Discussion on the hottest magnetic printing techno

2022-08-24
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Discussion on magnetic printing technology and machinery

3. selective adsorption of printing plates on water and ink

as mentioned above, the plates of lithographic plates generally choose metal aluminum and zinc with medium hydrophilicity and lipophilicity. In order to improve the hydrophilicity of the printing plate made of this kind of plate, a stable hydrophilic film of metal inorganic salt or metal oxide is formed on the surface of aluminum printing plate or zinc printing plate through chemical treatment, or chromium is plated on the blank part of the printing plate. In order to make the printing plate surface absorb enough water, the printing plate surface has a certain roughness by grinding, so as to increase the surface area of the plate and increase the water storage; In order to improve the lipophilicity of the printing plate, various non-polar films are coated on the graphic part or copper is plated on the graphic part by printing. Through mechanical or chemical treatment of the surface of the printing plate, the lipophilic graphic part of the printing plate has selectivity for the adsorption of water and ink, that is, it only adsorbs ink, not water. Through grinding, the roughness of the printing plate is increased, so that the blank part of the printing plate is also adsorbed to the ink. The hydrophilic blank part of the printing plate has no selectivity for the adsorption of water and ink, that is, it adsorbs both water and ink. In short, there is no such selectivity for the adsorption of water and ink. In the printing process, if you ink first, it will be absorbed by the lipophilic graphic part and the hydrophilic blank part on the printing plate at the same time, and the printing process cannot be carried out

in actual printing, the printing plate must be watered first. The water is adsorbed by the hydrophilic blank part, but not by the lipophilic graphic part, and then the printing plate is inked. The ink is adsorbed by the lipophilic graphic part, and is basically adsorbed on the blank part wetted by the wetting plate. This ensures the normal printing process

4. ink emulsification and water ink balance

the process of dispersing the fountain solution in the form of small droplets in the ink that is immiscible with it is called ink emulsification, and the resulting mechanical mixture is called "emulsion". There may be two forms of "emulsion" in lithography: if oil is the dispersed phase and water is the dispersion medium, it is called "oil in water emulsion has reached the domestic leading and international advanced level in several fields such as large-size monocrystalline silicon wafer, ultra-high purity metal sputtering target, optical film, new cobalt materials, rare earth permanent magnet materials, high-purity electronic chemical materials", which is represented by the symbol "O/W". If water is the dispersed phase and oil is the dispersion medium, it is called "water in oil emulsion", which is represented by the symbol "w/O". Theoretically, only when there is a strict dividing line between the water film in the blank part of the printing plate and the ink film in the graphic part, and oil and water do not infiltrate each other, the water ink balance of lithography is achieved. This ideal static balance of water and ink is actually impossible and unnecessary. The emulsification of ink is objective and necessary to a certain extent

when the lithographic press presses, there are four kinds of roller gaps between the water roller, inking roller and the blank part and graphic part of the printing plate. Water and ink are forcibly mixed between each roller gap, and then separated

the first is the roll gap between the landing roll and the blank part of the printing plate. There is fountain solution in the roll gap. After the water roller is separated from the printing plate, the blank surface is wetted by the injection solution, leaving a layer of water layer. If you use coated paper for printing, the fountain solution is an ordinary acidic fountain solution. Experiments show that the ink film on the printing plate is 3 μ m. The water layer is about 1 μ When it is m thick, the adhesion of ink on the blank part can be blocked

the second is the roll gap between the landing roller and the printing plate. There are two phases of fountain solution and ink in the roll gap. Under the strong extrusion of the printing plate and the water roller, a small amount of fountain solution is squeezed into the ink, causing the first emulsification of the ink. The greater the water supply, Kneath, who was squeezed into the ink, said: "We can reduce the cost, improve the material properties and shorten the delivery cycle. The more liquid we have, the more serious the ink emulsification will be. After the water roller is separated from the printing plate, fine droplets of fountain solution will be left on the surface of the ink film.

