The generation, harm and elimination of static electricity in printing

The generation, harm and elimination of static electricity in printing

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The effect of static electricity on printing is very serious. Eliminating static electricity generated in printing is one of the important topics in the printing industry at home and abroad. In the printing production, the phenomenon that the printed paper flies up and cannot be stacked is caused by the generation of static electricity.

First, the cause of printing static electricity

The generation of printing static electricity is mainly affected by four major factors: process, equipment, raw materials and environment.

1. Process factor

1.1 Squeeze and friction to electrify. The paper is strongly squeezed by the printing plate and the roller, and the paper is rubbed with the roller, the printing plate and the ink. The friction increases the surface temperature of the above components, which is prone to thermal decomposition of the molecules, resulting in redistribution of electrons, resulting in static electricity. The production.

1.2 Contact and stripping electrification. In printing, the paper and the tape, the paper guide wheel and the ink quickly contact and separate, especially after the paper is bitten by the gripper, quickly wrap around the surface of the drum, and after printing, the paper quickly passes from the surface of the drum. Peeling, the surface of the drum is wrapped with a blanket, cotton cloth, kraft paper, etc., so that static electricity is generated on the printed paper and the drum liner.

1.3 The effect of machine speed on the generation of static electricity. When the speed of the printing press increases, the friction speed of the paper and the printing plate, the ink, and the contact and peeling speed of the paper and the roller are accelerated, so that the electron movement is intensified, the energy of the charge migration is increased, and finally the static electricity is increased. For example, a web-printed letterpress printer that prints newspapers has a high speed, and the electrostatic potential generated is also high. Even if the printed newspaper is distributed and delivered to the reader, the newspaper print is still due to the presence of a large amount of static electricity. Bonded to each other, it can be seen that the speed of the machine has a great influence on the static electricity.

2. Factors of equipment and raw materials

The plates or rollers of the printing press are mostly metal objects, and the outermost electrons of the metal body are easily separated from the nucleus and become free electrons. Therefore, the metal object has the ability to conduct electric charges rapidly. In printing, the frictional electrification of paper, lining, tape, etc., is mainly a surface effect, and its surface condition has a great influence on the generation of static electricity. If the surface is smooth and smooth, the generated static charge is small. On the contrary, if the surface is rough or oxidized and corroded, the static charge generated will increase. In addition, due to the different structural components of the paper, the hygroscopicity is also different, and some papers generate static electricity during the production process, which has a great influence on the generation of printing static electricity.

3. Environmental factors

The air temperature and humidity in the printing place also have a significant effect on the printing static electricity. When the printing parts and materials rub against each other, they cause thermal decomposition and thermoelectric effects of the molecules, which increase or electrify the charged particles, thereby charging the printing parts and materials. When the temperature of the air at the printing place rises, the above various effects are made more intense, so that the accumulation of the amount of static electricity is greatly increased.

In the summer, due to the relatively high relative humidity of the air, the static electricity is not generated much, and in the spring, autumn and winter when the relative humidity of the air is small, it cannot be released quickly due to the generation of static electricity, and a large amount of static charge is accumulated. Temperature and humidity are interrelated. When the temperature rises, the air becomes dry, the relative humidity of the space decreases, and the effect of humidity on static electricity is much greater than the temperature.

Second, the harm of static electricity to printing

1. The harm of static electricity to printing quality

Due to the influence of static electricity, the paper is skewed on the paper feeding belt, and it is not easy to fall when it reaches the delivery table. On the delivery table, the papers are attracted to each other and fit together. The paper-feeding device cannot align them, which will give Process folding work has a certain impact. Since sheets are pasted to each other, double failures often occur. The paper is not easy to walk on the paperboard, and it will cause the front gauge to malfunction, resulting in inaccurate overprinting, resulting in ghosting, graphics and other phenomena. In addition, the paper with static electricity sucks a lot of impurities and dust, making the ink in the prints unclear.

In printing, the ink is transferred and pressed at a high speed by an ink roller, an ink table, and a printing plate, and high-speed contact separation and friction cause electrostatic generation of the ink, that is, ink generation. Some fly directly to the printed matter, so that the printed matter is splashed. Some fly to the printing plate and the blanket and then transfer to the printed matter. This is called transfer and dirty, and often forms a relatively fixed light-colored ink spot until the plate is printed. Or the flying ink on the blanket will disappear after it has been removed. This not only affects the quality of the print, but also contaminates the equipment.

2. The harm of printing static electricity to the human body

Printers are easily electrostatically charged during production operations. For example, in the dry spring and autumn seasons, clothes are generally worn more, and most of them are chemical fiber products. The friction between the human body and clothes, clothes and clothes will generate static electricity, and the friction between the sole and the ground will also charge the human body. On the other hand, the resistance values of shoes and shoes worn by ordinary people are all above 10gΩ. If it is plastic or rubber soles, the resistance value will be higher. Therefore, the static electricity on the human body and clothes is difficult to release (the static resistance of the human body when the static electricity is discharged, the sole resistance should be below 10gΩ), so the static electricity is accumulated to make the human body charged.

