Static electricity testing and removal in printing

Printing is carried out on the surface of an object, and electrostatic phenomena are mainly manifested on the surface of the object. The relationship between the two is very close. Due to friction, impact, and contact between different substances in the printing process, almost all substances involved in the printing process are electrostatically generated. However, the impact of static electricity in the packaging and printing process has not received enough attention. The author combines many years of work experience to discuss the following:

The harm of static electricity

1. Affect product print quality

The first is surface electrification of substrates, such as paper, polyethylene, polypropylene, cellophane, etc. They will absorb paper dust or a large amount of dust, impurities, etc. floating in the air, thus affecting the transfer of ink, causing the printed material to send flowers, etc., resulting in printed materials. decline in quality.

Second, the ink is charged. If the charged ink is discharged during the movement, it may appear as “static ink spots” on the printed matter. It looks like the ink is not fluid and the ink is uneven. This is often the case in thin-layer printing. In the field printing, if the charged ink is discharged on the edge of the printed matter, it is easy to appear as a "whiskers" at the edges.

2. Influencing safe production

　　 In the printing process, due to high-speed friction, peeling can generate high static electricity. When static electricity accumulates, it can easily cause air discharge, resulting in electric shock or fire. When the voltage is high, the charged ink may cause the ink, the solvent to catch fire, or the worker may directly shock the operator by the ink.

The mechanism of static electricity generation

The main cause of static electricity is friction. When two solid materials contact and rapidly leave, one of them has a greater ability to attract electrons to transfer electrons to the surface of the material, and the surface of the obtained electron shows a negative charge while the other one loses the surface of the electron. It has a positive charge.

The main factors causing static electricity:

　　 1. The material properties include the chemical composition of the material, the internal structure of the material, the mechanical characteristics 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 in a local position where the charge is generated. This distribution on the charge surface can only cause lower voltages. Any part of the conductive material in contact with the ground will immediately transfer the charge to the ground. Insulation materials are different from conductors, such as paper, plastic film, etc. in most printing materials, and generate high voltage due to static electricity, which cannot be eliminated by grounding.

2. Ambient conditions include the composition and pressure of the gas surrounding the exposed object (material), temperature, humidity, and so on.

3. The mechanical action conditions include the type of contact between 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, the greater the static electricity generated.

4. Materials with different types of materials friction will produce different polarity of static electricity. Different materials have different electrostatic strengths.

Static electricity test

　　 1. The main purpose of electrostatic testing in packaging and printing plants is to analyze the degree of harm; to study preventive measures; to judge the effect of static elimination. A special person must be appointed to be responsible for the regular electrostatic testing of anti-static shoes, conductive shoes, anti-static overalls, and various positions. The inspection results shall be filed and reported to the relevant departments at a higher level.

2. Classification of static electricity detection items: Prediction of electrostatic properties of objects when new types of raw materials are used; Detection of live status in the actual production process; Determination of the use of electrostatic safety measures.

(1) The static electricity performance prediction items for objects are as follows:

Object surface resistivity. Measured with a high resistance meter or ultra high resistance meter, with a range of 1.0-10 ohms.

(2) The static electricity condition detection items of the live body in the actual production process are as follows:

The electrostatic potential of the electrified body is measured. The maximum range of the electrostatic potential meter is 100KV, the accuracy is 5.0; the ambient air temperature and relative humidity are measured; the running speed of the charged body is measured; the concentration of the flammable gas is measured; The determination of the earth resistance value; among them, the ACL-350 introduced by Derui company is the world's smallest pocket non-contact digital electrostatic meter. The meter can measure the static voltage and static voltage polarity of the surface of the object.

(3) The use of safety measures to determine the test items with the same (2) electrostatic potential measurement instrument range is appropriate 0-10KV, instrument accuracy is 5.0, but the detection point must be selected behind the electrostatic safety device.

Static elimination methods and measures in the printing process

Chemical elimination method

The chemical elimination method generally applies an antistatic agent on the surface of the substrate to make the substrate conductive, forming a slightly conductive insulator. The application of chemical elimination is very limited in practice. For example, when chemical components are added to printing paper, it is likely to have an adverse effect on the quality of the paper, such as reducing paper strength, adhesion, tightness, tensile strength, etc. 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 properties of the material, and makes the most frequently used method in the existing process.

(1) "Earthing" elimination method, which makes the simplest method, use a metal conductor to eliminate the static electricity connected to the earth and make it equal to the earth potential, the charge leaks through the earth and does not leak, but this way for the insulator Almost no effect.

(2) Humidity control elimination method

The surface resistance of the printed material decreases with increasing relative humidity of the air. 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 shop are: the temperature is about 20 degrees, and the ambient humidity of the charged body is increased to more than 70% as a measure of electrostatic safety.

(3) Introduction and selection principles of static elimination equipment

Static-eliminating devices commonly used in printing plants include induction type, high-voltage corona discharge type, ion flow static eliminator, and radioactive isotopes. Among them, the first two are widely used due to their advantages of low price, convenient installation and use, and absence of atomic radiation. The analysis is as follows:

Inductive static eliminator stick: The induction electrostatic elimination brush, the principle is: When the tip of the electric appliance is close to the charged body, the electric charge whose polarity is opposite to the electrostatic polarity on the charged body can be induced at the tip and moves at the tip. This neutralizes static electricity. The use of this method is gradually decreasing.

High-voltage discharge static eliminator: According to the structure is divided into electronic and high-voltage transformer type, according to the polarity of the discharge can be divided into unipolar and bipolar, unipolar static eliminator only has an effect on a charge, bipolar Can eliminate any kind of charge. In addition, in the printing process, static elimination brush and high-voltage discharge can be combined to eliminate static electricity. Jinan Drake Co., Ltd. launched the 969 power supply 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 where there is a risk of explosion, an explosion-proof static eliminator must be selected according to the hazardous area class, medium class, and group when using high-voltage corona discharge and ion flow static eliminators.

The length of the discharge electrode of the induction type and high voltage corona type static eliminator should be greater than 10-12 cm of the width of the electrified body. The form and number of the ion current static eliminator ion shower head should be set according to the possible installation distance and the width of the electrified body.

The static eliminator should be selected based on the level of the electrostatic potential of my body, elimination requirements, operating characteristics, explosion hazard areas, media levels, and groups.

The principle of installation location of static eliminator: easy to operate, eliminate the effect; follow the subsequent part of the solvent, avoid the metal background.

3. Measures to prevent static electricity

The measures taken by the printing shop to prevent static electricity hazards are suggested in the following aspects:
Electrostatic safety measures must be applied to all process equipment and sites that have electrostatic hazards. In areas where explosive gasses may be present in the surrounding environment, ventilation measures must be intensified to keep the concentration below the explosive range. Preventing static insulators from causing damage to operators In case of electric shock, the static potential of the insulator should be controlled below 10 kV. In the event that the static electricity will cause secondary electric shock when the operator is shocking, in addition to the above, the corresponding measures shall be taken for the device or device.

In areas with explosion and fire hazards, operators must wear anti-static shoes and anti-static overalls. The operating area shall be provided with conductive ground. The conductive ground-to-ground resistance shall be less than 10 ohms, and it shall always maintain its conductive performance. It is strictly forbidden for operators to enter the above-mentioned area except for clothes with synthetic fibers (except clothes that have been regularly treated with antistatic solution). It is forbidden to take off clothes in the above areas.

In short, the printing industry workers must pay enough attention to the static electricity generated in the production process to ensure production safety.