Entrance length of polymer flow through a contraction die: Comparison of calculation from an equation and flow induced birefringence measurements

摘要

When a polymeric melt flows through a contraction die, it experiences a certain distance in the slit before a fully developed flow is achieved, which is defined as the entrance length. Here, an equation for calculation of the entrance length was proposed based on an assumption that the pressure drops in an exponential function way along the flow direction in the contraction die from the entrance to the fully developed flow. The entrance length could be calculated from the flow parameters, such as volumetric flow rate, the pressures at the entrance and at the defined point in the fully developed flow. The entrance lengths of a low-density polyethylene (LDPE) and a high-density polyethylene (HDPE) melt were calculated from this equation with the measured flow parameters at various flow rates. The results are quite close to those measured from flow induced birefringence and similar to that reported in a published study by Martyn et al., which suggested that the equation could be used to describe the relation between the entrance length and the flow parameters.