The Deceptive Dimensions of Web Handling

Presented by Dr. Dilwyn P Jones, Emral Ltd

Roll to roll processes rely on web handling principles to operate correctly and avoid defects such as scratches and wrinkles. These focus on the transport of the web at the correct speed and tension (machine direction), and its position in the cross-machine direction. In the third, through-thickness direction, the web is usually assumed to follow a path defined by the cylindrical rollers and tangents between them. However, the real shape of the web is more complex than this, with bending in the span caused by its weight (catenary) and bending stiffness resisting tension that attempts to pull it flat. In addition, wrinkles, which are waves of displacement in the through-thickness direction, may appear. In the direction across the wrinkles, wave amplitude increase or decrease accommodates dimension change, while that stress component remains small.

Equations used in web handling are ultimately derived from 3-dimensional elasticity theory, which is mathematically exact but difficult to work with. Structural models such as elastic strings, beams and plates simplify the behaviour and ignore wrinkles, and useful formulae can be derived. The stresses associated with the through-thickness direction, tension/compression and 2 shear components, are usually neglected.

This talk will review some topics of web mechanics where these effects are not negligible, based on the author’s 40 years of experience in the field. There are advantages from improving the physical understanding: better equations, defined limits of validity, and the web and machine variables that are important. Some ideas for advanced theoretical work and validation will be presented. These will cover:
• Tension change and microslip at web-roller contact
• Wrinkling in spans
• Wrinkles carried onto rollers
Hopefully, future work will put web handling tools onto a firmer foundation, even if some of our “fundamentals”, like wrinkling predictions, Normal Entry and Normal Strain Rules need cautionary notices.