Offset quality control - application of dampening solution (1)

Offset printing is a printing method that requires water. Utilizing the principle of oil-water incompatibility, water is supplied first, and then ink is supplied to make the graphic part ink-free, and the blank part is water-free. During printing, the ink at the top of the printing plate is transferred by pressure to the surface of the elastic rubber roller, and then the rubber roller transfers the image to the paper on the surface of the printing roller.

The printing process is the process of ink and water balance, and 75% of the failures during printing are caused by chemical factors. Among them, water is the most prone to problems in the production process. Therefore, the correct control and use of fountain solution is the quality control of printing. One of the important links.

First, wetting phenomenon

Wetting refers to an interface phenomenon that occurs when a liquid is in contact with a solid. Any contact between a liquid and a solid after the system has a reduced free energy is called wetting, and vice versa.

When the liquid and the solid contact with each other, the liquid molecules in the adhesion layer are subjected to the internal force of the liquid, namely, the cohesive force, and on the other hand, the force of the solid molecule, the adhesion force. Therefore, the liquid can wet the solid. Depending on the size of cohesion and adhesion.

The interfacial tension between liquid and solid is a physical quantity that cannot be directly and accurately measured. Therefore, to judge the wetting phenomenon, the contact angle θ is usually used as a measure of the degree of wetting.

The relationship between contact angle and wetting

1 When the contact angle θ is 0°, sufficient wetting phenomenon occurs.

2 When the contact angle θ < 90°, it is said to be able to wet.

3 When the contact angle θ>90°, it is called non-wetting.

The contact angle of the liquid to the solid is small, the liquid water beads can spread out, the surface tension is low, and the wetting performance is good.

Second, two ways to improve the wetting properties

1, increase the PS version of the fountain solution adsorption capacity

Selection of materials with good hydrophilic properties, high abrasion resistance, and stable chemical properties as the basis for the blanks. Starting with the enhanced hydrophilicity and oil resistance of blanks in the printing plate, advanced processing technology is used to form dense layers on the surface of the printing plate. Sand spots, thereby increasing the surface area of ​​the PS plate, forming the basis for the blank part of the plate, and in accordance with this to expand the adsorption capacity, the use of sand pores to store moisture, to create conditions for resisting ink expansion.

Choose a printing plate with better grain processing. The ink tolerance adjustment during printing is large and the printing press is easy to operate.

2, reduce the fountain solution surface tension

To improve the wettability of the dampening solution to the PS plate, it is necessary to reduce the contact angle. To reduce the contact angle, it is necessary to add a surface active material, an interface active material, a hydrophilic colloid, etc., thereby reducing the surface tension of the solution and achieving sufficient wetting performance. .

Select dampening fluid additives with good wetting properties, which are not easily affected by the inhomogeneous changes of ink, fountain solution and other related materials, can effectively reduce water consumption, reduce emulsification, accelerate drying, improve print gloss, and effectively reduce the printing failure rate. .

(to be continued)