Transforming Art into Science

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.

In wrap slitting, the web needs support as it is wrapped around the anvil section. This requires narrow grooves in the anvil for slitting, which limits the blade profiles to steep grind angles. The blades must be very thin to minimize potential slit edge damage. Tangent slitting, however, permits a much larger ­variety of blade profiles (Figure 3).

Can’t work without cant

Cant angle induces an angle between two blades to close the nip—creating the bite between the blades. For example, a pair of scissors has a cant angle that forces the two blades together to create a nip or cut point. Without a cant angle, there would be an open nip and no cut point. In this scenario, some brittle materials and low-elongation materials may fracture. However, many other materials may bend around the blades without fracturing. The goal is to have enough cant angle to create the necessary amount of bite to successfully fracture the material. Anything more is excessive, and can lead to blade chipping and increased blade wear. Suggested cant angles by material are: 0.0 to 0.25 degrees for metals, plastic sheet, non-fiberous webs, and hard and brittle materials; 0.25 to 0.50 degrees for paper products, laminates, and plastic films; 0.50 to 0.75 degrees for synthetic fiber products, materials with loosely bonded fibers, and stretchy films; and 0.75 to 1.0 degrees for fabrics, unbonded non-wovens, etc. (Figure 4).

Once the operator sets the ideal cant angle for the material, do not adjust it during the run. Changing cant angles during a production run will change the wear pattern on the blade (in effect, rounding the blade corner), making it impossible to get back to the original setting without changing blades. The cut point will shift and repeatability will rely on the art of the operator, rather than the science behind the equipment. As slit quality begins to deteriorate, the operator may try to increase cant angle to generate more bite. While this can temporarily improve slit quality, it will increase wear between the blades and result in a lower quality cut.

Watch the cut point

Overlap is the amount of penetration of the upper blade into the web line. The lower anvil ring supports the web as the upper blade plunges into the web. After shearing, the upper blade is an obstacle the web must find a way past. The greater the penetration, the larger the obstacle and the greater potential for edge damage due to rubbing against the side of the blade.

Overlap also affects the cut point location. As overlap increases, the cut point location is shifted forward, which can result in an unsupported slit (Figure 5). An unsupported slit can act as a rotary razor or burst slit instead of a true shear slit. This is prone to creating more dust and having damaged edges. A prevailing theory is that increased blade penetration will improve cut quality. Unfortunately this has detrimental effects, including slowing the rotational speed of the upper blade, which can create an under-sped nip. Recommended overlap settings are: 0.015˝ to 0.030˝ for tissue and fine papers; 0.020˝ to 0.040˝ for heavy papers and light boards; and 0.030˝ to 0.060˝ for boards.

Using the force

Side force—sometimes called side load force—is the lateral force applied between the upper blade and the lower anvil ring. The purpose is to maintain traction between the upper and lower blade, allowing the upper blade to be driven by the rotation of the lower blade. Heavier materials require more side force to maintain contact between the blades. Some operators consider forcing the upper blade and lower anvil together as much as possible to improve cut quality. However, excessive force increases blade wear and can cause the lower blade to tilt over the top of the anvil, creating an open nip with the flat side of the upper blade making contact with the sharp corner of the lower anvil ring.

In blade side load systems, overall system mass and harmonics play a significant role at higher speeds. Accuracy of the side loading is only as accurate as the active component (spring, air cylinder, electronic motor, etc.). Blade side load systems that allow the blade to tilt over the edge of the lower anvil blade will compromise slit quality unless dished blades are used. With a dished blade, the inner part of the blade is pushed in from the outer blade edge. The result is a blade that has an appearance similar to a dish or dinner plate. Recommended side force settings are: 2 to 3 lbs. for tissue and thin plastic film; 3 to 4 lbs. for communication papers; 3 to 5 lbs. for packaging laminates; 2 to 5 lbs. for non-wovens; 4 to 8 lbs. for light boards; and 6 to 10 lbs. for heavy boards.

The need for speed

Overspeed is referenced when comparing the web speed to the nip speed. The desire is to have the slitter nip speed moving faster than the web speed. It is similar to using scissors to cut wrapping paper. If the scissors are allowed to cut the wrapping paper without closing (acting like a razor cut), the result is an under-sped nip, and the scissors are likely to snag at some point. When the scissors are closed to cut the paper, the nip is moving faster than the wrapping paper and is then an over-sped nip. In shear slitting, the desire is to have the nip speed moving faster than the web speed.

While the upper blade is driven by the lower anvil ring, there is no equal match of rotational speed between the two knives. Some of the rotational force of the lower anvil ring is lost via radial friction between the two blades. The more overlap that exists, the more friction between the blades and the slower the upper blade will rotate compared to the lower anvil ring. Therefore, the lower anvil ring must rotate even faster than the web to compensate for this loss in speed caused by friction. Sometimes an under-sped slitter nip can be identified by the appearance of a “web bubble” ahead of the slitter.

Tangent slitters are better positioned to use overspeed, while wrap slitters may have problems with scuffing the material or altering the tension in the web. In most wrap systems the speed of the lower anvil ring will match that of the web. Typical overspeed guidelines are: +1 to +3 percent for foils, low elongation films, and delicate materials; +3 to +5 percent for general materials, papers, and boards; +5 to +10 percent for high elongation films, and loose fibered materials; and +10 to +100 percent for textiles, unbonded non-wovens, etc.

Finding the cut point

Slitter geometry determines the location of the cut point (Figure 6). The designs are different for wrap slitting and tangent slitting. In wrap slitting—typically used with materials that require additional support throughout the slitting process—the web is wrapped around the lower anvil section and is supported throughout the process. There is a small gap in the anvil to allow the upper knife, which is placed above the center of the shaft, to penetrate below the web line. The web material is deflected around the top blade after being slit.

In tangent systems, the ideal cut point is top dead center on the anvil ring. To achieve this position, the upper blade is “set back” from the lower anvil blade. The material may either be deflected around the top blade or under the top blade depending upon the blade profile. If the blade is too far forward, the web begins to fracture ahead of the nip point resulting in an unsupported slit. This can resemble a burst slit or rotary razor where the web is ripping instead of shearing. If the upper blade is too far back, the result is an excessive rub area between the web and the upper blade, which can decrease blade life. The slitter geometry is usually designed into the machine with an expectation for a certain overlap. Fixed knifeholder beams yield more consistency than rotating beam supports.

Converting art into science

Slitting is transformed from an art into a science by controlling the six factors of slitting so that they are consistently applied. Identify a blade profile that is well suited for the material and keep the blades sharp. Determine the best cant angle and never change the angle. Make sure the overlap is properly set across the entire web each time the material is slit. Use the appropriate amount of side force without overdoing it. Set the overspeed that is appropriate for the material, and maintain the geometry of the machine. The more these factors are able to be set and controlled, slitting will become a science and good slit quality can be achieved each time a material is run. pP

Peter Wood brings more than 10 years of experience in the converting and ­packaging industry to his position as Slitting Product Manager for Tidland Corp. Prior to joining Tidland, Wood worked for several Alcoa divisions in product ­management, engineering, sales and customer service. Wood has a B.S. in ­Mechanical Engineering, a B.S. in ­Business ­Administration, and a Master of Business Administration degree.