Kinematic waves and draw resonance in film casting process

Draw resonance, one of major instabilities frequently occuring in fiber spinning, film casting and film blowing processes, arises as the drawdown ratio is increased beyond its critical value and is manifested by sustained periodic variations in spinline variables such as cross-sectional area and tension. 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 applied to film casting in the present simulation study. It has then been revealed 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 emerged. Particularly, the nonlinear dynamics of the film width whose counterpart does not exist in spinning has been found quite complex and also very sensitive to process parameters like fluid viscoelasticity and the aspect ratio of the casting equipment. This contrasts dramatically with that of the film thickness which shows rather simple dynamic patterns, almost insensitive 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 critical the film width control is for the successful operation of extrusion coating of polymer films on paper substrates.