Static electricity in gravure printing workshop and its control method

Disturbing static electricity, also known as frictional static electricity, will bring many results that people would not like to see. For example, paper is electrostatically stuck together, some materials are easily vacuumed due to static electricity, and so on. This article focuses on how to avoid the combustion of organic volatiles due to static electricity in gravure printing. This is especially the case when using hydrocarbon solvent-based metallic inks, especially flash copper inks. The hydrocarbon solvents for such inks are mostly naphtha and/or toluene, containing small amounts of organic combustibles such as esters, ketones or alcohols.

In the production process, we also sometimes consciously use static electricity. For example, the use of electrostatic assisted process (ESA) to remove small stains in highlights and halftone areas of gravure printing, corona treatment of auxiliary plastic films, and improved ink flowability and adhesion, or ribbon welding for bookshelves, etc. . For these conscious use of static electricity, this article does not discuss.

Here we emphasize some conventional safety measures. The main goal is to make people have a certain understanding of the electrostatic phenomenon in the printing and packaging industry. It helps to have a correct understanding of electrostatic phenomena and can take some measures in practice, such as the use of static eliminators, to remove the adverse effects of static electricity.

Because the nature of static electricity is more complicated, and the technology and materials used in gravure printing are not the same, this article only discusses the principle issues related to static electricity, and it cannot guarantee the security of certain specific equipment or processes. When operating these devices (such as static eliminators), the manufacturer's recommended methods and principles should be followed.

1.0 basic safety precautions

When the concentration of volatile organic solvents in the air is high, a strong electrostatic discharge can cause fire. Therefore, the most basic preventive measure in the workshop is to reduce the concentration of organic solvents, volatiles and other flammable substances in the air. Methods include:

o Close the lid of the ink fountain and the ink tray to prevent evaporation of organic solvents. Sealed lids reduce the amount of combustibles that can evaporate. If you open the lid with a metal handle that is grounded, the electrostatic charge on the printer's body will be released to the outside of the ink container.

o Take care not to spill solvent or ink. Be careful not to fill the ink tank or other containers too full.

o Spilled liquid must be cleaned in time. If you need to use organic solvents, you must choose a high fire point cleaning solvent. It's best to clean it in small pieces.

o Store only solvents and inks necessary for production in the work area and store them in closed containers. Wipes soaked with organic solvents should also be packed in closed containers.

o All printed parts, dryers, and dust on the floor should be regularly cleaned. Care should be taken to maintain good ventilation during the cleaning process.

The following measures will help prevent sparks from static electricity:

o Insulation shoes (sports shoes, rubber sole shoes) and synthetic fiber rags are prone to generate sparks and should be disabled. The floor on which people walk cannot be covered with plastic sheeting. Dry ink or other debris will become insulators, which will prevent the body's electrostatic discharge to the earth and should be removed.

o All containers or nozzles for in and out of ink, solvents should be grounded and preferably interconnected directly.

o If the relative humidity in the operating room is equal to or higher than 50% (RH), the conductivity of paper, cotton, and concrete floors will be increased to reduce static electricity generation. The static eliminator must be installed in the correct position and kept clean to have the best effect.

o Stronger static electricity is generated when steam containing more water or compressed air containing condensed water flows out at high speed. See section 9.

2.0 generation of static electricity

The generation of static electricity must have three conditions:

(1) Separation of positive and negative charges. One of the common causes of the separation of positive and negative charges is the close contact between two objects of different properties. This close contact can be caused by friction, so the static electricity is also called frictional static electricity. Another reason for the separation of positive and negative charges is electrostatic induction. Electrostatic induction is not understood by most people, but it is one of the most important causes of static electricity generated during the printing process, so we will introduce it in detail in the following chapters.

(2) After the positive and negative charges are separated, voltage and electric energy accumulation will occur. This build-up of voltage and energy is due to the fact that the charged object overcomes the electrostatic attraction or repulsion forces during mechanical movement. This mechanical movement may be the movement of the web on the printing press, the rewinding of the web, the pouring of ink into the ink fountain or container, or the movement of a person in the vicinity of the printing press.

(3) When the charge accumulates to a certain extent, if the positive and negative charges cannot be neutralized with each other and the static charge gathers at one point, a strong tip discharge phenomenon can be generated.

