Magnetic Printing Technology and Mechanical Discussion--lithographic Printing (II)

3. Printing plate selective adsorption of water, ink

As mentioned before, the plate material of the typographic printing plate is generally selected to be moderately metallic aluminum and zinc in terms of hydrophilicity and oleophilicity. In order to improve the hydrophilicity of a printing plate made of such a plate material, a hydrophilic thin film of a stable metal inorganic salt or metal oxide is formed on the surface of an aluminum plate or a zinc plate by chemical treatment or The blank part of the printing plate is chrome-plated. In order to make the surface of the printing plate absorb enough moisture, the surface of the printing plate is provided with a certain degree of roughness by grinding to increase the surface area of ​​the plate and increase the water storage capacity; in order to improve the oleophilicity of the printing plate, Stencil printing, coating a variety of non-polar film layer in the graphic or copper in the text part. Through the mechanical or chemical treatment of the surface of the printing plate, the oleophilic graphic portion of the printing plate is selective for the adsorption of water and ink, that is, only the ink is adsorbed and the water is not adsorbed. Through grinding, the roughness of the printing plate is increased, and the blank part of the printing plate is also adsorbed to the ink. The hydrophilic blank part on the printing plate has no selectivity for the adsorption of water and ink, that is, it adsorbs both water and ink. In short, the adsorption of water and ink does not have such selectivity. In the printing process, if the ink is applied first, the oleophilic graphic portion on the printing plate and the hydrophilic blank portion are adsorbed simultaneously, and the printing process cannot be performed.

In actual printing, water must be supplied to the printing plate. The water is adsorbed by the hydrophilic blank and cannot be absorbed by the oleophilic graphic part. Then the plate is inked, and the ink is oleophilic. Adsorption is substantially adsorbed on the blank portion wetted by the dampening plate. This will ensure the normal printing process.

4. Ink emulsification and water, ink balance

The process of dispersion of the fountain solution in the form of fine droplets in an ink that is incompatible with it is called emulsification of the ink, and the resulting mechanical mixture is called an "emulsion." Two types of “emulsions” may be produced in plain printing: If the oil is a dispersed phase and water is the dispersing medium, it is called an “oil-in-water emulsion” and is represented by the symbol “O/W”. If water is the disperse phase and oil is the dispersion medium, it is called "water-in-oil emulsion" and it is represented by the symbol "W/O". Theoretically, it is only when there is a strict boundary between the water film in the blank part of the printing plate and the ink film in the graphic part, when the oil and water do not infiltrate each other, the water and ink balance of the plain printing is achieved. This ideal static water and ink balance is actually unachievable and unnecessary. The emulsification of ink is objectively present and is also necessary to a certain extent.

When the typography machine presses and prints, there are four kinds of nips between the water roller, the ink roller and the blank part of the printing plate, and the graphic part. Water and ink are forcibly mixed between the nips and then separated.

The first is the nip between the water roller and the blank part of the printing plate. There is a fountain solution in the nip. After the water roller is separated from the plate, the blank surface is wetted by the fountain solution, leaving a layer of water. If the use of coated paper printing, fountain solution is an ordinary acid fountain solution, experiments have shown that the ink film on the plate is 3μm, the water layer is about 1μm thick, in order to block the adhesion of ink in the blank part.

The second is the nip between the water roller and the plate image. There are fountain solution and ink two phases in the nip. Under strong squeezing of the printing plate and the water roller, a small amount of fountain solution is squeezed into the ink, and the first emulsification of the ink occurs. The greater the amount of water supplied, the more fountain solution that is squeezed into the ink and the more emulsified the ink. After the water roller is separated from the plate, fine droplets of the fountain solution remain on the surface of the ink film.

The third type is the nip between the ink roller and the wetted blank. There are fountain solution and ink two phases in the nip. Under the strong pressure of the printing plate and the ink roller, a small amount of fountain solution is squeezed into the ink and a second emulsification of the ink occurs.

The fourth type is the nip between the ink roller and the wetted blank. There are fountain solution and ink two phases in the nip. Under strong squeezing of the printing plate and the ink roller, a small amount of fountain solution is squeezed into the ink and a third emulsification of the ink occurs. After the ink roller was separated from the plate, an ink containing a fountain solution was obtained in the graphic part.

