Printing is carried out on the surface of the object, and the phenomenon of static electricity is mainly manifested on the surface of the object. During the printing process, due to friction, impact and contact between different substances, all substances involved in printing generate static electricity.
The harm of static electricity
1. Affect product printing quality
The surface of the substrate is charged, such as paper, polyethylene, polypropylene, cellophane, etc., will absorb paper dust or dust, impurities floating in the air, etc., affect the transfer of ink, make the printed matter flower, etc., resulting in a decline in print quality. Secondly, if the ink is charged and discharged during the movement, "electrostatic ink spots" will appear on the printed matter, which is often the case in gradation and thin printing. In the field printing, if the charged ink discharges on the edge of the printed matter, it is easy to appear "ink whiskers" on the edge.
2. Affect production safety
In the printing process, due to high-speed friction, peeling will generate static electricity. When the static electricity accumulates, it is easy to cause air discharge, causing electric shock or fire. When the voltage is high, the charged ink will cause the ink and solvent to catch fire, directly threatening the safety of the operator.
The generation mechanism of static electricity
The main cause of static electricity is friction. When two solid materials contact and leave quickly, one of the materials has a greater ability to adsorb electrons to transfer the electrons to the surface of the material, the electron surface presents a negative charge, and the other has a positive charge. The main factors causing static electricity:
1. Material properties include the internal chemical composition of the material, the internal structure of the material, the mechanical characteristics of stress and strain, the shape and conductivity of the material, etc. The conductivity of the material has a dual effect on static electricity. First, if the material is a conductor, the charge moves freely on the surface. The distribution of this charge on the surface causes a lower voltage. The conductive material contacts the ground and immediately transfers the charge to the ground. Insulating materials are different from conductors. The paper, plastic film, etc. in most printing materials generate a high voltage due to static electricity, which cannot be eliminated by grounding.
2. The surrounding environmental conditions include the gas composition and pressure, temperature, humidity, etc. of the surrounding objects (materials).
3. The mechanical action includes the type of contact of the two materials, the contact time, the contact area, the separation speed, and the nature of the material force. The closer the two materials are in contact or the faster the separation, the greater the static electricity.
4. The friction between different types of materials and materials will produce static electricity with different polarities. Different materials have different electrostatic strengths.
Static test
1. The main purpose of static testing in packaging and printing factories is to analyze the degree of hazards; study preventive measures; and determine the effect of static elimination. A special person must be appointed to be responsible for the regular static test of anti-static shoes, conductive shoes, anti-static work clothes and various positions, and report the results to the relevant departments.
2. Classification of static electricity testing items: prediction of static electricity performance of objects when using new types of raw materials; detection of electrified conditions in actual production processes; judgment of the effectiveness of static safety measures.
(1) The items for predicting electrostatic properties of objects are as follows: surface resistivity of objects. Use high resistance meter or ultra high resistance meter to measure, the range is 1.0-10 ohm.
(2) The testing items of the static electricity state of the charged body in actual production are as follows: the measurement of the electrostatic potential of the charged body, the maximum range of the electrostatic potential measuring instrument is preferably 100KV, and the accuracy is 5.0; the measurement of the surrounding air temperature and relative humidity; Determination of speed; determination of flammable gas concentration; determination of the resistance value of conductive ground-to-earth; Derrick's ACL-350 is currently the smallest non-contact digital static electricity measurement table.
(3) The safety measures use the same effect as the test items (2) The range of the electrostatic potential measuring instrument is preferably 0-10KV, the accuracy of the instrument is 5.0, and the detection point is selected behind the electrostatic safety device.
Method of eliminating static electricity in printing
Chemical elimination method
Apply a layer of antistatic agent to the surface of the substrate to make the substrate conductive and become a slightly conductive insulator. The application of chemical elimination has great limitations in practice. For example, when chemical components are added to printed paper, it adversely affects the quality of the paper, such as reducing the strength, adhesion, tightness, and tensile strength of the paper. Therefore, the chemical method is not Too widely used.
2. Physical elimination
It is the most commonly used method to eliminate static electricity without changing the material properties.
(1) The grounding elimination method uses a metal conductor to connect the substance to be eliminated to the earth and has the same potential as the earth, but this method has no effect on the insulator.
(2) Humidity control elimination method
The surface resistance of the printed material decreases with increasing air humidity, so increasing the relative humidity of the air can improve the conductivity of the paper surface. The suitable environmental conditions for the printing workshop are: the temperature is about 20 degrees, and the environmental humidity of the charged body is more than 70%.
(3) Principles for selecting static elimination equipment
The static elimination equipment commonly used in printing plants includes induction type, high voltage corona discharge type, ion current static elimination device and radioisotope type. Among them, the first two are cheap, easy to install and use, and do not have the advantages of atomic radiation. They are widely used:
Inductive static eliminator bar: Inductive static eliminator brush. The principle is that when the tip of the static eliminator approaches the charged body, it will induce a charge with the polarity opposite to that of the charged body, thereby neutralizing the static electricity.
High-voltage discharge static eliminator: divided into electronic type and high-voltage transformer type, which is divided into unipolar and bipolar according to the discharge polarity. The unipolar static eliminator only affects one charge, and bipolar can eliminate any A charge. In the printing process, static electricity can be eliminated by combining static electricity brushes and high-voltage discharge. The principle of the installation position of the static eliminator: easy to operate, followed by the subsequent part of applying the solvent.
3. Measures to prevent static electricity
All process equipment and places that have electrostatic hazards must strengthen the ventilation measures in the area where explosive gases may appear in the surroundings, so that the concentration is controlled below the explosive range; to prevent electric shock from the electrostatic insulator to the operator, the electrostatic potential of the insulator is controlled at Below 10KV. Where there is a danger of explosion and fire, the operator must wear anti-static shoes and anti-static work clothes. Conductive ground is laid in the operation area. The resistance of the conductive ground to the ground is less than 10 ohms. The conductive performance is maintained. It is strictly forbidden for operators to wear synthetic fiber clothing (except clothing that has been regularly treated with antistatic solutions) to enter the above area. Undress.
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