# Thermal Layer Design in Fused Filament Fabrication
## 4 Discussion
### Method and Results
Due to these voids, the true area is somewhat smaller, and there may be some differences between the nominal and true stresses. According to Figure 9, however, the average width decreases, and the average thickness increases with an increasing layer temperature; the same amount of filament was being extruded, which supports the use of nominal dimensions. The change in specimen geometry seen in Figure 7 majorly affected the width/thickness as seen in Figure 9. However, dimensional accuracy is dependent on component geometry and size, and an optimal layer temperature can be obtained in components depending on the design. In our experience, circular-shaped closed geometries can maintain optimal layer temperatures within a size range, while maintaining excellent surface roughness and dimensional accuracy. When testing specimens made by EFF, geometrical defects from the fabrication will affect the results creating a deviation in the data. As seen in Figures 5 and 7, these defects are highly temperature dependent. In the lower temperature range of 150 \({}^{\circ}\)C, the defects are arguably less forgiving as the material rapidly cools down, preventing material flow after extrusion, creating a stress concentration, and resulting in a deviation in the dataset. As layer temperature increases, the material starts to melt and flow a small amount after extrusion, causing the thickness and width to decrease in reverse due to the lower viscosity. Moreover, as layer temperature increases from 150 to 166-175 \({}^{\circ}\)C, a more melted surface of the specimens is observed. The low viscosity allowing the material to flow a small amount after extrusion, in addition to few internal voids, might cause defects to be more forgiving, resulting in a low deviation between the specimens. However, when surpassing 166-175 \({}^{\circ}\)C, more internal voids (Figure 7) are observed, significantly affecting both the UTS and deviation in the specimens. An argument can be made that during manufacturing, the outer perimeter was first extruded before the infill, and due to the width/thickness flow effect, the infill might not fully connect with the outer parameter as the temperature increases. Therefore, internal voids may be the cause of the decrease in UTS at higher layer temperatures. According to the manufacturer's datasheet [14] the crystallization temperature of 184 \({}^{\circ}\)C causes different levels of crystallization that may also cause the reduction in UTS at higher temperatures. The increased layer temperature did not affect the outside surface roughness, as the Ra had minor variations, as seen in Figure 8. Similar Ra values are a positive result and still important to present, as rough surfaces both affect stress concentrations in addition to reducing postprocessing if smooth surfaces are required. The transition between the grip and gauge sections of the specimens had uneven geometry, which may be further affected by the higher printing speeds. Higher speeds mean that the filament is extruded on a hot layer, while both the nozzle and bed accelerate and decelerate rapidly during the infill for such a small area of the specimen, leading to the melting as well as inaccuracy of the deposited material on the surface of the previous layer. Melted, uneven geometry causes specimens of higher temperatures to have more deviations during testing, as they are more prone to cracks initializing in the uneven surface. Using three specimens for each layer temperature shows the progression displayed in Figure 5, but more samples would clarify the trend and deviations. Specimens printed in various locations on the printer bed could influence the print quality, as the bed has irregularities. The bed mesh had to be manually calibrated twice in the time it took to produce all of the samples used in this study. Samples had different prerequisites before printing, producing minor differences in the printing quality between samples.