A look at razor slitting – Slitting Technology, part 1 – Column
Author: William E. Hawkins
One of the more common problems converters encounter during razor knife slitting on thermoplastic films is the thickening of the web material in the area of the cut. This occurs because the blade blocks the flow of the web, much like a protruding rock blocks and divides the flow of water in a stream. The stream thickness increases in front of and near the sides of the rock due to blocked and retarded flow.
During razor slitting of a plastic web, flow is retarded directly in front of the angled cutting surfaces of the blade. The retarded flow increases the thickness of the web edges, and the amount of thickness increase is dependent on how much resistance the blade presents to the web flow.
Thicker edges significantly increase the winding roll edge radius and are responsible for many winding defects. For example, the increased roll radius at the edges prevents the lay-on roll from applying sufficient contact force on the rest of the roll to properly exclude boundary air, which, in the case of films less than 2 mils thick and rolls wider than about 3 in., will result in machine-direction (MD) wrinkles in the wound roll.
Another problem caused by increased edge thickness on very smooth films is the generation of slip dimple distortion (resembling small bumps) on the roll edges due to excessive pressure in these locations. This distortion often results in the roll – and possibly the adjacent rolls – being scrapped because of single-sheet distortion in the wound rolls.
Still another problem that raised edges cause is the wearing of grooves in the lay-on roll surface. Wear is accelerated by the high pressure applied at these locations. While this wear may actually help the lay-on roll maintain contact with the rest of the roll, it will prevent using the lay-on roll for wider winding rolls. MD grooves in the lay-on roll will entrap boundary air, which distorts the web in a narrow balloon band at each site on the lay-on roll and results in permanent single-sheet distortion in the winding roll.
Razor slitting also generates debris during the slitting process. As the web is pulled through the knife, strands of polymer are torn from the web matrix by the blade-plowing action and are then stretched before shearing or breaking at their anchor points in the matrix. These free pieces then tend to curl into tiny balls as they are thrown free by elastic forces when the breaks occur. This debris often has static charge on it due to relative motion with the knife and adjacent matrix material. And, it tends to stick where it lands, near the slit edge. Rolls wound from smooth webs often have slip dimples near the slit edges due to slitter debris even when edge thickening is minimal. Slitter debris is not easy to remove from the web. Hence, prevention of debris by employing good web handling techniques during razor slitting is important for all converters who slit webs.
One very important parameter is web tension at the slitter knife. Generally, the web should be at less than 10% of yield stress in the slitter zone. Low web tension helps reduce the amount the web is elongated as it is pulled past the blade. This is because the blade drag force tension is added to the overall web tension in the slitter zone. Excessive web tension in the slitter zone will result in edges that are stretched beyond their elastic range, and the slit rolls will exhibit a wavy or waffle appearance. Thus, web tension should be isolated on each side of the slitter zone to provide optimum tension for razor slitting.
Razor slitting is best accomplished by slitting in air between close transverse support rollers. An ideal slitting zone is depicted in the figure below.
William E. Hawkins has 30 plus years of process and equipment development in web handling, including experience on all types of converting equipment. He specializes in thin web applications. Contact him at 740/4745840; fax: 740/474-3148.