- June 01, 2002, Timothy J. Walker, TJWalker & Assoc. Inc.
Lines make life more difficult. Remember coloring as a child. At first it was just you, your crayons, white paper, and free expression. Life was easy. Later, you graduated to coloring books. Now you were constrained; you were supposed to stay inside the lines. Life became more difficult. How about learning to drive a car as a teenager? It was easy practicing in the parking lot, but hitting the highway and staying in your lane was another story.
The same line constraint applies to slitting web products. Slitting a homogenous wide material into narrower rolls is relatively easy. However, slitting material with lines or stripes, where the stripe-to-slit edge dimensions are critical, will make you feel the same constraints as the child with a coloring book and the teen driver on the highway.
You may have experience with precision slitting, holding width tolerances to ±0.003 in. Be warned: The same specification for slit-to-stripe width position registration is at least ten times more difficult. Where precision width slitting is a college course, precision stripe slitting is graduate-level material!
What makes stripe slitting so difficult? Tolerance stacking. Precision slitting itself is not easy. Three tolerance factors stack up against you — tensioning, flatness, and knife setup. For precision stripe slitting, you add four more tolerance factors. Let's review all seven factors important to precision stripe slitting. Each factor has variability, and controlling variability is harder for some than others.
Constant tension is needed to create constant necking into the knives and consistent width recovery when tension is removed. Tension also will affect tracking and guiding.
The web should be visibly flat entering the shear knives. Many slitters include a web-spreading device immediately upstream of the slitter knives to ensure the web is taut laterally.
Knife Setup and Performance
Knife setup spacing will dominate the resulting slit width. Knife position doesn't stop at setup. Runout, wobble, deflection, and other factors may cause the slit point to differ from the accurate setup.
The input stripes must have consistent width and spacing. The material also should have consistent thickness, modulus, and splices. The input roll should not exceed the web guide's range or rate. Any web bagginess should be pulled out easily with normal tensions. Input material variability may be the most difficult of these factors, since the input quality is usually out of your control.
The sensor must consistently detect the target stripe edge. Any sensor detection error or dead band will affect stripe-slitting error directly.
The guide's mechanics and controls should be responsive and rigid. The guide should have a range and actuation rate to match input roll quality and web speeds.
Don't assume an accurately positioned stripe at the web guide will lead to consistent stripe-to-knife alignment automatically. Just as last month's column addressed slit-to-wind tracking, many of the same factors can create guide-to-knife tracking errors. A short web path, good tension, cylindrical rollers, good traction, and good roller alignment all lead to low transport tracking error.
It is a lot of work to stay between the lines. Doing doughnuts in parking lots and doodling may be fun, but the professional artist, the NASCAR driver, and the experienced converter all understand that staying between the lines can be a profitable approach to life and slitting.
Timothy J. Walker has 20+ years of experience in web handling processes. He specializes in web handling education, process development, and production problem solving. Contact him at 314/323-6256; firstname.lastname@example.org; tjwa.com