Flow visualization and residence time distribution in spiral mandrel dies

Flow visualization experiments were performed on a spiral mandrel die with a clear acrylic body by employing a tracer technique. Samples were collected and analyzed for the purpose of determining the residence time distribution (RTD), or the age distribution of the exit stream. The effect of the annular gap was studied by using two acrylic bodies having 1 mm and 2 mm annular gaps respectively. It was found that the larger gap resulted in a broader RTD. Also, it was observed how the gap affected the maximum distance travelled by the tracer in the spiral channel. A simple model was developed for the RTD in a spiral die. Simulation results compared favorably with the experimental data, predicting a larger RTD in the case of a larger annular gap. Visualization of the flow was found to be potentially useful in mathematical model development. The higher residence time of the material flowing near the surface of the spiral channel is believed to be the major source of the phenomenon commonly referred to as extrusion haze lines.