A Recipe for Scratching

This month we're having another session of “Do the Opposite.” I'm going to tell you how to make great scratches. The solution to stopping scratches is to “Do the Opposite.” Eliminate these factors, even just one, and your scratches will be gone.

There are five prerequisites to create a scratch or abrasive wear. All five are required to create the scratch. Let's review them.

Contact

The web must be touching something — roller, any nonmoving equipment, or itself — in the winding or unwinding roll.

Do the opposite: If you don't touch the web, you can't scratch it. Eliminate unnecessary rollers. Use air turns or air floatation nozzles in place of rollers. Use air or fluid to fully lubricate the web/roller interface. For narrow and stiff webs, use an undercut or dumbbell-shaped roller that makes contact only at the web edges. Use a tenter or edge nips to hold the web.

Force

The web must be pushing against the scratch-causing surface with sufficient force.

Do the opposite: The normal load or pressure is created by the tension web pull over a radius. Reduce pressure of contact by decreasing web tension or increasing roller radius. Reduce tension in a scratch-sensitive area by reducing the roller diameter in that area. For example, if the coated center of a web is most important, machine a profile into all coated-side contact rollers to have smaller diameter in the middle and carry most of the web tension at the uncoated edges.

Relative Motion

Two surfaces with matched surface speeds may cause a gouge or pick out, but they are unlikely to cause a scratch.

Do the opposite: Ensure the traction available is greater than the traction demand. Increase available traction with larger wrap angle, high tension, and higher coefficient of traction. Prevent lubrication by increasing roughness, increasing porosity, or increasing surface texture or grooving. Decrease the traction demand with low drag bearing performance, reducing roller inertia, minimizing acceleration rates, and reducing tension differentials across driven rollers.

Stress Concentration

Solid materials react to stress or force exerted over an area. High stress, which is easier to get when the area is small, is needed to cause abrading and scratching. Debris, burrs, or other surface imperfections act to focus normal forces as concentrators of available forces into high stress.

Do the opposite: Eliminate dust and debris particles from the incoming web, roller surfaces, and process environment. Specify and inspect the incoming web for cleanliness. Prevent roller slippage and associated debris generation. Design processes and equipment for cleanliness by eliminating debris sources or isolating the web from them. Ensure roller surfaces are even and free of burrs.

Relative Hardness

When abrasive wear occurs, it is usually the softer surface that loses more material. Diamonds are the top of the hardness scale and usually will cut or abrade anything else.

Do the opposite: The first four factors will lead to some nice scratching, with more severe scratching on the softer of the two rubbing surfaces. Change from steel or aluminum rollers to rubber-covered rollers, or wrap the rollers with a softer material such as masking tape, cheesecloth, fabric, or paper.

In my experience, the top factor in stopping scratches is to stop relative motion by using rough or textured roller surfaces. Many converters wrongly associate high roller roughness with more likelihood of scratching. Roughness without relative motion does not create scratching. Rough or textured roller surfaces include micro-grooving, dimpled tape, plasma sputtering, and many roller wrap options, such as fabric or cheesecloth.

Follow this plan and you will have the opposite of scratched webs: happy customers.

Web handling expert Tim Walker, president of TJWalker+Assoc., has 20+ years of experience in web processes, education, development, and production problem solving. Contact him at 651-686-5400; tjwalker@tjwa.com; www.webhandling.com.


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