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Benefits Of New Technology Ionizers For Web Handling

Scott Shelton Simco Industrial Static Control Presented at TAPPI 2005 PLACE Conference, September 27-29, 2005 Las Vegas, Nevada ABSTRACT Ionizers are critical to the web handling process, controlling electrostatic charges to enhance product quality and throughput. In solvent coating and printing processes, they are critical to safety, reducing the chance of fires and explosions. All ionizers require periodic cleaning and performance verification, but this is rarely done due to lack of understanding of the importance and little knowledge of the ionizers and how to evaluate and service them. New technology ionizers solve these problems, offering constant monitoring of system operating status and performance. Local display or remote computer display indicates when maintenance is required. INTRODUCTION Ionizers in the form of static neutralizing bars have commonly been used to neutralize static charges on web and sheet materials since the 1930's. The devices are mounted within the process machine in proximity to the moving material. As the material passes by the neutralizing bar, the surface charges are reduced or eliminated at that point in the process. The static neutralizing bar produces both negative and positive charged gas molecules called air ions. These are free ions that are attracted to the charged surface. Positive fields attract the negative ions and negative fields attract the positive until the surface is in electrical equilibrium. Periodic cleaning and evaluation of the static neutralizing bar is needed to ensure it is operational and performing at its optimum. Traditional systems require some degree of knowledge of how they operate so they can be properly evaluated. Additional instrumentation is also needed to perform the task. The new technology ionizers available today incorporate self-diagnosis circuitry with indicators that display system status and performance. Coupled with much higher performance, these features provide a considerable benefit to the web handling operation. There are various forms of energy used to generate air ions. They include electrical AC and DC, radioactive and passive (induction). This paper focuses on electrically powered ionizers.

REASONS FOR USING IONIZERS

Product Quality Electrostatic charges that accumulate on the surface of a web can have serious effects on product quality. This is especially apparent when the material is to be printed or coated. The electric fields associated with the surface charges can influence airborne particulate and attract them to the surface. Particles in the environment may also be charged due to triboelectric charging. Some may be positive, some negative and some neutral. The electric field from the charged web can attract particles of the opposite polarity should they enter into the range of the field. Neutral particles entering the field may become polarized and can also be attracted to the web. Either case results in particulate contamination of the surface adversely affecting the quality of the coated or printed product. Surface charges on the web can also result in uneven coatings and poor print quality. Safety The sudden discharge of static electricity can result in sufficient energy to ignite volatile gases from solvent coatings or printing inks. A highly charged web may have field strengths capable of inducing charges onto nearby conductive objects such as an ungrounded machine component or a human body. These conductive bodies

