Printing static electricity and its elimination

Printing is performed on the surface of the object, and the electrostatic phenomenon is mainly manifested on the surface of the object. The relationship between the two is naturally very close. The effect of static electricity on printing has a dual nature. It has a side that can harm the normal printing process, and it also has a side that can be used.

The printing process involves frequent impact, friction and contact-separation processes. The result is electrostatic charges on almost any object involved in the printing process.

First, the harm of static electricity
1, affect the quality of product printing.

The first is the surface of the substrate charged, such as paper, polyethylene, polypropylene, cellophane, etc., which will absorb paper dust or a large amount of dust, impurities, etc. that are filled in the air, thereby affecting the transfer of the ink, reducing the ink transfer rate, in the print There are "flowers." On the display, or due to mutual exclusion, it causes rolling and slipping and uneven rolling.

Followed by the ink charge. This situation occurs when the ink is in a state of lower viscosity, or when the antistatic component in the ink is insufficient. If it is discharged on the move, it may appear on the prints "static ink spots", it looks like the ink is very poor liquidity, ink is very uneven, like this is often the case in the level of thin printing .

But sometimes it also appears in the white version of the field, and it also shows another phenomenon: repulsion. Originally white ink was laid, but when some words were encountered (most of them black), a gap was formed around these words, and white ink could not be printed on it. The main reason is that the quality of white ink is not good, and then the viscosity is too small. If the charged ink is discharged at the edge of the line on the print, it may appear here that the "black whiskers" appear mostly in the field version. In addition, the ink may be scattered due to charging, causing serious contamination of the printed matter and the printing environment.

2, affect the safety of production
A charged substrate may even cause an air discharge due to an excessively high potential in severe cases, resulting in electric shock or fire. Charged ink may cause the ink, solvent to catch fire, or shock personnel through the ink. Many parts of high-speed printing presses generate large amounts of static electricity due to strong friction during high-speed operation. In the case of old-fashioned web rotary presses with imperfect current-removal devices operating at high speeds, the voltage on some parts of the machine sometimes reaches 15000V. There are also about 100uA, so the impact of such a powerful voltage and current on the printing process is complicated, and the hazard is also quite serious, especially the direct threat to the personal safety of the operator.

3. During the printing process, the paper on the paper stack is separated and conveyed under the friction between the paper feeding belt and the pressure roller. Between paper and paper, paper and machine are always in friction. Friction produces charge. When the charge is gathered, the paper is charged. When the paper has the same kind of polar charge, the papers are mutually exclusive, resulting in inaccurate overprinting during printing and a misfeeding of the paper; when the paper has a heterogeneous charge, it causes empty sheets, double sheets, and multiple sheets when the paper is fed. Improper overprinting and other problems; paper surface with static electricity is also easy to absorb some paper hair and dust, resulting in prints appear flowers or spots and other defects, so the elimination of paper static electricity is very important.

Second, the elimination of static electricity
1, grounding. This is the easiest way to eliminate static electricity. That is, the metal conductor is connected to the ground to make it equal to the earth, and the charge leaks through the earth. However, this method has almost no effect on the insulator. If there are two printers adjacent, although they each have a wire grounded, because of the difference in the degree of charge leakage, there is still a potential difference between them, and in order to avoid discharging each other, they must be connected by wires.

2, ion neutralization method. This is ionizing the air to produce two positive and negative ions to neutralize static electricity on the paper. Eliminators can be eliminated by static eliminators. The static eliminators commonly used in printing machines include induction static eliminators, high-frequency high-voltage static eliminators, and industrial frequency high-voltage static eliminators. They should be placed near the rollers when installed.

3, control the relative humidity. This is a simple and effective way. The surface resistance decreases as the relative humidity of the air increases. Therefore, by increasing the relative humidity of the air, the conductivity of the surface of the paper can be increased, and the leakage of charge can be accelerated. In the printing shop, a temperature of about 20 degrees and a relative humidity of about 65% are suitable. Therefore, sprinkle some water in the printing environment, and use an air humidifier as a condition, which can effectively eliminate static electricity.

4, adjust the ink viscosity. Increasing the viscosity of the ink can also effectively reduce (eliminate) static electricity in the ink and improve printing quality.

5. Add antistatic agent or isopropyl alcohol to the ink. It is best to use antistatic agents, because the addition of isopropyl alcohol will cause the ink to "anti-slag". Many books describe that the amount of isopropyl alcohol added is not more than 2%, but according to practice, it must be far more than this amount. Will have effect.

6, when the paper moisture content is low, the paper is easy to electrostatic. When the paper is charged with static electricity, it is necessary to hang the paper or place the paper stack in a relatively humid environment for a period of time. However, care must be taken to prevent the occurrence of paper ruffles.

Third, the use of static electricity
The above is about electrostatic hazards. Let's talk about how to use static electricity to improve the quality of printed products. Photogravure is compared with letterpress printing and lithographic printing. In addition, there is a large amount of ink in the print graphic. Larger printing pressure must be applied to transfer the ink in the ink hole to the surface of the printing material. Therefore, the load of the printing press is required. Compared with letterpress printing presses and lithographic printing presses, the load is large.

Despite the high printing pressure, only 60% of the ink on the plate can be transferred to the substrate. In order to increase the ink transfer rate of the photogravure, an electrostatic ink-absorbing device can be installed on the printing machine, and the ink on the printing can be sucked onto the substrate by means of electrostatic ink absorption, so that the ink transfer rate can be increased by 20%, and the machine is extended. The longevity of the printed matter has also made the reproduction of the subtle levels of the printed matter abundant, which has greatly improved the quality and grade of printed products.

The impression cylinder of the printing press is conductive. During printing, an electrostatic high-voltage generator is used to generate an electric field between the printing plate and the impression cylinder. The electric field attracts the ink in the printing ink hole of the printing plate onto the substrate by means of electrostatic attraction. This device, due to the presence of high-voltage electric field can eliminate the static electricity on the paper, overcoming the various problems caused by static electricity on paper.