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FLEXO Magazine : Maxcess University Special
Disturbances to the web, they are every printer's nightmare. Once they occur, they wreak havoc. Jobs are destroyed. Waste scrap begins to pile up. Precious time is lost. Potentially missed deadlines present a danger. It seems like a sorcerer will have to be called in to work his/her black magic to fix things. Panic sometimes fills the air, and it's easy to sense the tension in the pressroom. Transitioning that tension from man to machine improves performance, satisfies customers, but most importantly keeps the presses humming and churning out high quality work. Accurate tension control is important in any web application, including flexographic printing, where registration, print qual- ity and speed are vital to making the best product as efficiently as possible. In the past, manual systems required the operator to continually decrease the output as the roll got smaller. Advances in tension control, like automatic open loop systems (i.e. follower arms and ultrasonic sensors) have freed the operator, but only made tension corrections proportional to the roll diameter, there- by failing to consider any other disturbances of the web. For this reason, it is typical to find dancers and load cells or a combination of both on higher-end machines. These systems provide feedback, are the most accurate and provide the highest level of control when correctly applied. Today's customers and their product demands are pushing the limits of even the higher-end closed-loop controls. Increased speeds, more sensitive material, larger roll builds and faster The Evolution of Tension Control Speed, Sensitivity Demand Power & User Friendliness By Darrell Whiteside New tension control technology can eliminate common issues experienced by flexographic printers like out-of-round rolls. changeovers have all increased the need for more powerful, yet user-friendly controls. Fortunately, closed loop tension control systems have evolved to meet the demands of printers and customers. Gone are the days when every control loop was the same (whether it was ten- sion control or a temperature control) and you had to have a de- gree in control theory to program it. In this article, we'll outline how recent advances in tension control can provide significant benefits to the flexographic press, both for the product and operator. OUT-OF-ROUND-ROLL COMPENSATION In the past, there has always been a compromise to make con- cerning the use of load cell or dancer control. Do I use a dancer system to accumulate the velocity changes of a bad roll? Do I save space and go with a load cell system that also gives me direct ten- sion readout? Do I spend the money for both systems to have the ultimate tension control? Dancer systems (see Figure 1) have been around for many years and provide accurate tension control by providing posi- tion feedback. Dancer systems are typically used if there is a concern of the unwind rolls being "out-of-round." In most cases this occurs from improper storage, resulting in flat spots or even egg-shaped rolls. The downside of these systems is the amount of engineering required to correctly design the system and the amount of space needed in the machine. Load cell-based systems (see Figure 2) employ the use of sen- sors that directly measure tension providing feedback to the control. Although considered the highest level of tension control, they have always been limited by their ability to stabilize out-of- round rolls. Today's load cell controls can continually monitor the fre- quency of the flat spot on an unwind and adjust the output as it is about to happen rather than always chasing the velocity change, eliminating the need for expensive, large and difficult-to-design dancer control systems. 6 FLEXO FLEXO SUPPLEMENT/MAXCESS UNIVERSITY www.flexomag.com EDUCATION & TRAINING
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