Offset printing, also known as lithography, is one of the most widely used printing methods in the printing field. Offset printing plate of the graphic part and the non-graphic part is basically in the same plane, there is only a slight difference in height, the use of oil and water repel the principle of printing, the graphic part of the hydrophobic (pro-ink), non-graphic part of the hydrophilic (repel ink). Printing first with a fountain solution to wet the non-graphic part of the graphic part is repulsive fountain solution; and then by the ink roller ink transfer, the graphic part of the acceptance of the ink, the non-graphic part of the repulsive ink; graphic part of the ink and then transferred to the rubber roller, and then passed to the substrate, is an indirect printing. Offset printing process shown in Figure 3-10.
Offset printing plate
The printing plate used in offset printing is the presensitized plate (PS plate), which is an offset plate that is pre-coated with a photopolymerized layer on an aluminum base plate, and then stocked up after processing and used for solarization at any time. there are two types of PS plates, namely, the positive PS plate and the negative PS plate. In the 1980s, digital plate-making technology – computer to plate (computer to plate, CTP) technology, representing the latest development of printing technology, into the 21st century, CTP technology is becoming more mature, CTP equipment and plate materials continue to develop and make progress, is becoming the mainstream of offset printing products.
(1) Positive PS plate
The positive PS plate is coated with photodegradable o-diazo naphthoquinone photopolymer and linear phenolic resin on the surface-treated aluminum substrate, and then dried and made into a photopolymer resin layer.
The plate making process of the PS plate:
UV exposure → development → drying → retouching → printing plate
When the positive PS plate is used, the negative and the positive PS plate are vacuum laminated and exposed with a metal halide lamp (e.g., gallium iodide lamp) or a medium-pressure mercury lamp. Under ultraviolet light irradiation, the diazo group decomposes and nitrogen is precipitated, and at the same time, an intramolecular rearrangement occurs and the ketone becomes enone, and the ketone hydrolyzes under the presence of water and becomes carboxylate, as the light-sensitive portion of the carboxylate group, and is removed by development with alkaline developer solutions, such as sodium silicate, sodium phosphate, sodium carbonate or organic amines, and is exposed to the light. Developed and removed, revealing the hydrophilic aluminum substrate. The non-photosensitive part of the formation of graphic pro-ink, to get a positive picture. The plate is made into a printing plate by removing the dirt, repairing and drying the plate. Sometimes in order to improve the print resistance, can also use high temperature baking plate.
(2) PS plate with negative image
The negative PS plate is coated with photopolymerized photopolymeric resins, such as diazo resin and alkali soluble resin system, polymer system with azide group and polymer system with cinnamic acid group on the aluminum substrate after surface treatment, and their sensitivities are higher than that of the positive PS plate, but some of them are solvent developed, which is costly, harmful to human beings, and also pollutes the environment.
The plate making process of negative PS plate:
UV exposure → developing → drying → retouching → printing plate
When the negative PS plate is used, the negative is vacuum laminated with the negative PS plate and exposed with a metal halide lamp (e.g., gallium iodide lamp) or a medium-pressure mercury lamp. Under ultraviolet irradiation, polymerization and cross-linking occurs in the part of the light-seeing part, and the graphic part of the plate is formed to be pro-ink. For not see the light part, some systems with dilute alkali solution such as sodium carbonate development to remove, some systems to use a solvent development to remove, exposed aluminum substrate hydrophilic, get the negative picture. The plate is made into printing plate by removing dirty, trimming and drying.
(3) CTP plate
CTP technology is the use of digitized data through the laser plate making machine will be the image and text directly copied to the printing plate technology. ctp plate making process compared with the traditional offset printing plate making process, do not need silver salt color separation film, but also can save a number of plate making process, as shown in figure 3-11.
Comparison of CTP process and traditional offset plate making process
The CTP plate making system has different structures, different light sources and different CTP plates. At present, there are mainly violet laser CTP plate, thermal CTP plate and UV-CTP plate, and their plate structure, imaging mechanism and performance are different, each with its own characteristics (see Table 3-30).
Comparison of the performance of three common CTP plates
Offset Printing
The main features of offset printing are as follows.
① The thickness of the ink layer is very thin, generally the thickness of offset ink is about 2μm in one color.
② Since the ink roller and water roller contact the printing plate alternately, the printing suitability of the ink is extremely high, and the ink should be prevented from emulsification.
③ The printing speed of offset printing is fast, generally in the range of 100-400m/min.
④ Offset printing ink has high viscosity, high pigment concentration and high coloring power.
