The factors affecting ink drying, the causes of printing static electricity and the treatment methods

The drying process includes all the processes after ink transfer. In these processes, ink curing is the primary condition for post-press processing and the premise for printed matter to be used in daily life.

According to the composition of ink, drying methods can be divided into physical drying, chemical drying, and the comprehensive application of these two methods. The difference between ink and varnish requires different drying systems or the comprehensive application of several drying systems to get the best drying effect. If an infrared and ultraviolet drying system is installed on a printing machine, it can basically meet all the needs of printing, so it is very useful.

Considering its drying performance, the composition of Printing Ink needs to meet the following two contradictory requirements: first, when the printing machine is printing or temporarily shut down, the ink cannot dry on the ink roller; Second, after printing, the ink on the substrate needs to be quickly dried and fixed.

Several most important factors affecting the drying performance of ink:
① Composition of ink (pigment, binder and related additives of ink); ②Performance of substrate materials (such as permeability, etc.); ③ printing conditions (ink transfer amount, delivery pile height, printing speed); ④ Climatic conditions (indoor temperature and humidity); ⑤ Composition of drying system (such as air flow on ink surface, reaction time, type of energy supplied, etc.). Among them, temperature is a decisive factor. Generally speaking, the higher the temperature, the better. Because the higher the temperature, the faster the ink polymerization speed, the smaller the ink viscosity, the more conducive to penetration and the faster the solvent evaporation.
Understanding the causes of printing static electricity and mastering the correct countermeasures
In the printing process, sometimes paper or printed matter will adhere to each other, as if the paper has a strong vacuum suction and is difficult to separate. This phenomenon is printing static electricity. In the winter with dry climate, the static electricity of printed paper is particularly prominent, which may cause obstacles to paper feeding, paper receiving and paper alignment, or even lead to the failure of automatic paper feeding or frequent occurrence of double sheets, blank sheets, disordered paper receiving and increased loss, which not only reduces production efficiency but also affects product quality. Therefore, it is of positive significance to correctly understand printing static electricity and master the countermeasures to promote production.
When the paper contains a high amount of water, it can become a conductor of electricity; On the contrary, when the water content is very low, it is absolute, and paper is prone to static electricity: First, the paper making process is not suitable. That is, in the process of paper making, due to the low content of electrolyte (alum, sodium chloride, etc.) and the excessive content of sizing substances (high territory, etc.), the paper is affected by high temperature and pressure during drying and calendering, which makes the paper electrostatic with high voltage. Second, improper storage of paper. For example, when there is a great disparity between the temperature of the paper carriage and the temperature of the printing workshop, the paper does not expand and contract due to the variation of water content, which affects the registration, and it is easy to cause static electricity, which brings inconvenience to the brush drawing. In addition, before printing, the paper placement time is too short, or the stack height is too high, which makes the paper lack of effective balance time with the printing environment temperature and causes the paper to be charged with static electricity. Third, printing friction causes static electricity. When the water content of paper is obviously less, the paper will be excessively dry. In the printing process, due to the friction and pressure such as paper feeding and embossing, the electrons outside the paper nucleus will gain and lose, making the paper electrostatic.

Paper is charged with static electricity, and the situation is different. There is one reason, and there are also several bad situations. All kinds of paper, even the same kind of paper, this batch is not necessarily the same as the previous batch, which means that the voltage will be different. Another feature is that the electrical properties of the two sides of the paper are opposite after being charged with static electricity, while the stacking of the front and back sides of the paper is the same when printing, that is, the front side is up or the back side is up. Therefore, the paper whose front and back are in contact with each other is firmly attached to each other and is not easy to separate, which brings certain obstacles to printing, paper feeding, paper conveying and paper alignment.

To prevent the occurrence of printing static electricity, we must first control it from the source. The use of paper with static electricity can be suspended, and the storage time can be appropriately extended. The temperature and humidity of the paper warehouse should be controlled at 18℃-20℃, and the relative humidity of the air should be controlled at 60-65%. Under this condition, it is beneficial to weaken or eliminate the static electricity of paper. Secondly, the static electricity of paper is removed by air drying. When the paper is in urgent need, the paper dryer can be used to hang the paper with static electricity. When hanging and drying, attention should be paid to the coordination of machine speed and air humidity, that is, when the machine speed is fast, the relative temperature of ambient air is controlled at 80-85%, and conversely, when the machine speed is slow, the air temperature is kept at 70-80%. It should be noted that when hanging and drying, the thickness of each stack of paper should be controlled within 10㎜. Third, emergency paper drying to remove static electricity. When static electricity appears in the printed paper after cutting, the paper can be dried quickly by "steam method" to reduce the static electricity of the paper and complete the printing as soon as possible.

The comprehensive elimination method can also be used to eliminate paper static electricity, that is, the effect will be better by combining the improvement of paper temperature and water content with the use of electrostatic eliminator. The method is as follows:

1. Install air-conditioning equipment in the printing workshop, and adjust or maintain the temperature and humidity of the printing workshop according to the strength of paper static electricity. In general, the workshop temperature is adjusted between 18℃ and 20℃, and the relative air temperature is kept between 60% and 65%. If the paper is seriously charged, the relative humidity of the air can be adjusted to 80-85%.

2. Install the static eliminator on the printing machine. Eliminate paper static electricity by adjusting voltage or high frequency. If paper static electricity is large, use high voltage or high frequency. If the static electricity is not high, turn down the voltage and high frequency.

3. Eliminate printing static electricity with isotope emitter. The principle of isotope radiation is basically the same as that of electrostatic eliminator, which ionizes various molecules in the air into ions. Because ions have different electrical properties, they become good conductors of electricity. When paper passes through the ion space, part of the static electricity is neutralized and the other part is taken away by air ions, thus achieving the purpose of eliminating static electricity.

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