Experimental Characterization of Air Entrainment During High Speed Web Winding

As a web is wound at speed onto a spool or roll, a thin layer of air becomes entrapped between the incoming web stream and the roll. The spiral-shaped air bearing which results separates adjacent web layers and can extend many wraps into the roll. The air entrained during the winding process increases the propensity for interlayer slippage, stack shifting, damage to the edges of the web, and non-uniform internal stresses. A new technique is discussed for measuring the film thickness of the entrained air layers during the winding process, and parameter studies quantify the effects of such winding variables as tension, transport speed, and surface roughness. With a view towards evaluating different web transport designs and operating conditions, three measures of air entrainment are discussed: (i) the cumulative thickness of all air layers, (ii) the thickness of the outermost air layer at the nip, and (iii) the rate at which air bleeds from the wound roll once it is brought to rest.