For the flexible package printer or converter who works with extensible films and materials, the fight to prevent defect-rendering process deviations never ends. The key to reliable defect prevention is the production staff’s ability to measure and make web machine or press adjustments that eliminate unwelcome deviations, including tension transients.
While many film processors are challenged to overcome issues of excess tension, some suffer from slack web conditions. Given the opportunity to diagnose a web process problem, a knowledgeable application engineer at a reputable tension equipment manufacturer can offer solutions to the machinery deviations that lead to deformed or flawed output.
Tension Related Web Problems
Loss of color-to-color registration while running at speed, splicing or changing speed, inconsistent repeat length and slack web, which can cause web breaks and wrap-ups around driven rolls, are perhaps the most obvious consequences of inadequate tension control on a web press. Others include deformation of web due to stretching or wrinkling, variation of coating thickness, unwind or rewind core crushing, reduction of machine speed to accommodate web handling problems or any of the problems above, hard rolls, soft rolls, telescoping rolls, excessive waste of web material and the inability to run a wide range of web thicknesses, widths and materials.
Many of these problems are simply accepted as normal and are not attributed to inadequate control of web tension. However, printers and converters experiencing such trouble and recognizing the relationship can improve efficiency and profits by employing better tension control methods.
By reducing web scrap and allowing a web press to run at higher speeds with defect-minimized output, a reliable web tension control system will not only improve a press’ bottom line performance, but can pay for itself in as little as a few weeks.
Selecting a Web Tension Control System
Several process factors determine what tension components should be considered for an upgrade on a web press or converting line:
- The material nature of the flexible substrate.
- The level of print quality required of the jobs being run on the press.
- The magnitude of tension transients introduced into the system by the combination of all process variables.
- The speed at which the press should be run.
When characterizing a process to identify tension measurement or control requirements, the machine can be divided into three tension zones — the unwind zone (one or more), intermediate tension zones (typically multiple) and the rewind zone (possibly one or more). A tension zone is a length of machine in which the web is under the same tension. This occurs between driven rollers, between the unwind stand and the first driven roll or nip and between the last driven roll and the rewind stand. To properly control the tension in any of the zones, it must be measured, or approximated, by some means.
Tension Measurement Technology
There are three predominant tension measurement technologies in use today.
One is roll diameter measurement by optical or ultrasonic sensor. These sensors measure the change in distance from the fixed sensor head to the roll as it is either building up or winding down. As such, they are not used for intermediate zone applications. Sensor output, usually a 0-to-10 VDC signal, varies as the roll diameter changes at the unwind or rewind station.
The sensor output can be tied to a tensioning device such as a drive to trim the line speed. This technology may be suitable for non-critical winding applications, but the inferred tension measurement from diameter measurement is less accurate than either of the following technologies.
Other open-loop tension control systems such as roll followers, draw controls and diameter calculation by line and roll speed ratio may work well in printing and converting applications with forgiving substrates, but they cannot measure the actual tension on a moving web and compensate accurately for deviation from a preset value; nor can they compensate for speed changes, brake fade, temperature and humidity variations, non-uniformities in the web and other factors that affect web tension.
Another technology is a dancer system. In a dancer system, a controller signals the dancer roll (a.k.a. dancer arm) to move (vertically or horizontally) back to a start position in response to a significant change in position. The initial positional change is almost directly proportional to the variation in web tension.
A dancer system allows some forgiveness for web slack by creating web storage in its zone of operation. In an unwind zone, a dancer system may help to compensate for out-of-round rolls by supplying web accumulation as needed.
But dancers have certain drawbacks. Compared to tension transducer systems, they are expensive to design, build and install. They are difficult to properly engineer. They are prone to errors caused by mass, damping and friction. And they cannot measure tension directly nor read out tension or compensate for small but significant tension transients in the process.
Finally, there are tension transducers (a.k.a. load cells). This direct tension-sensing technology is the most accurate available. Using strain-gage-based sensors, tension transducers directly measure the force applied by the web on a selected idler roll. The sensors are calibrated against an electronic readout using a known weight over the tension roll in the web path.
Transducers that mount on idler roll shaft ends are typically used in pairs, with one installed on each end of an ordinary idler roll shaft. Or, in the case of the tension roll type transducer, the tension sensors are built into the roller mechanism of a dead-shaft idler roll.
The strain gauges detect slight beam deflection (of even a few hundred microns) under load and convert the mechanical deflection into a voltage that is proportional to web tension and highly accurate (~1%). A transducer pair’s 0-to-500 mV tension output signal is typically amplified to 0-to-10 V and displayed on a meter readout.
To achieve the desired running tension for a job being run, a machine operator may adjust a web-tensioning device. Or the transducer signal can be brought to a PLC or automatic tension controller to send a compensated output signal to a drive, clutch or brake.
The only way to achieve accurate web tension measurement and control is to use tension transducers combined with automatic tension control electronics in a closed loop system.