the third kind is the roller gap between the ink roller and the wetted blank part. There are two phases of fountain solution and ink in the roller gap. Under the strong extrusion of the printing plate and inking roller, a small amount of fountain solution is squeezed into the ink, causing ink Second emulsification

the fourth is the roller gap between the inking roller and the blank part after wetting. There are two phases of fountain solution and ink in the roll gap. Under the strong extrusion of the printing plate and inking roller, a small amount of fountain solution is squeezed into the ink, causing the third emulsification of the ink. After the inking roller is separated from the printing plate, the image and text part gets emulsified ink containing fountain solution

it can be seen from the transmission process of water and ink in lithography that there are three times of mixing and emulsification of water and ink in one water supply and ink supply, and it is impossible to maintain a strict dividing line between water phase and oil phase. Therefore, in the actual printing process, the ideal water ink balance does not exist, and the water ink balance of lithography is only a relative concept

emulsification of lithographic ink is not only inevitable, but also necessary for the transmission of protective printing ink in time. If the ink is not mixed with the fountain solution at all, the fountain solution beads forced into the ink will soon separate from the ink and adhere to the surface of the ink, forming a layer of water film on the ink surface, and the inking roller will not be able to transfer the ink to the graphic part of the printing plate. Therefore, non emulsifying ink cannot be used for lithography. Inkless and fountain solution must be immiscible, but to a certain extent, they must be miscible, that is, they must be able to form a mixture of fine water droplets dispersed in the ink. The emulsification of water in the ink provides a way to eliminate the fountain solution in the image and text part, otherwise the printed matter will appear blooming phenomenon; On the contrary, micro ink beads are not allowed to disperse in water, so as to prevent the blank part from getting dirty

o/W emulsion is very harmful to the quality of lithographic printing and the normal process of lithographic production. It will make the blank part of the printed matter dirty, and will make the ink roller deinking, and the ink cannot be transferred. In recent years, resin based inks have been used in lithography, with enhanced water resistance, and ink hydration rarely occurs. O/w emulsion ink is not easy to produce, mainly producing w/O emulsion ink. Light w/O emulsion ink is not only needed for the transmission of lithographic ink, but also the viscosity of ink decreases slightly. Lubrizol, headquartered in Wycliffe, Ohio, can improve the fluidity of ink and facilitate the transfer of ink to paper. However, serious w/O emulsion ink will make the ink viscosity drop sharply, the oil filament becomes shorter, and the ink transfer performance becomes worse. At the same time, the fountain solution immersed in the ink will also corrode the metal ink roller, forming a hydrophilic layer on the surface of the ink roller, which repels the ink, causing deinking of the metal ink roller

the use of fountain solution in lithography has the following purposes

① form a water film that repels ink in the blank part of the printing plate, resist the expansion of ink on the text to the blank part, and prevent dirty plates

② supplement the hydrophilic layer damaged in the printing process to maintain the hydrophilicity of the blank part of the printing plate. During printing, due to the friction between the rubber roller and the water roller on the printing plate, the paper powder and paper wool falling off the paper aggravate the wear of the printing plate. Therefore, with the increase of the number of prints, the hydrophilic layer of the layout will be destroyed. It is necessary to use the dielectric in the fountain solution to react with the exposed plate base metal aluminum or zinc to form a new hydrophilic layer and maintain the hydrophilicity of the blank part

③ reduce the temperature of the printing plate surface. After the offset press starts, the ink roller spreads the ink into an ink film at a high speed, and the temperature of the ink roller rises, resulting in a decrease in the viscosity of the ink. If the printing car is at 25 ℃ and does not supply the printing plate fountain solution, the lithography machine will continue to operate for 30min, and the ink temperature on the printing plate will rise to 40 ~ 50 ℃, the viscosity of the ink will drop sharply, the fluidity will increase, and the ink will spread rapidly, causing a serious expansion. Therefore, coating the blank part of the printing plate with a fountain solution with a temperature equal to or lower than room temperature can reduce the temperature of the printing plate surface

therefore, only by maintaining a certain thickness of water film in the blank part of the printing plate can we ensure the normal printing and obtain better prints. But this layer of water film is too thin to achieve the purpose of using fountain solution; If the water film is too thick, serious emulsification of ink will occur. Therefore, to achieve the balance of water and ink in lithography, we should control the thickness of the water film of the printing plate

in order to ensure the quality of printed matter and the normal progress of printing, the significance of the balance between lithography water and ink is: under a certain printing speed and printing pressure, adjust the supply of fountain solution, and use the least fountain solution to make the emulsified ink get the best printed matter and compete with the ink on the printing plate

experience has proved that when printing normally, the ink layer thickness of the graphic part of the printing plate is 2 ~ 3 μ m. The water film thickness of the blank part is about 0.5 ~ 1.0 μ m. The volume percentage of fountain solution in the ink is 15% - 20%, which basically realizes the water ink balance of lithography

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