The effect of static electricity on the printing of paper

Once the paper is charged with static electricity, it will cause a lot of trouble for printing. The first is that the paper cannot be hit. Under the action of static electricity, paper and paper are firmly sucked and uneven, and it is difficult for air to enter between the papers. In order to collide, it is sometimes necessary to open them one by one, which is a waste of time. During the printing process, due to the electrostatic attraction, the sheets are firmly stuck together, sometimes two, sometimes a few, and sometimes a sheet of paper cannot be separated, resulting in a paper suction nozzle that cannot absorb the paper. The brush is heavy, often producing broken sheets and empty sheets; the brushes are lighter in pressure and produce double sheets and more sheets. Multiple sheets of paper enter between the blanket cylinder and the impression cylinder, causing boring cars and crushed blankets and gaskets. The paper with static electricity is not smooth when it is conveyed forward at the delivery station, and it is misaligned and misaligned when it reaches the front gauge, causing the second overprint to fail to register, and the product quality is poor and wasteful. Even after passing through the embossing section, the delivery is very uneven, causing great troubles for the second entire paper and seriously affecting the production speed.

There is a certain relationship between static electricity and papermaking. Under normal circumstances, the original paper is less charged when shipped from the factory and the probability of coated paper is not high. Because printing paper (whiteboard, cardboard, etc.) and coated paper are reprocessed on the basis of the base paper, even if the base paper is charged, it will be eliminated during processing. Generally, paper with a basis weight of 80g/m2 or less has more static electricity. However, static electricity is not present before the paper is printed on the machine, or static electricity is not obvious before printing. The static electricity is often increased after the printing process. In the offset printing process, due to the presence of water, it is not common to see static electricity after printing. For offset printing, static electricity is mainly generated before printing.

Cause Analysis

The main cause of static electricity on paper is frictional electricity generation. Many objects are charged by the friction caused by friction, paper and calender friction paper, printing paper and rubber roller, the pressure between the printing roller is an important factor in the generation of static electricity.

The binding forces of the electrons of different substances are not the same, or they have different affinity for electrons. When two different substances are in close contact, the electron-releasing side is weak due to the binding force to the electrons, and some electrons will adhere to the surface of the electrophilic side through the interface. As a result of this electron transfer, a double layer is formed on the interface. At this time, if the two substances are rapidly separated, the electron-releasing side is positively charged due to the loss of some electrons, and the electrophilic side is negatively charged due to obtaining some electrons. This is the essence of triboelectricity.

The papers, blankets, inks, printing plates, etc. in print materials have different incomparable characteristics of electrons. The great pressure in the printing process makes them in close contact with each other, and they have high speeds to make them quickly separate and have full friction. Electrical conditions. Therefore, it is understandable that thousands of volts of static electricity appear in printing. In particular, some papers are electrostatically charged before printing. Paper has generated static electricity during the manufacturing process. When it is too late to be packaged and sent to the printing plant, some static electricity remains. This is also the reason why the paper is charged before printing. When using these papers, it is necessary to take a different approach to eliminate the static electricity they carry, in light of the actual production conditions.

Solution

Inventory method

After the paper enters the printing factory, storage time should be longer and the storage location can be better connected with the printing shop. The best temperature is 18 to 25°C and relative humidity is 60% to 70%. The consistency of the temperature and humidity of the printing shop with the paper bank will help change the moisture content of the paper. Changing the moisture content of paper is a process of releasing static electricity.

2. Drying method (wetting method)

The main method is to eliminate static electricity by adjusting the main humidity. When the relative humidity in the workshop is less than 50%, high static electricity is easily generated during the printing or plate making process, and the relative humidity of the shop and the moisture content of the paper are increased. Especially, increasing the indoor relative humidity when paper is being dried is very effective for eliminating static electricity. The humidity control equipment can be used to increase the relative humidity in the room. When there is no humidity control equipment, sufficient water can be sprinkled on the floor. The humidity control equipment is mainly a humidifier, and a centrifugal automatic humidifier can be installed on the ceiling or wall of the workshop. When the relative humidity in the room does not meet the requirements, the humidifier can automatically spray misty water vapor, increase the indoor relative humidity, and automatically stop spraying when the relative humidity in the room reaches the required level.

3. Static eliminator method

The ions generated by the static eliminator neutralize the charge on the electrified body to eliminate static electricity. There are three types of static eliminators: one is an applied-voltage static eliminator, which applies a high voltage to acicular or thin wire electrodes to generate ions due to corona discharge. The transistor static eliminators used in printing machines are generally classified as such. Second, the self-discharge type static eliminator, the conductive fiber, conductive rubber or conductive metal materials such as needle-like or thin wire electrode and well grounded, use the electric field of the charged body to generate corona discharge ion, And the charge on the electrified body; Third, the static elimination of radioactive elements, the use of ionization of radioactive isotopes, ionized air ions, and static electricity on the charged body.

4. Antistatic agent method

Antistatic agent is also called static elimination agent or deoxidizer. The principle is to give hygroscopic ions on the surfaces of charged objects such as paper and film to make them hydrophilic, absorb moisture in the air, reduce resistance, increase electrical conductivity, and make static charges less likely to accumulate. Antistatic agents are mainly surfactants, with hydrophilic and hydrophobic groups, or polar groups and non-polar groups. Hydrophilic groups have strong affinity for substances such as water, which are more polar, and hydrophobic groups have strong affinity for oils and other less polar objects. Antistatic agents are widely used in printing, such as the use of antistatic agents to produce anti-static soft roller and so on.

5. Process operation method

During the printing process, a damp towel may be added to the delivery section to hold the dipped towel on the pull rod so that the paper is contacted with a damp towel to eliminate static electricity. This is a temporary but effective way to eliminate static electricity. The disadvantage is that Frequent wet towels.