Static electricity near the printing nip

(STATIC ELECTRICITY) Generation of Static Electricity During normal printing, the transfer of static electricity between the web and the printing press member is very rare. Even in the vicinity of the nip line, how much pressure can not completely eliminate the static electricity in the blank area of ​​the web through the printing cylinder which has been grounded. The transfer of static electricity between the web and the printing cylinder is very limited at the top of the web page, especially when using a drier paper or printing on a roll of paper that has been patterned and the ink on it has dried. of. Electrostatic transfer on paper and paperboard depends mainly on the moisture and electrolyte content of the paper. Dry inks are usually insulators, which prevent electrostatic transfer. When printing, the ink is transferred to the paper and the charge on the ink is transferred. When the printing roller carries some kind of charge, the ink on the roller is induced to carry opposite charges. This induced charge will transfer with the ink to the web. The charge on the print cylinder and the ink charge on the graphic part of the paper below form an electric double layer. There will be no net charge before the web leaves the print cylinder. There are three mechanisms that cause static electricity when the web leaves the nip: (1) When the print cylinder is not grounded, insulated, or has a little electrical conductivity, tribo-static charges will cause static charge on the gravure cylinder and wet ink on the cylinder. When printing, most of the induced static electricity is transferred together with the ink to the graphic area on the web. (2) The web itself carries a static charge before it enters the nip. During imprinting, electrostatic discharge occurs in the area where ink transfer occurs, but the rest of the area is charged. (3) Some frictional static electricity is also generated during the imprinting process. The main cause of static electricity generated by the voltage build-up web is electrostatic induction during imprinting. When the web approaches the intaglio printing cylinder, the initial voltage generated is usually relatively low. As the web exits the nip and is separated from the grounded press component, voltage begins to build up. Here we look at two locations: one is where the web leaves the print cylinder and the other is where the web leaves the impression cylinder. When the web exits the nip, the web and the impression cylinder are separated from the grounded gravure cylinder. If the surface of the web or impression cylinder is electrically charged, then they are separated from the grounded gravure cylinder and cause a medium-sized voltage on the uninked blank area of ​​the web. Of course, if the web has a strong ability to attract opposite charges from the gravure cylinder, the voltage will also be less. In the printing area, the web accepts ink while accepting the charge, which neutralizes the opposite charge on the top of the web and on the print cylinder. As the web leaves the location of the nip line near the printing cylinder, the possibility of discharge-induced combustion is very small because the voltage at this time is far below its maximum value and the air gap is also small. When the air gap is small, the electrostatic voltage required to break through the air resistance is very high, and the thick printing cylinder can also play a role in extinguishing the fire. The printing roller absorbs heat and causes the fire to disappear. The principle is similar to that of a metal screen on a fire extinguisher or an old-fashioned safety miner's lamp. Through the study of electrostatic assisted processes, especially the stripping process, we have found that the most critical position for the generation of discharge-induced combustion is at the point where the web is separated from the impression cylinder. Printing fluff is a phenomenon that is very unfavorable for printing. It is characterized by the fact that the ink spreads from the graphic area to the blank area during printing, causing blurring of the boundary. When using non-conductive organic inks (such as publication gravure printing), this phenomenon often occurs. This phenomenon is even more pronounced when the moisture content is very low or using an electrostatic assisted process (ESA). At present, the research on this phenomenon has been deeply studied for a long time. In the electrostatic assisted process, the surface of the impression cylinder is made of a special conductive rubber and deliberately charged. In electrostatic assist devices, the voltage on the surface of the impression cylinder is controlled at a relatively low level. However, the charge on the rubber can flow toward the embossing area, which can result in a larger amount of static charge in the embossed area. The charge on the surface of the impression cylinder causes the ink in the cylinder and the ink hole to have opposite polarity induced charges. The electrostatic attraction of the ink surface will reduce or eliminate the dot loss of the ink hole. However, when printing, the induced charge will also transfer to the surface of the web with the ink, causing the web to be charged. However, in the printing of blank areas, there is not much charge transferred from the drum without the ink-receiving portion to the web. When the printed page comes out of the nip, there is a charge on the surface of the conductive impression cylinder. Its distribution curve is in correspondence with the printed image and the charge distribution on the back of the web. As long as the web is in contact with the impression cylinder, the charge on the surface of the impression cylinder will offset the influence of the charge on the back of the printed page. When the web is separated from the impression cylinder, a scattered electric field is generated. For poorly conductive inks, this scattering electric