Labelling Solutions
Web tension– Web control
By Tom Kerchiss, chairman of RK Print Coat Instruments
Centers of Innovation in Spain, India and North Carolina in the United States.
ension is created in a fast moving web by pulling or stretching the web against a resisting force during processing. Totally transient, it ceases to exist when that force is removed. Process variables such as ink and coating viscosities, drying tunnel temperature and web cooling over chill rolls will aff ect the web at selected points in the process. However it’s web tension that aff ects the accuracy and consistency of coating thickness, axial dimensional stability of the web, lamination quality and the overall integrity of the material being processed. It is impossible to control a web without proper tension being applied. Tension must be applied to webs to keep them in traction with idler and driven rollers. Factors aff ecting the longitudinal dynamics of the web include torque variations caused by an unbalanced roll, wound in tension variations and other periodic disturbances.
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Web tension must be managed within set limits from unwind zone and various processing zones on up too and including rewind. Inappropriate control of tension will impact on the accuracy and consistency of an applied barrier or surface coating or indeed any other passive, active or smart coating application. When tension is too high, webs may stretch in
the longitudinal direction or machine direction and compress in the cross machine direction. This narrowing of the web width can cause the web to wrinkle. Conversely when tension is too low, webs may shrink in the machine direction and web width will widen in the cross machine direction, this too may result in wrinkles. Dished rolls, wrinkles and even web breaks are symptomatic of poor control at the unwind. Similar problems occur equally at the rewind. Converters may fi nd that they are unable to run diff erent web thicknesses, widths or coat uniformly.
Closed loop load cell methods for controlling tension together with edge guide have been integrated in with the VCML pilot coaters supplied to Archroma Packaging Technologies
The term: closed loop comes from the fact that the output of the system (tension) is fed back into the input. This enables us to visualise the output forming a continuous path through the regulator or controller and back into the input, circulating in an unbroken loop. Load cells have no moving parts and can be integrated in at wherever they are required on the coater or other web based processing machine. Edge or web guides are hardly an unfamiliar concept, which is probably why they are often taken for granted. Web guides exist for a number of reasons, not least of which is the fact that substrates are not necessarily uniform in thickness and weight. Imperfections such as these are amongst the cause of web drift and the movement of the web from side-to-side. Edge or web guides keep high-speed webs on course. More compact, smarter and easier to use web guides tend towards being plug-in and run and provide precise control while streamlining the tasks of the operator. Many guiding systems allow for wide band, spot or ultrasonic detection. Spot and wide band sensors are generally available for use with transparent and opaque substrates while ultrasonic versions sense a wide range of webs and can be useful for light sensitive materials.
For organisations and departments charged with bringing new products to market or for the development of more sustainable materials and products the VCML pilot coating system with small-scale capability and selectable coat/print/ laminating and associated support technologies is an option.
An alternative is the VCM pilot/production coater. The VCM is a bespoke system, individually designed and constructed for specifi c requirements. For reasons of commercial confi dentiality details on machines supplied and other pertinent information is never released. The area of commonality though is that the VCM incorporates high specifi cation drives, tension and web control equipment with best available processing accessories. Rigid framework serves as a foundation for each machine, which enables the processing of wider web width and heavier substrates.
www.convertermag.com
April 2026
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