# Experimental Study of Compression Behavior on Monolayer FFF Samples
## 2 Materials and Methods
Due to buckling criteria, it was chosen to be relatively big (1 mm). However, as the specimens finally had to be manufactured at very low extrusion speeds due to heat dissipation constraints, the flow ended up being higher than specified. This excess material extrusion caused an average actual wall thickness of 2.2 mm. The minimum necessary diameter of the circular specimen was determined by buckling criteria using the actual wall thickness obtained for the manufacturing experiments. Out of the buckling cases established by [PERSON], the one with the smallest allowable maximum force Rc before buckling onset (for two ends that are free to rotate) was used to calculate the resulting minimum diameter of the specimen. Solving the resulting inequality numerically, a minimum diameter of 5.6 mm would be needed in order to avoid buckling of the specimen before the expected compression strength of about 100 MPa [7; 20] is reached. In order to account for other effects between the extruded filaments of the specimen, an about four times higher diameter of d = 22.2 mm is chosen to make sure that no inadvertent buckling will occur. When disregarding the material thickness, the diameter of the circle that the print head follows is d = 20 mm. This has the added advantage of being equal to the specimen height. The dimensions of the specimens can be seen in Figure 3.  
The dimensions of the S-shaped specimen follow directly from the dimensions of the circular specimen. The wall thickness of the monolayers, the specimen height, and the radius of the (half-)circles are kept the same. The resulting final design is essentially the same as two halves of the circular design being rotated and fused together again. All manufacturing was performed on the additive fabricator CubeX 3 Dsystems (3D systems, Rock Hill, SC, USA). It has a print head nozzle diameter of 0.75 mm. Seven specimens were manufactured of each geometry type due to relatively high standard deviations of the mechanical parameters compared to tensile tests. The compression specimens of each type were manufactured with an extruder temperature of 498 K and with a low extruder speed of 5 mm/s due to heat dissipation constraints. The layer thickness  
Figure 3: Dimensions of the specimens. (**a**) Circular shape (Type C and H). (**b**) S-shape (Type S). Figure 2: Experimental methodology. was 0.75 mm. The material used was commercial PLA from 3D systems (3D systems, Rock Hill, SC, USA) in a cartridge adapted to the machine used. The material was incorporated in a cartridge that guarantees its humidity conditions. After the manufacturing process, each specimen was subjected to a characterization stage in order to catalogue its macro-geometrical defects resulting from manufacturing. In order to catalogue the occurring defects in macro-geometry, images were captured with a high-resolution camera. For a more detailed characterization of the geometrical quality of the specimens, they were also evaluated using Nikon(tm) SM2800 (Nikon, Tokio, Japan) scanning optical microscope (SOM). For a more quantitative approach, all specimens were measured with a caliper gauge regarding their main dimensions. From there, the cross-section of the specimens was calculated and used as an input parameter for stress calculation during compression testing. The universal testing machine Shimadzu(tm) AG-X (Shimadzu, Kioto, Japan) series was used for all mechanical tests. It has a maximum load cell capacity of 50 kN and a displacement resolution of 0.0208 um. Data sampling occured at a frequency of 5 kHz. The platens used for mechanical tests were fixed compression plates with a flat surface. They have a diameter of 100 mm, which is sufficient for the specimens with a diameter of about 20 mm. The tests were performed with environmental conditions and specimen minimum age after manufacturing according to ISO 527-1 [21], and compression test conditions according to ISO 604 [22].