*2.3. Layer Thickness*

Layer thickness (or layer height [64]) represents the thickness of the layer printed by the nozzle tip, as shown in Figure 4. In general, it is smaller than the diameter of the extrusion nozzle (usually one-half), depending on the material and tip size. Layer thickness is directly related to the number of layers printed and hence print time. It has been verified that better accuracy of the component can be achieved by setting lower layer thickness.

**Figure 4.** Layer thickness.

Layer thickness is usually studied together with other parameters, most commonly with raster angle. Somireddy et al. [42] researched the influences of raster angle and layer thickness on the flexural behavior using classical laminate theory. Results presented that thinner layer laminates have higher loading capacity and flexural stiffness than thicker ones, except for the maximum deflection. Tymrak et al. [63] quantified the elastic modulus and tensile strength of PLA and ABS parts by comparing different layer thicknesses and bidirectional raster angles. Tests showed that tensile strength dropped with increasing layer thickness. In another study by Rankouhi et al. [62], the mechanical characterization of PLA by varying layer thicknesses and raster angles were analyzed. The maximum elastic modulus and ultimate tensile strength were obtained at lower values of both two factors. Similar results can be obtained for other materials, such as PEEK (Wu et al. [65]) and plaster-based powder (Vaezi and Chua [43]). Garg and Bhattacharya [66] considered layers of different thicknesses and rasters at different angles by simulation and experiment. FE analysis indicated that tensile strength, strain at yield, elongation, and developed stress first decreased with an increase in layer thickness and then increased. Layer thickness, build orientation, and raster angle were evaluated parameters to examine their effects on tensile strength by Nidagundi et al. [67]. Thinner layer thickness, 0◦ build orientation, and 0 ◦ raster angle were optimum for ultimate tensile strength.

In comparison, Rodríguez et al. [24] compared the effect of build orientation, infill density, and layer thickness on the mechanical characteristics of ABS and PLA test components. Regarding ABS, the mechanical strength results barely varied with respect to layer thickness. In contrast, tensile strength of PLA decreased as layer thickness increased. Chacón et al. [27] characterized the effect of layer thickness, build orientation, and print speed to determine the mechanical response of the PLA specimens. They observed that the increased print speed and layer thickness caused ductility to diminish. In addition, the mechanical properties for the upright orientation increased as layer thickness increased and as the print speed decreased, which however were of slight significance for on-edge and flat orientations. Alafaghani et al. [28] demonstrated that mechanical properties were significantly influenced by build orientation, extrusion temperature, and layer thickness; and less significantly on infill pattern, for high infill density specimens, and print speed. To improve the mechanical properties, higher extrusion temperature and larger layer thickness are needed in addition to appropriate build orientation. Carneiro et al. [68] mechanically assessed the influence of raster angle, layer thickness, infill density of PP and GRPP composites. The results showed the infill density had a linear effect on both mechanical properties. Instead, layer thickness had an insignificant effect on the performance of samples. Dong et al. [69] demonstrated that the number of layers was the only dominant factor in improving mechanical strengths of PLA and PLA/wood composites, compared with infill density and layer thickness.

In summary, layer thickness has a different effect on the strength. For a given total height, the thickness of a layer has an inverse proportional relationship with the number of layers. The thinner the layer thickness, the more layers. This response will lead to a high-temperature gradient towards the bottom of the component, which will improve diffusion between adjacent rasters, thus ultimately contributing to the load-bearing and enhancing the strength. In addition, this trend is heightened when at low print speed, which gives a better bonding with the previous layer. On the other hand, an increase in the number of layers also adds to the number of cooling and heating cycles, which in turn gives rise to residual stress accumulation. This behavior can result in distortion and inter-layer cracking, which will reduce the strength. Due to the interaction of these two different influences, in general, a moderate thickness value is obtained as the optimal parameter in some research [70].

### *2.4. Air Gap*

The air gap represents the space between two neighboring printed filaments on the deposited layer. In most cases, the air gap represents the distance between rasters, viz. raster to raster air gap. However, in some research, the air gap is distinguished as raster to contour air gap and contour to contour air gap, respectively. In general, there are three types of air gap, and they are zero, positive and negative. The zero type is generally the default configuration, which places beads just alongside each other. The positive type has a

loose place between beads which results in rapid building, while the negative type means that two beads partially overlap the structure, creating a denser component, as shown in Figure 5.

**Figure 5.** Air gap: (**a**) positive air gap (**b**) zero air gap (**c**) negative air gap.

Rodriguez et al. [71] observed three monofilaments with different air gaps made of ABS. From all arrangements tested, the highest stiffness and tensile strength values were found for the filament with rasters aligned in the loading direction and a small negative air gap. Too et al. [72] characterized that the air gap size had a profound impact on the porosity and compressive strength of FDM built part. With the increasing air gap of the test specimen, compressive strength decreased while porosity increased, respectively. Dawoud et al. [73] researched the impact, flexural and tensile strength of ABS components with different raster angles and air gaps. The air gap with a negative value proved to be the most significant factor in the enhancement of mechanical properties. However, in the case of a positive air gap, varying raster angles seemed to have a more significant effect on tensile strength. Masood et al. [74] presented experimental work on the effect of raster angle, raster width, and air gap on tensile properties of PC. They reported that the air gap was the only dominant parameter influencing tensile properties. This study also found that PC material by FDM had tensile strength in the range of 70 to 80% of the injection molded and extruded PC parts.

In the study of Slonov et al. [75], raster angle, air gap, and raster width on the mechanical properties of samples from PPSF were examined. The authors found that the elastic modulus generally depended on the air gap between rasters, independent of raster angle. On the contrary, the impact strength depended on the raster angle and the adhesion degree between filaments. Hossain et al. [76,77] modified raster width, raster angle, raster to raster air gap, and contour width to improve tensile mechanical properties of PEI material by visual feedback method. Using negative raster to raster air gap led to an average increase in ultimate tensile strength of 16%, compared to the default configuration. Montero et al. [78] examined five process parameters (raster angle, raster width, extrusion temperature, air gap, and color) to understand the ABS properties fabricated by FDM. They observed that the raster angle and air gap influenced tensile strength FDM printed part, while color, extrusion temperature, and raster width had little influence. Moreover, stiffness and shear strength between roads were lower than those measured between layers. Bagsik and Schöppner [79] considered the effect of build orientation, air gap, raster angle, and raster width based on the mechanical data of PEI. Based on their study, the air gap with a negative value contributed to the best results for all directions. With thicker filaments, better mechanical performance could be obtained for the on-edge and upright build direction, while a thinner filament enhanced the strength properties of the flat specimens.

In summary, air gap determines the area of force bearing as well as bonding between filaments. From the perspective of effect, the work of the former one on the mechanical property is more apparent than that of the latter one. In general, the positive air gap results in a loosely packed structure with weak bonding between adjacent filaments, leading to lower strength. In contrast, the negative air gap results in a denser squeezed structure with strong interfacial bonding, significantly improving the strength. Zero air gap may enhance the diffusion between the neighboring rasters, and cause the total bonding area to diminish as well.
