Rheology in Film Processing (2)-Flow Instabilities in Film Casting Process

Film casting is one of the important processes for obtaining the plastic film products. Molten polymer casting is conducted by cooling the extruded melts at the surface of chill roller laid below the extrusion dies. Flow instabilities in the film casting are divided into two categories. One is the shear flow instability and the other is the extensional flow instability. These instabilities are similar to those appearing in the melt spinning process. Among many parameters governing shear flow instability, shear rate and Deborah number defined as the ratio of polymer relaxation time to retention time in the land of dies or spinneret orifice are likely to be most important. The small value of Deborah number is found to be effective for depressing the shear flow instability. Regarding the extensional flow instability, draw resonance phenomena were summarized and discussed. Polymer parameters such as power low index in the power low fluid model and relaxation time in Maxwell fluid model were found to be important for onset of draw resonance instability. As process parameters, cooling effect of extruded melts, draft ratio defined as elongation ratio of melts and air gap length corresponding the distance from dies exit to chill roller were also important. Further study on transient response of disturbances having various frequencies was done. It was found that film thickness uniformity changed depending on disturbance frequency.