To maintain proper tension as web diameters change, motion controllers on converting equipment continually update the electronic gear ratio between the unwind spindle and pull roll. This calls for a controller that can dynamically change electronic gear ratios on-the-fly, adaptively increasing (or decreasing) servomotor speed. The better the controller does this, the better the converting machine will work.

More feedback

Encoders and web dancers provide much of the feedback on converting equipment, but a few more strategically placed sensors can make the controller's job easier and its actions more effective. Adding a dancer assembly with an analog output, for example, lets machines run faster and more accurately because it provides direct tension feedback. Here, the feedback from the dancer roll is brought into the controller where it factors into the dynamic electronic gearing algorithm.

Another technique is to monitor the current drawn by the servomotors that drive the unwind and pull rolls. As web tension varies, so does the torque (and current) applied to the servomotor. Like direct tension measurements, current feedback signals can be brought into the controller, where they serve as an additional input in the electronic gearing algorithm. Current monitoring may be used alone or together with direct tension feedback for better results.

Zone two

The second tension zone on the plastic-bag making machine lies between the two sets of pull rolls. Tension is typically maintained across this zone using a combination of electronic gearing, a web dancer, and servodrive current monitoring.

Usually, one pull-roll axis or the other is designated as the master, and all other axes within the zone adjust relative to the master to maintain the desired tension. Designating a single master prevents the various axes from trying to dominate each other, which usually results in erratic tensions around the zone.

Converting operations that occur in zone two include folding the film with mechanical plows, heat sealing the seams and, depending on the application, perforating the film by scoring the plastic with perforation cylinders or wheels. Perforation imparts operational qualities to the finished bag such as mounting tabs and breakouts. The success of all these mechanical operations hinges on maintaining proper servo-axis coordination and correct web tension.

Fine print

Printing is yet another operation performed in zone two, between the two sets of pull rolls. Naturally, it requires additional components — print cylinders and servomotors — though they're controlled in a manner similar to tension-control motors, using electronic gearing techniques. One difference, however, is that the controller must apply position offsets to the print cylinders to properly phase individual colors in multi-color images. Subsequent web operations, such as embossing or die cutting, may also require offsetting.

Another difference where printing is involved, weather the web is preprinted or printed on the machine, is the need for registration to account for offset variations or web compliance issues such as stretch or slippage at the pull rolls. Registration corrects for such errors as well as most minor disturbances, accurately re-aligning print areas with the machine.

High-speed product registration requires an additional sensor, a photo eye, to detect print registration marks on the web. When the marks are detected, the slave's axis position is latched or captured in the motion controller. The difference in position between current and previous marks is used to calculate the distance between registration marks.

In the same sampling period, this measured distance is also compared to a theoretical distance. The resulting error signal is used to generate a correction factor that's applied to the slave axis to shift the web to the desired machine location. The intense math behind the correction process calls for a high-performance motion controller.

Cut and run

After the web passes through the second set of pull rolls, the conversion process enters its final stage, where the web is cut to the defined bag length. The cutting tool might be an electronically geared rotary knife or sequenced shear. Both offer programmable control over cut length.

Variable-length cutting makes it possible to run different products without hardware changeovers. The controller need only adjust the electronic gear ratio (for machines using knives) or the initiating sequence (on machines using shears). In either case, the operation requires correct web tension as well as precise product registration to align the web with the desired cut point.

With a little imagination, the concepts discussed here can be expanded to other web operations such as unwind, rewind, lamination, embossing, cut to length, and so on. What's more, many of the techniques described also apply to nonweb machinery, with less emphasis (of course) on web tension and more on product positioning and registration.