The two more common Shear Top Knife types used in the converting industry are the Flat Machined and the Dished. There are variations in the shape and the metal from which they are made. Considerations for the choice include the web materials being slit, how fast the web is going to run, how much knife side-load force is needed and how much web disruption is acceptable to your customer.
Differences and similarities
Flat Machined Knives are cut from raw stock and typically are used for higher-web-speed operations, or high-density material slitting. Paper mills and their finishing departments almost always use high-quality steel Flat Knives.
Dished Knives usually are thinner than Flat Knives and can be stamped in high-volume production runs; therefore, Dished Knives are less expensive. They are well suited for medium and slower web speeds with lighter materials, such as films, foils, nonwovens, light to medium paper and more.
Both Knife types commonly have their circumferential edges machined with 1, 2 or 3 bevels with a thickness relief area. The purpose of a bevel is to present a sharp, thin cutting edge to the web and minimize dust generation and web deformation from web-to-knife side contact. The angles of these bevels are crucial for some applications (see Figure 1).
A third available Shear Top Knife design is the much thicker Wide Rim Knife with no side bevels and a very slight cutting bevel; 1° to 3° across the full knife O.D. width (see Figure 2).
This knife is very effective with coated materials and fractured edges requiring minimum web deformation. After fracture, the web travels under the top knife, avoiding any side contact. The cut after slitting practically mirrors the Flat Bottom Knife’s opposite cut that travels over the bottom knife O.D.
Alloy composition
All these knives typically are made from common steel composed of varying alloys. Depending on which alloys are present, various performance characteristics are noted. For instance, the amount of chromium in a knife determines how much wear resistance it has. Also, the cost to manufacture the knife can vary based on how easily it can be machined and how readily available the material is.
All knife metals are heat-treated to high hardness levels. Ovens will raise the knives to an Rc-65 hardness and then use a controlled annealing process to lower the hardness back to Rc-60 to Rc-62.5. They must be tempered properly to lessen how brittle they become from the higher hardness level.
With hardness levels and knife force loads being equal, it is the alloy content that determines why one knife lasts longer than others. For most applications, there are four metals that fit most shear slitting applications: 52100, D2, M2 and CPM-10V.
Final thoughts on cost and performance
The better the knife metal, the higher the cost. Companies must weigh the increased knife performance and longer life against the higher upfront costs. Typically, with high productivity and quality standards in finely controlled slitting operations, the higher upfront costs translate into longer, more efficient production runs. One caution to note is that as wear resistance increases, shock or impact resistance decreases. Proper knife contact force is important.
Finally, don’t forget to track the slitting setup and performance data with your Slitting Log Book.
Dave Rumson
860-256-5658;
drumson@maine.rr.com
ARC Member, ARC TV Presenter,
R2R Presenter, Converting
School Educator