field will attract the ink from the graphic part to the blank part, causing the ink to fluff, causing the graphic border to be blurred. For inks with good electrical conductivity, this phenomenon does not occur and there is little fluffing. Experience with static-assisted processes or thermal effects in the embossing area indicates that: (1) The mechanism of the liveness of web graphics is mostly due to electrostatic induction caused by frictional static electricity on the surface of the cylinder. (2) After the web is separated from the impression cylinder, the electrostatic voltage may be so high that arc discharge may cause combustion. Remedy The above considerations apply to all soluble inks. However, there may be some special problems when using metallic ink. This is mainly because the solvent of the metallic ink is not conductive, and the non-conductive solvent can separate the metallic particles, so the ink is not conductive at the beginning. However, since the metal particles themselves are conductive, they will suddenly become good conductors when electrostatic discharge is generated. This will make the discharge sparks very concentrated and generate high heat. The preventive measures that can be taken are: (1) Make the moisture content of the printing paper appropriate, keep the room humidity suitable, or use steam to humidify properly. (2) The static eliminator is installed about 1/2 to 3/4 inch away from the web. In a press using an electrostatic assist device, a static eliminator is usually placed between the gravure cylinder and the web surface. The static eliminator installed between the impression cylinder and the web is usually located at a position that both discharges the web and discharges the surface of the impression cylinder. (3) Use new ink. This method has now been applied, such as the silver glossy ink technology used in the publishing industry. A properly installed and properly maintained static eliminator can often solve the electrostatic problem of metallic inks. In special cases, try to ground the surface of the impression cylinder. It is very important to ground the roller surface because most static auxiliary rollers have an insulating layer between the outer surface and the inside of the roller. Special equipment must be used to ensure that the power supply is grounded through the electrostatic aid. Other remedial measures include reducing the volatilization of the ink solvent, keeping the ink reservoir sealed, and improving the ventilation performance in key areas. Basic Precautions: Electrostatic assistance must not be used on printing presses that use metallic inks or on printing devices that immediately follow the presses that use metallic inks. In this case, the electrostatic assist means does not improve the overall print quality, and the charge accumulation caused by the use of electrostatic assist means may cause severe discharge sparks in combination with the well-conducted metallic ink. Appendix Formulas for calculation of maximum energy value and electrostatic attraction All formulae herein use international standard units, such as length in meters (m), area in square meters (m2), grounding voltage in volts (V), and capacitor usage in pulling (F). The text also gives some common conversions of old units and international standard units for reference. Resistance is measured in ohms. One megawatt equals 106 ohms. The resistivity of the material is expressed in ohms x meters or ohms x cm. It represents the resistance when a one-meter (or one-centimeter) long volume is one cubic meter of material cross-section. The conductance is the inverse of the resistance and is measured with Siemens (S). In the past, we used mho to denote S. Conductivity is expressed in S per square meter (S/m) or S per square centimeter (S/cm). 1 (ohm x cm) = 100 (ohm x m) 100 (S/m) = 1 (S/cm) The voltage between the object and a grounded object is expressed in volts (V), 1 kV (KV) = 1000 volts (V). The electric field strength is expressed in volts/meter V/m (or volts/cmV/cm). 1 (V/cm) = 100 V/m The air breakdown strength between two capacitive plates several centimeters away is 2 x 106 to 3 x 106 volts per meter (20,000 to 30,000 volts per centimeter). What is interesting is that such a strong electric field is simply due to the absence or absence of an electron in the surface atoms per million objects. Capacitance refers to how much an insulated object can accumulate charge. Capacitance shows how close an object can be to a grounded object. Usage (F) indicates. 1 Farad = 106 microfarads 1 farad = 109 nanofarads 1 farad = 1012 picofarads Capacitance Ca of two parallel air-isolated capacitive plates can be calculated from the following equation: Ca = 8.85 x (A/d) x 10-12 (farad) where A represents the area of ​​the capacitor plate, expressed in square meters, and d represents the distance between the two capacitor plates, expressed in meters. When the capacitor plate is not separated by air but by other insulating substances, the capacitance should be the capacitance Ca multiplied by the dielectric constant of the partitioning substance. Table A.1 gives the dielectric constants of some substances. Table A.1 Permittivity of several substances Paper About 3.5 Polyethylene 2.6 Vinyl chloride About 3.5 Pure rubber About 2.5 Carbon rubber About 10 to 80 Titanium dioxide About 15 to 120 octane 1.9 Toluene 2.4 Ethyl acetate 6.0 Ethanol 24.3 Pure water 78.5 The energy stored in the electric field, ie, the maximum energy that can be converted, is: Stored energy = .5×C×V2 (joules) The attractive force between two capacitive plates separated by air is: Gravity=4.4×A× (V/d) 2 (Newtons) where 1 Newton = 0.225 lbs (gravity) Note that V/d refers to the field strength between the capacitive plates. When the capacitor plate is not separated by air but by other insulating substances, it should be multiplied by the dielectric constant of the insulating material. See Table A.1.