Influence of reservoir diameter on the melt elasticity in capillary flow

An experimental study has been carried out to investigate the influence of the reservoir‐to‐capillary diameter ( D R /D ) ratio on the elastic properties of polymer melts flowing through capillary tubes. For the study, a capillary rheometer, described in an earlier paper by Han, was used with D R /D ratios of 3, 6, 9 and 12. Measurements were taken of wall normal stresses in high density polyethylene along the axis of the capillary, and the axial pressure profile was used to obtain the “exist pressure,” which is believed to be an elastic phenomenon. The analysis of the experimental data indicates that, for a given capillary length‐to‐diameter ( L/D ) ratio, the exit pressures first increase and then level off, as the D R /D ratio is increased. This behavior of the exit pressure is in accord with that of extrudate expansion which has also been determined in the experiment. This dependence of the exist pressure and extrudate expansion ratio on D R /D ratio is explained with the concept that the amount of recoverable elastic energy stored in the melt flowing through the capillary depends on the strain history, which the melt experiences on entering the capillary.