How to eliminate the biggest killer in the printing process----static
Electrostatic phenomenon is mainly manifested on the surface of the object, and printing is carried out on the surface of the object. During the printing process, almost all objects involved in the printing process are electrostatically charged due to frequent impact and friction between different substances.
1. Electrostatic hazard
1) Affect the quality of product printing
If the surface of the substrate is electrostatically charged, they will adsorb paper dust or dust, impurities, etc. floating in the air, thereby affecting the transfer of the ink, and "flowers, dots" appear on the printed product, resulting in a deterioration in the quality of the printed product. If the ink is electrostatically charged and discharged during movement, it may appear as "electrostatic ink spots" on the printed product. It seems that the ink has poor fluidity and uneven ink loading, which is often the case in layered printing. In the field printing, if the charged ink is discharged at the edge of the line on the printed product, it is easy to appear "ink whisker" at the edge of the line.
2) Affecting production safety
In the printing process, high-speed friction generates high static electricity. When static electricity accumulates to a certain extent, it is easy to cause air discharge, resulting in electric shock or fire. At very high voltages, charged ink may cause ink, solvent to ignite, or electric shock to the worker through the ink, directly threatening the operator's personal safety.
2. Electrostatic generation mechanism
The main cause of static electricity is the presence of friction. When two materials are in contact and rapidly separated, one of the materials having a strong electron-adsorbing ability transfers electrons to the surface, and the surface of the material from which the electron is obtained exhibits a negative charge, while the surface of the other material is lost due to the loss of electrons. positive charge. The main factors for generating static electricity are as follows.
1) Material properties
Including the chemical composition of the material, the internal structure of the material, the stress characteristics, material shape and conductivity. If the material is a conductor, the charge is free to move on its surface, and the charge tends to be evenly distributed throughout its surface rather than staying at a local location where the charge is generated. This charge can only produce a lower voltage, and any part of the conductive material that comes into contact with the ground can immediately transfer the charge on the material to the ground. The insulating material is different from the conductor material. For printing materials such as paper and plastic film, static electricity generates a high voltage and cannot be eliminated by grounding.
2) Ambient conditions
Including the composition of the contact material gas, pressure, temperature, humidity, and the like.
3) Mechanical action
It includes the type, time, area, and separation speed of the two materials, the nature of the force, and so on. In general, the tighter the contact between the two materials or the faster the separation, the greater the static generated.
3. Methods and measures for static elimination during printing
1) Chemical elimination method
The chemical elimination method generally applies an antistatic agent on the surface of the printing material to make the printing material conductive. The application of the chemical elimination method has great limitations. If the paper is added with chemical components during papermaking, it may have adverse effects on the quality of the paper, such as reducing the strength, adhesion, tightness, and tension of the paper. Therefore, chemical elimination is generally not used. law.
2) Physical elimination method
The physical elimination method is a method for eliminating static electricity by utilizing the inherent characteristics of static electricity without changing the properties of the material, and is a static elimination method generally used in industrial production.
4. Introduction and selection principle of static elimination equipment
The static eliminators commonly used in printing plants include inductive, 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, convenient installation and no atomic radiation, and they are analyzed as follows.
1) Inductive static eliminator
Inductive static eliminator is an inductive static elimination brush. Its working principle is: when the tip of the consumer is close to the charged body, it can induce the opposite polarity of the electrostatic polarity on 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 separately under the action of the electric field, thereby neutralizing the static electricity.
2) High-voltage discharge static eliminator
High-voltage discharge type static eliminators can be divided into electronic type and high-voltage transformer type according to different structures, and can be divided into unipolar and bipolar according to the difference of discharge polarity. In the printing process, a combination of inductive and high-voltage discharge can be used to eliminate static electricity.
In addition, in areas with explosion hazards, when using high-voltage corona discharge and ion-flow static eliminators, explosion-proof static eliminators must be selected according to 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 by 10-15 cm; the type and number of ion nozzles 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 electrostatic potential of the object. , static elimination requirements, operating characteristics, explosion hazard area level, media level and group selection; static eliminator installation position selection principle: easy to operate, eliminate static electricity, close to the solvent coating, avoid metal objects .
5. Static test
The purpose of electrostatic testing in packaging and printing plants is to analyze the degree of damage, study the prevention measures, and determine the static elimination effect. Special personnel must be appointed to carry out regular static electricity detection on anti-static shoes, conductive shoes and anti-static overalls, and the test results will be archived and reported to the relevant departments at higher levels. The items for static electricity detection include the following:
1 Object with static performance prediction. The surface resistivity measurement of the object is measured by a high resistance meter or an ultra high resistance meter, and the range is preferably 10-1 OKQ.
2 In the actual production process, the charged body is detected with static electricity. Determination of air temperature and relative humidity in the surrounding space; measurement of the operating speed of the charged body; determination of the concentration of the flammable gas; determination of the resistance value of the conductive ground to ground. Among them, Derrick's ACL-350 is the world's smallest pocket-sized non-contact digital electrostatic meter, which can measure the electrostatic voltage and electrostatic polarity of the surface of the object. The watch is simple to operate, with a measurement hold button and a set button for a wide range of applications.
3 safety measures use the effect judgment test. The range of the electrostatic potential measuring instrument is preferably 0 to 10 kV, and the accuracy is 5.0. The detection point must be selected behind the electrostatic safety device.
6. Measures to prevent static hazards
Measures to prevent static electricity hazards in packaging and printing workshops are recommended from the following aspects: Any anti-static safety procedures, equipment and places must adopt corresponding anti-static safety measures; in areas where explosive atmosphere may occur in the surrounding environment, ventilation measures must be strengthened. The concentration of the insulator should be controlled below the explosion limit; after the electrostatic safety measures are taken, the electrostatic potential of the insulator should be controlled below 10kV to prevent the electric shock caused by the electrostatic insulator to the operator; the second accident may be caused when the static electricity strikes the operator. Occasionally, in addition to the electrostatic potential of the insulator should be controlled below 10kV, the corresponding safety measures should be taken for the equipment; in areas where there is danger of explosion and fire, the operator must wear anti-static shoes or conductive shoes, anti-static overalls; Conducting a conductive floor, the conductive ground resistance to ground should be less than 100, and must be tested frequently to maintain its electrical conductivity.
In short, the author of the packaging and printing industry should pay enough attention to the static electricity generated in the production process to ensure safe production.