Electrostatic Analysis in Printing (I)

1 Electrostatic hazard

1) Impact on product print quality

If the surface of the substrate is electrostatically charged, they will absorb paper dust or dust floating in the air, impurities, etc., thereby affecting the transfer of the ink, and “flowers and dots” appear on the printed product, resulting in a decrease in the quality of the printed product.

If the ink is electrostatically charged, if it is discharged during the move, it may appear as “static ink spots” on the prints. It seems that the ink has poor flowability and uneven ink application. This is often the case in thin-layer printing. In field printing, if the charged ink is discharged at the edge of the line on the print, it is easy to appear as a “must” on the edge of the line.

2) Impact on production safety

In the printing process, high static electricity is generated due to high-speed friction. When the static electricity accumulates to a certain degree, air discharge may easily occur, resulting in electric shock or fire. When the voltage is very high, the charged ink may cause the ink, solvent to ignite, or shock the personnel through the ink, directly threatening the personal safety of the operator.

2 Electrostatic generation mechanism

The main cause of static electricity is the presence of friction. When two materials are contacted and rapidly separated, one of the materials with strong electron-adsorbing ability transfers electrons to its surface, and the surface of the material that obtains the electrons exhibits a negative charge, while the surface of the other material is lost due to the loss of electrons. positive charge. There are mainly the following factors that generate static electricity.

1) Material properties

Including the material's internal chemical composition, material internal structure, stress properties, 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 in a local position where the charge is generated. This charge can only produce a lower voltage, any part of the conductive material in contact with the ground can immediately make the charge on the material to the ground. The insulating material is different from the conductor material. For example, the printing material such as paper and plastic film will generate a high voltage due to static electricity, and it cannot be eliminated by grounding.

2) Ambient conditions

Including the contact material gas composition, pressure, temperature, humidity and so on.

3) Mechanical action

Including the type, time, area and the separation speed of the two materials, the nature of the force. In general, the closer the two materials are in contact or the faster the separation, the greater the static electricity generated.

3 static elimination methods and measures during printing

1) Chemical elimination method

The chemical elimination method generally applies an antistatic agent to the surface of the substrate to make the substrate 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 paper strength, adhesion, tightness, tension, etc., so chemical elimination is generally not used. law.

2) Physical elimination

The physical elimination method is a method of eliminating static electricity by utilizing the inherent characteristics of static electricity without changing the properties of the material, and it is a static elimination method commonly used in industrial production.

1 "grounding" elimination method. This is the simplest method. It is to use a metal conductor to connect the object to eliminate static electricity with the earth and make a potential difference with the earth. The charge leaks through the earth, but this method has almost no effect on the insulator.

2 humidity control elimination method. The surface resistance of the printed material decreases with increasing relative humidity of the air. Therefore, increasing the relative humidity of the air can increase the surface conductivity of the paper and accelerate charge leakage. The suitable environmental conditions for the printing shop are: temperature is about 20°C and relative humidity is 70% or more.

4 Static Elimination Equipment Introduction and Selection Principles

There are several types of static eliminators commonly used in printing plants: inductive, high-voltage corona discharge, radioisotope, and ion flow static eliminators. Among them, the former two are more widely used because of their advantages such as low price, convenient installation and use, and absence of atomic radiation. Now they are analyzed as follows.

1) Inductive electrostatic eliminator

Induction static eliminator is an inductive electrostatic elimination brush. Its working principle is: when the tip of a dissipator is close to a charged body, it can induce a charge with a polarity opposite to the electrostatic polarity on the charged body, and is formed in the vicinity of the tip. An electric field that ionizes the air and generates positive and negative ions that move to the charged body and the tip of the dissipator respectively under the influence of an electric field, thereby neutralizing static electricity.

2) High-voltage discharge static eliminator

High-voltage discharge static eliminator can be divided into electronic and high-voltage transformer type according to different structures, according to the polarity of discharge can be divided into unipolar and bipolar. In the printing process, static elimination combined with inductive and high-voltage discharge can be used to completely eliminate static electricity. The 969 type power supply unit introduced by Jinan Drake Co., Ltd. and the DJ62 high-efficiency static electricity elimination stick have a better static elimination effect and has been widely used in the packaging and printing industry.

In addition, in areas with explosion hazards, high-voltage corona discharge type and ion flow static eliminators must be selected according to the hazardous area class, medium level and group, explosion-proof static eliminators; inductive and high-voltage corona The length of the discharge electrode of the discharge-type static eliminator should be greater than the width of the charged body 10-15cm; the type and quantity of the ion spray head of the ionization 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 , Electrostatic elimination requirements, operating characteristics, explosion hazard area rating, media level and group selection; static elimination device installation location selection principle: easy to operate, eliminate static effects, followed by brushing solvent parts, avoiding metal objects .

(to be continued)