Investigations Of Elongational Flow Behavior Of Polymer Melts Based On The Laser Doppler Velocimetry

The temperature field, stress field and pressure field along with the chemical reactions inthe polymer melts exert a significant influence on the appearance and performance of theplastic products while processing. Additionally, secondary flows exist in the corners of theflow channel while the polymer melts flow through an abrupt contraction channel even if theReynolds number of the melt is far below the turbulent critical value. Long residence time ofthe polymer in the secondary flow will lead to the formation of gel and black spots due to thedegradation thus reducing the integrated properties of the products. The source of thesevortexes attracted the attention of a large number of scholars and became a widely studiedsubject. So far there has been no comprehensive explanation for the phenomenon of inlet flow.Therefore, the research on the inlet flow behavior through the measurement of velocity field isof great significance in polymer processing.This paper was based on the previous work of other researchers, using laser-Dopplervelocimetry to detect the melt velocity field of low-density polyethylene, linear low-densitypolyethylene and high-density polyethylene under the same process conditions, the effects oftemperature, volume flow rate and contraction ratio on the flow behavior of the melts alongthe center-line and in die entrance plane were investigated at the same time. It wasdemonstrated that: LDV is suited to investigate the flow behavior of polymer melts with highspatial and temporal resolution; by comparing the measured values of the three materials atypical secondary flow and a pronounced overshoot was found in LDPE melts which does notappear for LLDPE or HDPE.By comparing the velocity field of LDPE melts under different conditions, it was foundthat the centerline velocity is not very sensitive to temperature change while the maximumvelocity and maximum tensile strain rate increase with the rise of temperature slightly in dieentrance plane; volume flow rate and contraction ratio influenced centerline velocityremarkably, basically, with the rise of volume flow rate and contraction ratio centerlinevelocity and tensile strain rate increased linearly with maximum tensile strength generallyequal under different conditions; the area of LDPE secondary flow changes with temperaturereaching its peak at180℃. According to time-temperature equivalence rules, a curve is plotted using superposition method after the processing of data of different temperature, itwas observed that the trend of the curve corresponds with the trend of the standardized staticviscosity which indicates strain hardening characteristics of the material; according toCogswell model and Binding model the vortex boundary line was calculated which wascompared to the results of LDV measurement. It was found that the calculated results byCogswell model revealed the same trend as the results of LDV measurement but the value ismuch bigger. However, using Binding model the calculated results were very close to those ofLDV test and the calculated vortex boundary line was consistent with the actual value withdifferent contraction ratio.