Stability windows for draw resonance instability in two-dimensional Newtonian and viscoelastic film casting processes by transient frequency response method

•Draw resonance onsets were determined from transfer function approach.•Transfer functions were analyzed in tension-controlled film casting systems.•Stability windows were established in 2-D viscoelastic film casting processes.•Stability patterns were closely related to flow residence times of fluid elements. Critical onsets for draw resonance instability occurring in two-dimensional (2-D) film casting processes with Newtonian and viscoelastic (Upper-Convected Maxwell and Phan-Thien and Tanner) fluids have been newly determined using a transient frequency response method. Under a constant tension condition, which guarantees always stable operation, the trajectory of a transfer function between the output take-up velocity and input tension in a Nyquist plot has been used as an indicator for finding draw resonance onsets. Various stability windows for Newtonian and viscoelastic fluids have been constructed, confirming that the onsets were well-predicted when compared with transient responses in actual velocity-controlled operating systems around the onsets. Interestingly, up-and-down stability patterns along the aspect ratio in the Newtonian cases were found to be closely related to the flow deformation features of the fluid elements within the film width. Dichotomous stability behaviors for extensional-thickening and extensional-thinning fluids were well addressed in the 2-D processes.