Mechanical Properties of Printed Epoxy-Carbon Fiber Composites

Despite the promise of additive manufacturing (AM) to bring unprecedented agility and design freedom to manufactured components, structural applications remain largely out of reach due to material restrictions – notably the lack of a mature AM process for reinforced thermoset composites. AM is also hindered by process-induced defects such as porosity and unfavorable microstructure. This research shows that a direct write AM process for epoxy / chopped carbon fiber composites can simultaneously achieve a high degree of fiber alignment and low degree of porosity, obtaining 90% of the theoretical tensile modulus and 66% of the theoretical tensile strength for a fully aligned composite. These values exceed those of compression molded properties for the same material. Transverse properties of AM samples were roughly half of the longitudinal properties but showed no statistically significant difference from the matrix material, suggesting that the process may not adversely affect microstructure. The addition of only 5.5 vol% carbon fiber more than doubled the strength and stiffness of the neat epoxy, and more than tripled the properties of ABS thermoplastic while achieving a higher glass transition temperature. Flexural properties show similar trends. SEM and CT imaging shows that fiber orientation is largely maintained in the print direction and cross-section micrographs show there is sufficient local material flow during deposition to achieve low porosity.