Analysis of Interaction between Ink and Fountain Solution for Offset Printing and Ink Balance (II)

1. Several basic concepts in surface chemistry theory

To really understand the interaction between the ink and the fountain solution, we must first understand several basic concepts in surface chemistry theory.

(1) Emulsion

In offset printing, the ink and the fountain solution are incompatible with each other, but they are required to be mixed to some extent. The completely water-repellent ink cannot be used in offset printing, and a certain amount of fountain solution (general requirements have 15%-30 The % water content) forms a water-in-oil emulsion (emulsion ink) due to the squeezing action of entering the ink to form droplets dispersed therein.

(2) Interface

For example, the fountain solution forms an interface with the non-graphical portion of the plate. The ink forms an interface with the graphic portion of the plate, and an interface is formed between the ink and the fountain solution.

(3) Surface tension

Different inks have different surface tensions; after the ink contains water, its surface tension will change; the composition and content of the fountain solution are different, and its surface tension is also different. For example, after adding alcohol, the 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 interface tension, the smaller the contact angle and the stronger the wetting ability. For example, the dampening droplets in the offset printing plate are only slightly changed after the printing plate graphic part and even the ink layer above the graphic part, and basically appear in the form of water droplets (large contact angle), indicating that the interface tension is large; If it is dripped on the non-graphic area of ​​the plate, the interfacial tension between the fountain solution and the non-graphic part of the plate is small, so it spreads quickly. We know that if we stop the water supply, we will quickly get ink on the non-graphic part of the printing plate and get dirty; if we stop the ink supply, the fountain solution will soon wash away the ink from the printing plate graphic part. The separation conditions for the ink and fountain solution are that the two have sufficient difference in interfacial tension.

(5) Viscosity

Water entering the ink, high ambient temperature, heat generated by the ink supply unit, etc. will cause the ink viscosity to decrease. If the film layer between the nips is composed of two mixed film layers of different fluids but the same thickness, when the rolls separate, the less viscous fluid will be pulled apart. The viscosity of the ink is 10 to 20 Pa·s, but the viscosity of the fountain solution is only 0.001 to 0.002 Pa·s. Therefore, assuming that the film thickness is the same, the mixed film of ink and fountain solution can be separated by the fountain solution.

2. Fountain solution transmission method

In the printing process, the dampening solution is continuously transferred to the printing plate. The water supply systems of the various types of printing machines are different. Each has its own advantages and disadvantages, but overall it can be summarized as the following two ways.

(1) Water supply for plate type (normal water supply type)

The device includes a water bucket, a fountain roller, a water transfer roller, a series of water rollers, and one or two water rollers that are in contact with the plate cylinder. The fountain solution is directly applied to the printing plate. In the old model, the water transfer roller can oscillate back and forth, so it is a non-continuous water supply method. The water supply is uneven, and the surface speed of the water 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 contact between the water roller and the water roller directly. The water roller is driven by the motor alone. The water supply can be changed by changing the rotation speed. The surface speed exists between the water roller and the water roller. Poorly, it is a continuous water supply method. When the printing speed is increased, the thickness of the printing plate water film will become thinner, as shown in Fig. 3(a).

(2) ink roller supply

This is the Dagrange water supply used on most new models. Separate motor-driven, variable-speed, chrome-plated water rollers apply fountain solution to the first inking roller for continuous water supply. Some models also have one or two water transfer rollers or transfer rollers placed between the inking roller and the fountain roller. The amount of water supply is determined by the rotational speed of a continuously variable paddle roller driven by a separate motor. The pressure between the transfer roller and the fountain roller determines the thickness of the water film transferred to the plate. The surface speed of the ink roller is the same as that of the printing plate.

3. Fountain film thickness

The fountain solution entering the machine consumes mainly the following ways: one is emulsified in the ink and transmitted to the graphic part of the paper; the other is that the non-graphic part of the plate is separated from the blanket and is again associated with the paper. Separated and transmitted to the non-graphic part of the paper; third, it is evaporated by the heat generated by the nip. Where only the dampening liquid film layer exists between the nips, the separation at the nip exit still follows the rule of only the ink (half-parts separation principle). After separation, the thickness of the film on each roller or roller is the nip inlet. Half the total thickness of the film. At the same time, since the appropriate amount of water entering the ink is 15% to 30% of the ink, and the ink layer thickness of the ink roller is 2.5 to 3.5 μm, it is easy to understand that the thickness of the water film kept by the non-graphic part of the printing plate is the plate graphic. The thickness of some ink layers and the thickness of the ink roll ink layer are 1/3 to 1/2, that is, 0.5 to 1 μm, which has been confirmed by data actually measured by an infrared detector.