Kinematic waves and draw resonance in film casting process

Draw resonance, one of major instabilities frequently occurring in fiber sp inning, film casting and film blowing processes, arises as the drawdown rat io is increased beyond its critical value and is manifested by sustained pe riodic variations in spinline variables such as cross-sectional area and te nsion. The approach which was introduced by Hyun and coworkers [1-6] based on kinematic waves traveling on the spinline to explain the physics behind this draw resonance in spinning and to derive its criterion, has been appli ed to film casting in the present simulation study. It has then been reveal ed that the same mechanism and criterion govern the draw resonance in film casting as in spinning, but at the same time, some differences also have em erged. Particularly, the nonlinear dynamics of the film width whose counter part does not exist in spinning has been found quite complex and also very sensitive to process parameters like fluid viscoelasticity and the aspect r atio of the casting equipment. This contrasts dramatically with that of the film thickness which shows rather simple dynamic patterns, almost insensit ive to the changing parameters values. The good control of film width is of as much importance as that of film thickness in many industrial processes including paper coating. An ingenious coating method [7] exemplifies how cr itical the film width control is for the successful operation of extrusion coating of polymer films on paper substrates. (C) 2001 Elsevier Science B.V . All rights reserved.