Physical Methods for Eliminating Static in Printing
Physical methods employ various types of static eliminators. When a charged object possesses a certain amount of charge, generating static voltage, the eliminator, based on the principle that like charges repel and opposite charges attract, generates ions of a certain polarity to neutralize the ions of the opposite polarity on the charged object. Based on the method used to generate ions, eliminators can be categorized as active, inductive, and radial (RI) types.
① Active Static Eliminators
This type of eliminator boosts the power supply voltage to tens of thousands of volts. The high voltage, transmitted through the eliminator electrodes, ionizes the air, eliminating the static voltage on the charged object. Depending on the type of voltage, it can be categorized as either DC or AC. The secondary voltage of the eliminator should ideally be 10-12 kV. Excessively high secondary voltages are detrimental to safety and maintenance, and can sometimes result in excessive discharge.
② Passive Inductive Static Eliminators
Passive inductive eliminators are also known as self-inductive eliminators. Self-induction static elimination typically involves arranging multiple sharp needles in a row and grounding them. The needle tips are held approximately 5 mm away from the charged paper surface. The charged surface (the charged paper surface) is then electrically charged, generating a large number of positive and negative ions, thereby eliminating static electricity. The basic principle is as follows:
The paper surface is charged (assuming a negative charge). When the needles approach the charged surface, they immediately induce a charge of the opposite sign (i.e., a positive charge). This charge is concentrated at the needle tips, resulting in an extremely high charge density and a high field strength. This generates electricity at the needle tips, ionizing the surrounding air and forming positive and negative ions. The negatively charged paper surface then attracts the positive ions of the opposite sign, eliminating the static charge on the paper surface and achieving the desired static elimination effect.
If the field strength acting on the gas is uneven, as the voltage increases to a certain critical value, a localized discharge occurs in the high-field intensity region near the tip. This is called a corona discharge (the light from the negative needle tip corona is light blue, and the light from the positive needle tip corona is light red). If the voltage is further increased, it will eventually become a spark discharge.
③ Radioisotope Static Eliminator
A radioisotope static eliminator is a new type of static eliminator that uses isotopes to generate radiation to ionize the air. A radioisotope is a nuclide that decays spontaneously, producing one or more radiation types, typically γ-rays or γ-rays. Currently, two types of radioisotope static eliminators are used in the printing industry.
④ AC Ion Wind Static Eliminator
AC Ion Wind Static Eliminator uses AC high voltage (12,000-13,000 volts) applied to a row of needles to generate a corona, ionizing the air and generating positive and negative ions. An air compressor then blows large amounts of these ions onto the charged paper. The charged paper (+ and -) charges are neutralized by air ions of opposite signs, eliminating static electricity. This device, once used on sheet-fed web presses, can effectively reduce static electricity potentials of 10,000 to 20,000 volts on the paper surface to essentially zero, achieving excellent results. However, it is highly dependent on the installation method, location, angle, ion air flow rate, and ion current density, requiring appropriate consideration and adjustment at all stages. It is also effective for eliminating static electricity in paper piles, eliminating static-induced sticking before paper is loaded onto the press. Blowing ionized air onto wrapping sheets can eliminate static electricity exceeding 2,000 volts. Air ionization devices can be installed in warehouses and workshops where conditions permit. Air ionization methods include corona discharge, pulsed electric fields, and the photoelectric effect. Ionized air reduces dust, improves work efficiency, and enhances health. Its application should be promoted in printing plants.
In printing shops, it is recommended to evenly distribute the devices, using multiple small units located close to the machines and paper piles. Large, evenly distributed devices are recommended for paper warehouses. Adjust the ion concentration and, if necessary, adjust the polarity (by reversing the discharge electrodes) according to the requirements of each application. Ozone is produced when air ionizes. Therefore, it's important to keep ozone concentrations below safe levels, ideally without any ozone odor. In areas like paper storage and drying machines, the ion concentration can be appropriately increased to facilitate static discharge and prevent accumulation. Once the paper is in use, the potential harm of static electricity naturally diminishes.
⑤ Increase Humidity to Eliminate Static Electricity
Dry air is prone to static electricity generation and accumulation. Properly controlling the relative humidity of the environment can reduce static electricity generation. Increasing humidity increases the conductivity of paper and other charged objects, allowing static electricity to be quickly dissipated. According to relevant data, static electricity generation is minimal at temperatures around 20°C and relative humidity of 60%. Therefore, humidification can be employed based on the specific working environment. Before paper is fed to the press, steam can be sprayed onto the paper surface using a hose with a 0.5-20mm diameter hole, 20-30mm in distance. Water can also be sprayed around the printing press, and damp cloths can be hung at the paper feed area to increase relative humidity. Humidification devices such as flat humidifiers, centrifugal humidifiers, jet humidifiers, and ultrasonic humidifiers can also be used. The ejector should be installed above the workshop and be able to change its angle.