# Strength Enhancement in Fused Filament Fabrication via the Isotropy Toolpath
## 2 Literature Review
### Strategies for Enhancing Mechanical Strength  
To date, several studies have investigated the process parameters' impacts on mechanical strength, such as building orientation, rotational angle per layer, filament deposition direction [30; 31; 32; 33; 34; 35; 36]. The common conclusion is to obtain a maximum strength is to keep the filament deposition orientation along the pulling direction. In addition, effects from extrusion rate and layer thickness are commonly studied [37; 38; 39; 40; 41]. However, the layer thickness is still a controversial parameter which varied results are concluded. In the meantime, increasing the feed rate could decrease the tensile strength [40; 41]. There are limitations of the strategies mentioned above to improve the build quality as listed:  
1. Process parameters (building orientation, layer thickness, extrusion rate, hatch spacing... ) are highly correlated. The individual adjustment on a single parameter would cause an unexpected quality change
2. Lack of quantitative relationship between process parameters and process outcomes (mechanical properties), the modification of the process parameters becomes more challenging  
Unlike any of the previous approaches mentioned, the method introduced by [PERSON] [42] involves the study of organic material as it aims to optimize the FDM process. This method is referred to as the Bacterial Foraging Optimization Algorithm (BFOA) and operates on the social and symbiotic relationships found in nature between organisms. [PERSON] [43] make the argument that utilization of computational tools and algorithms allows for a better comprehension of the strengths of the layers and fibers of the part. The magnitude of this strength can be significantly altered by the orientation of the build for each part. Thus, the utilization of computational algorithms to find this desired orientation is crucial to the FDM process. Besides, [PERSON] et al. [44] create a cylinder inside any given geometry and deposit a spiral toolpath upon the cylinder surface to increase the strength of the build. Some other studies on optimization on the deposition toolpath improve the product qualities [27; 28].  
Other methods, such as adding nanoparticles [45] or additional post-processing [46], can also improve mechanical properties. For example, [PERSON] et al. [47] used a new polymer such as PEEK to strengthen the 3D printed parts of physical and chemical properties. [PERSON] et al. [48] also presented extruding the same material (Z-pinning) in the structure to reinforce the element. Some other toolpaths are planar/non-planar to improve mechanical strength [22; 23; 49]. [PERSON] et al. [49] proposed a ZigZagZ toolpath that creates a zigzag pattern across the layers to increase the \(Z\)-axis strength. In addition, [PERSON] [22; 50] utilized the multi-axis slicing and curved layers to improve the part strength in a support-less manner. Altering the manufacturing strategy can enhance the mechanical strength, but using continuous carbon fiber [51] can also increase the print mechanical properties. However, the equipment and the material are relatively expensive compared with PLA or other thermoplastic materials.