Gentle wavy defects in knurled wound rolls

The manufacture of many products involves the winding of continuous thin, flexible webs into wound rolls. In many applications involving the use of plastic webs, it is beneficial to mechanically emboss the edges of the web prior to winding so as to provide a thickening of the web in these areas. During and after winding, radial pressures developed in the wound roll then are concentrated in the localized embossed areas. This reduces the sensitivity to the formation of web distortions due to stress concentrations that would otherwise develop due to lengthwise persistent widthwise thickness nonuniformities. One of the drawbacks of this process is that the wound roll is now more sensitive to buckling-type defects owing to reduced interlayer pressures in the bulk of the roll away from the embossed edges. In this paper, we present results for a particular type of web defect known as a Gentle Wavy Defect (GWD) that forms in the wound roll due to presence of axial corrugations that develop in the wound roll. We begin by presenting product and process information characterizing this situation along with potential theories as to the cause of the corrugations and GWDs. Next, we provide results from numerous experiments that provide guidance on the likely mechanism responsible for the formation of the roll and web defects. Through these experiments and the application of winding models, we then demonstrate the cause of the defects and provide guidelines on how to avoid them in future applications