The above points are introduced in the later chapters of this article. The following discussion will begin on the generation of static electricity in the printing shop.

2.1 Separation of positive and negative charges

Each substance consists of a positively charged nuclei and an extranuclear electron that rotates negatively around the nuclei. In an uncharged object, the positive charge of the nucleus in the object is just balanced with the negative charge of the electron, and the charge is neutralized, so there is no electrical property. However, if two objects are in close contact, the electrons can easily be transferred from one object to another, so that the power will be unbalanced and the objects will be electrically charged. Especially when two objects rub, this close contact is more obvious, so we often call it frictional static electricity. In the contact area, the object that loses electrons is positively charged, and the object that receives electrons is negatively charged. This is charge separation, but the voltage and power generated by this charge separation are very small.

The second most common charge separation mechanism is electrostatic induction. When an electrically conductive object is placed near a charged object, the object will have a charge separation. The same charge that is repulsed to one end by the original charged body may be directed to ground by the grounding switch. At this point, the uncharged object is charged with the opposite charge from the original charged body. At this time, if the inductive charged object is separated from the ground, a higher voltage and power will be generated. Inductive charging is considered to be the most important reason for the large amount of static electricity and electrical energy accumulated in the printing process. In addition, if the electrical conductivity between the shoe and the floor is not good, even if the printed part is connected to the ground, the human body will still bear the charge when it touches the printing press.
Similarly, if a charged object is close to a grounded metal component or container, the corresponding surface of the metal component or container is also sensed and charged with the opposite charge. In this way, although they are 0 voltage (because it is followed), they are charged. In the same way, the surface of the solution is also charged like this. Therefore, unless all the containers are grounded, static sparks will be generated when a part of the solution is poured out.

The third mechanism of charge separation is the separation of droplets. The molecular structure of liquid will cause the difference between the liquid surface and the internal voltage of the liquid. Even if there is no externally charged object, the liquid surface rupture will lead to the charge separation. Due to the strong polarity of water molecules, the static electricity generated by this mechanism is evident in high-speed flow of wet steam and wet compressed air.

2.2 Voltage and Electrical Energy Accumulation

The voltage and electric energy generated during the charge separation process are low enough to produce sparks. The higher and more dangerous voltage and electric energy accumulations are generated when charged objects move, overcome electrostatic attraction and repulsion forces. During gravure printing, voltage and electrostatic accumulation are caused by the web moving on the press. The flow of ink, and even the walking of workers wearing insulated shoes, is also a potential factor for high voltage and static electricity accumulation.

The voltage and electrostatic accumulation generated by the movement of a charged object can be explained by the fact that an insulating, charged metal plate leaves a piece of metal placed on the floor. Although the charge on the charged metal plate does not change, the voltage rises when it is raised. Its voltage can be measured by a gold foil electrometer (or a modern impedance electrometer).

Similarly, when two objects with the same charge approach, excessive voltage and energy accumulation will also occur. Although the web has only a small amount of charge, when rolled up, the situation is different. Similarly, when liquid with a small amount of static charge is poured into the container, high voltage and power savings can occur. Details in this area will be discussed in the next chapter.

2.3 Prevention of Neutralization of Positive and Negative Charges

If the positive and negative charges can be neutralized, voltage and energy accumulation will not occur. For example, when two different metals are in contact, electrons flow from one to the other, that is, the charge is separated. However, when they are separated, the charge is neutralized at the last point where their contact surfaces are separated, so voltage and power accumulation do not occur.

If at least one of the two objects is insulated, the charge will not neutralize at the last point of their contact separation, so voltage and energy accumulation will occur.

Paper materials are conductive to a certain extent because of their moisture content. The charge is neutralized when the relative humidity is greater than 50% RH or when there is steam on the web. This is also a way to eliminate static electricity. The degree of electrostatic hazard suppression depends on the amount of moisture, minerals, and discharge time in the paper. The faster the printing speed is (or the faster the solution is poured), the more likely it is that electrostatic problems will occur. In general, when printing paper or cardboard, proper air humidity can greatly reduce problems caused by static electricity.

Source: Shenzhen Printing Network