Rheological characterization of long fiber thermoplastics – Effect of temperature, fiber length and weight fraction

Isothermal squeeze flow tests were conducted on E-Glass/polypropylene long fiber thermoplastics (LFT) to obtain the rheological characteristics of the material over a range of squeeze rates (0.5–60 mm/min). A transversely isotropic power-law model has been incorporated to capture the combined effect of shear and extensional flow behavior. Scott’s approach [Bird RB. Useful non-Newtonian models; 1976. p. 13–34] was used to obtain the shear power-law parameters, which were then used to calculate the radial velocity in the r-direction. The continuity equation was used to calculate transverse velocity in the z-direction. Radial and through the thickness velocity profiles were determined to obtain the extensional and the shear strain rates. Finally the extensional and shear viscosities were determined at strain rates calculated. Good agreement between the experimental applied stress and the predicted curves from the model was achieved. Effects of mold separation, mold temperature, and fiber length on viscosity at constant fiber weight fraction were examined. Effect of fiber weight fraction on viscosity at constant fiber length, mold separation and temperature was examined. Results indicate that viscosities decrease with either increase in mold temperature or decrease in fiber length at constant mold separation and fiber weight fraction. Viscosities also decreased with decrease in fiber weight fraction.