## 2 Background  
### Fused Deposition Modeling (FDM)  
Fused Deposition Modeling (FDM) is the most popular additive manufacturing technique in desktop 3D printers. In FDM, the mechanical properties of resulting printed parts depend on the bonding between layers [14]. Recent studies have also shown several factors influencing the adhesion and mechanical properties. These include the raster angle, raster-to-raster gap, infill density, layer thickness, and print orientation. Parts printed using the FDM process are weaker in the build direction, primarily due to the anisotropic nature of the parts [15; 16; 17; 18; 4]. The most feasible technique to reduce anisotropy is increasing the bond strength of each bead with the neighboring ones. A large nozzle diameter to layer height ratio is a possible method to improve cohesion [18]. A study by [PERSON] et al. [19] shows that higher operating temperatures of the printer can lead to better cohesion between deposited beads. However, this is not a recommended technique. Higher temperatures can cause the polymer to degrade, thereby weakening the part. This can also cause a residue in the melt channel. If the temperature of an already deposited layer increases past the glass transition temperature, it can adversely affect the cohesion. Small temperature gradients between subsequent layers are better for improving cohesion. While depositing a molten bead, it assumes an oblong shape. The final shape of the bead is dependent on the viscosity and surface energy of the bead. The oblong shape of the deposited material causes voids within the layers. These voids also affect the cohesion and adversely affect the part's mechanical properties. The final bead width and the contact area between beads also define the resolution achieved in an FDM process [10]. The five parameters that create voids resulting from the toolpath are bead width, raster angle, percent overlap, bead height, and build orientation [20]. Voids can be classified into three groups: corner voids, edge voids, and contour center voids. Larger bead widths will create larger corner voids. [PERSON] et al. [21] showed that nozzle temperature and nozzle-bead distance have a statistically significant effect on bead width. As temperature increases, viscosity decreases, resulting in a larger bead width. A smaller nozzle-to-bead distance \"sandwiches\" the material to the print bead, causing the material to spread transversely, resulting in larger bead widths. Edge voids depend upon raster angle, bead width, and percentage overlap [22]. Gaps between beads of parallel tool paths are usually constant on one layer. Multiple gaps between path segments may bring overfill/underfill in some areas [23]. Porosity or the percentage of open space (voids) relative to the printed part varies considerably with different print settings. In [PERSON] et al. [24], solid blocks of PLA were printed using various print settings and tested for porosity using SEM microscopy. The blocks ranged in porosity from 1.46% to 13.54% volume pores depending on the layer height, extrusion temperature, and extruder speed. Larger layer heights were shown to produce more porous parts. Bond strength is explored in [PERSON] et al. [25]. According to this research, stronger 3D-printed parts correlate to faster print speeds because each layer gets deposited on top of the previous one before it cools off completely. This higher initial interface temperature might lead to more diffusion between layers. However, this work also cautions against extremely high print speeds, which might result in thinner extrusions and thereby reduce the printed parts' strength. Another prior work related to this study was done by [PERSON] et al. [10]. This work deals with the relationship between voids within the extrusion and the dimensional accuracy. A negative air gap indicates that the distance between adjacent bead centers is smaller than the bead width.