Measuring Fiber Length in the Core and Shell Regions of Injection Molded Long Fiber-Reinforced Thermoplastic Plaques

Abstract

Long fiber-reinforced thermoplastics are an attractive design option for many industries due to their excellent mechanical properties and processability. Processing of these materials has a significant influence on their microstructure, which controls the properties of the final part. The microstructure is characterized by the fibers’ orientation, length, and concentration. Many characterization methods can capture the fiber orientation and concentration changes through the thickness in injection molded parts, but not the changes in fiber length. In this study, a technique for measuring fiber length in the core and shell regions of molded parts was proposed, experimentally verified, and used on injection molded 20 wt.% glass fiber-reinforced polypropylene plaques. The measured fiber length in the core was 50% higher than in the shell region. Comparison with simulation results shows disagreement in the shape of the through-thickness fiber length profile. Stiffness predictions show that the through-thickness changes in fiber length have little impact on the longitudinal and transverse Young’s modulus.

An algorithm for generating microstructures of fiber‐reinforced composites with long fibers

“…which are typical for LFT, 35,88 and take a look at Figure 9. When generating straight fibers, the minimum edge length of the volume elements should exceed the fiber length.…”

“…For this purpose, we furnish the matrix material with the isotropic elastic properties of polypropylene with Young's modulus E = 1.8 GPa and Poisson's ratio 𝜈 = 0.35. 35,88 Computations of the effective elastic properties on the samples with fiber aspect-ratio r a = 240 yields the directional Young's moduli E 1 = 6.22 GPa, E 2 = 3.28 GPa, and E 3 = 2.71 GPa.…”

“…The situation changes, however, when longer fibers need to be considered. To illustrate the basic problem with straight fibers, let us consider a volume fraction of $$$ \phi =13\% $$$ and a fiber orientation $$$$ A=\mathtt{\operatorname{diag}}\left(0.71,0.26,0.03\right), $$$$ which are typical for LFT, 35,88 and take a look at Figure 9. When generating straight fibers, the minimum edge length of the volume elements should exceed the fiber length.…”

“…Last but not least, we would like to study how the generated microstructures compare to real ones in terms of the effective properties. For this purpose, we furnish the matrix material with the isotropic elastic properties of polypropylene with Young's modulus $$$ E=1.8 $$$ GPa and Poisson's ratio $$$ \nu =0.35 $$$ 35,88 . Computations of the effective elastic properties on the samples with fiber aspect‐ratio $$$ {r}_a=240 $$$ yields the directional Young's moduli $$$$ {E}_1=6.22\kern0.3em \mathrm{GPa},\kern1em {E}_2=3.28\kern0.3em \mathrm{GPa},\kern1em \mathrm{and}\kern1em {E}_3=2.71\kern0.3em \mathrm{GPa}.$$…”