Explore the application of offset printing ink in the printing process (2)

6. Viscosity control

Frequently used Bruckfeldt viscosity meter or domestic Hermite viscometer. Actually, this is because the pigment particles in the ink attract and flocculate each other when they are at rest, so that they are in a thick condensed state. When mechanically agitated, the flocculation is disrupted and becomes thin and easy to flow. But when it is still for some time, it will flocculate again. This is thixotropic. Therefore, the offset ink must determine its viscosity and yield value.

7. Determination of viscosity, measurement of fluidity

The determination of the viscosity and the measurement of fluidity are all related to the printing adaptability: the printing speed is slow, and the ink with higher viscosity can be used; the printing speed is high and the ink with lower viscosity can be used. For example, offset inks used in high-speed four-color offset presses are less viscous than inks used in monochrome or dual-color offset presses. However, the viscosity of the ink becomes smaller as the temperature rises. After the high-speed offset press is turned on, the ink roller has a high rotation speed and a high temperature rise, and the viscosity of the ink gradually becomes smaller, so the viscosity is slightly higher. Of course, the substrate is also related to the ink viscosity requirements, smooth and solid paper can use a higher viscosity ink, loose structure of the paper to apply a smaller viscosity ink, otherwise it is easy to cause off powder, hair and other phenomena.

8. Tack and viscosity

Viscosity and viscosity are related: that is, under normal circumstances, the viscosity of ink with large ink viscosity is also larger, but there are times when it is not relevant, so some inks have similar viscosity, but the viscosity is different, and some inks have similar viscosity and a large difference in viscosity. In this case, the ink should be selected mainly for stickiness. For example, a color offset ink having a viscosity in the range of 5 to 9 is more suitable, and its range is 5 to 11. Due to the fast printing speed, offset viscosity is 3-6. The viscosity of the ink has a great relationship with the printing speed, the separation speed of the ink layer at the printing speed is also fast, and the viscosity exhibited by the ink is also large. This is the three reasons for the particularly low viscosity of web offset inks. Viscosity has a great influence on the transfer of ink. In the applicable range, the ink with the higher viscosity is better to transmit ink. The ink layer printed on the paper is also thicker, and the dots are more standing. However, when the viscosity is too large, unevenness occurs on the ink roller, and even the roller stacker or the blanket is piled up. At this time, the outlets of the prints are also fuzzy and false. There is also a solid imported paper that can be printed well. Switching to a poorly made domestic paper can cause it to lose powder, lose hair, and even peel. This is caused by the stickiness of the paper that cannot withstand the separation of the ink layer between the blanket and the paper. There is also stickiness and temperature. When the temperature is high, the viscosity is reduced. After the offset printing machine pauses in the middle of printing, the paper will lose hair and peel off. This is because it has stopped for a while and turned cold. The viscosity of the ink has increased. . If some good paper waste paper is printed first, the car will be normal after a certain period of time. In a four- or more-color offset press, the mutual adhesion of the four color inks is also very important. In general, the viscosity of the latter color ink cannot be greater than the previous color, otherwise the previous color ink layer on the print will be pulled down. .

In addition, there is a relationship between the yield value and fluidity and the “body bone” of the ink, and it also has a lot to do with thixotropy.

9. The relationship between fixation and drying of ink

Offset printing inks are classified into three types: oil type, resin type, light fast drying solid type. However, the latter two types are generally used now. Therefore, the brief is as follows:

1 After the resin ink is printed on the paper, low-viscosity petroleum solvents and dry vegetable oil can quickly penetrate the pores of the surface of the paper, so that the pigment coated with synthetic resin and dry vegetable oil can be solidified on the surface of the paper. This state is called For fixing. At this time, a small amount of printing pressure will not be stained on the back, but the pressure will be too dirty. The solid ink layer is further oxidatively polymerized to form a strong ink film, which is truly dry. General resin inks are fixed and completely dry in two stages. The fast fixing is called fast fixing ink. The fixing property is actually measured in seconds, but due to the relationship between the test method and the instrument, the total value is now calculated. The dryness is comparable to the time when the print is completely dry, in hours.

2 The effect of fast fixing of ink on coated paper or coated paper or gold and silver card paper is remarkable because the pores on the surface of the coating are very small and large, and it can be fixed in an appropriate amount of petroleum solvent in a very short time. Because the ink cannot be fixed by inhaling a certain amount of petroleum solvent for a short period of time, the effect is not good. The fixing state of the ink is not really dry, so only a small number of printing parts can be stacked and can not be moved by hand, because the excessive pressure is too large to be stacked, or the local compression is too large by hand to move, only the solidification state. The ink layer will still be damaged and stain the back.

Whether fixed or not, the oxygen in the air makes dry vegetable oils unsaturated double bonds, first oxygen, followed by polymerization, gradually increased in molecular weight and finally become a solid film, which is the real dry.

10. Ink drying conditions

1 needs oxygen in the air;

2 Heat or chemical additives that cause and promote polymerization—that is, drying oil. In order not to cause end-of-life, the instrument may be used first to perform an ink drying test of 5 to 100 grams. For example, the amount of white dry oil is about 3% to 15% of the ink (too much drying does not increase, it will also lead to ink emulsification and prone to paste, etc.), red dry oil consumption is about 1% to 3% of the ink ( Although the drying effect of red dry oil is several times faster than that of dry oil, the surface is quickly dried and the internal drying is slow. In practice, the two should be mixed together as well.