typically have sufficient electrical capacitance to store significant energy. Another conductive body having a difference of potential or polarity approaching the charged isolated conductor will cause the charged conductor to instantly discharge all of its stored charge, possibly resulting in a spark between the two conductive bodies. If ignitable gas/oxygen mixtures are present, ignition can occur. A common problem experienced in virtually all web handling applications is operator shock. The shock may be mild and uncomfortable or it may be serious enough to result in burns or even cardiac or respiratory problems. Physical injuries such as cuts and bruises are common because, when receiving a shock, the normal tendency is to flinch and jerk back. Mild uncomfortable shocks usually result from charges induced onto the human body when in proximity to a charged web or material roll then discharging when contacting a conductive object. Severe shocks usually result from contacting a highly charged roll of material at the winder or unwind, or contacting an ungrounded idler roll that has become charged. Enhance Product Throughput Process speed is a major contributor to increased charges on the web. Typically, as web speed increases so does the charge. Some operators will reduce process speeds which results in reduction of the charges in order to enable the product to run properly. This is often as much as 50 or 60% less than desired speeds which obviously results in greatly reduced product throughput. Customer Requirements Many converters and end users receiving roll goods are requiring their suppliers to ship product as static-free as possible. A magic number of 5kV on a roll of material seems to have proliferated throughout the industry and many incoming inspectors are instructed to return shipments to the supplier charges exceed that level. REVIEW OF ELECTROSTATIC CHARGE GENERATION Electrostatic charges develop on the surfaces of films, papers, non-wovens, and fabrics as they contact and separate from various surfaces in the web transport system. These surfaces include but are not limited to extrusion dies, chill rolls, idler rolls, nip sets, coating and print cylinders, slitter and sheeter knives and lay-on rolls at the winder. Extremely high charges can also occur at the unwind as the web parts from the roll. The charge density and polarity depends upon many variable factors. Type of material and dielectric property, pressure and speed of contact and separation, frictioning, ambient temperature and humidity are all key elements affecting chargeability of the process material. Materials such as films and coated papers typically have high surface resistivities and are capable of generating significantly high charge levels and retaining them for long periods of time. Their surfaces are not very hygroscopic so they resist rapid absorption of ambient humidity, therefore, maintaining high surface resistivities. Since electrical current flow is very limited on highly resistive surfaces, charges cannot flow to ground as the web contacts grounded metal surfaces such as idler rolls or other conductive surfaces. Materials such as uncoated papers may have relatively high surface resistivities when processed in lower ambient humidity conditions and also generate and store significant charges. However, these same materials, when subjected to higher humidity environments tend to readily adsorb humidity which lowers the surface resistivity allowing some of the charge to dissipate to ground. The result is lower charge generation and shorter charge retention time. REVIEW OF TRADITIONAL TECHNOLOGY IONIZERS Non-Shockless (Hot) AC Static Bars The first active static neutralizing bars used in the early 1930's were high voltage, 60 Hz AC operated, of the nonshockless variety. They are still used today, but their popularity is diminishing. A voltage of 4kV to 5kV is applied directly to the sharp-pointed emitter pins. These ionizers produce high ion output and can function well in dirty environments; however, accidental contact with the emitter pins can produce a very uncomfortable electrical shock.

Short circuit to ground at the emitter pin could result in current draw as high as 5 milliamperes. Due to the high energy at the emitter, these types cannot be used in hazardous environments such as solvent printing and coating. In addition, excessive buildup of paper dust within the open design of the static bar can result in fires if not cleaned frequently. The neutralizing bar is usually mounted 25 to 50 mm from the web and can handle high charges and web speeds to about 305 m/min. Shockless AC Static Bar Capacitive-coupled shockless AC operated static bars have been in existence since the 1950's and are still popular and widely used. They are constructed similar to the non-shockless variety; however, each emitter pin is coupled to the 7 to 8 kV AC high voltage source via a capacitor. This reduces the short circuit to ground current draw at the emitter to a maximum of 10 to 12 microamperes making it shockless to the touch in the event of accidental contact. Due to the very low current at the emitter as well as other design and construction features, this type of ionizer is offered in Class I, Division 1, Group D approved configurations for use in high solvent gravure printing and coating applications. These ionizers require frequent cleaning to ensure optimum ion output and are usually limited to web speeds around 305 m/min. They are usually mounted 25 mm from the web. Low Frequency Pulsed DC Static Bars These ionizers were originally developed in the 1980's for use primarily in cleanrooms used for manufacturing, assembly and packaging of electronic components and medical devices. The advantage is vast quantities of ionization at a considerable range. The static bar uses few emitters compared to AC bars, with emitter pins spaced several inches apart. Unlike AC ionizers where each emitter produces positive and negative ions, the adjacent emitters in a DC ionizer produce opposite polarity ions. Typical pulse frequency of these systems range from as low as .2 Hz to 4 to 6 Hz. These slow duty cycles result in propelling ions an extended distance from the bar; however, the slow cycle and the wide emitter spacing, makes these systems unsuitable for moving webs. When positioned too close, they result in striping the web with alternate polarity charge patterns. These bars are typically light duty construction and are current-limited, operating on DC voltages in the range of 3.5 to 8 kV. They are not used in hazardous environments. Sophisticated variations of this technology are widely used today in cleanrooms and in medical and electronic device assembly and packaging. Some systems are available with more ruggedly designed bars for industrial applications and have pulse frequencies as high as 20 Hz and operating voltages upwards of 12 kV. Steady-State Dual DC Static Bars This style is similar to the pulsed DC systems described above, using the same basic bar design and operating voltage except the output of the high voltage supply is not pulsed. The ion output is high and the range is greater than the 60 Hz AC bar but much shorter than the low frequency pulsed DC type. When used properly, these bars can prove advantageous to neutralizing high charges on high speed webs, but placement can be critical. Mounting the bar too close to the web or near a grounded metal object can result in incomplete neutralization, charging the web or stripe charging with alternate polarity charge patterns. As with low frequency pulsed DC, positive emitter pins can experience severe premature erosion due to the high current draw present between the opposite polarity emitters. These current limited systems are also not available for use in Class I, Division 1, Group D applications. USER ISSUES WITH IONIZERS Static neutralizing bars can be critical to safety, product quality and machinery performance. Therefore, it becomes essential to ensure the ionizer is functioning and is operating at its optimum performance. High voltage present on the static bar's emitters causes them to act similar to electrostatic precipitators where airborne particles attract and accumulate on the sharp points. The sharpness and shape of the emitter tip is critical to efficient ion production. As contamination accumulates at the tip, in essence, it's geometry is affected and the result is lower ion production or possibly, in some cases, no ion production. Therefore, periodic inspection and cleaning is required. This is a task that seldom gets done due to lack of understanding of the importance, and shortage of time and manpower due to the trend toward fewer employees with less time to pay attention to secondary equipment .

