The present paper discusses the unique flow behavior in a new type of extruder. The extruder design is based on a hollow conical assembly of static (stators) and rotating (rotor) parts. Extrusion is achieved from each side of the rotor to provide a two-layer annular product. Flow is helicoidal in the vicinity of the rotor tip and die entry where the polymer layers merge. However, it becomes fully parallel to the main extrusion direction after a certain distance downstream from the rotor tip. Numerical simulations based on the resolution of Navier-Stokes equations using a Generalized-Newtonian viscosity showed that the length of the helicoidal flow depends on die design, viscosity and flow rate ratios between adjacent polymer layers. Analysis of flow variables along the interface showed that the helicoidal flow distance can be increased using a die design inducing lower interfacial shear rate and velocity. When the viscosity ratio and the flow rate of the most viscous polymer are increased, the helicoidal flow can be maintained over a longer distance in the die. Finally, experiments have shown that the advantage of the helicoidal flow is for short fiber orientation in pipes. The fibers are oriented along the streamlines in the hoop direction in the mid-gap of the die; however, they are parallel to extrusion direction in the vicinity of the stators.
© 2003, Carl Hanser Verlag, Munich