The Draw Resonance in A Two Layer Film Forming Process

Abstract

The forming process of a two-layer film is generally constituted by pulling through a variable air gap into a water-cooling bath after the two polymer melts are coextruded through a flat die. In this process, the film thickness often varied in a cyclic manner according to the process conditions. This phenomenon is referred to as draw resonance and is also observed in melt spinning and film processing.
An experimental study was carried out to investigate the draw resonance in forming process of a two-layer film. The two-layer film in which the first layer consists of poly (vinylidene fluoride) (PVDF) and the second layer consists of poly (methyl methacrylate) (PMMA) or PMMA/PVDF was prepared, and the total film thickness was about 50μm. As the take-up velocity or the die land gap was increased and/or the air gap was decreased, the instability of film thickness was in-creased.
The average elongational strain rate was calculated from the conditions of the film forming process. The instability of the film thickness increased abruptly at a critical point of the average strain rate, and the critical point changed with the composition of polymers. Through rheological mesurements on the polymer melts, we concluded that the instability is related to the elasticity of the second polymer layer.