Preliminary Efforts in the Simulation of Molding of a Polypropylene Melt Reinforced with Long Glass Fibers using Transient Shear Rheology

Abstract The purpose of this research is to understand the transient fiber orientation and the associated rheology of long glass fiber (> 1mm) reinforced polypropylene, in a well-defined simple shear flow, to extend the results and knowledge gained from these fundamental experiments to the use of simulating (more complex) molding processes. Specifically, we are interested in associating the rheological behavior of glass fiber reinforced polypropylene with the transient evolution of fiber orientation in simple shear flow in an effort to ultimately model fiber orientation in complex flow. A sliding plate rheometer was designed to measure stress growth in the startup and cessation of steady shear flow. Results were confirmed by independent measurements on another sliding plate rheometer. Two fiber orientation models were investigated to predict the transient orientation of the long glass fiber system. One model, the Folgar-Tucker model (FT), has been particularly useful for short glass fiber systems and was used in this paper to assess its performance with long glass fibers. A second fiber orientation model, one that accounts for the flexibility of long fibers as opposed to rigid rod models commonly used for short fibers, was investigated and results were also compared with experimentally measured values of orientation. The accuracy of these models, when used with the stress tensor predictions of Lipscomb, was evaluated by comparing the model predictions against experimental stress growth data. Samples consisting of 10% wt. glass fiber in polypropylene with an average fiber length of 4 mm were prepared with random initial orientation and were sheared at different rates. Model predictions showed that fiber flexibility has the effect of retarding transient fiber orientation but provides poor rheological predictions with the chosen stress model. Additionally, it was shown that the predictions of the Folgar-Tucker model are not able to capture the dynamics of neither the orientation evolution nor the stress growth evolution that was measured experimentally.