The Coefficient of Winding Trouble

In winding, the important coefficient of friction (COF) is always about the two surfaces that will come in contact as the entering layer hits the winding roller. The COF values that are troublesome to winding fall into three categories: low, high, and pressure-dependent.

Friction is the force that opposes the sliding of two solid surfaces relative to each other. The COF describes the ratio of the force of friction and the force pressing two solid surfaces together. Friction is never a property of one material or surface but a property that describes the interaction of two surfaces.

If the COF is too low, it will be difficult to transmit torque through a roll. Low COF products are prone to cinching and cinching-related telescoping. (For more on cinching, see “Belt Tightening Gone Bad: Part 1” and “Belt Tightening Gone Bad: Part 2”)

Winding a product with a COF of 0.1 relative to 0.3 will require three times the nip load to create the same roll tightness. But since low COF rolls need more, they usually need to be wound at higher tension and more than three times higher winding nip load.

If COF is high (defined as greater than 0.7), increased wrinkling is the first problem. All winding rolls have diameter variations and poor alignment — both are known wrinkling causes. Combined with high friction and long entry spans, winding high COF webs quickly becomes a trip to Wrinkle City.

High COF products often use controlled gap winding to avoid problems associated with long entry spans and winding. However, this solution is limited to low speeds since nip-less winding will let too much air in at high speed-to-tension ratios and large diameters.

The most problematic COF — what I call the coefficient of winding trouble — is when COF is a function of pressure. In high school physics, we learn that COF is independent of surface area of contact or pressure. It doesn't matter if you slide a brick across a table laying down or standing on end, even though the pressure under the brick will be higher if the brick is horizontal. Too bad this isn't true for all materials.

Many polymeric films, especially optically clear films, have the annoying property in which the side A-to-B COF is a function of pressure (a.k.a. non-Coulomb friction). A simple hand test can detect this in your product.

Take two sheets and slide them between your finger and thumb. They initially will appear to have a reasonable COF. For the annoying problematic films, if you push harder still, you will find they want to stick together and will not slide cooperatively.

Why is this a problem? If the pressure across a winding roll is uniform, it isn't, but in the case of a crossweb pressure difference, trouble is on the horizon.

The outer layers of a winding roll require a small amount of skidding or sliding as the layer first touches the winding roll and the air bleeds out or is rejected upstream. If the full width of the web skids as one, then there's no problem. But if one lane or spot in the outer layer sticks (due to a large debris particle or gauge band) while the rest slides, a local shear stress will develop near the sticking point.

This local shear may form a small buckle or soft wrinkle in the top layer. In non-Coulomb friction products, the next layer will not smoothly wind over a bump or ripple but instead will conform over the bump and form a slightly larger bump or ripple.

As additional layers are added, like a rolling snow ball, the defect often will get bigger with each turn. These defects sometimes are called slip knots or convolution wrinkles.

Smart products are designed with this in mind by using internal or external slip additives, engineered surface roughness, or winding interleave webs to avoid problems. High yield winding is dependent on avoiding the coefficients of winding trouble.

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