Slitting Education and Information

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

Step 2 – Determine the Best Slitting Method(s) for Your Process

Choosing the best method for your product is based on experience as much as engineering.

Nearly everyone makes slitting decisions in their everyday lives involving the three traditional mechanical slitting methods are razor, crush, and shear. When you wake up and shave your face or legs, you are probably razor slitting. When you us a chopping knife on a cutting board or a pizza cutter, you are crush slitting. Whenever you use a pair of scissors, you are shear slitting. Every time you want to cut something, you are deciding which slitting method will be best.

If you want to cut something out of the newspaper, you may tear is out or use a pair of slitters. Both methods rely on focusing a force over a small area to exceed the paper’s fracture stress. The sharp edges on a pair of scissors make a cleaner, more precise cut than tearing between your fingers. In tearing newsprint, you will also notice the ease of tearing with the paper fiber orientation and the ragged edge of tearing across the fibers.

If you want to cut a thick slice of cheese, you will likely put away your scissors and pick up a knife. Depending on the softness of the cheese, you may find that you have problems with the cheese distorting before it fractures or smearing and adhering to the side of your knife. Using a narrower blade or the wire in a cheese cutter reduces this problem. For individually wrapped cheese slices, you might find the plastic liner allows you to cut the cheese using scissors and avoid the cheese smearing problem.

If you want to cut a pizza, you can use a knife or scissors, but you may find that both have problems with the soft toppings smearing and sticking. A rotating pizza cutter or crush knife avoids cheese smearing with its limited contact length and narrow profile. 

If you want to cut a sheet of metal, such as 10-mil thick brass shim stock, you would find it difficult to fracture with a pocket knife or crush knife. Even if you could begin a cut with a razor-style blade, you would quickly dull the blade and fail to cut. A pair of tin snips will have no problem, using the leverage and sharpness of two blades to create focused shear stresses and fracture the metal.

In each of these every day scenarios, the material properties and geometry affect the decision and performance of the three slitting options. 

When is Razor Slitting the Best Choice?How is fracture stress created?
The sharp tip of the razor blade is used to focus the force of the tensioned web, creating sufficient stress to fracture the web.
When will it fail to cut a product? 
If the force is too low or blade is too dull to create high enough stress, the web will not fracture.
What causes poor slit edge quality?
In ductile materials, the web may stretch and deform near the web edge prior to fracture, creating a wavy edge. As a razor blade dulls, the fracture zone volume will increase, creating more debris and roughness of the slit edge.
What causes debris?
A crystalline material, such as diamond, can be crack cleanly along the crystalline structure, fracturing with minimal debris. Webs are not crystalline, so cracks tend to split and cross other cracks leaving free particles (dust and debris).
What causes width variations?
If the razor blade or blade holder bends or wobbles, the cut point will shift, changing the dimension from one slit point to another. As with any slitting method, tension variations will create a width difference between tensioned and untensioned web, especially with low modulus webs and many knitted or woven products.
When is razor slitting best used?
Razor slitting is best used with thin, easy to tear, non-abrasive materials. Razor blades are inexpensive and easy to change in non-continuous operations. However, in slitting extremely long rolls, the blade may wear before the roll is finished and show degraded slit edge quality at the end of the roll.

Hard-coated razor blades, such as the diamond-like coating (DLC) used in most commercial disposable razor blades, will maintain blade sharpness longer (but also cost more).  Some operations slowly oscillate razor blades perpendicular to the moving web to distribute the blade wear over a larger error, extending blade life and slit edge quality. Some razor slitting use circular razor blades, but this technique increases cost and side contact with the web, eliminating two of the main advantages of razor slitting.

When is razor slitting a poor choice?
Razor slitting is not a good choice for materials with high toughness, faster or longer processes where heat generation or blade wear may quickly degrade slit edge quality. Where the rotation of crush or shear slitting blades provide significant convective cooling, razor slitting is sensitive to heat generation since the web-to-blade contact point is consistently in the same small area.
What are additional advantages of razor slitting?
Razor blades are purchased pre-sharpened and usually thrown away after single use. When used in a tip-slitting orientation, each blade has two or four corners. Razor blades and their holders can create the narrower slit strands than crush or shear slitting. Razor slitting is simple and inexpensive. The simple design is easy to oscillate laterally for trim oscillation or out-of-plane to reduce blade wear.

