What about the static electricity generated by the paper in the printing factory? How to remove static electricity
Once the paper is electrostatic, it will bring a lot of trouble to printing. The first is that the paper cannot be aligned. Under the action of static electricity, the paper and paper are firmly absorbed and uneven, and it is difficult for air to enter between the papers. If you want to collide, you sometimes have to pull them apart by hand one by one, which is a waste of time.
In the printing process, due to electrostatic attraction, single sheets of paper are firmly stuck together, sometimes two, sometimes several, sometimes a stack of paper cannot be separated, resulting in the paper separation suction nozzle can not absorb the paper. When the brush is pressed heavily, it often produces broken tension and empty tension; When the brush is pressed lightly, it will produce double sheets or multiple sheets. Multiple sheets of paper entering between the rubber drum and the embossing drum will cause stuffy cars and crush the blanket and liner.
The paper with static electricity is not smooth when it is transported forward on the paper feeding table, and it is skewed and misaligned at the front gauge. As a result, the overprint cannot be aligned, and the product quality is poor, which is a great waste. Even after the embossing part, the paper delivery is also very uneven, which brings great trouble to the whole paper and seriously affects the production speed. Paper with static electricity has a certain relationship with papermaking. In general, the base paper is less charged when leaving the factory, and the probability of coated paper being charged is also small. Because the printing paper (white board, paper jam, etc.) and coated paper are reprocessed on the basis of the base paper, even if the base paper has been charged, it will be eliminated in the processing process. The general ration is 80g / m ² The following papers have more static electricity, but the paper has static electricity before machine printing or the static electricity before printing is not obvious, which is often a sudden increase in static electricity after embossing. 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 reason for static electricity on paper is friction. Many charged objects are caused by friction. The friction between paper and calender during papermaking, and the friction between paper and rubber roller and imprint roller during printing are all important factors that generate static electricity. The binding force of electrons in the atomic nucleus of different substances is different, or they have different affinity for electrons. When two different substances are in close contact, some electrons will pass through the interface and attach to the surface of the electrophilic party due to the weak binding force of the electron sparing party. As a result of this electron transfer, an electric double layer will be formed at the interface. At this time, if the two substances are rapidly separated, the electron sparing party will be positively charged due to the loss of some electrons, and the electron loving party will be negatively charged due to some electrons. This is the essence of electricity generated by friction. The paper, blanket, ink, printing plate and other printing materials have different affinity for electrons. During the printing process, a lot of pressure makes them close contact, and a high speed makes them separate rapidly, which fully meets the conditions of friction electricity generation. Therefore, it is understandable that there are thousands of volts of static electricity in printing, especially some papers have static electricity before printing. Static electricity has been generated during the manufacturing process of the paper. When it is packaged and sent to the printing plant before it escapes, some static electricity is still retained, which is also the reason why the paper is charged before printing. When using these papers, we should take different measures to eliminate the static electricity in combination with the actual production situation.
Solution:
1. After entering the printing plant for storage, the storage time of stock method paper should be appropriately longer, and the storage location can be better connected with the printing workshop, with the temperature of 18 ~ 25 ℃ and the relative humidity of 60 ~ 70%. Make the temperature and humidity of the printing workshop consistent with that of the paper warehouse, which is conducive to changing the water content of the paper. The change of paper moisture content is a process of releasing static electricity.
2. Paper drying method (humidification method) mainly uses the method of adjusting the main humidity to eliminate static electricity. When the relative humidity in the workshop is less than 50%, it is easy to generate high static electricity in the process of printing or plate making. Increasing the relative humidity in the workshop and the water content of the paper is very effective to eliminate static electricity, especially when drying paper. Humidity control equipment can be used to increase the indoor relative humidity. If there is no humidity control equipment, enough water can be sprinkled on the ground. Humidity control equipment is mainly humidifier. Ultrasonic atomization 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 like water vapor to increase the indoor relative humidity. After the indoor relative humidity meets the requirements, the spray will be automatically stopped.
