# Continuous Fiber-Reinforced Material Extrusion with Hybrid Composites of Carbon and Aramid Fibers
###### Abstract  
An existing challenge in the use of continuous fiber reinforcements in additively manufactured parts is the limited availability of suitable fiber materials. This leads to a reduced adaptability of the mechanical properties to the load case. The increased design freedom of additive manufacturing allows the flexible deposition of fiber strands at defined positions, so that even different fiber materials can be easily combined in a printed part. In this work, therefore, an approach is taken to combine carbon and aramid fibers in printed composite parts to investigate their effects on mechanical properties. For this purpose, tensile, flexural and impact tests were performed on printed composite parts made of carbon and aramid fibers in a nylon matrix with five different mixing ratios. The tests showed that the use of hybrid composites for additive manufacturing is a reasonable approach to adapt the mechanical properties to the loading case at hand. The experiments showed that increasing the aramid fiber content resulted in an increase in impact strength, but a decrease in tensile and flexural strength and a decrease in stiffness. Microstructural investigations of the fracture surfaces showed that debonding and delamination were the main failure mechanisms. Finally, Rule of Hybrid Mixture equations were applied to predict the mechanical properties at different mixture ratios. This resulted in predicted values that differed from the experimentally determined values by an average of 5.6%.  
continuous fiber; fiber-reinforced additive manufacturing; hybrid composites; design for additive manufacturing; material extrusion; hybrid fiber-reinforced polymers 2022 22 2022 2021  
parts compared to short-fiber reinforcements, due to their significantly improved mechanical properties [9, 10]. Continuous fiber-reinforced MEX combines the unique potentials of additive manufacturing processes with the mechanical properties of composite parts. The process principle is based on conventional MEX. In principle, all thermoplastic materials that are also used in conventional MEX can be used as matrix materials. Synthetic carbon, glass and aramid fibers are usually used as reinforcing fibers [11, 12, 13]. In addition, new research approaches are being pursued to also use natural reinforcing fibers such as flax or jute fibers [14, 15, 16, 17]. The incorporation of continuous fibers can be achieved by two different methods. The first method is to merge the continuous fibers in the printhead with the surrounding thermoplastic matrix and then deposit them on the print bed (see Figure 1a). The second is to use pre-impregnated continuous fiber filaments that are already embedded in a thermoplastic matrix and can be extruded immediately (see Figure 1b) [18, 19]. Commercial printing systems and corresponding fiber filaments are available for both approaches. In most systems, dual extruders are used, which allow both the application of continuous fiber-reinforced strands and the extrusion of pure thermoplastic material (see Figure 1).  
The mechanical properties of continuous fiber-reinforced parts depend on a large number of factors and parameters. In addition to the number and size of the defects and the interfacial adhesion, the selection of the fiber and matrix materials and their orientation in the part have a major influence on the mechanical properties [6, 20]. Two of the biggest challenges in establishing continuous fiber extrusion in industrial applications are severe anisotropy and low print quality. Various research activities have investigated the damage mechanisms of printed composite parts. It was found that delamination between the layers or the fiber strands on the one hand and debonding, i.e., detachment of the matrix from the fibers, on the other hand, are the main types of failure [21, 22, 23, 24]. In this context, [PERSON] et al. [6] point out that another major challenge is the very limited choice of fiber and matrix materials with the associated limited adaptability to the load case at hand.