Electrostatic Systems-Charge And Discharge
The principles of electrostatics and how electrostatic print assist systems can improve printing technology
When processing materials which have insulating properties – like paper, textiles, glass, chemicals or synthetics – electrostatic build up is almost inevitable. In most cases it is generated when processing the materials (e.g. combining, separating, bonding, welding, pressing). Lightweight materials with high insulating values often raise almost unsolvable problems.
Therefore, in the plastics industries almost nothing works without an in-depth knowledge of electrostatics. However, it is not only the materials which generate electrostatic charges but also the kinetic energy applied to set them in motion. As a rule of thumb, the higher the speed the higher is the charge. In contemporary manufacturing environments determined by high performance and cost pressure ever larger amounts of materials must be handled at ever higher speed. As a consequence faults and dangers caused by electrostatic charges rise accordingly. An example of such a fault is the electrostatic attraction of materials. This attraction is an annoying effect of electrostatic charging, which must be removed in a controlled way before it causes damage. However, this physical phenomenon also offers creative, innovative and productive potential when following its rules. For example, Eltex ESA (electrostatic print assist) systems for gravure printing are installed worldwide in more than 6000 printing units. Applying targeted charging results in improved ink transfer and the reduction of missing dots. (Eltex is one of the leading suppliers worldwide of such devices.)
Examining uncontrolled charging
The impact of energy on the boundary layers of physical bodies may disturb the electron balance, which results in electrostatic charging. This excess or lower amount of negative charged electrons attempts to balance out: this means discharging. Electrostatic systems keep such phenomenon under control and use charging/discharging processes in a useful way. Electrostatic processes are fuelled by the smallest elements of matter: atoms and electrons. Prior to charging the number of positively charged protons in the nucleus and the number of negatively charged electrons in the shell is the same. Therefore, the atom is in a state of electronic neutrality (figure 1).
If the number of electrons is lower than that of the protons, the atom is charged positively. One of the bodies involved in this move must be a capacitor, which is an isolated body capable of electrostatic charging and discharging respectively. For example, this could be a running film web (figure 2).
When the electrons skip from the one body to the other caused by processes like the contact and subsequent sudden separation of the boundary layers of two bodies, this unbalances the neutral condition and is called charging. In the case where the number of negatively charged electrons exceeds the number of positively charged protons the atom in general is negatively charged (figure 3).
Figure 1(left): Electrostatic processes are fuelled by the smallest elements of matter, which are atoms and electrons. Figure 3(right): In the case where the number of negatively charged electrons exceed the number of positively charged protons the atom in general is negatively charged
The ever faster industrial production processes require a maximum of reliability with the discharging of static electricity as a key factor. High running speeds of the increasingly compact machines present the discharging systems with steadily increasing challenges and conventional systems often fail. For example, the mechanical construction of a machine might not facilitate the installation of the discharging components at the desired position. Therefore it is crucial, that discharging is also operational over larger distances. Eltex calls this a depth-effect on high-speed applications, which defines the unique characteristics and advantages of their discharge technology against comparable systems (figure 4).
The Eltex discharge components meet the highest quality requirements and have proven their reliability for decades in day-to-day operations at machine manufacturers and users in many different industry segments. The discharge electrode R50/51 offers a significant higher active discharge performance compared to previous discharge technologies, as insulated ground conductors amplify the electric field at the active tips and therefore contribute to increased generation and speed of the load carriers. This result in considerably higher amounts of positive ions and free negative electrons compared to conventional electrodes. At the same time the electrodes offer a high passive discharge performance. Decoupled from electric resistance the tips ensure very good discharge performance and make the electrode absolutely safe against short-circuiting and are also contact proof. Eltex systems are used with laminating units, slitter/rewinders and film processing machines.
