Testing and cleaning of static electricity in printing
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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. The relationship between the two is very close. During the printing process, almost all substances involved in the printing process generate static electricity due to friction, impact and contact between different substances. However, the impact of static electricity in the packaging and printing process has not received enough attention. The author combines years of work experience to explore the following:
Electrostatic hazard
1. Affect the quality of product printing
The first is that the surface of the substrate is charged, such as paper, polyethylene, polypropylene, cellophane, etc., they will absorb paper dust or a large amount of dust and impurities floating in the air, which will affect the transfer of ink, make the prints on the print, etc., resulting in printed matter. decline in quality.
Second, the ink is charged. If the charged ink is discharged during the movement, it may appear as "electrostatic ink spots" on the printed matter. It seems that the ink has poor fluidity and uneven ink loading, which is often the case in 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
Due to the high-speed friction during the printing process, the peeling generates a high static electricity, and when the static electricity accumulates, it is easy to cause the air to discharge, causing an electric shock or fire. When the voltage is high, the charged ink may cause the ink, solvent to catch fire, or shock the worker through the ink, directly threatening 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 a large ability to adsorb electrons to transfer electrons onto the surface of the material, the surface of the electron obtained exhibits a negative charge, and the other due to the loss of the surface of the electron, And with a positive charge.
The main factors that generate static electricity:
1. Material properties include the inherent 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 its surface, and the charge is evenly distributed on its surface rather than staying at a local location where the charge is generated. This distribution on the surface of the charge can only cause a lower voltage. Any part of the conductive material that comes into contact with the ground will immediately transfer the charge to the ground. The insulating material is different from the conductor. Like the paper or plastic film in most printing materials, static electricity generates a high voltage and cannot be eliminated by grounding.
2. Ambient conditions include the composition of the gas surrounding the object (material) and the 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. In general, the tighter the contact between the two materials or the faster the separation, the greater the static generated.
4. The friction between the materials and the different materials will produce static electricity of different polarities. 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 carry out static electricity testing on anti-static shoes, conductive shoes, anti-static overalls and various positions regularly, and the test results will be archived and reported to the relevant departments at higher levels.
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 project with static performance prediction for objects 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) The items of static electricity detection of charged bodies in the actual production process are as follows:
Determination of electrostatic potential of charged body, the maximum range of electrostatic potential measuring instrument is 100KV, the accuracy is 5.0; the measurement of ambient air temperature and relative humidity; the measurement of running speed of charged body; the determination of flammable gas concentration; conductive grounding The determination of the ground resistance value; the ACL-350 introduced by Derui is the world's smallest pocket-sized non-contact digital electrostatic meter. The meter measures the static and static voltages on the surface of the object.
(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, and the instrument accuracy is 5.0, but the test point must be selected behind the static safety device.
Methods and measures for static elimination during printing
Chemical elimination method
The chemical elimination method generally applies an antistatic agent on 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, it is likely to have adverse effects on the quality of the paper, such as reducing the strength, adhesion, tightness, and tensile strength of the paper. Therefore, chemical methods are not widely used.
2. Physical elimination method
The physical elimination method is a method that is most frequently used in the prior art by eliminating the inherent characteristics of static electricity without changing the properties of the material.
(1) "grounding" elimination method, which makes the simplest method, using a metal conductor to connect the substance to be electrostatically removed to the earth, so that it is connected to the earth's equipotential, and the charge is not leaked through the earth, but this way for the insulator Almost no effect.
(2) Humidity control elimination method
The surface resistance of the printed material decreases as the relative humidity of the air increases. Therefore, by increasing the relative humidity of the air, the conductivity of the surface of the paper can be increased and the charge leakage can be accelerated. The suitable environmental conditions for the printing workshop are: the temperature is about 20 degrees, and the ambient humidity of the charged body is increased to more than 70% as an electrostatic safety measure.
(3) Introduction and selection principle 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 have been widely used due to their low price, convenient installation and use, and the absence of atomic radiation. The analysis is as follows:
Inductive static eliminator bar: the inductive static elimination brush, the principle is: when the tip of the eliminator is close to the charged body, the tip can sense the opposite polarity of the electrostatic polarity on the charged body and move at the tip. This neutralizes the static electricity. The use of this method is gradually decreasing.
High-voltage discharge type static eliminator: divided into electronic type and high-voltage transformer type according to structure, can be divided into unipolar and bipolar according to discharge polarity, unipolar static eliminator only acts on one kind of electric charge, bipolar Can eliminate any kind of charge. In addition, in the printing process, the combination of static elimination brush and high-pressure discharge can completely eliminate static electricity. Jinan Derrick Company introduced the 969 type power supply unit with DJ62 high-efficiency static eliminator, which has been widely used in the printing and packaging industry due to its good static elimination effect.
In areas with explosion hazards, explosion-proof static eliminators must be selected for use in hazardous area grades, media grades, and groups when using high-voltage corona discharge and ion-flow static eliminators.
The length of the inductive and high-voltage corona static eliminator should be 10-12 cm wider than the charged body. The form and number of ion-flow static eliminator ion nozzles should be set according to the possible installation distance and the width of the charged body.
The static eliminator should be selected according to the level of the electrostatic potential of the body, the elimination requirements, the operating characteristics, the explosion hazard area, the medium level and the group.
The principle of the installation position of the static eliminator: easy to process the operation, the elimination effect is good; the subsequent part of the solvent is applied to avoid the metal background.
3. Measures to prevent static electricity
The measures to prevent static hazards in the printing workshop are recommended from the following aspects:
In the process equipment and places where there is electrostatic hazard, the corresponding electrostatic safety measures must be adopted; in areas where explosive gas may occur in the surrounding environment, ventilation measures must be strengthened to keep the concentration below the explosion range; prevent electrostatic insulators from causing the operator In the case of electric shock, the electrostatic potential of the insulator should be controlled below 10KV. In the case where static electricity may cause a secondary electric shock when the operator strikes electric shock, in addition to the above, corresponding measures should be taken for the equipment or device.
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 should be less than 10 ohms, and the conductive performance must be maintained frequently. 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. It is strictly forbidden to undress in the above areas.
In short, the printing industry workers should pay enough attention to the static electricity generated in the production process to ensure production safety.