- August 10, 2012, Timothy J. Walker
WebHandlersQuiz 18 continues the Q&A format I began this past February with our quest to address fundamental questions concerning web handling. To see past Quiz questions and answers, visit: pffc-online.com/blog/walker.
So this blog posting continues to focus on the subject matter we began in July, which is Winding–If you wind a paper and a film product of equal thickness and roll geometry at the same combination of winding conditions (tension, nip, taper, speed), which will likely have higher internal roll pressures or greater tightness?
The mostly likely answer is the film product will wind a much tighter roll, especially if it is a lower modulus film, such as polyethylene or polypropylene.
Paper products tend to wind looser rolls since they have a high relative compressibility in the radial direction, and the web stretch (a.k.a. strain) in the paper layers can lose their tension from radial shifting or compressing of the roll’s core and inner layers.
Film products tend to wind tighter rolls since they have a low relative compressibility in the radial direction, and the web strain (especially in stretchy films) in the film will have minimal tension loss from radial shifting or compressing of the roll’s core and inner layers.
One exception might be high speed winding of a smoother film without control of the entrained air layer compared to a more porous paper. While paper can absorb an entrained air layer, films will capture the air between layers. Over time, the air can bleed out of a film roll, causing it to loosen and potentially giving rise to a looser condition than paper on the same winder.
More on this topic was covered in this archived Web Lines column: The Pressure of Winding Rolls, May 2006.
Stay tuned to this blog for more Q&A. My next Quiz will feature still another facet of Winding involving the advantages of using a nipped or gap-controlled roller ahead of winding.