By Dave Rumson, slitting educator and consultant

Shear Slitting is one of three common slitting methods, and all three operate with a common challenge: the Web Material. The material’s different physical characteristics – thickness, density, toughness, etc. – affect the types of defects that can occur. In addition, winding and slitting processes provide their own challenges, including knife or blade life, the type of operation and finished roll metrics. In this article, 10 web defects that can be caused by slitting and winding factors are discussed.

In 2013, Duane Smith of Davis-Standard asked me to work with Reiny Schable, the well-known slitting expert from Maxcess-International Tidland fame, to help organize the Slitting Process section of the then to-be-published “The Ultimate Roll and Web Defect Troubleshooting Guide.” Because there were a lot of different roll and web defect terms used by web producers, converters and their customers throughout the converting industry, Smith, the program director, rightly felt a group of roll-making industry experts could organize and define these roll defects with causes and potential remedies. This troubleshooting detail then would help process personnel find the cause of production defects more quickly.

Thirty-four other experienced industry professionals also contributed to different web and roll-making sections of this document. The finished project was published by the Technical Association of the Pulp and Paper Industry, The TAPPI Press. Ultimately, pun intended, 254 roll and web defects were identified. I do strongly recommend this book to all roll-making companies.

Schable and I found 18 web defects that can be caused by slitting and winding geometric and dynamic factors. This article will present 10 of those 18.

Slitting and winding processes

The three more common slitting methods are Razor Slitting, Crush/Score Cutting and Shear Slitting. They all operate with a common challenge, the Web Material. And there are all kinds of web materials with many different physical characteristics. One must consider the material’s thickness, density and toughness. Is it soft or hard? How much force is required to slit it? How brittle or stiff is it? Do burrs and jagged edges develop when it’s fractured? Does it stretch easily and is the stretch permanent or does it have a memory to return it to its original shape? Is the material coated with something harder than the knife to negatively affect knife life? Is an adhesive present that can gum up the slitting and winding process?

Besides the web material, there are two process factors that play a major role in roll quality: the Winding Process and the Slitting Process. In the Winding Process, knife or blade life varies with the roll-making process. Is the process a roll-to-roll start/stop operation, or is it a 24/7 non-stop operation with automatic web cut to core transfer? What are the finished roll widths, diameters and weights?

Winders are designed and manufactured to handle the dynamic challenges of running a material at the required web speed and web tension to meet finished roll specifications. If the operator is told to increase the web speed for higher product throughput, then both slitting and roll-building quality dynamics can change. Winding different materials usually means different winding tensions to maintain web stability.

How often does one need to change slit widths or roll material? Well…gotta run a new material? Darn sales department! Working in the “cleanroom” world of roll making? A very special place for sure. And some processes may have an explosive potential that bring extremely special needs.

The Slitting Process usually has some unique and different factors when Razor, Crush or Shear Slitting. The number of cuts, minimum slit widths, required slit-width accuracy and slit-edge quality are four of these factors. Then, is there a better knife-life metal or a better profile for a cleaner slit web edge? Is there a better size to run faster? Are knife holders manually set or can operators look into getting a semi-automatic or fully automatic positioning system? And, what do operators mean when they say the trim might break and shut the process down? What’s involved with that?

And there’s more… but, enough of that for now. Let’s look at 10 of 18 Shear Slitting defects found in “The Ultimate Roll and Web Defect Troubleshooting Guide.”

Slitter scratch line

A Shear Slitting marking or scuffing line, caused by excessive contact of the Top or Bottom Knife, can be seen on either the upper web surface or the lower web surface (see Figure 1). The Top Knife’s outer face chamfered edge, opposite the Cut Point Bottom Knife contact face, can cause this defect on the upper web surface if there is excessive knife Overlap Setting. It’s important to limit the amount of Overlap Setting to only what’s needed and to ensure that the impacting edge be chamfered properly.

