appropriately increased. Inking pressure is the pressure between the ink transfer roller and the plate cylinder, which determines the thickness of the ink layer of the plate. Only by accurately adjusting the inking pressure can we ensure the uniformity of ink color and the clarity of graphics and texts, and better improve the printing plate's printing resistance. This requires that the plate cylinder and the embossing roller be in a horizontal position and kept tangent to ensure that they are in a good parallel state. If the contact pressure is too small, the printing ink will be thin and uneven. The control of printing pressure is also very important. Due to the soft material of flexo, if the printing pressure is too large, the printed graphics, lines and dots are prone to deformation and expansion; If the printing pressure is too small, the ink transfer of the layout is insufficient and uneven. Therefore, the adjustment of printing pressure is based on the premise that the embossing cylinder is in uniform and full contact with the layout, and the contact pressure should be kept as small as possible, which is beneficial to the quality and machinery

IV. structure and types of flexographic inks

flexographic inks are divided into water-soluble inks, alcohol soluble inks and mixed solvent inks, which are mainly composed of pigments and blending agents. Pigment is a general term for dye or pigment. Dye is a pigment soluble in water, oil and alcohol. Some use organic pigments such as lake pigments, azo pigments, phthalocyanine pigments, etc., and some use inorganic pigments plus carbon black and oxygen to develop iron and titanium dioxide from the standpoint of automobile manufacturing. Blending agent is to use solvent and resin to blend pigment particles, so that the ink can maintain a certain fluidity and meet the requirements of printing performance. After printing, it also has the characteristics of rapid drying of conjunctiva. Flexographic inks can be divided into water-soluble inks, alcohol soluble inks and mixed solvent inks according to the printing process requirements and different ink blending agents. Water soluble inks use water as solvent, add a certain amount of ammonia and alkali, and dissolve with soluble resin, shellac, corn, protein, and add an appropriate amount of alcohol soluble substances. The alkaline component of the ink disappears after it is transferred to the paper during the printing process, forming an insoluble dry conjunctive, so it is suitable for paper substrates with easy absorption on the surface. Alcohol soluble ink is made by dissolving with resin, nitrocellulose, shellac, polyamide, etc. with ethanol, isopropanol, n-butanol, ethylene glycol, etc. this ink has the characteristics of fast drying. It is not only suitable for paper printing, but also for printing plastic films. Mixed solvent ink is to add an appropriate amount of ethyl acetate or hydrocarbon into alcohol soluble ink to make the ink dry quickly and strengthen its adhesion. This kind of ink will have a strong corrosive swelling effect on the printing plate and ink roller. Non absorbent substrate is suitable for this kind of ink

v. proper storage and use of flexographic plates

resin plates have become the development direction of flexographic printing process due to their convenient plate making process and good printing quality. Good storage and use of plates is an important part of quality control. Therefore, the printing plate should be placed in a sealed place, kept away from light and prevented from heavy pressure. The printing plate must be stored in a cool and ventilated place to avoid moisture. The storage time of the printing plate shall not be too long, and it is better to use it within one year. The printed plates must be cleaned. When cleaning, gently wipe with soft cotton cloth. Do not use a hard brush to clean the layout to prevent scratches. After the printing plates are cleaned and dried, they are sealed with plastic bags and placed in a cool and ventilated place for future use

to sum up, flexographic printing process combines the characteristics of letterpress, lithography and gravure processes. It has a wide range of printing and stable quality. It will have a broader development prospect in the field of packaging. Therefore, a correct understanding of the flexographic printing process is of the great significance

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