Water and ink balance
From the above analysis, it can be seen that the ink and the fountain solution have the basic separation state and the film transfer thickness respectively when they reach the normal printing, and both phases of the printing material exist simultaneously and work together. Controlling the amount of ink is an important factor in ink-ink balance in offset printing, so the following two basic dampening systems are still discussed to discuss the issue of ink-water balance.
1 for the layout of the water
The plate feeding mode covers the following four different roll gap states.
(1) The nip between the water roller and the non-graphic part of the plate
The non-graphic part of the plate is wetted by the fountain solution so that there is only a single dampening film layer between the water roller and the plate, and the water film layer splits at the exit of the nip so that it is left on the plate. Water film.
(2) The nip between the water roller and the printing plate graphic part
The graphic portion containing the residual ink film on the plate is also wetted by the fountain solution, and the ink emulsification occurs. The rest will leave fine water droplets on the surface of the ink roller instead of the ink film, so that the ink roller and There is a mixture of fountain solution and ink in the plate gap.
(3) The nip between the ink roller and the graphic part of the plate
The surface of the plate containing the fine water droplets, after the ink roller passes, forces the water droplets to emulsify in the ink. The ink layer (containing the emulsified fountain solution) is split at the nip exit, and finally the ink is made up in the graphic part.
(4) The nip between the ink roller and the non-graphic part of the plate
The wetted non-graphic portion of the plate produces a mixed film of fountain solution and ink in the already emulsified ink gap when passing through the ink form roller. At the nip exit, the thinner layer of dampening fluid splits, leaving a thinner layer of dampening fluid on the non-graphic part of the plate, leaving fine droplets on the inking roller as it is passed up again. In other nips, these water droplets emulsify in the ink.
After the above four processes or conditions are completed, the printing plate cylinder and the blanket cylinder are in contact with each other. Due to the splitting effect of the film layer, the inking graphic is transferred to the blanket, and the non-graphical portion of the fountain solution is also transferred together. At the same time, we still follow the semi-ratio separation theory.
The above explanation about the platelet-type dampening system is based on the fact that the relative film thickness or relative amount of the ink and the fountain solution matches the actually measured thickness on the printing press. The thickness of the water film on the printing plate can be regarded as half the thickness of the ink layer on the printing plate. That is, the water film thickness is 1 Î¼m, and the ink layer thickness is 2 Î¼m. This proportion is particularly important for offset printing, which is commonly referred to as water-ink balance. If the water supply is increased, the amount of the fountain solution in the graphic part is excessive, and it is either emulsified in the ink or left on the surface of the ink, and the result is not water flushing or flowering. Conversely, if the water supply is reduced even below the limit level, the non-graphical water film layer is too thin, it will destroy the ink and ink balance, and will inevitably cause dirty plates.
2. Ink roller water supply
The plate-type dampening device relies on the water surface of the printing roller to have good hydrophilicity. However, if the rubber roller is used, it will be wetted by the ink due to its good ink receptivity and strong adhesion. Wet effect. If the surface tension of the fountain solution is reduced and the adhesion between the rubber roller and the fountain solution is improved, it is possible to change the plate roller to a rubber roller. The first inking roller accepts the ink on the ink fountain roller and the fountain solution on the chrome roller. The dampening system can only function when the fountain solution expands on the ink. Adding alcohol to the dampening solution can reduce the surface tension of the fountain solution, improve the wetting ability, and improve the ability to supply water to the printing plate through the ink roller to quickly achieve the purpose of water-ink balance. The state between several nips is analyzed below by using FIG. 4 .
(1) The nip between the first ink roller and the water roller
The inking roller has a polyurethane-like coating layer, so that the ink roller is preferentially wetted by the ink and coated with a layer of emulsified ink, while the water roller is chrome-plated and preferentially wetted by the dampening solution. Due to the low surface tension and viscosity of the fountain solution, the fountain solution is split between the two nips, and some of it is transferred to the inking roller. The surface speed of the inker roller is faster than that of the fountain roller. some.
(2) The nip between the first ink roller and the ink roller
Both rollers can be preferentially wetted by the ink. The ink conveyed on the ink fountain roller is always the emulsified ink, and the ink conveyed on the ink roller is a layer of water film attached on the surface of the ink layer. Where the dampening water film is continuous, the entire water film is split between the nips and the water film on the discontinuous inked ink layer cannot be kept intact, so the fountain solution will emulsify in the ink and the entire film layer will Split. In this way, this nip condition can be kept constant, the film between them is always equal to the thickness of the emulsified ink, and its composition is also only the emulsified ink.
(3) The nip between the first inking roller and the plate
Regardless of whether the graphic part and the non-graphic part of the plate pass the inker roller, the above two nip states remain constant, so the state of the plate and the ink roller can be simply discussed here. Glenn water delivery device status. In the non-graphic part of the printing plate, since the ink roller adheres the fountain solution, the printing plate is preferentially wetted by the fountain solution, and the water film layer is split to achieve the purpose of wetting. The remaining fountain solution on the ink application roller passes through the screed roller. It will emulsify in the ink and it will no longer remain intact. In the graphic part of the printing plate, the fountain solution is emulsified by the simultaneous extrusion of the fountain solution into the ink of the printing plate. In this nip, the entire emulsified ink film layer is split to achieve the purpose of supplying ink to the graphic part. . The corresponding film thickness required to maintain the proper ink-water balance in these two states is also proportional.
(4) The nip between the other ink roller and plate
Between the non-graphic portion of the printing plate and the nip of the ink form roller, the ink film on the ink form roller draws moisture from the fully wetted non-graphical portion (splitting the water layer) and comes into contact with the ink layer of the ink stringer. , thus transferring the water to the inking system. In the printing plate graphic part, since the first ink roller has transmitted the emulsified ink, there is no water film layer on the surface, and the ink roller is only emulsified ink film layer between the following ink roller and the formed nip, so the ink film is split. .
From the above analysis, it can be seen that the ink roller water supply type has many advantages compared with the supply type dampening system, such as high reaction sensitivity and small water consumption. Among them, the responsiveness refers to the ability of the dampening device to dirty the printing plate. Strong. The first inking roller is provided with a continuous water film so that more and more fountain solution can remove dirt from the non-graphic portion of the plate by the inker roller. The printing apparatus conveys the fountain solution to the dirty surface through the water roller in the form of small water droplets, so the dampening fluid for supplying the printing plate is less, and the speed of removing the dirt is slow.
(to be continued)