Slitting Education and Information

What slitting equipment is right for my process?
Specifying the web slitting needs of your process takes three easy steps.

Specifying the web slitting needs of your process takes three easy steps.

Step 3 – Identify Slitting Equipment to Meet Your Needs

Slitting requires proper control of both slitting knives and the web. Similarly, slitting equipment should be viewed as a two-part system: knife control and web control.

Slitting knife control equipment includes slitting knives or blades, the knife or blade holders, knife speed control (if needed), knife load control (for crush and shear), and knife positioning system.

Slitting web control includes web guiding and spreading, entry and exit angles and span, web speed control, web entry and exit tensioning, and the overall slitter web path.

Slitting knife control equipment options:

Razor slitting: razor-in-air
This is the simplest slitting options, commonly used for trimming films and thin papers. A set of razor blades are place to interfere with the web path between two rollers, fracturing the web like an icebreaker. Since web flutter will shift the cut position along the blade, razor-in-air usually uses a 2-3” long double-edged blade, positioned so the web passes by the entire blade width, 20-30 percent up the knife’s length.
Razor slitting: razor-in-groove
This process is usually creates a better edge than razor-in-air. The web wraps a roller with uniformly spaced grooves of 0.050- 0.100” wide by 0.1-0.25” deep. The blade tip or center of the blade edge is positioned in a groove center with small amount of penetration into the groove. Razor-in-groove will typically have better edge quality than razor-in-air due to reduce web flutter, shorter contact length, and less deflection or deforming of the roller-supported web edge.

For more on razor slitting, see Four Factor to Consider in Razor Slitting.
Crush slitting: common shafts
Crush slitting presses a rotary blade against a hardened anvil roller. Crush knives are rarely mounted on a common shaft, since even small diameter variations or shaft deflection would lead to insufficient load and failed cuts. The main exception to common shaft crush slitting is the use of rotary dies to make continuous crush cuts, but is usually limited to relatively narrow widths (16” or less).
Crush slitting: individual knife holders
Most crush slitting uses a common anvil roller, but individually positioned and loaded crush knife holders. Crush slitting blades use different profiles and radius for different web properties.

For more on crush slitting, see table and text Crush Slitting Blade Profiles from Five Factors to Consider in Choosing Score Slitting.

Shear slitting: top knives on single shaft, bottom knives on single shaft
Many low-end shear slitting systems have the top knives on a common shaft. Though this may work with some products, it hinders any significant adjustment of one critical shear slitting factor, the cant angle, the slight toe-in of the blade to ensure the knife ‘nip’ is closed at the initial overlap point. Without cant angle adjustment, shear slitting may be similar to using left-handed scissors in your right hand.

Common shaft top knives may also have problem with deflection in wider slitting processes. The shear slitting process requires lateral load to engage the knives, but many people forget the normal load that pushes perpendicular to the cut point, especially with thicker and stiffer products. Top knives on a common shaft will tend to have less beam stiffness and more deflection compared to individual knife holders on solid cross beam. To compensate for deflection, common shaft shear is often force to run at higher overlap settings, which avoids top knives jumping out of engagement, but makes sacrifices in edge quality and blade wear.

For easy to slit products, the cant angle may not be critical.

For more on shear slitting blades, see Factors to Consider in Choosing Shear Blade Profiles.

Shear slitting: symmetric top and bottom knives, each set on single shafts
Most shear slitting has top and bottom knives of differing geometry, creating detectable differences between top and bottom side shear slit edges. Some processes use matched identical top and bottom knife assemblies, sometimes called meshed knives. Meshed knives are used in diverse applications such as pasta cutting and the metal industry.
Shear slitting: bottom knives on common shafts, top knives with individual knife holders
This is the most versatile and robust shear slitting option. Bottom knives are mounted on a large diameter shaft (with diameter based on width for minimal deflection). The top knives have individual knife holders mounted on a rigid cross shaft. Each top knife includes the control for position and side loading of the top blade its corresponding bottom blade.

For more on shear slitting holders, see The Role of the Knifeholder in Productive Shear Slitting.

Shear slitting: top and bottom knives with individual knife holders.
This option closely resembles the individual top / common bottom knife system, but allows for the both knives to be repositioned as a set. This option is typically used for slitting edge trim of thicker and stiffer webs, especially at the end or a paper or film maker.

Slitting web control options Web guiding and spreading:
Web guiding is most critical on slitting process with small edge trim widths. Whether the trim is handled by a tensioning system or pneumatic take-away, the width needs to be sufficient to carry tension without breaking. The average trim width will be the input web width minus the distance between the outermost knives, divided by two. A web that is off center or wandering will create trim width variations. When these variations are too great, the web trim may fail to cut or break causing downtime and waste.

Any non-flatness of the web as it enters the slitting knives will directly create a slit width error. If the web has a hard wrinkle or crease, the slitting knives may fail to cut or create a double cut in the doubled or tripled web thickness. Many slitters include a web spreading roller just upstream of the slitting process to ensure flat, wrinkle-free web at slitting.
Entry and exit angles and span:
Wrapped slitting methods are less sensitive to entry and exit angles and spans. Shorter exit and entry spans will usually be more stable with less wrinkling or tracking problems. Wrap slitting wrap angles should be sufficient to prevent slippage of the over the wrapped roller. The entry and exit rollers should ensure the web contacts the wrapped knife or roller before the cut point and doesn’t pull the web up into the knives at the exit.

Tangential shear and razor-in-air slitting have more critical web span requirements. For razor-in-air, the spans should be short and stable. (Also, not the comments on slitting and span orientation below.)

The entry span on tangential shear is critical to present the web exactly at the shear slitting entry nip point. Entry and exit spans should be relatively short, but some length will help reduce the stress from web twisting to pass by the top knives after the cut point.

Web speed control:
Web speed control can affect slit edge quality and debris generation. Speed variations may lead to instabilities in the fracture zone, leading to poor edge quality, tear outs, or failed cuts. If web speed is too fast or too slow relative to shear knife speeds, the web may ‘crash’ or gather in the slitting zone, creating failed cuts, ragged cuts, or double cuts. Large differences in web and knife speed will create more abrasion, leading to faster knife wear and more web debris.
Web entry and exit tensioning:
Consistent web tension is critical to maintaining precise slit width tolerance. Tension variations may lead to instabilities in the fracture zone, leading to poor edge quality, tear outs, or failed cuts. Relatively high entering web tension usually improves any slitting methods. Whenever possible, tension should be high enough to pull out all center or edge slackness, so each lane of the web is under some tension. In razor slitting, sufficient entering web tension is required to create the force to reach fracture stress.

In all slitting method, high entering web tension will help ‘open’ up the slit point after fracture, reducing the contact or load on the side of the blades.

Good control of the exit web tension is also important. In slitting baggy input material, slit strands cut from baggy lanes will tend to have low or no tension after slitting. If exit tension is controlled by individual or differential rewinding, then each web can be properly tensioned. If the multiple slit strands are handled over common high traction rollers, baggy lane strands may go slack due to the traction isolating their independent tensioning.

Overall slitter web path:
For products that are sensitive to slitting debris, it is best to orient the slitting process and overall web path to avoid slitting debris falling onto the web. Razor-in-air and tangential-shear slitting should have relatively horizontal spans to allow debris to fall down away from the web, where vertical spans would drop the debris onto the incoming or exiting web. Wrap slitting (whether razor, crush, or shear) should have the knives on the bottom half of the wrapped roller or knives, allowing debris to fall away from the web. In any case, the web line layout should never have an open web passing underneath a slit point, again to avoid debris falling on the web.

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