⑤ Offset printing is indirect printing, which is transferred to the substrate by elastic rubber rollers, so the printing pressure is relatively small and even, with high overprinting precision, good reproducibility of graphics, clear dots and rich layers.
Traditional offset printing ink is a rosin modified phenolic resin and other synthetic resins as the connecting material solvent-based ink, polluting the environment, the use of UV offset printing ink, offset printing has made new progress.
(1) Improvement of the general offset printing process
① offset printing in the use of UV inks, prints in the arrival of the receiving part has been cured, can be free of powder spraying, not only conducive to the cleanliness of the printing environment, to protect the health of the operator, but also to avoid the spraying of powder to the post-press processing of the trouble brought about by the effect, such as varnishing, laminating effect of the effect of the continuous processing and can be carried out. Not only improve the operating environment, but also improve production efficiency, and shorten the delivery time.
② UV ink cured by UV light source irradiation, not irradiated before the ink tank does not crust, in the printer or the plate will not dry knot, therefore, offset printing machine downtime without cleaning the ink tanks and rubber rollers, to solve the offset printing machine downtime worries, reduce the waste of ink, saving cleaning time.
③ Curing device is simple, easy to maintain, do not need infrared drying device like the drying channel, even if you add the equipment associated with it, the footprint is still a lot smaller than the footprint of ordinary infrared drying equipment.
(4) UV curing equipment can make UV ink curing quickly at low temperatures, its energy consumption than the traditional ink printing through hot air drying or infrared drying way of drying the energy consumed by much lower.
(2) Make offset printing applicable to a variety of substrate materials, broadening the scope of application of offset printing
① composite paper substrate printing. Composite paper is made of two or more of the two or more of the different properties of paper or other materials combined together to form a paper material, such as common daily life of milk, cigarettes, alcohol, most of the outer packaging box made of composite paper. Composite paper materials with ordinary ink printing adhesion, friction resistance and poor drying and other issues. UV ink photosensitizer in the UV irradiation, the resin in the unsaturated bonds open, crosslinked to form a mesh structure, greatly improving the physical properties of the ink layer surface, fundamentally solving a series of problems.
② non-absorbent material printing. Such as polyethylene film, PVC film, polypropylene film, metal foil and non-absorbent special materials. These substrates if ordinary solvent-based ink printing, in addition to the need for a certain drying time, usually also need to use spray powder, drying rack device or in the ink mixed with other additives and other means of processing, and UV ink curing without penetration, completely free of ordinary ink drying slow trouble. In addition, UV ink can improve the physical properties of the ink layer surface, and can be used to offset UV ink printing fine sand, gold, silver, etc., while ordinary offset printing is difficult to achieve printing.
③ Metal material printing. UV ink printing on the surface of metal materials, can shorten the curing process and simplify the original curing device. At the same time, UV ink surface layer of good curability and film can not only improve the characteristics of post-press processing, but also improve the appearance of the product quality.
(3) UV offset printing has excellent printing results
Offset printing is mostly used for fine product printing, UV offset ink curing and drying immediately after printing, the ink does not penetrate into the substrate, so the color is more vivid, higher clarity, greatly improving the color saturation of the printed matter, and can reduce the loss of small dots, and improve the ink transfer rate of the color overprinting. In addition, UV offset prints in the surface texture, protection, abrasion resistance and production efficiency, etc. with the traditional offset prints can not be compared to the advantages of prints with durability, anti-scratch and scratches, etc., but also has a chemical resistance and corrosion resistance.
(4) UV offset printing is green printing
Environmental issues are the main reason for the massive use of UV ink. The main reason why the vast majority of ink users choose UV printing technology is that the application of UV ink is in line with environmental regulations.
① No solvent volatilization: UV printing has almost no volatile organic compounds (VOC) emissions, solving the problem of environmental pollution in the drying process of solvent-based inks.
② Energy saving. Conventional solvent-based inks have a significant proportion of solvent evaporated in the coating process. Therefore, if the same area, the same film thickness of the printing area for comparison, it is clear that the amount of solvent-based ink energy consumption is much larger.
(5) UV offset printing to expand the application of offset printing in the field of special printing
As the application of offset printing technology continues to mature, many equipment manufacturers have introduced a variety of models of UV offset presses. With these offset presses, simple, high-quality lenticular three-dimensional printing can be performed. As a result, three-dimensional printing can be produced in large quantities and the quality of the product is further improved, broadening the application of offset printing in the field of special printing.
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