# Strength Enhancement in Fused Filament Fabrication via the Isotropy Toolpath
## 1 Introduction
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Process Variable Adjustment** & **Process Properties Enhancement** & **Key Findings** & **Reference** \\ \hline \hline \multirow{2}{*}{Parallel directional toolpath} & \multirow{2}{*}{Porosity} & Minimizing porosity by adaptive & \multirow{2}{*}{[1]} \\  & & path width & \\ \hline \multirow{4}{*}{Material deposition sequence} & Ultimate yield strength, flexural & \multirow{2}{*}{Altering deposition sequencing in} & \multirow{4}{*}{[2]} \\  & strength & linear toolpath to minimize the & \\ \cline{1-1} \cline{3-3} \multirow{2}{*}{Material deposition sequence} & Minimize Airtime & airtime, overall building time, and & \\ \cline{1-1} \cline{3-3} \multirow{2}{*}{Minimize travel time for lattice} & Minimize extrusion-less travel & & \\ \cline{1-1} \cline{3-3} \multirow{2}{*}{Interlayer bonding} & \multirow{2}{*}{Part strength} & Building orientation affects the & \multirow{2}{*}{[6]} \\  & & intra-layer and inter-layer bonding, & \\ \cline{1-1} \cline{3-3} \multirow{2}{*}{Interlayer bonding} & \multirow{2}{*}{Part strength} & ultimately affect part strength & \\ \hline \multirow{2}{*}{Processing condition} & The bonding strength between & \multirow{2}{*}{Adjoining polymer filaments are} & \multirow{2}{*}{[7]} \\  & layers & investigated to strengthen the & \\ \cline{1-1} \cline{3-3} \multirow{2}{*}{Adaptive slicing} & \multirow{2}{*}{Part surface quality, geometric accuracy} & \multirow{2}{*}{Non-uniform layer thickness can} & \multirow{2}{*}{[8]} \\  & & improve the surface finish & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Optimization Strategies in FFF processes. Figure 1: Workflow for AM builds preparation. However, the homogeneously laminated filament bars can cause the in-plane anisotropy material property [13]. Considering the material deposition manner, the manuscript targets to provide a material deposition strategy to achieve the in-plane isotropy, ultimately improving the mechanical strength. Existing techniques in improving mechanical strength regarding altering toolpath can be classified as bio-inspired structures and non-planar toolpaths. Bio-inspired structures such as honeycomb [14], cross-linked [15], and Bouligand structures [16; 17] have been studied to improve mechanical properties. In addition, the other strategies by altering the toolpath to reduce the in-plane anisotropic property caused by longitudinal filament are shown in Figure 2. These strategies are mostly adopted in metal AM processes for reducing the thermal distortion by evenly distributing the heat. However, the continuity of the in-plane filament cannot be maintained through these. Compared with these structures, our proposed in-plane isotropy toolpath can achieve a solid component in a more flexible filament arrangement to reduce the in-plane void region and increase mechanical strength at the same time. Recently, several researchers [20; 21] studied the rotational angle \(\alpha\) per layer to improve the mechanical properties; however, the rotational angle can create voids on the 2D geometry based on the deposited filament shape, which can further induce the unsatisfactory mechanical properties. The non-planar toolpath [11; 22] effect towards improving mechanical strength is still ambiguous; the non-planar layer creation and modeling towards better print performance are not developed. In this research, an isotropy in-plane material deposition strategy is provided to fulfill the solid density with strong interfacial bonding. Our developed toolpath strategy is targeting to achieve a randomized material distribution in one layer while maintaining the material deposition in a continuous manner. The proposed toolpath generation workflow is presented in Figure 3.  
Figure 3: Workflow of generating in-plane isotropy toolpath. Figure 2: Existing strategies of handling anisotropy created by longitudinal filament, (**a**) adaptive toolpath width [18], (**b**) island toolpath strategy [19], (**c**) hatch parallel toolpath strategy [19].