The Influence of Processing Conditions on the Polypropylene Apparent Viscosity Measured Directly in the Mold Cavity

Abstract

Measurements of rheological properties of polymeric materials play an increasingly important role in the polymer plastics industry. Knowledge about the plasticized material flow behavior is essential at the injection molds design stage. The polymeric viscosity curves usually are determined by a capillary rheometer tests. Nevertheless, the rheological properties determined this way differ from those observed during actual processing conditions. This paper presents a comparison of apparent viscosity measurements of polypropylene conducted by three methods. The first one was plastometer measurements made at different masses of test load. The second method evaluated the viscosity in the injection mold cavity equipped with pressure and temperature sensors. Capillary rheometer was used as the third method. It was observed that polypropylene viscosity curves, determined by different methods have a similar pattern. Comparing changes in apparent viscosity values obtained with the capillary rheometer and directly in the mold cavity, the largest differences were found for the shear rate below 1000 s −1. This proves the non‐isothermal measurement in the injection mold cavity. In the range of shear rate above 1000 s−1, the apparent viscosity determined directly in the injection mold was about 40 Pa · s greater than that measured by the rheometer, regardless of the test temperature. The results recorded by the plastometer reflect the real polypropylene viscosity curve at low shear rate values in a better way.