Anisotropic modeling of isothermal squeezing flow of glass-mat reinforced thermoplastics (GMT)

Lubricated isothermal compression molding of Glass-Mat Thermoplastic (GMT) was carried out using a circular mold on a 100 kN test machine to characterize the rheological parameters for squeezing flows of the material. Flow was found to be anisotropic with the main flow direction perpendicular to the main fiber direction. In-plane flow of circular specimens was found to follow an elliptical shape, which allows a relationship to be derived between extensional strain rates in the two main in-plane directions. A transversely isotropic incompressible Newtonian fluid model was successfully applied to interpret the squeezing data parallel and perpendicular to the main flow direction. Longitudinal and transverse extensional viscosities were calculated for two squeezing temperatures, 180 degrees C and 200 degrees C. The results were compared to the standard isotropic power law interpretation. shaving the largest anisotropic viscosity to be 20% higher than the isotropic viscosity at low closing speeds and 9.5% higher at fast closing speeds. A very significant difference in the viscosities was found between the two different temperatures as the viscosity at 200 degrees C dropped to about 65% of its value at 180 degrees C. In this work, original material sheets were also confirmed to display anisotropic mechanical properties. The final squeezed specimens were found to have different fiber volume fractions in the same specimen.