3. Electrostatic eliminator method neutralizes the charge on the charged body with the positive and negative ions generated by the electrostatic eliminator, so as to achieve the purpose of eliminating static electricity. There are three types of electrostatic eliminators: one is the externally applied voltage electrostatic eliminator, which applies a high voltage to the needle or thin wire electrode to generate corona discharge and generate ions. Generally, the transistor electrostatic eliminator used on the printing machine belongs to this kind; Second, self discharge electrostatic eliminator, which makes conductive fiber, conductive rubber or conductive metal materials into needle or thin wire electrodes and well grounded, generates corona discharge by using the electric field of the charged body itself to generate ions and neutralize the charge on the charged body; The third is the electrostatic separator of radioactive elements, which uses the ionization of radioisotopes to ionize the air to generate ions and neutralize the static electricity on the charged body.
4. Antistatic agent method antistatic agent is also called electrostatic eliminator or electrostatic eliminator. Its principle is to give hygroscopic ions on the surface of charged bodies such as paper and film, make them hydrophilic, absorb moisture in the air, reduce resistance, increase conductivity, and make static charges difficult to accumulate. Antistatic agents are mainly surfactants, including hydrophilic and hydrophobic groups, or polar and nonpolar groups. Hydrophilic groups have strong affinity for substances with larger polarity such as water, and hydrophobic groups have strong affinity for substances with smaller polarity such as oil. Antistatic agents are widely used in printing, such as using antistatic agents to make soft rubber rollers to prevent static electricity.
5. Process operation method in the printing process, you can add a wet towel to the paper receiving part, fix the towel dipped in water on the pull rod, and make the paper contact with the wet towel to eliminate static electricity. This is a temporary but effective way to eliminate static electricity. The disadvantage is to wet the paper often.
Printing is carried out on the surface of objects, and electrostatic phenomena are also mainly manifested on the surface of objects. The relationship between the two is naturally very close. Electrostatic phenomenon has dual effects on printing, which is harmful to the normal operation of printing and can be used. The process of printing involves frequent impact, friction and contact separation.
As a result, almost all objects involved in the printing process have static electricity.
yi、 Harm of static electricity 1. Affect the printing quality of products. First, the surface of the substrate is charged, such as paper, polyethylene, polypropylene, cellophane, etc., which will absorb paper scraps or a large amount of dust, impurities, etc. diffused in the air, thus affecting the transfer of ink, reducing the ink transfer rate, and causing 'flowers' on the print. Point ', or caused by mutual exclusion, the winding is slippery and uneven. Secondly, the ink is charged. This occurs when the ink is in a low viscosity state or when the antistatic component in the ink is not enough. If the discharge is in motion, there may be 'electrostatic ink spots' on the print, which looks like the fluidity of the ink is very poor and the inking is very uneven. This often happens in hierarchical thin plate printing.
But sometimes it also appears in the white version of the field, and it also shows another phenomenon: exclusion. Originally, white ink is paved, but when you encounter some words (mostly black words), a gap is formed around these words, that is, white ink cannot be printed there. The main reason is the poor quality of white ink, and the viscosity is too small. If the charged ink discharges at the edge of the line on the print, there may be 'ink whiskers' here, which often occurs in the field version. In addition, the ink will be fogged due to electrification, causing serious pollution to the printed matter and the printing environment.
2. The charged substrate that affects production safety may even cause air discharge due to ultra-high potential, resulting in electric shock or fire. Charged ink may cause ink and solvent to catch fire, or shock workers through ink. Many parts of the high-speed printing machine will generate a lot of static electricity due to the strong friction during high-speed operation. When the old web wheel transfer brush machine with imperfect electric dissipation device is running at high speed, the voltage at some positions on the machine is sometimes as high as 15000V, and the current is also about 100ua. Such a strong voltage and current has a complex impact on the printing process, and the harm is also quite serious, In particular, it directly threatens the personal safety of operators.
