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, and the relationship between the two is very close. During the printing process, due to friction, impact and contact between different substances, almost all substances participating in the printing process generate static electricity. However, the impact of static electricity in the packaging and printing process has not received enough attention. The author discusses as follows based on many years of work experience:
The harm of static electricity
1. Affect product printing quality
The first is that the surface of the substrate is charged, such as paper, polyethylene, polypropylene, cellophane, etc., they will adsorb paper dust or float a large amount of dust, impurities, etc. in the air, which affects the transfer of ink, causing flowers on the printed matter, etc., resulting in printed matter decline in quality.
Secondly, the ink is charged. If the charged ink is discharged while moving, "static ink spots" may appear on the printed matter. It seems that the fluidity of the ink is not good, and the ink is not uniform. This situation often occurs in gradation thin plate 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
Due to high-speed friction in the printing process, peeling will generate high 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 may cause the ink, solvent to catch fire, or shock the worker through the ink, 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 and the electrons are transferred to the surface of the material. The surface that obtains electrons shows a negative charge, while the other surface loses electrons. And with 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 substance, 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 its surface, and the charge is evenly distributed on its surface, rather than staying in the local position where the charge is generated. This distribution on the charge surface can only cause lower voltages. Any part of the conductive material that comes into contact with the ground immediately transfers the charge to the ground. Insulating materials are different from conductors. Like paper and plastic films in most printing materials, a high voltage is generated by static electricity, which cannot be eliminated by grounding.
2. The surrounding environmental conditions include the composition and pressure, temperature, humidity, etc. of the gas surrounding the object (material).
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. In general, the closer the two materials are in contact or the faster the separation speed, the greater the static electricity generated.
4. The friction between different 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 conducting static tests on anti-static shoes, conductive shoes, anti-static work clothes, and various positions on a regular basis.
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:
Object surface resistivity. Use high resistance meter or ultra high resistance meter to measure, the range is 1.0-10 ohm.
(2) In the actual production process, the static electricity charged state detection items 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 space temperature and relative humidity; the measurement of the operating speed of the charged body; the determination of the concentration of flammable gas; Determination of ground resistance value; ACL-350 launched by Derui Corporation is the world's smallest compact non-contact digital static electricity meter. The meter can measure the static voltage value and static voltage polarity on the surface of the object.
(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, and the accuracy of the instrument is 5.0, but the detection point must be selected behind the electrostatic safety device.
Methods and measures for eliminating static electricity during printing
Chemical elimination method
The chemical elimination method generally applies 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 is very limited in practice. For example, when adding chemical components to printed paper, it is likely to have an adverse effect 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
The physical elimination method is a method that eliminates the inherent characteristics of static electricity without changing the material properties, making it the most frequently used method in the existing process.
(1) "Grounding" elimination method, which is the easiest method, is to use a metal conductor to connect the substance to be eliminated to the earth and make it equipotential with the earth, and the charge does not leak through the earth, but this way is 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, increasing the relative humidity of the air can increase the electrical conductivity of the paper surface and accelerate the charge leakage. The suitable environmental conditions for the printing workshop are: the temperature is about 20 degrees, and the ambient humidity around the charged body is increased to more than 70%, as a static electricity safety measure.
(3) Introduction and selection principles of 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 widely used because of their low price, easy installation and use, and the absence of atomic radiation. The analysis is as follows:
Inductive static eliminator rod: the inductive static elimination brush, the principle is: when the tip of the consumer is close to the charged body, the tip can induce a charge with a polarity opposite to the static polarity of the charged body and move at the tip This neutralizes static electricity. The use of this method is gradually decreasing.
High-voltage discharge static eliminator: It is divided into electronic type and high-voltage transformer type according to the structure, and it can be divided into unipolar and bipolar according to the discharge polarity. Can eliminate any kind of charge. In addition, in the printing process, two methods of static elimination brush and high-voltage discharge can be used to completely and effectively eliminate static electricity. Jinan Drake launched the 969 power supply with DJ62 high-efficiency static eliminator. Due to the good static elimination effect, it has been widely used in the printing and packaging industry.
In areas where there is a risk of explosion, when using high-voltage corona discharge and ion current static eliminators, explosion-proof static eliminators must be selected according to the hazardous area level, medium level and group.
The discharge electrode length of inductive and high voltage corona static eliminator should be greater than the width of the charged body 10-12 cm. The form and number of ion spray head of the ion current static eliminator 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 static potential of our body, elimination requirements, operating characteristics, explosion hazard area, medium level and group.
The principle of the installation position of the static eliminator: it is convenient for the process operation and the elimination effect is good; it is immediately followed by the subsequent part of painting the solvent to avoid the metal background.
3. Measures to prevent static electricity
The measures to prevent electrostatic hazards in the printing workshop are recommended to be considered from the following aspects:
All process equipment and places that have electrostatic hazards must adopt corresponding electrostatic safety measures; in areas where explosive gases may appear in the surrounding environment, ventilation measures must be strengthened to keep their concentration below the explosion range; prevent electrostatic insulators from causing operations to operators In the case of electric shock, the electrostatic potential of the insulator should be controlled below 10KV. In cases where static electricity may cause secondary electric shock to the operator, in addition to the above, corresponding measures should be taken for equipment or devices.
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 should be less than 10 ohms, and its conductive performance must be maintained frequently. 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 It is strictly forbidden to take off clothes in the above area.
In short, workers in the printing industry should pay enough attention to the static electricity generated in production projects to ensure production safety.
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