Basics of Web Processes

Keywords

Cross-web variation; dimensional analysis; down web variation; nip pressure; rewind designation; roll length estimation; torque units of measure; web tension; web winding

Chapter Outline

Essentially all flexible packaging converting processes involve rolls of web materials (thin materials, manufactured and processed in the form of a continuous, flexible strip). The full length of the strip represents the “machine direction” and its width, the “cross direction”. Equipment pulls material from the roll and then modifies it in some way that increases its suitability for use as a package. If the eventual fit-for-use packaging material requires several converting processes, the equipment will rewind the modified material into roll form again. The basic flexible packaging converting processes are printing, laminating, and slitting. The modifications at each stage are generically called “value-adding” processes and they form the basis for converters’ selling margins over their costs of purchased raw material.

Web handling in general reflects a dynamic, but otherwise simple, model of Newton’s laws of motion:

These and related “laws” of classical mechanics make web handling a model system for mechanical engineering science to understand and control web processes. The physical and mathematical relationships developed by this science to describe and design web processes involve several sophisticated concepts. This chapter can only highlight some of the powerful insights of the science when designing and operating web handling equipment, but the interested reader can find more detailed sources in the further reading chapter.1

Web Tension

“Pulling” a web off an unwinding roll and rolling it onto a rewinding one presents major mechanical issues. A force must be applied to the unwinding web. The general term for this force is called “tension.” Tension in web converting is often expressed in terms of “pounds per linear inch (PLI).” The units reflect the actual force pulling the web divided by its width (without regard to the thickness of the web material). Web process conditions typically report only this value. To better understand the physical effects of tension on a material, its “tensile stress” must be recognized. Instead of force per unit width, this measure addresses force per unit area, “pounds per inch2.” This value relates directly to laboratory measures of tensile properties, an “Intrinsic Property” of the web material (Chapter 31).2

Tension applied to a web may not only pull it off the unwind roll, but also stretch it, or even break it (depending on the web’s tensile properties). Flexible plastic films in particular have tensile and elongation properties that can result in diversion of some of the applied unwind force to stretching the film (Figure 1.1). When cross-web length variation (called “bagginess”) is present, the stretching force can sometimes “pull out” the bagginess, so that the web appears to lie in a flat plane to observers as well as to the mechanics of the value-adding processes.

In addition to moving the web through the equipment to the unwind, tension on the web helps to resist side to side movement, to reduce drooping (“catenary” effect) in horizontal spans between supports, and to establish friction against rollers along the web path and in the rewinding roll itself.

Web Winding

The rewind roll of a web process represents a protective means of storing the web for subsequent use in converting or at an end user. Consideration of the winding step itself reveals many of the additional mechanical considerations critical to successful web processes. Consistent winding of an excellent roll involves three critical factors at the rewind: Tension of the web as it wraps onto the roll; Nip pressure of drum or roller that presses down on the winding web; Torque of the rotating roll as it winds more web material onto itself. Controlling various combinations of “T N T” factors at different points along the whole web process provides the essence of its design and operation. Tension was described above. Torque is simply a “turning” force, which is the one acting in a clockwise or counterclockwise, rather than a linear direction. Nip represents a point along the process at which two rollers contact the web at the same time. One or both of these rollers are “driven,” that is, having torque applied to them (using a DC motor, a fluid motor, or a slip clutch). Surface friction between the roll(s) and web’s surface controls the web’s speed, lateral position, tension, etc.

Excellence for web winding (called “good roll formation”) implies an overall cylindrical shape (i.e., circular cross-section), lateral alignment of web edges on both sides (i.e., even from core to top of roll), and centered placement on the core. Because the roll itself represents a convenient interim storage state for the web, it must of course be “unwindable.” “Blocked” is the term used to describe the condition in which one wrap of the web on a roll adheres to an adjacent one. Blocking prevents unwinding and often tears the web. Block-prone webs require a thin layer of air between wraps of a roll, generally referred to as winding a “soft” roll. Tensile and surface properties of the web mean different TNT combinations provide optimum roll formation for a particular web. Three types of winding processes can adapt to the range of properties anticipated in a particular industry:

Table 1.1 summarizes how these winding processes apply the TNT factors to webs.

All along web processes, TNT factors control movement of the web through the overall process. Many value-adding steps themselves represent nip points (e.g., applying inks, adhesives; laminating webs to one another). The combination of friction, adhesion, and lubrication between a web and any roller surfaces is called “traction.” Traction between a web and a roller along the process can transfer some of the web’s energy to the roller and cause it to rotate if the roller is properly lubricated. Traction at a nip changes the tension of the web relative to the force between nip rollers, the...