High velocity winding of plastic films : influence of suface topography on the air interlayers

One challenge when processing flexible media such as plastic films is to obtain rolls without any aspect defect : if one considers that a “defect” (i.e. wrinkling or buckling) is due to the fact that the stress generated within the roll is greater than some “plasticity yield”, then it is crucial to predict the internal stress state.Several process parameters must be carefully mastered. One important effect, though difficult to evaluate, is that of surface topography, and its influence on the roll quality.Several authors [1,2] assumed that the roll is a composite material, resulting from the accretion of circular film layers separated by air layers. It is suggested that the radial elastic modulus of the roll is a nonlinear function of the compressive stress, depending on the amount of air entrapped. This is an indirect way to take into account the influence of surface topography on the stress state generated within a roll wound at high speed.In this article, we propose to connect some pertinent static roughness parameters of plastic films to their dynamic effects. The main features of our experimental set-up are recalled, see [4] : it consists in an air layer squeezed between the rough flexible surface to be characterized (i.e. film sample) and a rigid smooth one (i.e. glass substrate).In order to establish a relationship between the thickness reduction of the air layer and the displacement of the front representing the air layer evacuation, a mathematical model has been developed [5]. This model is based on homogeneization techniques, considering that the flow domain (gap between the rough film and the substrate) is composed of a regular array of cylinders. The cylinders heights, radii and periods of spatial distribution are connected to the actual characteristics measured by a 3D roughness profilometer.The statistical data processing consists in selecting the peaks higher than some prescribed threshold.The corresponding parameters are introduced into the mathematical model. They represent respectively : cylinders heights, spacing between cylinders and diameters.The experimental data are compared to the theoretical predictions, for several values of the threshold, and an optimum value of the threshold is found