## 3 Results and Discussion
### Effects of Infill Patterns on Mechanical Properties
All specimens cracked at the gauge length area but the cracked lines in zigzag patterns generated by CAD were perpendicular (see Figure 11). The cracks started from failure of the internal structure and subsequently the bottom layers due to the poor binding that was mentioned before. The lower strength in CAD samples might happen due to the non-optimized design and poor binding of patterns in the internal structure. Subsequently, the top layer in samples cracked because of strong binding between line width. Hence, this issue was investigated in the form of a microscopic view inside the samples in the following section. Results showed that by increasing infill density, samples' strength increased as well. Lower density resulted in weaker parts but with lower weight. Undoubtedly, solid or 100% density is the strongest way to 3D print products but weight is maximum in the final sample. Different crack lines occurred due to the pattern structures and various binding behaviors. However, other elements are vital to enhance stiffness and strength, in this study, infill effects were examined for two different production methods. In brief, patterns that were generated by the FDM printer showed better strength and mechanical properties compared to the CAD specimens. This means adhesion between layers was a little bit weaker in CAD products. According to the infill structure, the microscopic view of printed patterns was analyzed to identify issues inside the specimens. Concerning the mechanical properties and strength of the 3D-printed sample, infill structures were examined throughout the cracked areas. As shown in Figure 12, all CAD and FDM printed samples follow the same structure. Layer printing structures were totally visible inside the samples, which showed the reason why each part cracked at specific loads. The reason why the concentric pattern tolerated higher tensile load was the pattern generated in the same direction as the tensile load. The same thing happens in FEA procedure, and internal layers started to crack in both procedure due to poor binding. Apart from this, cracks occurred in internal layers in the FDM process due to poor binding. This drawback might cause weaker samples in the CAD procedure. Besides, defects in zigzag pattern generated by CAD with 50% density are shown in Figure 12b. Gaps and voids were greater in the CAD process compared to the FDM printing procedure. This obstacle led to lighter and weaker final products. Also, layer adhesion in the internal structure was affected by this defect in CAD samples. Figure 11: Failure areas in (**a**) grid, (**b**) triangle, (**c**) concentric, and (**d**) zigzag patterns of the printed parts. In contrast, the triangle and grid patterns which are shown in Figure 12c,d are neat and clear in both procedures. Finding an appropriate design and density was effective in both ways. This means layer classification is crucial in achieving the highest stiffness. Figure 12: Microscopic view of crack areas in (**a**) concentric, (**b**) zigzag, (**c**) triangle, and (**d**) grid patterns in CAD (left) and FDM (right) specimens. If each layer sticks to the previous one properly, the strength of the printed part becomes better as well. This task can be done with optimum temperature and melting process. Poor layer adhesion affected surface quality and strength due to the rough and not aligned structure. Also, the attachment between internal patterns and wall thickness is important. The gaps between these two sections lead to poor mechanical properties and weak samples. Weight divergences were not excessive but density had a direct effect on weight. Further, SR was slightly better in CAD samples compared to the FDM, but samples generated by FDM were moderately stronger. Both techniques were useful for building infill patterns. The differences between simulated and printed items were not excessive, and both procedures resulted the same strength and stiffness. As an example, FDM printers are not able to combine patterns in one sample.