Analysis of influencing factors of ink balance in

Analysis of influencing factors of offset ink balance

wetting system

1 Several basic concepts in surface chemistry theory

in order to truly understand the interaction between ink and fountain solution, we must first understand several basic concepts in surface chemistry theory

(1) emulsion

in offset printing, ink and fountain solution are incompatible with each other, but they are required to be miscible to a certain extent. Completely water repellent inks cannot be used in offset printing. A certain amount of fountain solution (generally required to have a water content of 15% - 30%) enters the ink due to extrusion, forming beads to disperse between them, thus forming water in oil emulsion (emulsion ink)

(2) interface

for example, the fountain solution forms an interface with the non graphic part of the printing plate, the ink forms an interface with the graphic part of the printing plate, and an interface is also formed between the ink and the fountain solution

(3) surface tension

different inks have different surface tension; The surface tension of ink will change after it contains water; The surface tension of fountain solution varies with its composition and content. For example, after adding alcohol, its surface tension is significantly reduced, the contact angle with the printing plate is reduced, and the wetting effect is better

(4) interfacial tension

the smaller the interfacial tension, the smaller the contact angle, and the stronger the wetting ability. For example, in offset printing, there is only slight change after the fountain droplets on the image and text parts of the printing plate, even on the ink layer of the image and text parts, which basically appears in the form of water droplets (large contact angle), indicating that the interfacial tension is large; However, if it drips on the non graphic part of the printing plate, because the interfacial tension between the fountain solution and the non graphic part of the printing plate is small, it also shows that its operation is spread out quickly. We know that if the water supply is stopped, the non graphic parts of the printing plate will soon be inked and dirty; If the ink supply is stopped, the fountain solution will soon wash away the ink in the graphic part of the printing plate. The separation condition of ink and fountain solution is that they have sufficient interfacial tension difference

(5) viscosity

water entering the ink, high ambient temperature, heat generated by the ink feeding unit, etc. will reduce the viscosity of the ink. If the film between the rolls is composed of two mixed films of different fluids with the same thickness, the fluid with low viscosity will be pulled apart when the rolls are separated. The viscosity of ink is 10 ~ 20Pa · s, while the viscosity of fountain solution is only 0.001 ~ 0.002pa · S. therefore, assuming that the film thickness is the same, the mixed film of ink and fountain solution can be separated by fountain solution

2. Transmission mode of fountain solution

in the printing process, fountain solution is continuously transmitted to the printing plate. The water supply systems of various types of printing machines are different, with their advantages and disadvantages, but generally speaking, they can be summarized into the following two modes

(1) water supply for layout (ordinary water supply)

the device includes a water bucket, a water bucket roll, a water transfer roll, a water string roll and one or two landing rolls in contact with the printing plate cylinder, which directly applies the fountain solution to the printing plate. In the old model, the water transfer roller can swing back and forth, so it belongs to the discontinuous water supply mode. The water supply is uneven, and the surface speed of the water transfer roller is the same as that of the printing plate. The new model is no longer equipped with a water transfer roller, but the water bucket roller is in direct contact with the water landing roller. The water bucket roller is driven by the motor alone, and the water supply can be changed by changing the speed. There is a surface speed difference between the water bucket roller and the water landing roller, which belongs to a continuous water supply mode. When the printing speed is increased, the water film thickness of the printing plate will be thinner, as shown in Figure 3 (a)

(2) ink roller water supply type

that is, the daglen type water supply device with small volume is adopted on most new models. A variable speed transmission chromium plating water roller driven by a separate motor applies the fountain solution to the first inking roller for continuous water supply. Some models also set one or two water transfer rollers or ink mixing rollers between this inking roller and the water bucket roller. The water supply is determined by the rotation speed of the stepless variable bucket roll driven by a separate motor. The pressure between the ink mixing roller and the water bucket roller determines the thickness of the water film transmitted to the printing plate. The surface speed of the inking roller is the same as that of the printing plate

3. The water film thickness of fountain solution

the consumption of fountain solution entering the machine mainly has the following ways: first, emulsify in the ink and transfer it to the graphic part of the paper; Second, the non graphic part of the printing plate is transferred to the non graphic part of the paper by separating it from the blanket and then from the paper again; Third, it is evaporated due to the heat generated when passing through the roll gap. When there is only the fountain liquid film layer between the roll gaps, its separation at the roll gap outlet still follows the law when there is only ink (half ratio separation principle). Common faults of each roll after separation 5: the film thickness on the roller or roller is half of the total film thickness at the roll gap inlet. At the same time, since the appropriate amount of water entering the ink is 15% - 30% of the ink, the ink layer thickness of the inking roller is 2.5 ~ 3.5 μ m. It is not difficult to understand that the water film thickness maintained by the non graphic part of the printing plate is 1/3 ~ 1/2 of the ink layer thickness of the graphic part of the printing plate and the ink layer thickness of the inking roller, that is, 0.5 ~ 1 μ m. This has been confirmed by the actual data measured by the infrared detector

(to be continued) the typical starch/degradable high molecular composite is starch/pva alloy