Once the paper is charged with static electricity, it will bring a lot of trouble to printing. The first is that the paper cannot be aligned. Under the effect of static electricity, the paper is firmly absorbed and uneven, and the air is difficult to enter between the paper. To collide it, sometimes it is necessary to open it one by one by hand, which is a waste of time. In the printing process, due to electrostatic attraction, single sheets of paper are firmly stuck together, sometimes two sheets, sometimes a few sheets, sometimes a stack of paper can not be separated, resulting in the separation nozzle can not absorb the paper. When the brush is heavy, it often produces broken and empty sheets; when the brush is lightly pressed, it produces double and multiple sheets. When multiple sheets of paper enter between the rubber cylinder and the impression cylinder, it will cause stuffiness and crush the blanket and liner. Electrostatically charged paper is not smooth when it is transported forward on the paper conveying table. The paper reaches the front gauge where it is skewed and incorrectly positioned. As a result, the second overprint cannot be registered, and the product quality is poor and wasteful. Even after passing the embossing part, the paper collection is very uneven, which brings great trouble to the second whole paper and seriously affects the production speed.
The static electricity of paper has a certain relationship with papermaking. Under normal circumstances, the base paper is less charged at the time of shipment, and the probability that the coated paper is charged is not large. Because printing paper (whiteboard paper, 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 the processing. Generally, the paper with a certain weight below 80g / m2 has more static electricity, but the paper has static electricity before printing on the machine or the static electricity before printing is not obvious. Due to the presence of water in the offset printing process, those who carry static electricity after printing are generally rare. For offset printing, static electricity is mainly generated before printing.
Cause Analysis
The main cause of static electricity in paper is triboelectricity. The charging of many objects is caused by friction. The friction between the paper and the calender during papermaking, and the friction between the paper and the rubber cylinder and the impression cylinder during printing are important factors that generate static electricity.
The nucleus of different substances has different binding power to electrons, or different affinity for electrons. When two different substances are in close contact, the electronophobic side has weaker binding power 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, an electric double layer is formed at the interface. At this time, if the two substances are separated quickly, the electronophobic side is positively charged due to the loss of some electrons, and the electronophile side is negatively charged due to the acquisition of some electrons. This is the essence of triboelectricity.
Papers, blankets, inks, printing plates, etc. in printing materials have different affinity for electrons. The great pressure in the printing process makes them in close contact, and the high speed makes them separate quickly, fully equipped with friction. Electricity conditions. Therefore, it is understandable that thousands of volts of static electricity appear in printing, especially some papers carry static electricity before printing. The paper has generated static electricity during the manufacturing process. When it is too late to be packaged and sent to the printing factory, some static electricity is still retained, which is why the paper is charged before printing. When using these papers, it is necessary to take different measures to eliminate the static electricity carried in combination with the actual production situation.
Solution
1. Inventory method
After the paper enters the printing factory and enters the warehouse, the storage time should be appropriately longer. The storage location can be better connected with the printing workshop. The temperature is 18-25 ° C and the relative humidity is 60% -70%. Making the temperature and humidity of the printing workshop consistent with the paper library is conducive to changing the moisture content of the paper. The change of paper moisture content is a process of discharging static electricity.
2. Paper drying method (humidification method)
Mainly to eliminate static electricity by adjusting the main humidity. When the relative humidity in the workshop is less than 50%, it is easy to generate high static electricity during printing or plate making. Increasing the relative humidity of the workshop and the moisture content of the paper, especially increasing the indoor relative humidity when drying the paper, is very effective in eliminating static electricity. The humidity control equipment can be used to increase the indoor relative humidity. When there is no humidity control equipment, sufficient water can be sprinkled on the ground.
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 indoor relative humidity does not meet the requirements, the humidifier can automatically spray mist water vapor to increase the indoor relative humidity. After the indoor relative humidity reaches the requirements, the spraying will be automatically stopped.
3. Static Eliminator Method
Use the ions generated by the static eliminator to neutralize the charge on the charged body to achieve the purpose of eliminating static electricity. There are three types of static eliminators: one is the externally applied voltage static eliminator, which applies a high voltage to the needle or thin wire electrodes, and corona discharge generates ions. The transistor static eliminators used in general printing machines belong to this category. The second is a self-discharge static eliminator, which uses conductive fibers, conductive rubber or conductive metal materials as needle-shaped or thin-line electrodes and is well grounded. The electric field of the charged body generates corona discharge to generate ions. And the charge on the charged body; The third is the radioactive element destaticizer, which uses the ionization of radioactive isotopes to ionize the air to generate ions, neutralizing the static electricity on the charged body.
4. Antistatic agent method
Antistatic agents are also called static eliminators or antistatic agents. The principle is to give hygroscopic ions on the surface of charged objects such as paper and films to make them hydrophilic, absorb moisture in the air, reduce resistance, increase conductivity, and make it difficult for static charges to accumulate. Antistatic agents are mainly surfactants, with hydrophilic and hydrophobic groups, or polar and non-polar groups. Hydrophilic groups have a strong affinity for water and other polar materials, while hydrophobic groups have a strong affinity for oil and other less polar objects. Antistatic agents are widely used in printing, such as the use of antistatic agents to make soft rubber rollers that prevent static electricity.
5. Process operation method
During the printing process, a damp towel can be added to the paper receiving part, and the towel dipped in water can be fixed to the pull rod to make the paper eliminate static electricity after contact with the damp towel. This is a temporary but effective method to eliminate static electricity. The disadvantage is to Wet towels often.
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