# Enhancing Structural Performance of Short Fiber Reinforced Objects through Customized Tool-Path
## 1 Introduction  
Additive manufacturing (AM) technologies have been rapidly advancing and widening its applicability to complex geometries and range of material choice [1; 2; 3; 4; 5; 6]. Fused deposition modeling (FDM) is one of the widely used in AM technologies for the thermoplastic material. In contrast to traditional subtractive manufacturing, FDM parts are built by adding materials layer by layer. This layered nature of FDM causes some defects of the printed objects, such as staircase effect, coarse surface, and anisotropic mechanical properties [3; 4]. To address these challenges, improvement of the quality of FDM parts has been an active research area. For example, printing process techniques, such as heat treatment [7], machining [8], and chemical treatment [9; 10; 11; 12], were investigated to have a better surface quality of the parts. Printing parameters, including the effect of raster angle [13] and building direction [14], were optimized to obtain better bonding strength. However, there is a lack of design strategy to address the anisotropic characteristics of FDM parts. Moreover, the development of carbon fiber-reinforced polymer (CFRP) for FDM requires the optimization of material anisotropy for its best performance. For CFRP, extensive research has been carried out to investigate the anisotropic mechanical properties of CFRP, including ABS, PLA, and nylon. Generally, the anisotropic structural property of FDM parts is highly dependent on the building direction. Short carbon fiber provides 3D printing flexibility to improve the mechanical properties of parts than those printed by continuousfiber-reinforced filaments [15]. The effect of build orientation has been studied to enhance structural performance [16; 17; 18]. For FDM 3D printing, commercial slicer programs provide several limited infill patterns to choose a building direction. Once a pattern is selected, it is not allowed to edit its route. This is because the tool-path for FDM was originally developed to control the movement of the CNC machine cutter. When the tool-path algorithm was developed for FDM, it was for printing process improvement, structural strength enhancement of final products was not recognized. In this research, a novel design methodology for short carbon fiber composite built by FDM 3D printing is proposed. It starts with FEA stress analysis to determine principal stress directions of the FDM part with which customized tool-paths are developed. Structural performances of FDM parts built by the proposed tool-path method are verified by both FEA analyses and experiments. Tensile tests and related microstructural analyses using scanning electron microscope (SEM) for CFRP-Nylon were performed to investigate mechanical properties and fiber orientations. The framework for this methodology is shown in Figure 1. The primary contributions of this research are as follows:  
1. A novel design methodology for FDM parts assisted by finite element analysis;
2. A customized tool-path algorithm for FDM that maximizes the effect of fiber reinforcement under the given loading and boundary conditions.