1 The danger of static electricity
1) Affect product printing quality
If the surface of the substrate is charged with static electricity, they will attract paper powder or dust, impurities, etc. floating in the air, which will affect the transfer of ink, and "flowers and dots" will appear on the printed product, resulting in a decline in print quality.
If the ink has static electricity, if it is discharged during movement, it may appear "electrostatic ink spots" on the printed product. It seems that the ink has poor fluidity and uneven ink application. This situation often occurs in gradation thin plate printing. In the field printing, if the charged ink discharges on the edge of the line on the printed product, it is easy for "ink whiskers" to appear on the edge of the line.
2) Affect production safety
Due to high-speed friction during the printing process, high static electricity is generated. When static electricity accumulates to a certain degree, it is easy to cause air discharge, causing electric shock or fire. When the voltage is very high, the charged ink may cause the ink or solvent to catch fire, or shock the worker through the ink, directly threatening the personal safety of the operator.
2 The generation mechanism of static electricity
The main cause of static electricity is the presence of friction. When the two materials are in contact and quickly separated, one of the materials with strong ability to adsorb electrons transfers electrons to its surface, the surface of the material that obtains electrons shows a negative charge, and the surface of the other material is charged due to the loss of electron positive charge. The factors that generate static electricity are mainly the following.
1) Material properties
Including the internal chemical composition of the material, the internal structure of the substance, the stress characteristics, the shape and conductivity of the material, etc.
If the material is a conductor, the charge moves freely on its surface, and the charge tends to be evenly distributed over its entire surface, rather than staying in a local position where the charge is generated. This charge can only generate a lower voltage, and any part of the conductive material that contacts the ground can immediately transfer the charge on the material to the ground. Insulating materials are different from conductor materials. For printing materials such as paper and plastic films, static electricity generates a higher voltage and cannot be eliminated by grounding.
2) Surrounding environmental conditions
Including the composition, pressure, temperature, humidity, etc. of the gas in contact with the material.
3) Mechanical action
Including the type, time, area and separation speed of the two materials, the nature of the force and so on. In general, the closer the two materials are in contact or the faster the separation speed, the greater the static electricity generated.
3 Methods and measures for eliminating static electricity in the printing process
1) Chemical elimination method
The chemical elimination method generally applies a layer of antistatic agent to the surface of the printing material to make the printing material conductive. The application of the chemical elimination method is very limited. If the paper is added with chemical components during papermaking, it may have an adverse effect on the quality of the paper, such as reducing the strength, adhesion, tightness, and tensile strength of the paper. law.
2) Physical elimination method
The physical elimination method is a method for eliminating static electricity by using the inherent characteristics of static electricity without changing the properties of the material. It is a static elimination method commonly used in industrial production at present.
â‘ "Ground" elimination method. This is the simplest method, which is to connect the object to be static electricity to the ground with a metal conductor to make a potential difference with the ground, and the electric charge will leak through the ground, but this method has almost no effect on the insulator.
② Humidity control elimination method. The surface resistance of the printed material decreases with the increase of the relative humidity of the air. Therefore, increasing the relative humidity of the air can improve the electrical conductivity of the paper surface and accelerate the charge leakage. The suitable environmental conditions for the printing workshop are: temperature is about 20 ℃, relative humidity is more than 70%. [next]
4 Introduction and selection principles of static elimination equipment
The static eliminators commonly used in printing plants include induction, high voltage corona discharge, radioisotope and ion current static eliminators. Among them, the first two are widely used due to their advantages of low price, easy installation and use, and absence of atomic radiation. Now they are analyzed as follows.
1) Inductive static eliminator
The inductive static eliminator is the inductive static elimination brush. Its working principle is: when the tip of the consumer is close to the charged body, it can induce a charge with a polarity opposite to the static polarity of the charged body, and form a strong near the tip After the electric field ionizes the air, the generated positive and negative ions move to the tip of the charged body and the consumer under the action of the electric field, thereby neutralizing the static electricity.
2) High-voltage discharge static eliminator
High-voltage discharge static eliminators can be divided into electronic and high-voltage transformer types according to different structures, and can be divided into unipolar and bipolar according to the difference in discharge polarity. In the printing process, a combination of inductive and high-voltage discharge static elimination methods can be used to completely and effectively eliminate static electricity. Hangzhou Ansuo (ANSOLL), (svcor) company's AS6503 power supply with AS6401 high-efficiency static elimination rod has a good static elimination effect, has been widely used in packaging and printing industry.
In addition, when using high-voltage corona discharge and ion current static eliminators in areas where there is a danger of explosion, explosion-proof static eliminators must be selected according to the hazardous area level, medium level and group; inductive and high-voltage corona The discharge electrode length of the discharge static eliminator should be greater than the width of the charged body 10-15cm; the type and number of ion spray heads of the ion current static eliminator should be determined according to the installation distance and the width of the charged body; the static eliminator should be based on the level of the electrostatic potential of the object 1. Static elimination requirements, operating characteristics, explosion hazard zone level, media level and group selection; selection principles for the installation position of static eliminator: easy to operate, good static elimination effect, close to the part where the solvent is applied, and avoid metal objects .
5 Static test
The purpose of static testing in packaging and printing plants is to analyze the degree of hazards, study preventive measures, and determine the effectiveness of static elimination. A special person must be appointed to be responsible for conducting regular static tests on anti-static shoes, conductive shoes, and anti-static work clothes, and sorting and filing the test results and submitting them to the superior department. Static testing items include the following:
①Prediction of electrostatic properties of objects. The surface resistivity of the object is measured with a high-resistance meter or an ultra-high-resistance meter, and the measuring range is preferably 10-10 KΩ).
â‘¡Detect the static electricity of the charged body in the actual production process. The maximum range of the instrument for measuring the electrostatic potential of the charged body is preferably 100kV, and the accuracy is 5.0; the measurement of the surrounding air temperature and relative humidity; the determination of the operating speed of the charged body; the determination of the concentration of flammable gas; the resistance value of the conductive ground to ground Determination. Among them, the pocket-size non-contact digital static electricity meter can measure the static voltage value and static polarity on the surface of the object. The watch is easy to operate, with a measurement holding button and a setting button, and is widely used.
â‘¢ Judgment and detection of the use effect of safety measures. The range of the electrostatic potential measuring instrument is preferably 0-lOkV, and the accuracy is 5.0. The detection point must be selected behind the electrostatic safety device.
6 Measures to prevent electrostatic hazards
The measures to prevent electrostatic hazards in packaging and printing workshops are recommended to be considered from the following aspects: all processes, equipment and places that have electrostatic hazards must adopt corresponding anti-static safety measures; in areas where explosive gases may appear in the surrounding environment, ventilation measures must be strengthened, The concentration should be controlled below the explosion limit; after taking electrostatic safety measures, the electrostatic potential of the insulator should be controlled below 10kV to prevent the electrostatic insulator from causing electric shock to the operator; when the electrostatic shock to the operator may cause a second accident On occasions, in addition to the electrostatic potential of the insulator should be controlled below 10kV, corresponding safety measures should be taken for the equipment; in areas where there is a risk of explosion and fire, the operator must wear anti-static shoes or conductive shoes, anti-static work clothes; the operating area should be Lay the conductive ground, the resistance value of the conductive ground to the ground should be less than 10Ω, and it must be tested frequently to maintain its conductive performance.
In short, workers in the packaging and printing industry should pay enough attention to the static electricity generated in the production process to ensure safe production.
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