Consistent repeatability is a critical goal in slitting. However, if slitting is approached as a work of art, where uniqueness is valued, such repeatability will be extremely labor-intensive, if not impossible. Instead, slitting should be a product of science, where the value is in understanding the parameters and being able to replicate the results again and again. Variability and uniqueness in the final product is not valued in science and shouldn’t be valued in the world of slitting.
Knifeholders offer direct control over six critical factors—blade sharpness/profile, cant angle, overlap, side load force, overspeed, and slitter geometry—limiting variation and improving slit edge quality in the final product. The more these variables are understood and controlled, the better operators are able to create consistency in slit quality and turn slitting from less of an art into more of a science.
Understanding the six critical factors in slitting
Blade sharpness plays a critical role in edge quality controlling dust. As blade sharpness is lost, an open nip forms between the blades. The nip is the place of intersection or contact of two contiguous surfaces. In slitting, the nip is where the upper blade contacts the lower anvil ring creating the cut point. Nip speed is the rotational surface speed of the upper blade (
Figure 1). An open nip results in the material ripping, which frees more particles and increases dust. Wear can be caused by blade-to-blade friction and web-to-blade friction, and as the blades wear, the contact point between the blades shifts, moving the nip point. The impact against a blunt tip or an open nip allows dust to form.
Blade profiles impact the way the web moves past the knives after being slit. Shear slitting uses two blades to shear the material. These blades are often referred to as the upper blade and the lower anvil ring. In shear slitting there is a nip point where one side of the web is supported by the lower anvil ring while the other side is left unsupported and sheared, creating a fracture in the material. Immediately after shearing, the web has to move past the upper blade, which is positioned as an obstacle in its path. The web will move around the upper blade, under it, or in a combination of the two paths. It is the blade profile that determines this path (
Figure 2). For example, a steep grind angle such as 35 to 60 degrees will push the material to the side. As the web moves around the blade, the web will rub against the side of the upper blade, creating friction that results in blade wear. Minimizing the blade overlap and using proper geometry will minimize the rub area. As a contrast, a low grind angle, such as 5 to 15 degrees, will push the material down causing the web to move under the blade.
