Influence of fiber breakage on flow behavior in fiber length- and orientation-dependent injection molding simulations

Injection molding is one of the most important processes for manufacturing discontinuous fiber reinforced
polymers (FRPs). The matrix of FRPs shows a transient chemo-thermomechanical behavior and the fibers create
anisotropy influencing physical properties. Hence, FRPs are complex materials, but also likely used in volume
production. In this work, the fiber-induced anisotropic behavior during mold filling is modelled with an
anisotropic fourth order viscosity tensor. The viscosity tensor takes second and fourth order fiber orientation
tensor, fiber length and non-Newtonian matrix viscosity into account. In this way, the macroscopic simulation
captures the influence of the flow field on the fiber re-orientation and vice versa. The fiber orientation tensor is
used to determine reference fibers in every element for calculation of hydrodynamic forces. This information is
used in a novel fiber breakage model, based on buckling of fibers in Jeffery’s orbit. The result is a macroscopic
molding simulation with not only transient fiber orientation distribution, but also fiber length distribution. Due
to the anisotropic viscosity tensor, the predicted fiber breakage influences the material’s viscosity and flow
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behavior, which is also visible in the simulated cavity pressure. The results are validated with injection molding
experiments, performed with a glass fiber reinforced phenolic compound, showing good agreement.