Web Lines: Wrinkling of Foils

In our everyday lives, our main experience with foil likely comes from wrapping a potato in kitchen-grade aluminum foil. Wrinkling thin aluminum foil is easy. If you are new to foil-based products, such as solar cells, batteries, packaging laminates, or flexible electronics, you may fear that handling foils in manufacturing will be a similar wrinkle-filled venture.

The most significant difference between foils and papers or films is the much greater modulus of elasticity. Foil may have machine direction Young's modulus 10-30 times higher than many papers and polyesters.

Does the high modulus of foils mean increased or decreased sensitivity to roller misalignment and diameter variations? On the anti-wrinkling side, higher modulus increases stiffness and resistance to buckling. On the wrinkling side, higher modulus increases stress variations created by the same roller misalignment or diameter variation.

Will copper and aluminum foil webs run wrinkle-free on equipment designed for paper and film handling? What is the recommended roller alignment and diameter tolerances for aluminum and copper foil web handling?

If you are a manufacturer of copper or aluminum foil, you would know some of the answers to these questions, having solved them by necessity. If you are new to handling copper, aluminum, or other foils, you easily could be quite fearful of the prospect, plus you may face an additional challenge that you foil suppliers do not — baggy foil. Even if you have manufactured foils for many years, you may find moving into thinner and wider foils can make you wrinkle-phobic.

Roller alignment and diameter tolerance specifications are one driver of equipment cost. If roller alignment and diameter tolerance specifications are loosened, then machining accuracies can be relaxed and machine alignment procedures will take less time. If roller alignment and diameter tolerances are tight, then equipment design, fabrication, assembly, and maintenance all will cost more.

Misaligned rollers and diameter variations can create lateral shifting in processes. However, most roller alignment specifications are and should be driven by wrinkling, specifically, shear wrinkles.

Based on some experimental work in which I was fortunate to participate, I have good news and bad news.

The Good News | Aluminum and copper foils are less sensitive to roller misalignment than theory would suggest. Two interesting things keep foil wrinkles at bay:

  • Foils are more difficult to buckle, so they need high web-roller friction (from friction, tension, and width) to apply the load needed to make a wrinkle.
  • The stiffness of foils means that misalignment effects quickly create large crossweb tension differential that cannot be contained in a single web span, spreading the forces out over a longer length of web and preventing wrinkles.

The Bad News | Foils are more sensitive to roller deflection and diameter variations. Where misalignment creates an asymmetric tension variation that can pivot and transfer into an upstream span, deflection creates symmetric tension variations that are contained within a span.

Conclusion | Don't overspend on ultra-precise roller alignment, but do invest in rollers with minimal deflection, accurate diameters, and web paths that avoid the effects of gravity (especially avoid long vertical spans).

“Wrinkling of Foils” was a paper and presentation I co-authored for the 2011 Intl. Conference on Web Handling held at Oklahoma State Univ. I would like to thank my co-authors, Dr. Kevin Cole of Optimation Technology Inc. and Jeffrey Quass and Stephen Zagar of Megtec Systems.

Web handling expert Tim Walker, president of TJWalker+Assoc., has 25 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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