By David Rumsom
The obvious major web-slitting production objective is to maintain meeting acceptable finished roll quality specifications for the longest possible time. This requires knowledge of the slitting process relating to the web material’s characteristic, the web fracturing method, the tooling in use, the knife positioning geometric setup requirements, and controlling the dynamic forces in play. A partial overview follows.
The three more common web-slitting methods are razor slitting, crush cutting, and shear slitting. These three fracture webs in different ways.
Razor slitting creates a tensile stress fracture by placing its very sharp beveled edge into a moving web to fracture the material as it is pulled into it.
Acceptable blade production life depends on the material the blade is made from or coating it has (see note), the amount of dynamic contact force developed from the web material’s density, toughness, and running speed during contact. (Note: I’ve seen a slitting operation where diamond-coated razor blades extended razor slitting life from one day to six months.)
The blade’s useful non-stop slitting life can be extended with a setup technique that slowly relocates the very sharp cutting edge, up and down, against the web. The oncoming web being fractured can be unsupported at the blade contact point or slightly supported over a motor-driven grooved roll.
Crush cutting (also called score slitting) fractures a web by a compressive force. A blunt knife tip is forced through the web to contact a rotating anvil roll. The amount of force required depends, again, on the web material’s toughness and thickness. Available knife holder force levels can vary from approximately 50 lbs. to 250 lbs. Knife bluntness level choice depends on the quality of finished cut edge required. It is extremely important that the knife engagement to anvil roll never has high-speed shock impact, especially with small blunt-tipped knives. Major shut down results can happen (see Figure 1).
The most complex web-slitting method, shear slitting, creates a shear stress tearing-type fracture with two sharp rotating knife edges. There are minimally 18 critical operating factors to be aware of that can affect how long acceptable finished roll quality will last (see Figure 2).
During web fracturing, there are two knives in side-to-side contact: the bottom knife and top knife. The bottom knife is motor-driven with the web in contact with its circumference. The knife circumference contact lifts the web slightly to establish a firm location for where the two knives meet the web—the cut point. The web path can be tangential or wrapped over the bottom knife.
Shear slitting knife life also depends on the material it is made from, the web material’s toughness and the amount of knife contact force friction. Proper control of the amount of knife contact force and rotational speed is very important to establish the longest knife life. The top knife is non-driven for most materials but can be driven for some web materials that require a very clean-fractured web edge.
If your slitting process quality run time lessens, you might seek more knowledge to review your operation’s procedures. One of the first things you might consider is the material your knives are made from (see Figure 3). █
Dave Rumson is a slitting educator and consultant. He can be reached at 860-256-5658, drumson@maine.rr.com, or www.drumson.wordpress.com.