Closed Loop Tension Transducer Systems
The simplest type of complete closed-loop transducer system consists of a pair of transducers, an enclosure with a display meter, a circuit card that provides excitation voltage to the transducers and amplifies their output, and a pair of interconnecting cables. The circuit card will typically have voltage and current outputs proportional to tension that can be fed to variable speed drives, recorders, or PLC’s or computers.
In addition to being displayed on the meter, the tension signal can also be fed to a PID-controlled regulator circuit where it is compared to a desired tension signal set by the machine operator. The regulator sends a continuously-updated compensated output voltage or current to a servo valve, motor, brake or clutch to automatically control tension in a closed-loop control scheme.
A tension control system that supplies control output to a pneumatic brake in the unwind zone is perhaps the most commonly found tension-sensor control loop in flexographic printing. Rewind tension control is unnecessary on press lines that are strictly web-to-sheet.
Trends in Tension Control Product Development
With the exception of flexible packaging printers and converters, and the nonwovens industry—the two web-processing industry segments that are still growing at a healthy pace—most machinery markets for web tension control products in North America have reached a state of maturity.
Product development from tension measurement and control equipment manufacturers has evolved slowly and incrementally over the past decade, only changing with corresponding technological changes in the web converting and printing markets they serve. Tension equipment suppliers have predictably moved to microprocessor-based, software-driven devices and away from the older analog-style electronic circuitry and outdated user interfaces with toggle switches, buttons and dials. Manufacturers are shaping the look of industrial controls to follow the styling of user interfaces that are being used in the consumer electronics markets.
Even without monumental developments in the tension control industry, there are some trends in flexible packaging today that are sure to have an impact on future product developments from the tension controls manufacturers.
One of the recent drivers of continued growth in the flexible packaging industry, as mentioned in a recent report from BNP Media’s Market Research Division and Flexible Packaging magazine, is the shift by segments of packaging users from rigid containers to flexible packaging solutions, particularly in the food and beverage sectors.
The intense level of competition in many consumer packaged goods categories has manifested itself in a proliferation of micro-segmented brands and customized regional promotions for popular products at the retail level. This marketing differentiation fever has forced increased demand for shorter print runs, faster time to market, brighter colors and more new and exciting packaging options in general.
Another trend: Producer cost-containment measures and the green movement have driven the demand for thinner films, improved film barrier characteristics, embedded security technology and package size minimization.
Flexible packaging printers and converters have been forced to respond with faster turnarounds, greater process versatility, increasing sub-process capabilities, the ability to print on a wider variety of substrates and more economical methods of completing short print runs.
Tension control equipment manufacturers have likewise had to respond to the fast turnaround and substrate variety by developing and offering measurement and control devices that are quick to install, simple to set up and easy to use. The tension sensors and electronics must operate over a wider substrate tension range, for a wider offering of web widths, in a broader scope of industrial environments and with more input and output capability.
On the electronics side, the setup storage and recall features of digital controls are obviously desirable for web converters who run a wide variety of substrates or even the same substrate at various thicknesses. Other programmable features like automatic zero and calibration, auto-PID-tuning, gain scheduling, diameter compensation, variable wrap-angle technology and tension profiling offer benefits that allow flexible packaging converters to maintain output quality at speed.
Another evolutionary development in tension management has been the result of changes in web press technology. The wider-style narrow web printing presses, which are enjoying steady growth as an economic alternative to wide web for shorter print runs, has spurred demand for a new type of tension transducer. Tension equipment manufacturers have been called on to produce cantilevered tension transducers of various lengths for the single-sided frames of narrow web presses. Transducer design in general will always have to adjust to accommodate the presses and machines they will run on.
But with no revolutionary changes expected in tension sensing or control algorithm technology over the next few years, real growth in tension measurement and control is now occurring in only a few ways:
Individual manufacturers are working to drive down component and system costs in order to stay competitive. They must. With so many tension equipment manufacturers saturating the competitive field, tension measurement and control devices are now perceived as commodities. But as a consequence of the resulting lower component prices, the potential market for tension equipment is expanding. Now, smaller printers and converters, in North America and internationally, are starting to be able to afford the technology that was too expensive 20 years ago.
Tension controls manufacturers are differentiating the user interfaces and mounting styles of their measurement and control electronics. Standardized digital communications features are being added to control electronics so that customers can integrate the sensor outputs from their various process parameters into a supervisory control center. This should allow for more efficient monitoring and control of individual press components.
- Competition to dedicated tension controllers is also growing from the D.C. drives manufacturers who have integrated a PID control function for tension control right into their drive packages. This solution may be more cost effective than to purchase a dedicated tension controller and a separate drive, but the downside is the difficulty in determining which part of the system needs to be tuned if the customer’s web process becomes unstable during operation. A dedicated tension control system with one supplier having technical support responsibility throughout may be preferable for customers with limited technical staff or experience.
Dover Flexo Electronics