Electrostatic induction can induce an uncharged conductor to electrify and become a charged conductor. The human body is also equivalent to a conductor for static electricity, and the human body also induces electricity in an electrostatic field. When the printer is operating, it often comes into contact with charged paper and paper piles. The human body induces electricity under the action of the electrostatic field of paper and paper piles. Since the human body is well insulated from the earth, it becomes an independent charged body.

After the human body accumulates static electricity, when the finger in operation touches the outer casing or metal part of the printing machine, since the outer casing of the machine is grounded, the static electricity of the human body will discharge from the finger to the casing, so that the static electricity will leak into the earth, and the human body will be affected. electric shock.

When the human body does not accumulate static electricity and wears shoes with low insulation resistance when standing on the ground, if you touch the paper with high potential or the delivery pile by hand, you will also receive electric shock. The reason is that the human body and the ground form a good grounding, which is static electricity. The release provides access.

Printing electrostatics is a shocking electric shock caused by electrostatic discharge. The discharge current is only at the micro-ampere level. Although the electrostatic voltage is as high as several thousand volts or tens of thousands of volts, it is not fatal to humans. It is worth noting that sudden electric shocks bring acupuncture and numbness to people, which makes people nervous and affects the operation of the printing workers. In serious cases, it can cause product or personal accidents.

3. Static electricity causes fire and explosion

In the printing of high-speed web, the electrostatic potential generated is very high, and spark discharge is easy to occur. When the spark discharge amount reaches the minimum ignition energy of gasoline and alcohol, there is a danger of fire and explosion.

Third, the method to prevent static electricity

In order to avoid the danger of static electricity and ensure the normal operation of printing, on the one hand, it is necessary to eliminate static electricity on the paper lining and the tape, and more importantly, prevention is to prevent or minimize the generation of static electricity.

1. Condition the paper

Since paper generates static electricity during papermaking, it should be air-dried when transported to a printing house to reduce the amount of static electricity on the paper. At present, the methods of drying paper are as follows: 1 natural hanging method. It refers to hanging the paper in the printing workshop or in the hanging paper room with the temperature and humidity of the workshop, which is called the natural hanging method. Hanging time is generally more than 2 days. The longer the hanging time, the better the humidity control effect, and the smaller the paper expansion rate during printing. 2 mechanical hanging method. The method of using a special paper machine is called mechanical hanging method, and the general one has a circular paper machine and a bridge paper machine. The humidity control of the 3 rolls can only be done by standing, and the conditioning time takes several months. The hanging of the sheet of paper not only adjusts the temperature and humidity of the paper, but also makes the paper moisture content uniform, so that the paper can be flattened to avoid wrinkles caused by uneven water content in the printing process. In addition, the paper is suspended. After drying, it can remove surface dust and foreign matter. Due to the influence of the composition of the paper material and the manufacturing process, the impurities contained in the paper are different from the surface smoothness. When printing, paper with less impurities and smooth surface should be selected to reduce the static electricity generated by the paper due to friction and peeling. In addition, the raw material composition of the paper affects its hygroscopicity. From the viewpoint of reducing static electricity, paper with strong hygroscopicity should be selected, so that the surface resistance of the paper can be rapidly lowered, and the discharge of static electricity of the paper can be accelerated.

2. The influence of printing pressure and machine speed control on static electricity

The size of the printing pressure has a significant impact on product quality, printing durability and even machine maintenance. The pressure is too small, the ink color of the printed matter is light, no spirit, no dullness; if the pressure is too large, the expansion and deformation of the dot will be reduced, the level will be reduced, the normal operation of the drum will be damaged, and the bearing wear will be accelerated, so the pressure should be scientifically controlled. The printing process is to transfer the ink to the paper by the pressure of the roller. The pressure is large enough to make the ink in the print firm and the pen tip is clear, but at the same time a large amount of static electricity is generated. Reducing the printing pressure can reduce the paper charge migration and reduce the static electricity of the paper, but it will affect the quality of the printed matter. Therefore, under the premise of ensuring normal printing, the printing pressure should be minimized.

The high speed of the printing press will increase the static electricity accumulation. If the machine speed is reduced, the contact separation and friction between the paper and the machine parts will be weakened, the charge migration movement will be weakened, and the static electricity generation will be reduced, but the reduction of the machine speed will also increase the printing cycle. Consider increasing the number of shifts or increasing the number of presses.

3. To control the temperature and humidity of the printing workshop and place

The effect of temperature and humidity on printing is great. When the temperature of the workshop is high, the flow performance of the ink increases due to the increase in temperature and mechanical force, and the molecular attraction between the inks becomes weak. The amount of water used in printing will also increase, which will accelerate the occurrence of emulsification. It appears as an easy-to-dry paste in printing. When the temperature is low, the viscosity of the ink is large, and it is difficult to apply the ink. When printing, an appropriate amount of additives must be added to the ink to reduce the viscosity of the ink to ensure the printing progress. When the humidity is high, the paper is easy to absorb and deform, the water volume is difficult to control during printing, and the water is easy to be large, so controlling the temperature and humidity of the workshop is necessary for printing. In the printing process, the temperature and humidity of the printing workshop can be reasonably mastered, and the temperature change does not exceed 3 °C. The overprinting caused by the expansion and contraction of the paper will be improved. According to the actual situation in China, the temperature and humidity of the printing workshop are controlled in Table 1. , can make production tend to be stable.

