# Experimental Study of Compression Behavior on Monolayer FFF Samples
## 3 Results  
### Geometrical Characteristics of the Specimens  
The qualitative analysis of specimen macro-geometry was in line with the quantitative measurements. Maximum values for all dimensions were expected (and confirmed) to occur at the starting point, where the transition zones between all layers were located, leading to a material accumulation. As the starting points were all external to the part contour for compression specimens, they did not affect specimen height. For the Type C specimens, the transition point between layers was always located in the same place. The extruder started and stopped the material deposition in the same x-/y-coordinates for each layer. Without careful considerations, this could have led to an extrusion of an excess of material, since the FFF machine tends to extrude more material at the starting point and end point of each layer. This could be avoided by careful trajectory design, leading to almost no differences in height between the starting point location and the rest of the circumference of the specimens. Still, an anisotropy of the material properties due to the non-continuous rasters at the starting point of the extrusion is very likely. Figure 4 shows the transition zone of one circular specimen. Type H specimens had a fundamentally different trajectory design that resulted in different geometrical deviations. As the specimens with helicoidal routes still had to have a flat contact area with the print bed, the layer thickness of the first \"layer\" was not uniform by definition. In fact, no clear layers can be defined using this trajectory type. Instead, each point along the trajectory is displaced slightly in z-direction compared to the previous point. The geometry can be grouped into loops instead of layers. The starting point slightly disturbed the geometrical accuracy of the subsequent loop (Figure 5), as the beginning of the first loop could not be manufactured with infinitesimal thicknesses. In fact, the starting point was experimentally found to have to have a minimum thickness of about 0.2 mm. The end point of the extrusion also slightly disturbed the uniformity of the monolayer. At this location, the extrusion thickness was not reduced, so the end point of the extrusion protruded at least one-layer thickness in z-direction. As can be seen in Figure 5, the end point often extended slightly above the last filament due to the print head moving up after the end of manufacturing. This protruding end point influences the compression properties of the monolayer because the compression platens do not have uniform contact with all parts of the cross-section at the start of the mechanical test. As the first loop of the geometry was extruded with less layer thickness and material flow, the second filament normally encompassed the first, as shown in Figure 6.  
Figure 4: Geometry surrounding the layer transition (build direction left to right). Figure 5: Type H specimen (build direction bottom to top). (**a**) End point, (**b**) starting point. Figure 6: Type H specimen viewed from below. This overlap of the second layer caused a more uniform thickness and diameter of the helicoidal monolayer across the specimen height. Otherwise, the slightly lower extrusion width in the first loop might have provoked an early onset of buckling. This means that the overlap shown above actually improved the suitability of the specimen type for mechanical testing. For all specimen types, the walls were slightly thicker than specified. This also led to a higher outer diameter of the specimens than specified. For S-shaped specimens, the wall thickness deviated the most from the nominal value compared to the other compression specimen types. The back-and-forth trajectory likely caused some excess material being extruded at the beginning and end of each layer. The S-shaped routes that were deposited in a back-and-forth manner caused some other interesting macro- and micro-geometrical effects.