Open Access

In an effort to broaden the engineering applications of material extrusion based additive manufacturing (MEAM), new materials are being developed. Adding carbon-fibers (CF) has been one strategy to increase the mechanical performance of different thermoplastics. One challenge is to determine the amount of CF needed to increase the mechanical performance without affecting the “printability” of the compounds. In this paper, different amounts (10, 15, and 20 vol.%) of CF were added to recycled polypropylene (rPP) and polyamide 12 (PA12). A compatibilizer was used for rPP, but not for PA12. Filaments for MEAM were extruded from the different compounds and the viscosity as well as the tensile properties were measured and compared to the processed polymeric matrices. It was observed that the viscosities at the angular frequencies relevant for MEAM (100 to 200 rad/s) were not significantly different for rPP+CF compounds, but it was higher for PA12+CF compounds. As expected, the elongation at break significantly decreased with the addition of CF for all compounds. For the composites with an rPP matrix, the Young’s modulus and the ultimate tensile strength (UTS) continuously increased as the CF content increased to 20 vol.%. For PA12-based materials, the Young’s modulus and the UTS increased with CF content, but adding more than 15 vol.% did not further improve these values. Therefore, it was concluded that for PA12 the maximum amount of CF that should be added was 15 vol.%. Using scanning electron microscopy, it was observed that the CF were homogeneously dispersed in the rPP matrix, but not so well in the PA12 matrix, with fibers being more concentrated towards the rim of the filament. Finally, filaments of rPP, rPP+20CF, PA12 and PA12+15CF were used to print complex geometries by means of MEAM, and it was observed that CF helped to reduce the warpage compared to the unfilled filaments. A potential application of this phenomenon could be the reduction of the bed temperature to develop a more energy efficient MEAM process for semi-crystalline polymers.