## 5 Validation: Design for Additive Manufacturing and Multi Jet Fusion
### Design for Additive Manufacturing  
Design for additive manufacturing (DFAM) is a type of design for X applied to this new technology, full of opportunities in terms of design. It is a tool that optimizes functional performance and  
\begin{table}
\begin{tabular}{c c} \hline Power Melting Point & 187 \({}^{\circ}\)C \\ Particle Size & 60 \(\upmu\)m \\ Density & 1.01 g/cm\({}^{3}\) \\ XY Tensile Strength & 48 MPa \\ Z Tensile Strength & 48 MPa \\ Elongation at Break XY & 20\% \\ Elongation at Break Z & 15\% \\ Short Hardness D & 80 \\ \hline \end{tabular}
\end{table}
Table 5: Mechanical properties of PA 12.  
other product lifecycle considerations, such as manufacturability, reliability, and cost. This process emerges thanks to the enormous design freedom provided by additive technologies. Typical DFAM methods or tools include topology optimization, multi material design, mass customization, and lattice structures. As stated, the DFAM should \"maximize product performance through the synthesis of forms, dimensions, hierarchical structures and material compositions, subject to the capabilities of AM technologies\".  
The DFAM can be seen as a version of the design for manufacturing and assembly pushed to the limit. In the DFMA, which was introduced in the 90s to improve the production of components, the most common solutions are:  
- Minimize the number of components  
- Remove the connections  
However, it is easy to understand that by decreasing the number of components, the cost of assembly decreases but the cost for geometric complexity increases exponentially. For this reason, the DFM presents this technological limit. The DFAM, instead, goes further thanks to the freedom of design offered by additive manufacturing. This philosophy focuses mainly on three issues (Figure 16):  
(1) Topology optimization: The layer-by-layer process allows to have complex sections up to the maximum allowed resolution. Moreover, it is possible to define the shape re-design, meaning that it is possible to optimize the shape of the product solely based on the functions it meets. Topology can be optimized in terms of stress and functionality and not in terms of technological limits. As for the fan, we have already seen that the central body cannot be produced by conventional technologies a part from additive manufacturing.  
(2) Lattice structure: It is possible to have a control of the different levels of structure. It generates much lighter and less expensive structures. Although the compact structures are stronger, the specific strength of the samples of latex structures is higher. They provide a higher force per kilogram of material. The reticular structures provide sufficient strength with reduced mass. During the project, both full and lattice structures were designed.  
(3) Functional complexity: It is possible to print assembled parts in one operation, avoiding assembly costs.  
As for machining, HP suggests some general rules to apply during the design process to avoid printing defects. The most important are:  
- Wall thickness: Printing resolution is strongly anisotropic. However, HP suggests wall thickness of 0.3 mm for XY planes and 0.5 mm for Z planes.  
- Lattice structure: Since HP uses powder selective technology, the not melted powder need some allowance to be removed. For closed components, it is necessary to leave holes of 2 mm diameter to let powder go out during cleaning.  
Figure 16: Design for additive manufacturing.  
Functional complexity: When printing assemblies, it is compulsory to allow at least 0.5 mm clearance between components.
* Part orientation: As we already said, the process is strongly anisotropic and cooling strongly affects the shape of components, It is important the orientation of surfaces to reduce cooling defects.