## 4 Conclusions  
In this research, the influences of four main FDM printing process parameters including the infill percentage, printing temperature, layer thickness, and raster angle on the material anisotropy of the ABS material have been investigated systematically, and the obtained experimental and numerical results demonstrated that these four machine parameters proved to be very influential on the degree of mechanical anisotropy for FDM-printed ABS. According to the results obtained, the following conclusions can be drawn:  
* Although the best mechanical properties for the ABS polymer in the LBO and TBO were recorded at the 0.25 mm and 0.15 mm layer thicknesses, respectively, the minimum mechanical anisotropy for this polymer was recorded at 0.15 mm. * In terms of raster angle, samples made of ABS and printed with a \(0^{\circ}/90^{\circ}\) raster angle showed the best results with lower mechanical anisotropy values compared to the \(+45^{\circ}/-45^{\circ}\) raster angle. * The infill percentage can enormously influence the integrity of the structure of the FDM-printed parts and consequently the degree of mechanical anisotropy. By increasing the infill percentage, ABS showed an improving trend in mechanical properties, as well as a lower degree of mechanical anisotropy to a point that at 100% infill, the values of mechanical anisotropy reached a very similar value. * The layer adhesion and mechanical integrity of the FDM-printed parts are very sensitive to print temperature changes and if the proper temperature can be selected, the mechanical anisotropy can be reduced noticeably. For both the LBO and TBO, 260 \({}^{\circ}\)C was found to be the best temperature to print samples. Considering the numerical analysis on the effect of the infill percentage on the UTS of the 3D-printed ABS polymers in the two build orientations--LBO and TBO--the following research finding can be listed as follows:  
* The UTS of the samples printed in the LBO can be predicted using the ROM method with very accurate results. By contrast, the UTS of those samples printed in the TBO only can be predicted by introducing additional functions with the modified ROM with an increase of 10% in accuracy. * It is worth mentioning that the developed MROM provides an effective tool to predict the material proprieties of FDM-printed ABS compared to the original ROM validated by experimental data. While the primary goal of this study was to systematically introduce and elaborate on the key influential factors contributing to the generation of anisotropy phenomena, further research is warranted to explore additional FDM printing parameters that may have a significant impact. Investigating other types of polymers other than the ones stated in this study could help to provide a better understanding of the formation of the intrinsic and extrinsic air voids within the printed samples. A further investigation can be conducted by incorporating other monomers into the ABS structures in the form of in situ polymerisation to introduce some of the favourable elements into the ABS polymer to increase mechanical isotropies. As for further future work, in terms of the modelling of the results, the other machine parameters need to be investigated more, and developing an accurate model to represent the results for the parameters not covered in this study is crucial. Until then, the outcome of this research can be extremely useful and can be relied on in terms of the effect of the anisotropy on the mechanical properties of the printed samples and will be useful in the material and process design of the 3D-printed polymeric parts. Conceptualisation: R.Y. and N.Z. ; methodology: R.Y., N.Z. and P.Q.K.N. ; investigation: N.Z. and P.Q.K.N. ; writing--original draft and conducting the numerical modelling: N.Z. ; conducting the experimental testings and editing: N.Z. ; assisting numerical modelling: R.Y. and P.Q.K.N. ; visualisation: N.Z. ; supervision: R.Y. ; project administration: R.Y. ; funding acquisition: R.Y.