Stress & Strain

A properly tensioned web always will lead to better slitting. Good tensioning is important up to, through, and out of the slitting blades. A poorly tensioned web may try to bypass razor-in-air slitting; or deflect and deform more in razor-in-groove slitting; gather and buckle ahead of the nip point of a crush knife; and flutter out of plane, contacting shear knives ahead of the overlap point. In all these scenarios, a perfect knife setup can be ruined by poor tensioning.

The Benefits of Good Slitting Tension

A tensioned web will play a better role by keeping a stable cut point. A tensioned web has the force to drive into a razor blade, creating the stress required to fracture the web. A tensioned web will have minimal flutter, contacting the shear knives at, not ahead, of the overlap point.

Tensioning a perfect web is relatively easy; it's tensioning baggy webs in and out of slitting where knowledge eliminates waste. Tension is the first line of defense against bagginess.

A baggy web under no or low tension will show its crossweb length variations in loose lanes or edges. Any looseness into slitting is an edge quality killer. For moderate bagginess, medium to high tension will pull out the short lanes equal to the long lanes and greatly aid slitting quality.

Bagginess that can't be pulled out with even high tension is grounds for complaining to your supplier (even if you are your own supplier). Web spreaders commonly are used immediately upstream of slitting where the lateral tensioning will prevent wrinkles and looseness at slitting. Through the Poisson's effect*, the lateral pull of a spreader will provide some help to pull out bagginess.

For solid materials, tensile and compressive stresses do not significantly change density. Increases in length (MD strain) is offset by decreases in the width and thickness.

Besides affecting slit edge quality, a tensioned web is important to accuracy of slit width. Any lateral buckles in the web between slitting positions will create web width variations. Tensioned webs are more likely to be flat, wrinkle-free, and the correct width.

Slit width accuracy also is dependent on good tensioning to maintain a consistent relationship between knife spacing and final web width. What is the correct knife spacing to create a 50-in. wide web? For stiff materials, the answer is usually 50 in., but not so for other webs. Stretchy webs like many fabrics and nonwovens will elongate the web in the machine direction by 2% or even 10% under tension. The tension also will reduce the web width (through the Poisson's effect, also known as necking) by 1%-5% or more. For stretchy materials, tension will reduce a 50-in. web down to 49 in. or less, so knife spacing must account for necking.

Slit width accuracy not only is dependent on setting the right knife spacing for a given tension and necking but also relies on the minimum tension variations. Many slitting processes rightfully try to reduce waste with small trim widths, but uneven tensioning will cause the web to neck in away from the trim knives. No web at the knives means no trim, improper web width, waste, and downtime.

Tension in slitting can help reduce abrasion of the slit edge in stretchy materials. A stretchy material elongates more under tension and will also neck in more. Inserting a blade into a stretchy tensioned web can be like cutting a ripe watermelon. The web will open a gap at the slit point as the necking width loss is divided between slit strands. This slit gap can pull the web away from the knife edges, reducing abrasion-related deformation and debris generation.

How is Slitting Tension Controlled?

If your slitting zone has closed-loop tension control, in which tension is constantly corrected in response to load cell roller or dancer roller feedback, then it's easy to understand how your average slitting tension is controlled. However, though closed-loop control is common on coaters, laminators, and other multi-drive converting lines, it is fairly rare to see it in the slitting section of a slitter/rewinder.

The logic behind how tension is controlled in slitter/rewinders may be the result of “how we've always done it” more than engineering or economic analysis. However, the simple designs that are repeated in most slitter/rewinders are logical when you dig into them.

There is an economic argument to keep the cost down for the tension control system in a slitter/rewinder. A high-speed coater or laminator line may need two or more slitter/rewinders to keep pace with the output, so any equipment design cost is doubled or tripled.

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The biggest difference between slitter/rewinders and most other converting equipment is the constant speed changes. Most slit rolls are one-third or even one-thousandth the length of the input roll. But due to the nature of finishing and unloading a slit roll and loading and starting a new core — possibly tens of new cores — the slitting process has many stops and starts. This can lead to a good amount of process time spent in accelerating and decelerating.

During rapid acceleration designed to increase slitter productivity, roller inertia can create large torques and steal or add tension to the web. Inertia-induced tension variations would lead to all the problems of poor tension control at slitting. The solution is to drive many of the rollers in a slitter/rewinder slitting zone.

Because of cost constraints, the driven roller must be controlled in open draw mode (also known as speed ratio control). Open draw is a common tension control strategy, especially when space and budget limitations make it the only alternative. But draw control is the least understood of the tensioning options.

Understanding draw control of slitting tension requires an understanding of how the stretch (a.k.a strain) of the web creates tension. Elastic webs are like springs. To elongate them requires force proportional to how much you stretch them and their spring constant. A web's spring constant is the product of Young's modulus of elasticity†, web thickness, and width. In a draw zone, there are two dominant variables that control tension. The obvious one is the draw ratio. If you drive the web with a succeeding roller with either a speed increase or decrease, you will change the stretch of the web. The less obvious factor is the input tension. A draw zone only modifies the web's existing stretch or tension.

In a traditional slitter/rewinder, the slitting tension is nominally the unwind tension plus or minus the change induced by the draw ratio of the driven rollers. If the driven rollers create a speed increase, the slitting tension will be higher than the unwind tension.

Tension Exiting Slitting

This is where many slitting processes get into trouble. The mostly likely problem is the tension of the two trimmed edges. Pneumatic take-away systems are a great way to manage the difficult-to-wind narrow trim but usually are lousy at creating a consistent slitting exit tension. Pneumatic trim systems often compound the tension problem by pulling the trimmed strand laterally out of the knives.

The best practice to reduce trim tensioning problems is to have the trimmed strand follow the other slit strands for at least one roller after slitting. This one post-slit roller rule will buffer the pneumatic tension variations and lateral bending from the knives that degrade slit quality.

Another problem with post-slit tensioning is caused by slitting a baggy web without sufficient post-slit tension or differential length compensation to keep the strands from baggy lanes or edges tight. Differential shaft winding would compensate for slit strand length variations if the path from slitting to winding was short and nearly roller-free. However, too many rollers between slitting and differential winding will create a buffer between the differential shaft and slitting. The shaft still will help with slit roll uniformity but lose the ability to create uniform slitting exit tension.

Many of the features of good slitter tensioning are not things you can change after you purchase a slitter. Thankfully, many slitter/rewinder manufacturers have learned through experience what works and include these features in their standard machine designs. Understanding how tensioning and slitting work together puts you in the best position to choose the best slitter design or maintain your slitter's high quality, productive life.

Tim Walker, president of TJWalker + Assoc., has 25 years of experience in web processes, education, development, and production problem solving. He is also the author of PFFC's “Web Lines” column. Contact him at 651-686-5400; tjwalker@tjwa.com; www.webhandling.com.

*Poisson effect: When material is stretched in one direction, it tends to contract in the other two directions perpendicular to the direction of stretch. This phenomenon is called the Poisson effect, named after French mathematician Simeon Poisson.

†Young's modulus of elasticity: Young's modulus, named after British scientist Thomas Young, is the ratio of tension stress to the resulting strain.


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