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FLEXO Magazine : Maxcess University Special
www.flexomag.com FLEXO SUPPLEMENT/MAXCESS UNIVERSITY FLEXO 3 The challenge to the web-guide system magnifies with in- creasing velocity of lateral error. Looking at this equation, the time of an error is important. If a position error is always off to one side, and is consistent in direction. This is called a steady-state or offset error. They are most common at unwinding. The web is off-center on the core, the core is off-center on the chuck or shaft, the unwind stand is off-center from the machine centerline; each of these are com- mon steady-state/offset errors. After the web exits the unwind, and heads for the first print station, a steady-state error may be introduced in several ways. A misaligned roller or a diameter variation on a roller due to un- even ink or adhesive build up is a common cause. Curvature (or camber) of the web, caliper and coating variation, or bagginess in the web are other possible culprits. External forces, such as air flow, also can contribute to this type of error. As long as the web guide range is greater than the steady-state error, the guide system accuracy is not usually challenged by such errors. Errors that happen in short timeframes or in a periodic manner will challenge web-guide accuracy. Position errors that hap- pen over short timeframes are called transient errors, or web wander. If an error happens over zero time, such as a misaligned splice, it is a step-change error. This is the most challenging type of error for any control system. When the web shifts in a step, it moves at infinite lateral velocity. There is no web guide that travels at infinite veloc- ity, so there will always be a delay, or following error, in correcting for step changes. Many flexo press operators know that when they splice two different web widths together, the guide will perform better if the wide web is trimmed to create a tapered, not stepped, width change. Any guide will have a smaller following error when the web has a finite lateral velocity, giving the guide time to respond to a width change without crashing into the edge sensor. Non-step transient errors can be caused by material, equip- ment, or process variations. Web characteristics can also cause wander; shifted layers in an unwinding roll, variations in web bagginess or skew are examples. Rollers, web guides, and oven air flow can all contribute to wander when they exert uneven forces on the web. Press conditions, such as improper tension, faster line speeds---which can result in air entrapment between idler and web---lubrication, and temperature changes may all affect web tracking and cause wander. The most common guide-induced wander results from poorly tuned control loops (usually the gain is too high), dead band in the web sensor, and backlash in the actuation mechanics. In ad- dition, wide-web flexographic presses may use steering guides. Steering guides are often mounted just prior to the first print station underneath the dryer on central-impression (CI) flexo presses. These guides sometimes present special cases where the guide is set up in violation of recommended geometry that can lead to web wander and potentially wrinkles (another topic altogether). Such conditions as abbreviated guide entry spans, excessive pre-entry spans and improper sensor placement can contribute to poor accuracy at the first print station. MAXIMIZING ACCURACY To maximize accuracy at the web sensor, the guide should have a stiff structure and minimal to zero backlash in the actuator and other mechanical components of the control system. Backlash can result from many things, including mechanical clearances in the actuator mounting, roller bearing axial clearance, and me- chanical deflection of the guide structure. The sensor and controller are designed to sense the web edge and provide a high gain and wide bandwidth signal to the actua- tor. The combined performance of sensor, controller, and actuator will determine the gain of an au- tomatic web guide and ultimately, any guide's theoretical accuracy. It should be noted that all of the process-related inducers of web wander discussed still apply after the web leaves the guide zone. Therefore, placement of the guide as close to the point of needed accuracy (typically the first print station) is critical. In other words, minimize the elements with which the web interacts between the guide zone and the first print station or point where accuracy is a concern. How accurate are web guides? As you can see, web-guide ac- curacy is the result of controlling multiple variables, many of which are web and process related. While there can be many more variables to consider, such as roll wobble, the main points for most flexo press applications have been addressed. As you take these into consideration, you now have insight into gaining more control of your registration and with your new- found knowledge and skill, hopefully you'll find that those costs of misalignment are ones that you need not experience in your process. ABOUT THE AUTHOR: Mike Flannigan, sales manager for cor- porate user accounts, Maxcess International, has more than 25 years of industry experience, including several engineering, technical and sales-related positions. He recently served as district sales manager for Maxcess International, where he provided local sales and support to Fife, MAGPOWR and Tidland customers on the East Coast. Flannigan holds a Bachelor's degree in Speech Communications with a minor in Business Administration from the University of Central Oklahoma. There are two primary characteristics of web behavior that influence accuracy: error magnitude and web lateral velocity. EDUCATION & TRAINING
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