## 6 Conclusions  
Processing parameters are of paramount importance in every manufacturing process. This study reviewed the influence of 3D printing parameters on the fatigue performance of polymeric samples produced by the fused filament fabrication technique. Since parametric studies on the fatigue properties of FFF prints are rarely found in the literature, it is hard to draw a general conclusion on the impact of each parameter. However, it has been shown that each parameter may play a positive or a negative role when the loading type or material changes. It was found that under tensile fatigue, FFF prints with cross-over infill patterns perform better than rectilinear patterns, and among various rectilinear raster orientations, 0\({}^{\circ}\) and 45\({}^{\circ}\) angles result in higher fatigue strength. However, increasing the raster angle would lead to lower flexural and rotating bending fatigue resistance. The infill density and layer height were shown to be directly proportional to fatigue life in all previous studies. However, their effect on all fatigue types is not fully understood. It was also found that the nozzle diameter has a positive correlation with flexural fatigue strength. Interestingly, the nozzle diameter was inversely proportional to the rotating bending fatigue performance of PLA specimens. However, this effect should be further investigated to draw a general conclusion. In all previous studies conducted on the tensile and flexural fatigue performance of FFF parts, Y prints possessed the highest fatigue strength, followed by X and Z prints. Printing speed exhibited the least significance among the other studied parameters, yet showing an inverse relationship with tensile fatigue life.  
Apart from the unstudied relationships, as shown by blank cells in Table 3, the following research gaps were identified during the reviewing process that can be addressed in future works:  
* How do strain rates and the loading frequency influence the fatigue life of FFF prints?
* How do FFF prints behave when subjected to a random or combined fatigue loading?
* How and to what extent do the temperature, environment, and surface finish impact the fatigue performance of FFF parts?
* The fatigue behaviour of composite FFF prints is in the initial stage and needs further investigation.
* In some applications, such as biomedical implants, a low infill density is required for tissue growth and osteoinduction. Thus, a compromise between infill density and fatigue strength should be made when an FFF print is used in such applications.
* The efficacy of other infill patterns, such as hexagonal and honeycomb, on the fatigue performance of FFF prints can be studied as they have shown great potential under static loading.
* Due to the anisotropic nature and layered configurations of FFF prints, they do not fully comply with the existing standards for the fatigue testing of polymers. Thus, new standards need to be developed.  
Conceptualization, H.B. and M.T.-R.; investigation, H.B. and M.T.-R.; writing--original draft preparation, H.B., M.A. and M.T.-R.; writing--review and editing, H.B., M.A. and M.T.-R. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The data presented in this study are available on request.  
The authors declare no conflict of interest.