Harm of static electricity in printing and cleaning methods
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Printing is performed 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, all substances involved in printing generate static electricity due to friction, impact and contact between different substances.
Electrostatic hazard
1. Affect the quality of product printing
The surface of the substrate is charged, such as paper, polyethylene, polypropylene, cellophane, etc., which will absorb paper dust or dust, impurities, etc. floating in the air, affecting the transfer of ink, making the printed matter, etc., resulting in a decline in the quality of the printed matter. Secondly, if the ink is charged, it will discharge during the movement, and "electrostatic ink spots" will appear on the printed matter, which is often the case in the layered printing. In solid printing, if charged ink is discharged on the edge of the print, it is easy to have "ink whiskers" on the edges.
2. Affect safety production
In the printing process, due to high-speed friction, the peeling generates static electricity, and when 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, which directly threatens the safety of the operator.
Electrostatic generation mechanism
The main cause of static electricity is friction. When two solid materials contact and leave quickly, one of the materials has the greater ability to adsorb electrons to transfer electrons to the surface of the material, with the electron surface exhibiting a negative charge and the other with a positive charge. The main factors that generate static electricity:
1. Material properties include the intrinsic chemical composition of the material, the internal structure of the material, the mechanical properties of stress and strain, the shape of the material and the electrical conductivity. The conductivity of the material has a dual effect on static electricity. First, if the material is a conductor, the charge is free to move on the surface, the distribution on the surface of the charge causes a lower voltage, and the conductive material contacts the ground and immediately transfers the charge to the ground. The insulating material is different from the conductor. Most of the printing materials, such as paper and plastic film, generate static electricity to form a higher voltage, which cannot be eliminated by grounding.
2. Ambient conditions include the composition of the gas surrounding the object (material) and pressure, temperature, humidity, etc.
3. Mechanical action includes the type of contact between the two materials, contact time, contact area, separation speed, and the nature of the material's force. The tighter the contact between the two materials or the faster the separation, the greater the generation of static electricity.
4. Material matching type The friction between different materials will produce static electricity of different polarity. Different materials are attached with different electrostatic strengths.
Static test
1. The main purpose of electrostatic testing in packaging and printing plants is to analyze the degree of damage; to study preventive measures; to judge the effect of static elimination. Special personnel must be appointed to take charge of anti-static shoes, conductive shoes, anti-static overalls and regular static electricity detection in each position, and report the results to relevant departments.
2. Classification of static electricity detection items: Prediction of electrostatic properties of objects with new raw materials; detection of live conditions during actual production; and judgment of static safety measures.
(1) The item with static performance prediction of the object is as follows: surface resistivity of the object. Measured with a high resistance meter or an ultra high resistance meter, the range is 1.0-10 ohms.
(2) In the actual production, the static electricity detection condition of the charged body is as follows: the measurement of the electrostatic potential of the charged body, the maximum range of the electrostatic potential measuring instrument is 100KV, the accuracy is 5.0; the measurement of the ambient temperature and relative humidity; the operation of the charged body Determination of velocity; determination of flammable gas concentration; determination of conductive ground-to-ground resistance; Derui's ACL-350 is currently the smallest non-contact digital electrostatic measurement meter.
(3) Safety measures The use of the effect judgment test item is the same as (2) The electrostatic potential measurement instrument range is 0-10KV, the meter accuracy is 5.0, and the test point is selected behind the static safety device.
Static elimination method in printing
Chemical elimination method
Apply an antistatic agent to the surface of the substrate to make the substrate conductive and become a slightly conductive insulator. The application of chemical elimination is very limited in practice. For example, when chemical components are added to printed paper, the quality of the paper is adversely affected, 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 method
Eliminating the nature of materials without the use of static properties is the most common method available.
(1) The grounding elimination method uses a metal conductor to connect the substance to be electrostatically removed to the earth and to 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 as the humidity of the air increases, so increasing the relative humidity of the air can increase the conductivity of the surface of the paper. The suitable environmental conditions for the printing shop are: the temperature is about 20 degrees, and the ambient humidity of the charged body is more than 70%.
(3) Principles for selection of static elimination equipment
The static elimination equipment commonly used in printing plants is inductive, high-voltage corona discharge, ion current static eliminator and radioisotope. Among them, the first two are cheap, easy to install and use, and no atomic radiation is widely used:
Inductive static eliminator bar: Inductive static elimination brush, the principle is that when the tip of the eliminator is close to the charged body, the polarity opposite to the electrostatic polarity on the charged body is induced, thereby neutralizing the static electricity.
High-voltage discharge static eliminator: divided into electronic and high-voltage transformer type, divided into unipolar and bipolar according to discharge polarity, unipolar static eliminator only acts on one kind of charge, bipolar can eliminate any An electric charge. In the printing process, static electricity brush and high-pressure discharge can be used to eliminate static electricity. The principle of the installation position of the static eliminator: easy to operate, next to the subsequent part of the solvent.
3. Measures to prevent static electricity
Where there are electrostatic hazard process equipment and places, it is necessary to strengthen the ventilation measures in the area where explosive gas may be present, so that the concentration is controlled below the explosion range; when the electrostatic insulator is used to cause electric shock to the operator, the electrostatic potential of the insulator is controlled. Below 10KV. In areas where there is a risk of explosion and fire, operators must wear anti-static shoes and anti-static overalls. Conductive ground is laid in the operation area. The resistance of the conductive floor to ground is less than 10 ohms, and the conductive performance is maintained. It is strictly forbidden for the operator to wear synthetic fiber clothes (except clothes that have been regularly treated with antistatic solution) to enter the above area. Undress.