The Bottom Knife Scratch Line is created on the lower web surface by the non-slitting edge opposite the sharp slitting edge. Too much web wrapping over the Bottom Knife and excessive Bottom Knife over-speed can cause this defect. Minimal over-speed and web wrapping over the Bottom Knife is important. It also is helpful to have a very smooth knife surface finish (eight or lower RMS) where the web meets these knives.

Cracked edge

A Cracked Web Edge usually is created by a lateral stress tearing of the web during the slitting process. It typically has a short length but can cause web breaks or turned edges during further operations (see Figure 1). Causes include uncontrolled trim removal that doesn’t prevent trim flapping at the Cut Point, excessive Top Knife overlap, knife under-speed and lateral stress-developing spreader bars.

Do not let the trim flutter after fracturing and prevent it from being pulled diagonally away from the Cut Point. Also, reduce the Top Knife overlap setting and make sure there’s sufficient knife over-speed. If the spreader bar is causing web lateral stress at the cut point, have a talk with an Association for Roll-to-Roll Converters member company for potential improvement.

Fuzzy edge

Roll faces with Fuzzy Edges have a high percentage of torn fibers rather than being cleanly fractured (see Figure 1).
There are at least seven causes for the defect: dull knife edges, knife disengagement, insufficient Shear Angle, incorrect knife mounting geometry, excessive knife Overlap, low Bottom Knife over-speed with Top Knife rotation that is too slow and excessive Bottom Knife axial run-out that causes high-frequency Top Knife engagement and disengagement with the Bottom Knife.

To counter these issues, know the top-knife life and schedule resharpening. Increase knife contact pressure. Ensure the Shear Angle is positive. Double-check for excessive knife Overlap and Bottom Knife radial run-out that can cause Top and Bottom Knife disengagement.

Slitting dust in a roll

Fine particles of web material are developed and deposited on the web and wound into the finished roll (see Figure 1).

There are 10 common reasons for excessive dust creation: dull knives, excessive knife Overlap, incorrect Top Knife bevel or shear angle, excessive Top Knife over-speed or Bottom Knife under-speed, rough knife surface finishes, worn slitter bearings, moving the web over a stationary surface, loss of Top and Bottom Knife contact due to excessive run-out or vibration, incorrect knifeholder mounting location and web or trim strip flutter.

Top Knives develop wear bands from operating contact. These must be totally removed during resharpening. Other suggestions include reducing the amount of knife overlap and making sure Top Knives have the correct bevel and are set up at the correct shear angle. Check Bottom Knife over-speed for the proper web fracturing. The Top Knife surface and cutting edge must have a fine finish to reduce dust; ensure a maximum eight RMS finish. With the machine power off, check non-driven roller bearings by hand for freewheeling or to see if they have detrimental staggered rotation.

Replace any non-rotating web-support devices with rotating devices. Check Bottom Knife axial and radial run-out for 0.002 in. to 0.004 in. (0.05 to 0.1 mm) accuracy and check knifeholder integrity for side-to-side looseness. Check for Bottom Knife web support where the Top Knife engages the web for a proper Cut Point. Avoid any long, unsupported web or trim to prevent fluttering through the slitting zone and ensure the Bottom Knives penetrate up and into the web line minimally one-half the intended Top Knife overlap.

Slitter rings

Wobbling Bottom Knives form a cambered edge to a roll’s web that has a sine wave shape when the roll is unwound. When wound into a finished roll, this cambered edge creates a concentric ring-shaped pattern on the roll face (see Figure 1).

The slitter-related causes of these rings are Bottom Knife run-out in excess of 0.002 in., to 0.004 in. (0.05 to 0.1 mm) due to non-concentric knife resharpening or having a sloppy fit to a shaft or mounting hub. Very narrow Bottom Knives may be prone to tilt on a shaft when locked in place, or a mounting shaft may develop a slight bend to create the wobbling. Other non-slitting Slitter Ring causes could include varying web tension levels, excessive bowed-roll cross-machine (CD) forces or machine roller misalignment.

