The harm caused by static electricity in printing and its removal method

Printing is performed on the surface of an object, and electrostatic phenomena are mainly manifested on the surface of the object. During the printing process, all substances involved in printing are electrostatically charged due to friction, impact and contact between different substances.

The harm of static electricity

1. Affect product print quality

Substrate charged on the surface, such as paper, polyethylene, polypropylene, cellophane, etc., will absorb paper dust or dust, impurities floating in the air, affecting the transfer of ink, so that the printed flowers, etc., resulting in reduced quality of printed matter. Secondly, if the ink is charged and discharged during the movement, “static ink spots” will appear on the printed matter, which often occurs in thin-layer printing. In the field printing, if the charged ink is discharged on the edge of the printed matter, “magenta” can easily appear on the edge.

2. Influencing safe production

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

The mechanism of static electricity generation

The main cause of static electricity is friction. When two solid materials contact and quickly leave, one of the materials has a greater ability to attract electrons to transfer electrons onto the surface of the material, the surface of the electrons exhibit a negative charge and the other a positive charge. The main factors causing static electricity:

1. The material properties include the internal 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 the surface. This distribution on the surface of the charge causes a lower voltage. The contact of the conductive material with the ground immediately transfers the charge to the ground. Insulating materials and conductors are different. Paper, plastic films, etc. in most printing materials generate high voltage due to static electricity and cannot be eliminated by grounding.

2. Ambient conditions include the gas composition and pressure surrounding the object (material), temperature, humidity, etc.

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. The more closely the two materials are in contact with one another or the faster the separation, the greater the generation of static electricity.

4. Materials with different types of friction between different materials 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. Special personnel must be assigned to take charge of anti-static shoes, conductive shoes, anti-static overalls, and regular electrostatic testing of various positions. The results should be collated and reported to relevant departments.

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 electrostatic property prediction of the object is as follows: surface resistivity of the object. Measured with a high resistance meter or ultra high resistance meter, with a range of 1.0-10 ohms.

(2) The static electricity condition of the charged body in the actual production is as follows: The electrostatic potential of the charged body is measured. The maximum range of the electrostatic potential meter is 100KV, the precision is 5.0; the ambient air temperature and relative humidity are measured; and the charged body is operated. Determination of speed; Determination of flammable gas concentration; Determination of conductive ground-to-ground resistance; Deere's ACL-350 is currently the smallest non-contact digital electrostatic meter.

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

Static elimination method in printing

Chemical elimination method

An antistatic agent is applied on the surface of the substrate to make the substrate conductive, making it 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 will have an adverse effect on the quality of the paper, such as reduced paper strength, adhesion, tightness, tensile strength, etc. Therefore, the chemical method is not Too widely used.

2. Physical elimination

It is the most commonly used method that does not change the properties of materials by using the characteristics of static electricity.

(1) The ground elimination method uses a metal conductor to connect the earth to eliminate static electricity and earth potential, but this method has no effect on the insulator.

(2) Humidity control elimination method

The surface resistance of the printed material decreases as the air humidity increases, so increasing the relative humidity of the air can increase the conductivity of the paper surface. The suitable environmental conditions for the printing shop are: the temperature is about 20 degrees, and the charged body's ambient humidity is more than 70%.

(3) Selection principle 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 cheap, easy to install and use, and do not have the advantages of atomic radiation. They are widely used:

Sensor-type static eliminator stick: The induction-type static elimination brush is based on the principle that when the tip of a dissipator approaches a charged body, an electric charge having a polarity opposite to the electrostatic polarity on the charged body is induced, thereby neutralizing the static electricity.

High-voltage discharge static eliminator: divided into electronic and high-voltage transformer type, divided into unipolar and bipolar according to the polarity of the discharge, unipolar static eliminator only has effect on one charge, bipolar can eliminate any A charge. Elimination of static electricity can be achieved by the combination of an anti-static brush and a high-voltage discharge in the printing process. The principle of the installation location of the static eliminator: easy to operate, followed by the subsequent part of the solvent.

3. Measures to prevent static electricity

All equipment and places with electrostatic hazards must be ventilated in areas where there may be explosive gas around, so that the concentration can be controlled below the explosion range; where static electricity is prevented from causing electric shock to operators, the electrostatic potential of the insulator is controlled. Below 10KV. In areas with explosion and fire hazards, operators must wear anti-static shoes and anti-static overalls. The operating area is laid with conductive ground. The conductive ground-to-ground resistance is less than 10 ohms, and the conductive performance is maintained. Operators are strictly forbidden to wear synthetic fiber clothes (except clothes that have been regularly treated with anti-static solution) to enter the above areas. Undress.