Temperature and humidity control in the production site, temperature and humidity control can be achieved in the case of air conditioning, and for the workshop without air conditioning, necessary measures should be taken to mitigate the impact of temperature and humidity changes on the printing process.

3.1 One of the measures to minimize the temperature and humidity of the workshop is affected by external climate changes. This does not mean that the requirements are consistent across the four seasons, but rather the change control within a certain time frame. Mainly the same batch of printed matter is completed from white paper to printing. The temperature and humidity of the workshop should not change greatly. It is necessary to reduce the direct exchange and exchange between the workshop and the outdoor air as much as possible. Generally, the workshop window should be closed and the structure of the house should be closed. To be able to scatter the sunlight in summer, reduce the direct impact of sunlight on the temperature and humidity of the workshop. Good insulation performance in winter, to prevent the impact of cold air on the temperature of the workshop. The door of the workshop should not be directly external, there should be a turning point, leaving a passage, and the door should also be blocked by a cloth curtain.

3.2 In the second measure, in case of weather changes, the semi-finished products to be printed should be covered with plastic sleeves to prevent and slow down the outside air to promote the deformation of the semi-finished paper.

Fourth, ways to eliminate static electricity

In the actual printing work, since the static electricity cannot be all suppressed, but still occurs, the elimination of static electricity is very necessary. The printing static electricity mainly has two methods of release elimination and neutralization elimination.

1. Printing electrostatic discharge elimination method

After the printing static electricity is generated, the electrostatic charge will be naturally released through various channels, but the rate of release is very slow. To prevent static buildup, it is necessary to accelerate the release of static electricity, usually using the following release method.

1.1 equipment grounding and antistatic agent application, the road tank truck, there is always a chain at the tail of the chain dragged to the ground, this is because the rubber tire of the tanker is an insulator, the static electricity generated on the body can not enter the earth, and The role of the iron chain is to provide a pathway for the release of static electricity to the earth. This method of eliminating static electricity is the ground release elimination method.

Antistatic agents, also called antistatic additives, spray antistatic agents on the surface of insulating materials such as paper, lining, and tape. The surface of these materials can easily absorb moisture in the air to form a water film, which reduces the surface resistance of these insulating materials. The conductive property is strengthened, so that the static charge is quickly released, and static electricity accumulation is avoided. However, since the effective use time after spraying or painting the antistatic agent is short, it is inconvenient to use for a large amount of printed matter, and the cost is high.

1.2 humidification, the humidity in the air refers to the degree of humidity of the air. The air contains a lot of water, the air humidity is large, and the air humidity is small. Increasing the humidity of the printing place can make the surface of the insulating parts and materials absorb moisture and accelerate the release of static electricity. However, if the humidity is too high, the tension, compressive strength and expansion amount of the paper will change, which will cause paper breakage and paper feeding difficulties. The overprinting is not allowed to be faulty. Therefore, the humidity of the printing site must be measured and controlled to keep it within a certain range. It is known from actual production experience that when the relative humidity of the workshop is 60% and the temperature is 20 °C ± 2 °C, the paper produces a sufficiently strong electrical conductivity, so that the amount of static electricity is greatly released, and normal printing can be performed. Nowadays, a variety of new types of temperature and humidity meters have been sold in the market. They are small in size, light in weight, and easy to carry and install.

2. Neutralization elimination method for printing static electricity

While the printing static electricity is generated, the electrostatic charge is naturally released through the air. Usually, there are a certain number of charged particles in the air, that is, electrons with negative ions and electrons lacking positive ions. The charge on the charged body such as paper, lining, and tape will neutralize the positive and negative ions in the air, but since the amount of charged particles in the air is extremely small, this natural neutralization is very slow, however, This has been inspired by the use of the basic characteristics of positive and negative charge phase absorption neutralization to eliminate printed static electricity. The static eliminator uses high voltage to ionize the air and increase the charged particles in the air, thereby accelerating the neutralization of the electrostatic charge on the printed paper. Therefore, the static eliminator is also called an electrostatic neutralizer. It is the most important and the most common method used by current printing plants to eliminate static electricity.

Static eliminators basically have the following types: (figure omitted)

Electrostatic phenomena are widespread in printing. Effectively preventing and eliminating static electricity is of great significance for improving the quality of printed products and ensuring production safety. With the development of science and technology, the way to overcome static electricity will also be constantly updated, let us work on the ground, adopt scientific and scientific working methods, and continue to work hard in practice to make our production safer and products more beautiful.