# Recent Advances in Additive Manufacturing of Fibre-Reinforced Materials: A Comprehensive Review
## 4 Research Gaps and Future Steps  
Although this review focused mainly on categorising the available existing technologies in AM and the mechanical performance they can produce, it could not cover the whole spectrum of CFC. Some future steps are proposed for a better understanding of all of the aspects that could elevate AM procedures. The main emphasis was given to tensile, flexural, and creep tests, while DMA and anisotropy were also described. Another issue that should be addressed is the printhead angle when the reinforced fibre is laid on the platform. The possible rheological effect of the ortho angle that the nozzle gives to the fibre while being extruded might provide a different performance than if the nozzle had a 20\({}^{\circ}\) angle with the building platform; therefore, nozzle to platform angle should be studied more. Moreover, there is a research gap in printhead nozzle extrusion technology, in which nozzles could be investigated to have different characteristics in order to work better with fibres and mixed composites. This could offer the best user-suitable solutions for each technology and each thermoplastic-thermosetting-to-fibre combination. Prediction models with the involvement of finite element (FE) models, and how these correspond to experimental data, is another division that in future reviews deserves to be mentioned, since FE models can give accurate predictions before creating the final part. Figure 11: (**a**) Tensile strength for different build orientations [114; 115; 116]; (**b**) Flexural strength for different build orientations [113]. Figure 10: Building orientation of tensile specimens: A1 along the x-axis, A2 along the y-axis, A3 along the z-axis [110]. In the mechanical performance section, there could be further investigation into the properties of printing variables like speed, layer thickness, raster angle, and temperature involved in the part. Except for the printing variables, an investigation into the mechanical properties of voids and porosity implements in the printed object, which are caused by the performance of each method, also needs to be categorised. The lamination strategies and the material grade for every method can also be described. In the experimental testing, more emphasis can be given in the areas of plane shear, bending fatigue, fatigue limit, and fracture toughness in order for mechanical performance to be a more complete subject. Moreover, quantification of how the environment degrades polymers under conditions like exposure to UV light or water absorption [118; 119] can also be reviewed in the existing literature. One of the major issues of CFC is the adhesion between the fibres and the matrix. Each of the described methods tried to overcome this by changing the route of fabrication. In addition, the classic methods of manufacturing CF need time or heat curation for the epoxy resins. Thus, postprocessing is an area in CFC that also needs more characterization [120]. Annealing, chemical, or even ultrasound postprocesses can be reviewed to see if they have any impact on layer-to-layer adhesion or matrix-to-core adhesion. A further step would be the implementation of recycled fibres to be used as reinforcing fibres [121], but the recycling technology is not yet perfected. Finally, more emphasis can be given to the integration of piezoresistive and piezoelectric properties into structural materials, where the evolvement of such innovative materials is pivotal for next-generation structures, especially in high-stress environments like bridges or tunnels, where embedded sensing capabilities can enhance safety and longevity. Future studies ought to take under consideration how to evolve the production technologies of reinforced additive manufacturing. The integration of AI for process optimization and real-time quality control is one study area, enabling enhanced production efficiency and predictive maintenance.