3. In the printing process, the paper on the stack is separated and transported forward under the friction of the paper conveyor belt and the paper pressing wheel. There is always friction between paper and machine. Friction produces an electric charge. When the charge accumulates, the paper carries an electric charge. When the paper has the same polarity charge, the paper repels each other, resulting in inaccurate overprint and uneven receipt during printing; When the paper has a heterogeneous charge, it causes problems such as blank sheet, double sheet, multiple sheets, inaccurate overprint, etc. when feeding paper; The surface of paper with static electricity is also easy to absorb some paper wool and dust, resulting in printing image blooming or spots and other faults. Therefore, it is very important to eliminate paper static electricity during printing.
二、 Elimination of static electricity
1. Grounding. This is a simple way to eliminate static electricity. That is, connect the metal conductor with the earth, make it equipotential with the earth, and the charge will leak through the earth, but this method has little effect on the insulator. If there are two printing presses near each other, although both of them have wires grounded, there may still be potential differences between them due to different degrees of charge leakage. In order to avoid discharging each other, wires should also be used to connect them.
2. Ion neutralization method. This is to ionize the air to produce positive and negative ions to neutralize the static electricity on the paper. It can be eliminated by electrostatic eliminators. Generally, electrostatic eliminators used on printing machines include induction electrostatic eliminators, high-frequency and high-voltage electrostatic eliminators, power frequency and high-voltage electrostatic eliminators, etc., which should be placed near the drum when installing.
3. Control the relative humidity. This is a simple and effective method. The surface resistance decreases with the increase of air relative humidity. Therefore, increasing the relative humidity of the air can improve the conductivity of the paper surface and accelerate the leakage of charge. In the printing workshop, the temperature is about 20 degrees and the relative humidity is around 65%. Therefore, sprinkle some water around the printing environment, and if possible, use an air humidifier, which can effectively eliminate static electricity.
4. Adjust the ink viscosity. Increasing the viscosity of the ink can also effectively reduce (eliminate) the static electricity in the ink and improve the printing quality.
5. Add antistatic agent or isopropyl alcohol to the ink. Use antistatic agents, because too much isopropyl alcohol will lead to ink 'anti slag'. Many books introduce that the addition amount of isopropyl alcohol is no more than 2%. However, according to practice, it is necessary to far exceed this amount to have an effect.
6. When the water content of the paper is low, the paper is easy to carry static electricity. When the paper is seriously electrostatic, it is necessary to hang the paper or stack the paper in a relatively humid environment for a period of time, but pay attention to prevent the phenomenon of paper ruffles.
三、 The use of static electricity is one aspect of the harm of static electricity.
Now let's talk about how to use static electricity to improve the quality of prints. Compared with relief printing and lithography, gravure printing has a large amount of ink left in the printed pictures and texts, and a large printing pressure must be applied in order to transfer the ink in the ink hole to the surface of the substrate. Therefore, the load of the printing machine is greater than that of relief printing machine and lithography printing machine.
Despite the great printing pressure, only 60% of the ink on the plate can be transferred to the substrate. In order to improve the ink transfer rate of gravure, an electrostatic ink absorption device can be installed on the printing machine, and the ink on the printing can be absorbed onto the substrate by using the electrostatic ink absorption method. In this way, the ink transfer rate can be increased by 20%, which not only prolongs the service life of the machine, but also enriches the reproduction of the subtle levels of the printed matter, greatly improving the quality and grade of the printed matter. The embossing cylinder of the printing machine is conductive. During printing, the electrostatic high voltage generator is used to generate an electric field between the printing plate and the embossing cylinder. With the help of electrostatic attraction, the ink in the ink hole of the printing plate is absorbed onto the substrate. Due to the existence of high-voltage electric field, this device can eliminate the static electricity on the paper and overcome various faults caused by static electricity on the paper.