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.