Choosing the best slitting techniques: a better understanding of each slitting method will yield better results, no matter what you're slitting.
Converters must choose the slitting method for each particular product to be slit. It is a three-step procedure:
* Identify the characteristics of each slitting method.
* Identify the characteristics of the material to be slit.
* Match the material to the best slitting method.
The three most common slitting methods used today are shear, crush (score), and razor (see above). Each method separates web materials using distinctly different principles. We must understand these principles before determining which method is preferred for a given material.
SLITTING METHOD CHARACTERISTICS
Razor slitting is the simplest and cheapest method. It can be easily adapted to almost any machine, in almost any location. It is potentially the cleanest method of slitting, assuming the appropriate materials are being slit. A "cutting" or "slicing" action is created by pulling the material past the stationary blade. The resultant edge depends on the characteristics of the material thickness, density, rigidity, plasticity, coating, bonding, and other factors.
At issue are blade life, slit edge quality, and safety. Since a very. small portion of the razor blade is engaged in the slitting zone, the extremely thin edge wears rapidly. The edge frequently fails at critical times, causing downtime in the process lines. Edge quality for thicker, denser materials may display a typical "raised edge," surface coatings may be disrupted, and tearing or splitting may propagate from the cutting point. Safety is a constant problem--exposed razors and their handling during replacement produces frequent accidents.
Crush (score) slitting is a common method of separating the web. It is more complex than razor slitting, requiring a hardened anvil roll and wrapping the web over the roll. The slit occurs in the crushing nip between the anvil and the slitting wheel. Changing slit widths is relatively easy, since only the slitter is repositioned over the fixed anvil roll. It is the dustiest of the slitting methods, delivering the poorest edge quality.
Issues include blade life, slit edge quality, and slitting speed. Since the blade exerts considerable force against the anvil roll, the metals suffer repetitive stress cycle failure. The slitter blade edge begins to chip, the anvil surface becomes grooved, and slit quality deteriorates. Increasing the nip pressure to improve slit quality results in even faster metal failure. Blade life is therefore fixed by the parameters of metal durability, nip stress, and number of stress cycles (actual number of slitter revolutions at any given nip stress). Mills may need to experiment to find an optimum profile.
Slit edge quality is variable and depends on the material being slit, blade edge profile, blade edge finish, and anvil smoothness. Slitting is a "crushing" action in the nip between the slitter blade and the anvil surface. Extremely dense or thick materials may need nip forces beyond the yield strength of the blade steel, making crush (score) slitting impractical.
Slitting speed is limited by two factors: the critical speed of the anvil roll, and the ability of the blade steel to absorb high repetitive stress cycle rates. Increasing the diameters of the anvil rolls and slitter wheels, where possible, is usually the only solution.
Shear slitting is the most versatile method, and can accommodate a wider variety of materials than any other method. A true shear stress is created within the material as it passes through the nip between two rotating disks (the upper blade and the anvil ring, or counter blade). The web path through the slitters may be either wrap or tangent; each has advantages and disadvantages. When the shearing nip is properly configured, however, the slit quality is unsurpassed.
At issue are the complexity of changing the slit width, precision of the nip configuration, and blade wear. When changing slitter positions, both the lower and upper slitters must be repositioned, and the nip precisely reestablished. Operator skill and/or knifeholder design become important repositioning factors. Automating the process of slit changing eliminates the operator skill variable.
Establishing the correct nip configuration is more complex than with either razor or crush (score) methods, and is not generally well understood. Nip configuration also includes factors such as web path (wrap vs. shear), shear (cant) angle, blade overlap, and blade edge profile. Good design in the planning stage will consider such factors as web path, blade diameters, slitter table geometry, nip placement, slitter overspeed, and trim removal path. Initial installation must be precise.
Shear slitting uses two blades rotating in contact with each other, and is a source of blade wear, although the amount of wear is far less than in crush (score) slitting. Slitter system stability and proper operator set-up have profound effects on this inter-blade wear. A second source of blade wear is edge abrasion caused by the material being slit. Changing the blade profile and/or using more abrasion resistant steels reduces abrasion wear.
Today, we have a wider variety of flexible web products than ever, and the future promises even greater variety. To effectively slit today's webs, we must identify the dominant properties of any given web, then select the optimum slitting method. Web properties include:
Caliper--how thick or thin is the web? This will determine what sort of blade profile to use. How will the caliper affect slitter speed?
Density--what is its density? Low density materials "compress" in the slitting nip. How will this influence edge distortion? High density materials may benefit from radically different blade profiles.
Elongation--does the material stretch? How critical is nip geometry with high elongation materials? Will the material recover after being slit?
Stiffness--it seems obvious that an extremely stiff material will have a conflict with a slitter blade. What can we do about it? What tactic do we use on low stiffness webs?
Tensile--metals are at one end of the tensile scale, polyethylene is at the other end. What are the best blade grind angles for each?
Abrasiveness--this is what wears out the extreme tip of the slitter blade. What options are available to compensate for abrasiveness?
Compressibility--does the material recover from compression in the slitting nip? If not, how can we minimize edge distortion?
All web materials possess all of these properties to some degree. To determine the optimum slitting method, technicians must identify the dominant property and design the slitting system accordingly.
MATCHING MATERIAL TO METHOD
There are some general guidelines for making the final match:
* It may be possible to use razor slitters successfully if the material has low values in caliper, density, elongation, tensile, and abrasiveness.
* It may be possible to use crush (score) slitters successfully if the material has low values in caliper, density, stiffness, and tensile; and high values in elongation, abrasiveness, and compressibility.
* It is usually possible to use shear slitters successfully for any material that can be classified as a "flexible web," regardless of the properties listed above.
The three slitting systems discussed--razor, crush (score), and shear--represent the majority of slitting methods used by the flexible web converting industry. While razor slitting is low cost, it is the most limited. The crush (score) slitter is very common, but frequently misapplied due to its deceptively simple demands on operators during slit size changes. The shear slitting method offers the greatest versatility and productivity, but requires greater precision and operator skill.
IN THIS ARTICLE, YOU WILL LEARN:
* Strengths and limitations of the most popular slitting methods
* Critical web material characteristics to consider before slitting
* How to make the match that brings the best results
* For books and CD-ROM materials about slitting and converting, visit www.tappi.org and click on TAPPI Press.
Reinhold Schable is applications technology manager at Tidland Corporation, Camas, Washington, USA. He has more then 35years experience the paper finishing and converting industry. Contact him by e-mail at firstname.lastname@example.org
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|Publication:||Solutions - for People, Processes and Paper|
|Date:||May 1, 2002|
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