Steps to take include making sure the mounting surfaces are clean and securing screws are tightened evenly. Also, check to find any bend in the mounting shaft.

Slitter skip

A slitter skip can be described as a small, incomplete web-fracture area located erratically along the web length (see Figure 2). Common causes are dull knife-cutting edges, insufficient knife side-loading or insufficient shear angle.

So, know when to have the knives resharpened, make higher knife-contact forces and look into how to possibly increase the shear angle on the Top Knife holder.

Rippled edge

This is a web slit edge condition where the web material has been distorted enough by the Top Knife physical interference to cause spaced, curved ripples to appear on the roll face. These also are called a Stretched Edge or a Puckered Edge (see Figure 2). The key factor here is the web material’s elastic limit when fractured. If it is stretched above the limit, it won’t return to its original length and the ripple will remain on the roll face.

The first potential cause is excessive Top Knife overlap that increases web-to-Top Knife contact distance. Secondly, this often results from the introduction of a new web material to the slitting system that the machine was not designed to handle. Thirdly, the Bottom Knife diameters may be too small and create severe compound bending of the web.

The first potential corrective action would be to try reducing the top knife overlap setting. The second action might be to contact the machine manufacturer about a possible retrofitting to meet the new web’s elastic limit when slitting, not an exciting action for sure relating to potential cost.

Slitter edge curl

This defect is more commonly seen on metal webs and gives the slit edge a continuous curly look, which makes the roll face look shaggy. It’s also called a Wavy Edge or Shaggy Edge roll (see Figure 2).

Some causes include excessive Top Knife overlap, excessive web tension and an incorrect Top Knife blade bevel.

Remedies are to reduce the amount of overlap and use the appropriate knife bevel. Most flexible web products slit well with 0.030 in. to 0.040 in. (0.762 mm to 0.1.016 mm) overlap settings.

Also check the before slitter spreader roll orientation and alignment of the web support components. These might require hiring a roller alignment company to check it out.

Slitter picks

This slit edge defect consists of small particles of web material protruding out of the slit edge in the horizontal plane of the roll face (see Figure 2).

The more common cause is having a chipped Top Knife cutting edge that fails to totally fracture the web. Another is a two piece Bottom Knife that’s not accurately assembled.

A cotton ball or swab slid over the full cutting edge of the knives can quickly identify these problems.

Flared edge

This common defect shows the outside edge of the finished roll to be larger in diameter than the rest of the roll body (see Figure 2).

This happens from elongation or distortion of the slit edge during the fracturing process. The material is stretched or distorted to increase the effective length or thickness. When wound into the roll the edges become larger in diameter. It’s more common when slitting foils, films and laminated webs.

When slitting with Razor Blades, be more active to ensure the cutting edge is always sharp. Blade oscillation is a good procedure. Crush Knives with a smaller tip radius or increased grind angle can work best. Shear Top Knives with an increased shear angle or bevel and less overlap setting work well. A near super Top Knife surface finish could be considered.

Lastly, start a machine slitting defect log book with dates, causes, remedies and photos for future issue quick responses. 

Dave Rumson has 15 years of experience as a slitting educator/consultant and 35+ years of roll/shaft handling, unwind/rewind equipment and web-slitting experience. The past 22 years include 10 years with Dienes USA and Sigmala, Ltd., the latter having developed a computer-/servo-motor-controlled shear knifeholder. For almost 30 years, Dave has developed and conducted technical presentations for CEMA, ARC (formerly AIMCAL) and TAPPI. He currently serves on the ARC Technical Advisory Panel and its Web Handling Committee, and he writes the “Cut Points” Q&A column for Converting Quarterly. Dave also manages the 1,500-member Slitting Community Group on LinkedIn. He can be reached at 860-256-5658, email: drumson@maine.rr.com, www.drumson.wordpress.com.