Study on extrudate swell of polypropylene in double-lumen micro profile extrusion

One of the basic problems in the design of polymer extrusion dies for multi-lumen micro profiles is the previously unknown extrudate swells, since they are more complex than those in axisymmetric extrusions. To investigate the three-dimensional extrudate swell of such geometries, numerical simulations and experiments are performed for the extrusion of a double-lumen micro profile using polypropylene. The selected profile exhibits typical non-axisymmetric geometry, uneven thickness and micro-scale dimension. A double convected pom-pom model considering the micro size effect is proposed to characterize viscoelastic properties of the polymer melt. Predictions are compared with experimental results as well as simulations using the power law model, and show the capacity of the proposed model for assessing the anisotropic swelling quantitatively. Influences of micro size effect, volumetric flow rate and material parameters on extrudate swell are investigated by parametric studies. It is found that swell ratios of various positions on the extrudate cross section are significantly different. The swelling anisotropy degree is enhanced with increasing volumetric flow rate as well as material parameters of second normal force coefficient and branching degree, while it is suppressed as the ratio of orientation to stretching relaxation time increases. The influence mechanisms of material parameters on extrudate swell are also discussed in detail. It is suggested that the anisotropic extrudate swell induced by micro size effect is significant and should be taken into account in high accuracy micro profile extrusions.