Knowing the symptoms of unnecessary blade wear can mean the difference of thousands of dollars a year in blade replacements, blade sharpening expenses, and downtime incurred from removing and replacing blades. By identifying simple wear patterns on your upper and lower knife blades, you can find clues that will help you optimize the life of your blade and reduce your maintenance budget.
The first step is to determine what a healthy blade looks like. Healthy blade wear can be identified in the wear band as a smooth, straight, plane surface that is normal and uniform. Any other type of wear will indicate a problem. There are typically four major types of unnecessary wear issues: elliptical, concave, burrs or chips in wear, and smooth or rounded blades that are too dull for a proper slit. Each type of blade wear has its own cause and solution.
Elliptical wear
An elliptical wear pattern on the upper or lower blade suggests extreme run-out of that blade. The run-out is caused by an inconsistent contact point between the upper and lower blade as the lower blade oscillates back and forth or up and down. All slitter blades have some level of run-out that contributes to blade wear. A small amount of run-out may be acceptable at slower speeds, but causes more aggressive blade wear at higher speeds.
Solution—Make sure your upper and lower blades are attached in a way that does not allow them to oscillate from side-to-side or up and down. You can maintain the ideal geometric settings of the upper blade by incorporating an in-house sharpening system that keeps the upper blade in the cartridge during the sharpening process, which reduces variability of remounting the blade.
Concave wear
When you see a concave wear ring around the blade, it is typically caused by excessive side-load established during operator set up, vibration from the speed of the material running between the blades, or from excessive run-out. Side-load, the most common cause, is the amount of force used to get the upper and lower blade to come into contact. Side-load is often determined by operator instinct, where the operator has a tendency to increase side-load whenever a slitting issue is encountered or an adjustment is desired. This can lead to excessive side-load, which actually makes your slit quality worse and degrades blade life.
Solution—Check your side-load. Here's a general rule: apply just enough force so that the upper blade will rotate because of the force applied to the lower blade. Anything more than that is excessive. In packaging, we typically see 3-5 lbs. of side force.
Chipped, burred edges
By investigating the point of the blade for imperfections such as burrs or chips, you can quickly find ways to improve blade wear. Chips or burrs can be caused by improper cant angle, highly acute grind angles, or excessive sideload. Simple adjustments to these settings during set up can have quick, positive impacts.
Cant angle is the process of inducing an angle between the upper and lower blades so that the cut point is at its most effective. An overly aggressive cant angle will cause chipping between the blades and create a burr (displaced metal) around upper and lower slitter edges. Angles greater than one degree are especially prone to chipping and burring.
Acute grind angles are also prone to chipping and burring because less metal is present at the extreme tip edge and the contact pressures between the slitters cannot be supported.
Solution—Try to use as little cant angle as possible to achieve the nip point, because the greater the angle, the shorter the blade life. In packaging (films and laminates), the ideal angle is typically between 0.25 to 0.50 degrees.
Check your grind angle against this basic rule: the smaller the angle, the stronger the blade (and thus, the longer the blade life). The ideal grind angle is dependent upon how much your material can give, or how susceptible to damage it is.
Though you may feel like experimenting with different blade materials to correct these patterns of excessive wear, the typical causes are operational.
Smooth, blunt blade edges
On the upper blade, a smoothly polished radius (rather than a crisp edge at the blade tip) indicates excessive wear on from the material. This wear is typically hard to avoid, especially when working with recycled content that often contains "grit" that will grind the blade down.
The cause for blunt edges is typically excessive overlap. The further the upper blade projects into the web, the more it is subject to excessive erosion from the web. When we hear that someone is struggling with slit quality, this is the first factor to check. An operator will often increase overlap to ensure that the blades have good contact. Not only will this have a negative effect on slit quality, too much overlap decreases blade rotational speed and increases friction (or traction) between the upper and lower blade, slowing the upper blade in relation to the lower blade and subjecting the upper blade to increasingly more abrasion from the web.
With pneumatic knifeholders, overlap is usually determined by the operator, who visually adjusts the blade depth. New electronic knifeholders can automatically determine the proper overlap with every run.
Beyond overlap, take a look at the blade. Some common factors include:
• Grind angle—A grind angle of 45 degrees or sharper is very vulnerable to abrasion from the web because there is less metal at the very tip of the blade.
• Blade alloy—Consider using a more resistant alloy blade such as those containing 12 percent chromium (known as D-2), Tungsten, or Vanadium content steels. The right alloy can also allow set-up grind angles to last longer.
• Surface finish—An abrasive edge will blunt a coarsely ground upper blade more quickly than a smoothly finished blade. A good test to determine if the blade edge is truly free of grinding-induced irregularities is to wipe a cotton ball around the ground edge. Cotton fibers will hang up on projections caused by rough grinding.
This article provided by the Tidland Slitting Group and Kasey Morales, Maxcess brand manager.
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