The generation and solution of static electricity in offset printing
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Once the paper is electrostatically charged, it will cause a lot of trouble for printing. The first is that the paper can't be hit. Under the action of static electricity, the paper and paper are firmly sucked. It is difficult to get into the air between the papers. If you want to hit it, sometimes you have to pull it one by one, which is a waste of time. During the printing process, due to the electrostatic attraction, the sheets are firmly adhered together, sometimes two, sometimes several, sometimes a sheet of paper can not be separated, resulting in the paper nozzle can not absorb the paper. The brush is heavy and often breaks. Empty sheet; the brush is light, and it produces double sheets and multiple sheets. A plurality of sheets of paper enter between the blanket cylinder and the impression cylinder, causing the vehicle to suffocate and crush the blanket and the liner. Paper with static electricity is not smooth when the paper is transported from the forward direction, and the front gauge is not skewed. The positioning is not allowed, resulting in the second overprinting can not be registered, the product quality is inferior, waste is great. Even after passing through the embossed part, the delivery is very uneven, which brings great trouble to the second whole paper and seriously affects the production speed.
Paper with static electricity has a certain relationship with papermaking. Under normal circumstances, the base paper is less charged at the factory, and the probability of the coated paper being charged is not large. Since printing paper (white paper, cardboard, etc.) and coated paper are reworked on the basis of the base paper, even if the base paper is charged, it is eliminated during processing. Generally, paper with a basis weight of 80g/m or less has a lot of static electricity, but the paper is electrostatically charged before the machine is printed or the static electricity before printing is not obvious, and the static electricity is often increased after the imprinting. In the offset printing process, due to the presence of water, it is not common to carry static electricity after printing pressure. For offset printing, static electricity is mainly generated before printing.
(1) Reason analysis:
The main cause of static electricity on paper is frictional electricity generation. Many objects are charged by friction, the friction between the paper and the calender during papermaking, and the paper and blanket cylinder during printing. The friction between the impression cylinders is an important factor in generating static electricity. |
The nucleus of various substances has different binding forces to electrons, or has different affinity characteristics for electrons. When two different substances are in close contact, the electron-eliminating side is weak due to the binding force to the electrons, and some electrons are attached 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 rapidly separated, the electron-eliminating side is positively charged due to the loss of some electrons, and the pro-electron side is negatively charged by obtaining some electrons, which is the essence of triboelectric generation. |
Paper in printed materials. Blanket. Ink. Plates and the like have different affinity characteristics for electrons. The great pressure in the printing process makes them in close contact, and they have a high speed to separate them quickly, and fully have the conditions of triboelectric generation. Therefore, it is understandable that thousands of volts of static electricity appear in printing, especially that some papers are electrostatically charged before printing. The paper has generated static electricity during the manufacturing process. When it is not ready to be packaged and sent to the printing house, some static electricity is still retained. This is also the reason why the paper is charged before printing. When using these papers, it is necessary to take different measures to eliminate the static electricity they carry in combination with the actual production conditions.
(2) Solution:
1. Inventory method
After the paper enters the printing factory, the storage time should be 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%. Make the temperature of the printing shop. The humidity is consistent with the paper library, which helps to change the moisture content of the paper. The change in paper moisture is a process of releasing static electricity.
2. Drying method (humidification method)
Mainly to eliminate the 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 printing or plate making, and the relative humidity of the workshop and the water content of the paper are increased. Especially when the paper is dried, it is effective to eliminate static electricity. The humidity control equipment can be used to increase the relative humidity of the room. When there is no humidity control equipment, enough 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 the misty water vapor to increase the relative humidity in the room. After the relative humidity in the room reaches the requirement, the spray is automatically stopped.
3. Static eliminator method
The ions generated by the static eliminator are used 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 a voltage-type static eliminator, which applies a high voltage to the needle-shaped or thin-line electrode, and generates corona discharge to generate ions. Generally, the transistor static eliminator used on the printing machine belongs to this type; The second is a self-discharge static eliminator that puts conductive fibers. Conductive rubber or conductive metal material is made into a needle-like or thin-line electrode and is well grounded. The electric field of the charged body itself generates corona discharge to generate ions, neutralizing the charge on the charged body. Third, the radioactive element destaticizer Electricity using radioisotopes
The ionization of the air, which is ionized, neutralizes the static electricity on the charged body.
4. Antistatic agent method
Antistatic agents are also known as static eliminators or de-energizers. The principle is to give paper. A surface of a charged body such as a film absorbs ions, making it hydrophilic, absorbing moisture in the air, reducing electrical resistance, increasing conductivity, and making static charge less likely to accumulate. The antistatic agent is mainly a surfactant having a hydrophilic group and a hydrophobic group, or a polar group and a non-polar group. The hydrophilic group has a strong affinity for a substance having a relatively high polarity such as water, and the hydrophobic group has a strong affinity for a relatively small object such as an oil. Antistatic agents are widely used in printing, such as antistatic agents to make soft rubber rollers that prevent static electricity.
5. Process operation
In the printing process, a wet towel can be added to the delivery part, and the water-repellent towel is fixed on the tie rod, so that the paper is in contact with the moist towel to eliminate static electricity, which is a temporary but effective method for eliminating static electricity, and the disadvantage is that Frequently paving wet towels.