When is Crush (Score) Slitting the Best Choice

How is fracture stress created?
The web is forced through the nip point of a sharp, but radiused circular blade loaded against a hardened anvil roller. The perpendicular force creates high stresses under the focused contact area or footprint, creating sufficient stress to fracture the web.
When will it fail to cut a product?
If crush knife doesn’t create high enough stresses, due to either too little load, too large of contact area (dull knife), the web will fail to fracture. If either the knife is chipped or anvil roller is damaged, the cut will fail if the blade loses contact with the anvil roller.
What causes poor slit edge quality?
Relative to razor and shear slitting, crush slitting is a duller process, tending to create more slitting dust and a more ragged edge. Crush slitting blades are much wider than razor blades, requiring more web deformation as the blade plows through the web towards contact with the anvil roller and as the web is pushed lateral as it passed by the contact zone.
What causes debris?
Since webs are not crystalline, cracks tend to split and cross other cracks leaving free particles (dust and debris). Crush slitting has a relatively large fracture zone (compared to razor and shear slitting), tending to create more dust from the larger fracturing volume.
What causes width variations?
If the crush blade or blade holder bends or wobbles, the cut point will shift, changing the dimension from one slit point to another. As with any slitting method, tension variations will create a width difference between tensioned and untensioned web, especially with low modulus webs and many knitted or woven products.
When is crush (score) slitting best used?
Crush slitting has limited side contact, making it a good choice for products that are easy to smear or want to stick to the blade.
When is crush (score) slitting a poor choice?
Crush slitting is a poor choice for product highly sensitive to dust created at the fracture point.
What are additional advantages of crush (score) slitting?
If crush slitting is used in a controlled gap or low force mode, it can be used for control-depth cutting and scoring. Individual crush knife holders are generally narrower and can cut to narrower target widths than individual shear knife holders.

Crush knives a relatively simple to setup. Slit width is set from one side of the web (no anvil roller changes), so width changes are easier than for shear sitting.

Crush knives have a rounded tip, so crush knife blade handling is less of a safety hazard than shear knives or razor blades.

When is Shear Slitting the Best Choice

How is fracture stress created?
The top and bottom blade overlap, pinching the web at the initial overlap point. The top knife is side loaded and angled to create high stress and continuously fracture the web at this initial pinch point.
When will it fail to cut a product?
Shear slitting requires sufficient side loading and controlled closing at the initial overlap point to properly fracture the web. Shear slitting has more problems when top knives are mounted on a common shaft, increasing the likelihood of variations in side load and preventing the use of cant angle to force contact at the overlap initial contact point.
What causes poor slit edge quality?
Dull, chipped, or nicked knives will increase debris and ragged edges. Poor geometry or low side load will fail to create the sufficient stresses to fracture the web.
What causes debris?
Some debris is created at the fracture point, depending on the web thickness, knife sharpness, and control of the fracture point. Abrasion may create debris or scratching as the web passes through the overlap zone of the shear knives. Larger overlaps will create longer side contact zones and greater web/knife slippage.
What causes width variations?
The shear slitting bottom blades set the spacing between cut points. Any error in position or axial runout of the bottom blades will contribute directly to width variations. As with any slitting method, tension variations will create a width difference between tensioned and untensioned web, especially with low modulus webs and many knitted or woven products.
When is shear slitting best used?
Shear slitting is the most diverse slitting method, able to function over a wide range of materials, thicknesses, and speeds.
When is shear slitting a poor choice?
Shear slitting may have problems with materials that are thick and have high friction or adhesion when contacting the side of a blade. Shear slitting may not be a good choice for products that can damage the fine edges of the shear blades. 
What are additional advantages of shear slitting?
Shear slitting knives will last longer than razor or crush knives due to their relatively low loads (compared to crush slitting) and distributing the wear over the circular blade edges. Shear knives can be reground several times. Due to the overlap of the shear blade, shear knives mounted on a common shaft have more tolerance for knife diameter variations than common-shaft crush knives.

Slitting Methods:

1. Razor Slitting
Polyester less than 2 mils thick
Thin paper products/

Thin “household” aluminum
Many thin polymer films (PE, PP, LDPE, LLDPE, HDPE, Acetate, TCA, OPS, Nylon)
Most flexible packaging films (less than 3 mils)

2. Crush Slitting
Tissue paper
Non-wovens
Pressure sensitive adhesive tapes
Labelstock
Abrasives (sand paper, emory cloth)
Gauze
Textiles (woven and knitted)
Newsprint
Paperboard

3. Shear Slitting

Polyester greater than 2 mils thick
Precision polyester films product less than 2 mils thick
Magnetic tape
Photographic film
Most steel, aluminum, and other metal webs

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