- October 01, 2001, Claudia Hine, Senior Editor
Whatever your slitting method, blade life can be improved dramatically if you have the right knife in the right holder, properly angled, explains Keith Fordham, chief engineer at Ashe Converting Equipment, Ipswich, Suffolk, U.K.
Blade materials range from steel, the most economical, through chromium steel, chrome vanadium steel, tungsten carbide, and ceramic. “Each blade material will have benefits over another to do the same job,” says Fordham, “but invariably it's the materials being slit, the volume of material, the number of slits, and the knife setting arrangements that determine if the choice of blade material is justified. The accumulated costs resulting from downtime in changing, resharpening, and replacing blades because of a below-spec purchase make the choice an important one for consideration.”
In fact, says Tom Vanden Heuvel, VP of Valley Grinding Service and Supply, Little Chute, WI, “Tungsten carbide as well as zirconia ceramic have been making an impact as sometimes they can extend blade life 40-fold and can be resharpened.”
But hardness is just one piece of the puzzle, Vanden Heuvel says. “Impact strength, wearability, as well as hardness, all play key roles in the selection of the proper knife material. Too hard, and the knife is brittle. Not enough wearability, and the edge wears away too quickly. Not enough impact strength, and the blade can chip or break easily. Depending on the material being slit, as well as the mechanical condition of the equipment, different blade and band materials need to be chosen to ensure top performance.”
A blade that wears longer, even if it costs more, can save huge amounts of money by increasing the amount of time between blade changes, adds Jim Kivligan, key account manager, Speciality Blades, Staunton, VA. “You want a blade that lasts longer so you can increase your roll/sets. You also want the quality of the cut to last longer so you get better, more consistent cuts. Therefore, you need a razor-sharp, longer-lasting blade. Steel blades usually have a good initial sharpness, but the sharpness tends to fade quickly. Solid zirconia ceramic blades tend to keep their sharpness much, much longer. The zirconia should be hipped (hot isostatic processed) for longer lasting, better wear characteristics.”
Solid ceramic blades “absolutely shine,” adds Kivlighan, when slitting thin stretchy film such as LDPE.
“As technology has become more sophisticated,” Vanden Heuvel explains, “so has steel manufacturing. Many steels that were not available a short time ago are now readily available to knife manufacturers. Powdered metals have made a major impact on knife life as well as a multitude of different grades of tungsten carbide used to manufacture bottom slitters. For converters that use industrial razors for their slitting needs, tungsten carbide as well as zirconia ceramic have changed the way these tools are being looked at.”
He continues, “Another major change is the availability of abrasives and machinery that allow manufacturers to produce finishes that were not available a few years ago. Using the same equipment with the same slitters, a smoother finish will produce more and better product than blades and bands done on more traditional equipment.”
Vanden Heuvel says the Ra (roughness average) finish of the knives should be very smooth to ensure that no undue dusting is produced. This Ra should be 8 or less.
“The further development of slitting technology lies in the hands of metallurgists,” Fordham says. “Stronger, cheaper, more readily available exotic materials will filter through from high-tech industries to general engineering. This will improve blade life and give sharper cutting edges for longer periods of time.”
Fordham predicts other technologies — such as laser, water jetting, and ultrasonic — are all very expensive for multiple slits at the moment but will become more affordable over time. “The materials to make these more available, such as gemstone water jet nozzles, high-power laser sources, and horn/anvil tooling, will make the difference in the future.”
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