Figure 4(left): High running speeds of the increasingly compact machines present the discharging systems with steadily increasing challenges
Figure 5(right): RX3 Ionstar
Far-reaching, intelligent and contact-proof
Wherever huge or changing distances must be bridged and very strong electric charging must be eliminated, the discharge electrode RX3 Ionstar proves its strength and performance capability. Its intelligent sensor continually detects the distance and the actual electric charging of the object to be discharged and adjusts the output performance to the current condition. Therefore, the system offers outstanding discharge options with slitter rewinders. Integrated into the electrode, the powerful high-voltage generator is capable of compensating even large electrical charges within a very short period of time.
The output voltage of 2 x 50 kV facilitates discharge ranges not possible so far and without any additional air assist required. Despite this impressive performance capability the RX3 Ionstar discharge electrode is fully contact-proof and even in the on mode there is no danger to the life or health of people (figure 5).
To achieve optimum discharge effects during the reel winding, its diameter, frequency and duty factor have to be continually adjusted. Uniquely the RX3 Ionstar performs this fully automatic by ultrasonic distance measurement, whereas other systems require continual re-adjustment.
Figure 6: Residual Charge in Winding Station
The Dual Task of Electrostatic Systems
The beneficial aspects of specifically applied electrostatic charging is described
Figure 1:Charging on in-mould labelling.
When specifically applied, electrostatic charging is highly beneficial and increases production performance (through blocking or precise fixation) during the production process. Electrostatic charging offers real benefits in terms of quality and efficiency, such as:
– Higher production speed;
– Increased efficiency;
– Optimised quality;
– Reduced energy consumption;
– Lower costs;
– Smoother processes;
– Minimised failures, stops, and waste.
Charging systems consist of a high-voltage generator and charging electrodes. They are mainly applied to speed up and protect processes, where materials have to be firmly combined for a certain period of time. The Eltex high voltage generators KNH35, KNH65and KNH124 generate up to 100 kVin both positive and negative polarity.
Voltage differences of 200 kV may occur in the alternating electrical fields (+,–). It is also possible to generate charges with a polarity (+or –) against the ground potential. The high-voltage electrodes are highly loadable, and due to the optimised arrangement of the electrode tips, the distribution of charge and electrical fields has been significantly improved. In addition, current- limited tips avoid any increase in air ionisation and brush discharge.
Attractiveness through performance
Charging technology for process optimisation is used in many industries. This includes laminating and coating (adhering or fixation of films), in-mould labelling (fixation of labels in the tool), and film extrusion (accurate, limited or continuous electrostatic fixing of films during processing; firm powder tacking on foil surfaces; exact core winding without (neck-in) and telescoping at the winding unit) (Figure 1 and 2).
The attractiveness of the technology is mainly based on advantages like low power consumption, compact power adaptors, single electrode for all applications, high efficiency, interference resistance and economical pricing.
Electrostatic printing assistance
Packaging and decoration gravure printing are some of the most important fields of application for electrostatic printing assist systems (ESA). ESA technology is based on the principle of the plate-type capacitor. This means that a homogeneous electric field is generated between two plates, in which a dielectric is pulled to one side. ESA applies this principle at the printing nip – precisely at the point where the ink is transferred onto the web (Figure 3).
This principle of ESA supported ink transfer works evenly over the entire web width and operates reliably at both very slow and very high production speeds. The application of ESA technology results in accurate printing on cardboard, paper, and film without any missing dots. Eltex ESA systems for gravure printing are installed worldwide in more than 6000 printing units and the company cooperates with all the well known press manufacturers. Although the majority of new gravure lines are already equipped with ESA, there is also a strongly increasing tendency for retrofitting already installed presses. The GNN75 system (DC charge, no charging electrodes) especially enjoys growing popularity. In particular, systems with air assisted charging electrodes are replaced by Eltex systems.
A promising future
The beneficial aspects of specifically applied electrostatic charging are increasingly used in a growing number of industrial areas which is also due to the fact, that applied high-voltages and low currents in the mA-range elicit low energy costs. For continuous further development of the ESA technology, Eltex intensively cooperates with machinery manufacturers and research institutes.
Figure 3: ESA applied at the printing nip to improve ink transfer.