*4.5. Interaction with Composites Factors*

As the characteristics of a pure polymer may not satisfy requirements sometimes, people turn their attention to FDM-based composite materials [145], such as polymer matrix composites [146,147], bio-composites [148,149], nanocomposites [150,151], and fiberreinforced composites [152,153], which have advantages of high mechanical performance and multi-function. However, the intrinsic properties of different composite materials, such as flow and fiber orientation, solidification behavior, and deformation [142], make it difficult for process parameters optimization related to composite materials. For example, Camineroa et al. [154] examined the influence of fiber volume, layer thickness, and build orientation on the impact properties of continuous fiber-reinforced composites. They noted that the interaction between fiber orientation and build orientation significantly led to different impact strengths for on-edge and flat specimens. In the study of Osman and Atia [155], a significant reduction of tensile modulus was observed for specimens with 45◦ raster angle, with the increase of rice straw content in the ABS-rice straw composite material. However, this phenomenon was insignificant overall for specimens with a 0◦ raster angle. In a word, the complicated influences of process parameters on the properties of composites, which are coupled with material factors, remains a big challenge for future research.

### **5. Summary, Recommendations, and Perspectives**

In summary, the research of FDM process parameters is critical for improving the characteristics and quality of parts. Different process parameters may have similar or opposite influences on the mechanical properties and behavior of components, which are also affected by other factors such as materials, printers, experiments, etc. Therefore, a compressive investigation of various process parameters is necessary and helpful. Despite existing research gaps, the future of research on FDM process parameters is the most appealing, and a number of innovative explorations await newcomers in the field. The following contents, though certainly not comprehensive, point out some potential future directions and areas that require attention from the field.

### *5.1. Condition of Printing*

FDM parts are printed in diverse conditions, which inevitably affect the mechanical characteristics of printed samples. The function of environmental parameters such as platform temperature [117], envelope temperature [115,156], humidity [157,158], and oxygen [159]; other print parameters such as nozzle parameter [92] and filament diameter [160], on mechanical behavior has been more or less studied, although not very extensive. In addition, how these factors impact process parameters remains a challenge and only attracts a few researchers' attention. For example, Mohd et al. [161] found that the diameter of the ABS filament increased as it was exposed to prolonged moisture with a certain absorption rate. However, this physical change would not cause nozzle clogging, which would directly affect the print speed. The influence of the FDM process condition could be a potential future research direction in this field.
