Derivation of a Qualitative Model for the Spatial Characteristic Wavelength of Extrusion Flow Instabilities: Investigation of a Polybutadiene Rubber through Capillary, Slit and Complex Geometry Extrusion Dies

The extrusion flow instabilities of commercial polybutadiene (PBD) are investigated as a function of the different extrusion die geometries, such as round capillary, slit, and complex cross-section profile slit dies via capillary rheology. Qualitative models are used to fit the experimental data for the spatial characteristic wavelength (λ) of the appearing extrusion flow instabilities. A new qualitative model for the slit die geometry, rectangular cross-section, is derived based on the theoretical concept of the “two layers” extrudate and the force balance at the die exit region. The proposed qualitative model for the slit die geometry is used to predict the spatial characteristic wavelength (λ) for extrudates obtained by complex cross-section profile slit die geometries similar to industrial manufacturing. Correlation between the ratio of the extensional (Y$_s$) and shear (σ$_x$) stress at the die exit area and the characteristic dimension, height H for slit dies and diameter D for round capillary dies, is presented. Moreover, a geometry-dependent model is used to predict the spatial characteristic wavelength (λ) of the extrusion flow instabilities from a round capillary die to a slit die and vice versa.