Unfortunately, the static control equipment gets attention after product quality is affected, customers complain or a fire occurs in a coater or gravure printer. Evaluating a static control system is not particularly difficult but most end users have little knowledge of how they work. Traditional ionizers have no integral means to indicate performance level or even if the system is working or not. One crude method to verify high voltage present at the ionizing bar requires approaching the emitter pin with the sharp tip of a grounded screwdriver blade while looking for an arc. This is difficult to accomplish when the web is running and the faint arc from the emitter on a shockless bar is difficult to see under normal lighting conditions. High voltage present at the emitter, however, does not indicate it is producing ions if contamination is present. Another method requires the use of a high impedance, high voltage probe coupled to a voltmeter. The probe is used to measure the presence of high voltage at the emitter but, like the arc test above, does not ensure the static bar is creating air ions. This method too is difficult to accomplish when the machinery is webbed-up and running. A more reliable method utilizes a hand-held electrostatic fieldmeter. The meter is used to measure the field strength of the static charge on the web upstream and downstream of each static bar. If the reading downstream is only slightly reduced or not reduced at all, the bar is dirty or voltage is not present. If the downstream reading is still high after cleaning the bar, then trouble shooting may be in order. As stated earlier, the downside of all of the above methods is that time and manpower is required and it must be done on a frequent basis, especially in critical applications. FEATURES AND BENEFITS OF NEW TECHNOLOGY IONIZERS During the last five or six years, new technologies have emerged which provide many distinct advantages to the end user over the traditional style ionizers. These include higher performance and faster web speed capability along with self-monitoring of system performance with local and remote indication capabilities. Versions of these systems can also communicate with a PC or PLC for visual display and data logging in spreadsheet format to meet quality control record-keeping requirements. UL/CUL listed systems are also available for use in Class I, Division 1, Group D environments. Performance The new technology ionizing bar systems offer considerably higher ion output than the traditional systems (Figure 1). These bars use resistively-coupled emitter pins as opposed to capacitively-coupled pins which, along with different power supply technologies, can neutralize higher charges, handle higher web speeds and can be positioned much further from the web. Depending upon the conditions of the application, some bars may be positioned as far from the web as 25 cm and up to 50 cm from unwind or rewind rolls without using air assist to move the ions to the target.

