*2.6. Infill Density*

The outer region of AM part is usually solid, but the interior area, generally known as the infill, is the inner component covered by the skin, which has different geometries and sizes. Infill density (or infill degree [68], infill ratio [82], infill percentage [90], fill density [88]) refers to the percentage of filament material printed in the given part, where 0% is a shell and 100% is a solid. FDM technology allows users to control the infill density through parameters such as air gap or raster width.

Alvarez et al. [90] observed that the maximum impact resistance, tensile stress, and tensile force were obtained with 100% infill density. Martikka et al. [91] revealed that the increment in infill density enhanced the tensile properties of PLA and PLA/wood composites. Gomez-Gras et al. [92] carried out the Taguchi method to investigate the impact of four process parameters and their intersections—layer thickness, infill density, nozzle diameter, and print speed, on fatigue response. It was concluded that infill density showed the strongest influence in fatigue performance, followed by nozzle

diameter and layer thickness, whereas print speed showed no relevant effect in PLA specimens. Aw et al. [93] looked at relating process parameters to tensile properties of CABS/ZnO composites with infill density and infill pattern. Results revealed that tensile strength of CABS composites was little affected by the change of infill density, while the increased infill density caused Young's modulus to increase, resulting in higher stiffness. Line pattern possessed better tensile properties. Kerekes et al. [94] pointed out that with an increase in infill density, Young's modulus, initial yield stress, ultimate strength, and toughness increased, while elongation at break decreased. Layer thickness showed a moderate influence affecting the specimen's properties, where an increasing layer thickness apparently increased Young's modulus, while it decreased elongation at break. Lužanin et al. [95] experimentally analyzed flexural properties depending on the infill density, layer thickness, and raster angle. The researchers reported that layer thickness was the most important parameter affecting flexural force, and the interaction between infill density and raster angle was significant as well. The mechanical effect of printing parameters for carbon fiber-reinforced polyamide was studied by Toro et al. [13]. The most dominant parameter was found to be infill density. Layer thickness and infill pattern played importantly in flexural and tensile behaviors, respectively.

In summary, the mass and strength of FDM produced parts are dependent on the infill density. Lower density requires less print time and material, thus saving cost and reducing the weight. However, more voids are generated within the structure simultaneously, leading to increased porosity. As a result, the dimension of the bonded region between filaments decreases and so as well to the mechanical properties. In contrast, the denser component possesses better mechanical properties but takes much more time to be complete. For example, the specimen built with 100% infill density usually exhibits maximum strength. Generally, infill density ranging from 50% to 98% is recommended, since the improvement in mechanical resistance is countered by longer manufacturing times [90].