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

The emulsification of offset inks is not only unavoidable, but is also required for the transfer of offset inks. If the ink is not completely mixed with the fountain solution, the dampening solution beads forced into the ink will quickly separate from the ink, adhere to the surface of the ink, and form a film of water on the surface of the ink. The ink cannot be transmitted to the graphic part of the plate. Therefore, inks that are not emulsified cannot be used for plain printing. The ink-free fountain solution must be immiscible with each other, but to a certain extent it must be miscible, that is, it must be able to form a fine mixture of water droplets dispersed in the ink. The emulsification of water in the ink provides a way to exclude the fountain solution of the graphic part, otherwise the printed matter will appear to have a phenomenon of flowering; on the contrary, the fine ink droplets are not allowed to disperse in water, so as to prevent the blank part from becoming dirty.

O/W emulsions pose great hazards to the quality of plain prints and normal production of offset prints. It will make all the blank parts of the print dirty, and will deink the ink roller, ink can not be transmitted. In recent years, the use of resin-type inks for plain printing has enhanced water resistance and ink watering rarely occurs. O/W emulsified inks are not easily generated, and mainly form W/O emulsified inks. The slight W/O emulsified ink is not only required for the transfer of the plain ink, but also the viscosity of the ink is slightly reduced, which can improve the fluidity of the ink and facilitate the transfer of the ink to the paper. However, severe W/O emulsified inks cause a sharp drop in the viscosity of the ink, shortening of the oil film, and poor ink transfer performance. At the same time, the fountain solution immersed in the ink will also corrode the metallic ink roller, forming a hydrophilic layer on the surface of the ink roller, and repelling the ink, resulting in deinking of the metallic ink roller.

The use of fountain solution in plain printing has the following purposes.

1 Form a water film that repels ink in the blank part of the printing plate, resisting the expansion of the ink on the graphic to the blank part, preventing the dirty plate;

2 Supplement the damaged hydrophilic layer in the printing process to maintain the hydrophilicity of the blank part of the printing plate. During printing, due to friction between the blanket cylinder and the water roller, the paper powder and paper that fell off the paper exacerbated the wear of the printing plate. Therefore, with the increase of the number of prints, the hydrophilic layer of the plate will be destroyed. It is necessary to use the dielectric in the fountain solution to chemically react with the exposed aluminum or zinc metal of the plate base metal to form a new hydrophilic layer and maintain the blank space. Part of the hydrophilicity;

3 Reduce the temperature of the plate surface. After the offset press is actuated, the ink roller spreads the ink into an ink film at a high speed, and the temperature of the ink roller increases, resulting in a decrease in the viscosity of the ink. If the printing car is at 25°C, the plate fountain solution is not supplied, the typography machine is continuously turned on for 30 minutes, the ink temperature on the plate will rise to 40-50°C, the viscosity of the ink will drop sharply, and the fluidity will increase. Rapid expansion, resulting in a serious expansion of outlets. Therefore, applying a fountain solution having a temperature equal to or lower than room temperature at a blank portion of the printing plate can reduce the temperature of the printing plate surface.

Therefore, a certain thickness of water film is always maintained in the blank part of the printing plate to ensure the normal printing, and obtain a better print. However, the water film is too thin to achieve the purpose of using the fountain solution. If the water film is too thick, serious emulsification of the ink may occur. Therefore, to achieve the balance of water and ink for flat printing, the thickness of the water film should be controlled.

In order to ensure the quality of the printed matter and the normal printing, the significance of the balance between the water and the ink is: under a certain printing speed and printing pressure, adjust the supply volume of the fountain solution, and use the least amount of fountain solution to emulsify the The ink gets the best print and competes with the ink on the plate.

Experience has shown that in the normal printing, the thickness of the ink layer of the printing plate graphics part is 2~3μm, and the water film thickness of the blank part is about 0.5~1.0μm, and the volume percentage of the fountain solution in the ink is 15%. ~20%, which basically achieves the balance of water and ink in the plain printing.