The residual fiber length in a molded part is one of the most important microstructural properties of discontinuous fiber‐reinforced composites. While there have been several research studies characterizing the process‐induced fiber length reduction, the measurement procedures vary substantially, calling into question the comparability of reported results. This article introduces a newly developed measurement procedure that aims to provide accurate, repeatable, robust, and time efficient fiber length analyses. A comprehensive study of measurement techniques was performed comparing commercially available systems and the conventional approach of measuring the fiber length manually. The results emphasize the need for a standardized procedure to characterize the fiber length distribution and the risk of generating inadequate results through improper sample preparation. The developed measurement technique was tested and compared for an experimental study of fiber breakage in injection molding. For a simple plaque geometry, the residual fiber length along the flow path was obtained for a long glass fiber‐reinforced polypropylene at 30 and 40%wt for varying process conditions. The new measurement technique showed accurate and repeatable results. The results of the injection molding study showed that screw speed and back pressure are important factors that drive fiber breakage. An increase in back pressure from 13 to 50 bar and screw speed from 27 to 35 rpm reduces the weight‐average fiber length by 37.5%. POLYM. COMPOS., 39:4058–4070, 2018. © 2017 Society of Plastics Engineers

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Abstract:As the applications for additive manufacturing have continued to grow, so too has the range of available materials, with more functional or better performing materials constantly under development. This work characterizes a copper-filled polyamide 6 (PA6) thermoplastic composite designed to enhance the thermal conductivity of fused filament fabrication (FFF) parts, especially for heat transfer applications. The composite was mixed and extruded into filament using twin screw extrusion. Because the fiber orientation within the material governs the thermal conductivity of the material, the orientation was measured in the filament, through the nozzle, and in printed parts using micro-computed tomography. The thermal conductivity of the material was measured and achieved 4.95, 2.38, and 0.75 W/(m·K) at 70 • C in the inflow, crossflow, and thickness directions, respectively. The implications of this anisotropy are discussed using the example of an airto-water crossflow heat exchanger. The lower conductivity in the crossflow direction reduces thermal performance due to the orientation in thin-walled parts.

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Extrusion and injection molding processes of filled polymers are widely used in industry due to their high strength‐to‐weight ratios and for their ability to manufacture a variety of geometries while improving the overall mechanical and thermal properties. However, filler migration and filler–matrix separation during processing are phenomena that are not fully understood. To gain an improved understanding of these phenomena, polypropylene samples with different glass bead concentrations were manufactured using extrusion, injection molding and a customized screwless extruder that was built in‐house. Computed tomography was performed on the samples to observe particle position and distribution after material solidification. For all three processes, filler migration and filler–matrix separation was observed. In the extrusion and screwless extrusion processes, particles migrated towards the wall, contrary to current theories and believes. During the injection molding process, filler–matrix separation was manifested as bead‐free zones in the center region and the walls of the spiral mold at the entrance region of the spiral mold. These bead‐free zones were later filled at the far end of the spiral mold suggesting migration of particles along flow with almost no bead‐free zones at the end of the mold. Particle redistribution towards the wall and the center during flow could possibly have happened due to migration and fountain flow effects. POLYM. COMPOS., 40:2165–2177, 2019. © 2018 Society of Plastics Engineers