Figure 1: Static Bar Comparison The highest speed at which traditional ionizers function efficiently is approximately 305 m/min and is primarily limited by the operating frequency of the high voltage supply to the bar and the inherent design of the bar itself. These bars are typically constructed of an inner bar assembly which contains the emitters and is located within a metal outer casing which must be grounded. The ionizing emitters peer through holes or a slot in the outer casing. The electric field from the powered emitters is contained within the grounded metal casing and is only external to the structure through the holes or slot directly in front of the emitters. Since this electric field which carries the ions is somewhat confined, the bars must be positioned close to the web so the ions can be attracted by the electric fields from the charged web. The presence of the surrounding metal casing also attracts and absorbs many of the ions generated by the emitters, thus reducing their availability to the charged web. Most of the newer technology bars are constructed of an epoxy-filled fiberglass extrusion. Each emitter is attached to a small circuit board containing a high value resistor which provides the current limiting to render the emitter shockless to the touch. The emitters protrude through the epoxy potting compound and are usually in line and unobstructed for easy cleaning. On 60 Hz. AC operated systems, the ground reference, which is necessary for its operation, is buried within the bar so an outer metal casing is not required. Pulsing style DC operated systems use a similarly constructed bar but do not require a ground reference. Both the 60 Hz pulse DC, and the 60 Hz resistively coupled AC ionizers described above offer efficient neutralization of high charges on high speed webs up to approximately 900 m/min and offer the convenience of mounting the bars further from the web than the traditional bar. Since these newer styles do not have the metal casing, the ion field plumes around the bar covering a considerable area along the web as compared to traditional bars. System Feedback Information The power supplies and control modules for these static control systems are quite sophisticated and provide much valuable system operation status information which is very beneficial to the user. Indicators on the control module show at a glance if the ionizer is on or off, is functioning properly and when cleaning is required. This greatly reduces the time requirements of the machine operator or maintenance department to periodically evaluate the static control equipment. These feedback features also compensate for the lack of understanding of how to properly test and evaluate such a system. "Bar ON" ­ LED indicator verifies system is operational and neutralizing bar is energized. "Bar OFF" ­ LED indicates neutralizing bar is not energized due to power module not turned on, incomplete wiring connection or system fault. "Fault" ­ LED indicates system fault or neutralizing bar mounted too close to ground reference such as an idler roll. "Neutralizing OK"­ LED indicates system is operational, bar does not require cleaning and is producing air ions. "% Bar Efficiency" ­ Multi-segment LED scale is calibrated to 100% when the bar is properly installed and clean. As emitters become contaminated with particulate, the LED scales indicate proportional decrease in neutralizing efficiency. This scale also shows the bar's neutralizing output efficiency status in relation to the "Clean Bar" triggering point. "Clean Bar" ­ LED illuminates when contamination accumulates on the emitter pins alerting the operator that the bar requires cleaning. The trigger point for this function is observed on the " Bar Efficiency" scale and can be adjusted by the user. "Neutralizing Current" scale ­ This is a multi-segment center zero LED scale which indicates the polarity and relative ion current being drawn by the charged web.

Remote Alarm Outputs ­ Contact closures are available for triggering remote "Clean Bar" and "Fault" alarms. In critical safety applications such as Class I, Division 1, Group D hazardous locations, the "Fault" alarm is sometimes used to shut down the process line. Computer interface/data logging ­ Optional systems are available with various communications protocol to connect to a PC or PLC. This enables viewing all the operational data described above on one or all of the compatible static control systems in the plant (Figures 2 and 3). A digital address is assigned to each control module. Each control module can be scanned at a user selected, predetermined time interval. All the operational status data from each can be logged in C.S.V. spreadsheet format and archived for quality control and statistical process control requirements (Figure 4).

Figure 2: Example of PC screen showing status of all systems

Figure 3: Example of display of individual system on PC

Figure 4: Example of data from individual system logged in spreadsheet format CONCLUSION The newer technology static neutralizing systems available in today's market offer many distinct benefits to the end user in web handling applications. The neutralizing efficiency far surpasses that of the traditional ionizers, while still offering the shockless features, allowing more efficient neutralizing of higher charges at higher web speeds and at greater distances from the web. The feedback of operational parameters of these systems proves invaluable to the operator. At a glance, the operator is assured the bar is operational, can view its relative efficiency level and knows if it requires cleaning. These systems are essential to ensure product quality and safety in the manufacturing environment with the least amount of personnel time involvement.

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REASONS FOR USING IONIZERS

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REASONS FOR USING IONIZERS