Tensile Mechanical Properties of Polypropylene Composites Fabricated by Material Extrusion

In the material extrusion additive manufacturing process, a thin filament of material is deposited in a layer-by-layer manner to fabricate a three dimensional part. The filament deposition pattern can result in voids and incomplete bonding between adjacent filaments in a part, which leads to reduced mechanical properties. Further, the layer-by-layer deposition procedure typically results in mechanical property anisotropy, with higher properties in the layer compared to those across layers. The study reported in this paper explored various polypropylene composite formulations to address these issues: low residual stress and warpage, good mechanical properties, and reduced anisotropy. The reduction in anisotropy will be the focus of this paper as a function of thermal properties and process variable settings. A series of process simulation models was developed to explore ranges of thermal properties and process settings, which provided insights into tensile specimen behaviors. Results demonstrate that anisotropy can be reduced almost completely if the material can be formulated to have low crystallinity, low coefficient of thermal expansion, and moderate to high thermal conductivity (for a polymer).