# A Computational Geometry Generation Method for Creating 3D Printed Composites and Porous Structures
## 3 Results-Discussion  
Three cubic geometries were generated by the method described above. Each geometry was split on two segments of approximately equal volume and materials were assigned for each segment. The produced materials were porous PLA (Figure 4A), porous TPU (Figure 4B), PLA and TPU composite (Figure 4C) and PLA and Nylon composite (Figure 4D). The materials in the composite configuration were intertwined within the structure. An infill setting of 20% was set during the printing process. The resulting specimens were within a density range of approximately \(\rho=\) 500-1000 kg\(\cdot\)m\({}^{-3}\).  
Figure 4: Cube specimens (side of approx. 30 mm) (**A**) 3D printed porous PLA specimen, (**B**) 3D printed porous TPU specimen, (**C**) 3D printed PLA and TPU specimen, (**D**) 3D printed PLA and Nylon specimen.  
Figure 3: (**A**) Boolean operation producing porous geometry specimens, (**B**) producing composite structure specimens.  
Figure 5A shows the compressive stress strain behavior profile of the tested specimens. Composite and porous PLA specimens exhibited an elastic plastic behavior i.e., a steep elastic part followed by plastic region of lower gradient. TPU specimens were of significantly lower stiffness.  
Figure 5B shows the stress response at 0.1 nominal strain in relation to density for each of the tested specimens. The same three geometries were used for all specimens but  
Figure 5: (**A**) Nominal stress and strain compressive response of porous PLA, porous TPU, PLA and TPU composite, PLA and Nylon composite. (**B**) Stress at 0.1 strain and density for the tested specimens.  
with different materials. Density and stress response appears to be more consistent in the porous specimens rather than the composite.  
Table 2 provides with the values of the stress response at 0.1 and 0.2 nominal strain in relation to the resulting density of the specimens. The relation of density to stress response in the porous structures and by comparison to fully solid specimens found in literature, appears to be non-linear [29; 30].  
The combination of soft and hard material in multiphase materials (e.g., TPU and PLA) can result in synergistic effects. Similar approaches in literature, using regular structures, found that while the hard phase can endure larger portion of the load, the softer phase can confine cracks. This could be promising in applications that require damage tolerance and vibration damping. Additionally, phase topology can have an important effect on mechanical properties [31]. Material combinations can also add to the efficiency of the materials. The stress response of the TPU-PLA composite can be more than twice the sum of the stress response that the two porous materials have when used separately (Table 2), and for less than the sum of their densities.  
The same three geometries were used for all specimen types. The consistency in the density and stress response of the produced specimens varied between types (Figure 5B). The produced porous structures were generally more consistent in the resulting density, rather than the composite specimens of which PLA--Nylon seemed to be more consistent than PLA--TPU. This might have been due to equipment limitations, such as resolution, nozzle size, in combination with geometrical features of the structures and the specific materials. Infill percentage was set to 20%, however this might not have been consistent between all cross-sections and materials. Cross-sectional variation has been found to affect the resulting properties and possibly infill, which should be considered if scaling is applied [32].