# Strength Enhancement in Fused Filament Fabrication via the Isotropy Toolpath
## 4 Results  
In this section, fractured specimens (layer thickness = 0.2 mm) are presented in Figure 7, with the geometry of each fracture surface. Compared among these printed specimens, the proposed toolpath specimen has a shear crack that behaves better. From Figure 7, we can see that all PLA specimens by using traditional linear toolpath crack under the tensile test. The contour toolpath specimen did not crack, but each path has necking. The bonding between each parallel path is weak, thus resulting in the failure. However, the specimen which is fabricated using the proposed toolpath has a shear crack. This implies the specimen developed tangential displacement, thus resulting in a better ultimate tensile strength. The load extension graphs of all printed specimens are shown in Figures 8 and 9. In general, specimens with proposed in-plane isotropy toolpaths present better tensile strength than all traditional paths. The ultimate tensile strength of all printed specimens is shown in Table 3.  
Figure 8 and 9 indicate the proposed toolpath strategy can successfully increase the tensile strength compared with the same layer thickness specimens. The ultimate tensile strength can be improved by at least 19% compared to the traditional contour toolpath. In the meantime, the proposed toolpath would lead to a minimum extension before the fracture. Table 3 shows the average of ultimate tensile strength for all printed specimens at each printing strategy; the proposed toolpath shows a 20% increase in the tensile strength  
Figure 8: Load extension graph for all print specimens at layer thickness = 0.1 mm. Figure 9: Load extension graph for all print specimens at layer thickness = 0.2 mm. compared with the other specimens under the same layer thickness condition. From Table 3, we can see that the Young's Modulus for the proposed toolpath specimen yields at least 1.3 times than the other traditional toolpath specimen. Therefore, the proposed toolpath can result in a higher modulus and a denser component. The higher ductility of the proposed manufactured components also indicates the lower voids fraction than the traditionally manufactured specimens. The relative density of each specimen is obtained. This also indicates the tensile strength is correlated with relative density. The proposed toolpath generates a higher density component, thus creating a strengthened geometry. Figure 10 presents all print specimens with each printing strategy with their average ultimate tensile strengths and associated standard error bars. From this figure, we can see that the 0.1-mm layer thickness can strengthen the part of the 0.2-mm layer thickness. However, such layer thickness effect is not significant on the printed specimens using the proposed toolpath. This further proves the proposed toolpath can create a stronger tensile property, disregards the layer thickness. To verifying the significance level of each variable to the tensile strength, an ANOVA analysis of variance (Table 4) is conducted for \"layer thickness\" and \"toolpath strategy\" factor with \"tensile strength\" as the response. The \"rotational angle\" is not included in this study because the contour and proposed toolpath do not have any rotational angle value. From Table 4, the P-value for toolpath strategy is 0.001\(<\)0.005, which means that the toolpath strategy is a significant factor for the tensile strength in the Fused Filament Fabrication process. Table 3 and Figure 10 show that the specimens printed with contour toolpath strategy at 0.1 mm and 0.2 mm show a better tensile strength and a higher density component compared with other specimens printed with the traditional linear toolpath even with a rotational angle. However, the specimen with the proposed toolpath that is printed with a 0.1-mm layer thickness behaves a 19.49% larger tensile strength than the 0.1-mm layer thickness specimen with a contour toolpath. The tensile test of the specimen with the proposed toolpath that is printed at a 0.2-mm layer thickness performs similarly.