# Continuous Fiber-Reinforced Material Extrusion with Hybrid Composites of Carbon and Aramid Fibers
### Hybrid Composites  
Hybrid composites are increasingly being used in conventional fiber-plastic composites in order to improve and adapt the mechanical properties to the existing load case. These composites are characterized by the combination of different fiber materials in one part in order to achieve unique material properties that could not be achieved by using one fiber material [25, 26, 27]. In conventional manufacturing processes, the hybrid composites with  
Figure 1: Schematic representation of the established approaches for introducing continuous fibers in the MEX process: (**a**) with pre-impregnated fiber filament; (**b**) with fiber impregnation in the print head. continuous fibers are usually processed in the form of mats. There are different approaches for hybridization. According to [PERSON] [25] a distinction is made between the following approaches in this context:  
* _Interply_, alternating layers of different fiber materials;
* _Intrapply_, regular or irregular mixture of fiber materials on laminate level;
* _Intrayarn_, mixed fiber materials on filament/roving level. This categorization for conventional manufacturing processes of fiber-reinforced plastic composites can also be applied to additive manufacturing in the broadest sense (see Figure 2). All three approaches can be implemented using continuous fiber-reinforced MEX. A mixed approach is also conceivable. The simplest approach is the alternating use of fiber materials between the layers (interply). The number of fiber cuts required per layer and the duration for a fiber or tool change are thus reduced to a minimum. However, this leads to a low dispersion of the different fiber materials. An approach involving the combination of different fiber materials in a layer plane can also be implemented. However, this is associated with increased production time and increased effort for path planning, but on the other hand leads to better adjustability of the mixing ratio--especially for thin, flat parts. Compared to the interplay approach, the intraply approach leads to improved dispersion. An intrayam approach requires the use of different fiber materials in one roving or fiber filament. Accordingly, this approach does not require tool changes or additional fiber cuts, but is associated with a loss of flexibility. Consequently, the mixing ratio for machine code generation cannot be set in a defined way, but depends on the ratio of the semi-finished product used. However, the best dispersion can be achieved with the help of the intrayarn approach [28]. Research in the field of conventional hybrid composites has focused, among other things, on improving elongation at break or creating pseudocultic material behavior [29; 30], improving fatigue strength [31] and impact strength [32; 33; 34] and implementing low-cost fibers [35; 36]. Since the combination of different fiber materials in a  
Figure 2: Approaches to hybridization in the context of continuous fiber-reinforced MEX: (**a**) non-hybrid composites with carbon fibers; (**b**) non-hybrid composites with aramid fibers; (**c**) combined fiber materials within a fiber strand (intrayarn); (**d**) layer-by-layer hybridization (interply); (**e**) strand-by-strand hybridization (intrapy). crease in certain mechanical properties, a compromise in hybridization is necessary [37; 38]. This implies an optimization criterion for the hybridization ratio that must be addressed in the design. Carbon fiber-reinforced composites are characterized by comparatively high strength and stiffness with low elongation at break and impact strength [39; 40]. Aramid fibers are therefore frequently added in hybrid composites with carbon fibers. Aramid fibers are usually added to carbon fiber-reinforced plastics due to their lower density and positive effects on impact strength [32; 41]. It was shown by [PERSON] et al.