# Comparison of CAD Software for Designing Cellular Structures for Additive Manufacturing
## 3 Results
### nTop  
The lattice structure module of the nTop design software is used to either build strut-based or surface-based base cells on a rectangular, cylindrical, spherical, or chosen periodic pattern [57; 58; 59; 60]. The program also offers functionality to build conformal lattice structures from a surface mesh or a volume mesh. Apart from the possibility of forming constant lattice structures, nTop offers the option to implement stochastic structures such as 2D and 3D Voronoi lattices as well as branched-, Delaunay-, and Orthofoam-Lattices. nTop is therefore seen as an engineering design software specifically developed to aid in the design and optimization process of products for AM. The software is developed by nTopolgy Inc. in New York City. Unlike proven CAD tools that are used in the industry today, which  
Figure 8: Different types of tessellations of the bike seat’s design space: (**a**) non-conformal BCC lattice structure (**left**) and conformal tessellation with quadratic lattice structure (**right**) and (**b**) mesh representation of the upper bike saddle surface (**left**) and quadratic mesh (**right**). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Criteria** & **BCC** & **FCC** & **Relative Change** \\ \hline Lattice processing time & 21 min 48 s & 25 min 33 s & 17.2\% \\ Resolution & 0.1 mm & 0.1 mm & \\ Export time & 17.59 s & 21.45 s & 21.9\% \\ File size & 33.1 MB & 39.5 MB & 19.3\% \\ \hline \hline \end{tabular}
\end{table} 
Table 3: BCC and FCC lattice generation values of the bike seat in Rhino7/Grasshopper®. use B-Reps and NURBS to represent a specific design, this software is based on the use of implicit model representations, which is advantageous with respect to increased computing speed, scalability, reliability, and control [61]. Nonetheless, the software tool offers all possible interfaces to import CAD and other representations of geometry such as meshes, voxels, and point clouds. The formulation of the lattice structure in nTop is similar with respect to model treatment, although the handling inside the tool is very different from the other investigated software tools. The outer and inner volumes of the helical gear (Figure 9) were considered as solids, and the design space in between was populated with the previously defined lattice characteristics. The overall process within nTop, although intuitive, requires additional time to manage correctly. The formulation of the lattice structure was achieved without complications. Because of the fact that nTop works with implicit body representations, calculation times are often longer compared with other software tools. The computing time of the BCC lattice structure required 2 min and 41 s, whereas the calculation of the FCC lattice required 3 min and 22.1 s, both of which were the longest of all the investigated software tools (see Table 4). The building of the \"implicit body view\" for the lattice representation was broken down into the following steps: outer solid, design space, inner solid, full body, and implicit body. In order to export the generated lattice structure into the desired STL-File, the formulated implicit body had to be converted into a mesh. For the BCC lattice, this additional operation took a further 3 min 44.41 s. After the conversion, the mesh was finally exported as binary STL, which took 30.53 s, adding to a grand total of 4 min 14.95 s of export time for the helical gear with the BCC lattice structure (see Table 5). The same approach was used to formulate the lattice structures within the design space of the bike seat (Figure 10). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Criteria** & **BCC** & **FCC** & **Relative Change** \\ \hline Outer solid & 20.83 s & 16.4 s & 27.0\% \\ Design space & 14.04 s & 13.4 s & 4.8\% \\ Inner solid & 13.99 s & 18.3 s & 30.8\% \\ Full body & 46.05 s & 1 min 2 s & 34.6\% \\ Implicit body & 1 min 6 s & 1 min 32 s & 39.4\% \\ Total & 2 min 41 s & 3 min 22.1 s & 25.5\% \\ \hline \hline \end{tabular}
\end{table}