(217f) The Extrusion of a Model Yield Stress Fluid Imaged by MRI Velocimetry

Extrusion tests were performed by forcing a well-characterized model yield stress fluid from a cylindrical cartridge through various cylindrical extrusion dies using a variety of different piston velocities. For our tests the Bingham number in the die ranged from 0.1 to 10. MRI techniques allowed for the non-invasive determination of the local velocity within the extruded material in the range. The velocity profile within a very long die was determined by MRI and agreed very well with the Herschel-Bulkley theory of flow in a conduit using the parameters determined from independent rheometrical tests, validating both the rheological approach and the accuracy of the MRI techniques. Although the velocity was determined by MRI in the upper and lower zones separately, the intersection of these zones showed great agreement, providing an entire view of the extrusion process. In our range of Bingham number the velocity field for a given contraction ratio appeared similar, with an heart of acceleration above the die and lateral dead zones varying negligibly with the piston velocity. Finite element simulations employing two different models were used in separate simulations: a biviscous model and an elastic solids model. It was found that to accurately reproduce the flow characteristics of extrusion this elastic stress model should be used. The MRI results show that for our range of values the piston velocity has little effect on the size of the weak velocity regions, and with an appropriate scaling the central, longitudinal velocity follows a master curve. A decreasing contraction ratio, on the other hand, appears to increase the size of the weak velocity region, in strong contrast with the simulation results.