Measurement of the flow of molten polymers through short capillaries

A method of treating experimental flow measurements on high polymers to obtain the basic shear diagram is described. Shear stress data are corrected to eliminate capillary end effects, and shear rate data are modified for the non-Newtonian behavior of the materials. Differences in flow behavior of commercial low-density polyethylenes are illustrated, and the effects of the two corrections are demonstrated. The method is applicable to other polymer melts. Although the data could be expressed by the power law, the fact that logarithmic plots of shear stress versus shear rate are not linear suggests that such relationships would be approximate. The data are expressed better by the threeconstant Powell-Eyring equation, although the significance of the three parameters is uncertain. The need to obtain the rheological constants from the basic shear diagram rather than from an uncorrected or partially corrected capillary flow diagram is emphasized. In support of previous investigators, irregularities in the extrudate were observed at high shear stresses. However, it was not possible to determine at what pressure the roughness was initiated. The shear diagrams calculated from the experimental data do not exhibit any point of inflection as reported in some studies.