## 2 Engineering Principles for Clinicians
### EBM vs. SLM  
It is useful to understand the main properties of the technology and to be aware of its limits and criticalities while designing a piece with particular resistance, finishing or precision features to be produced with 3D printing, since SLM and EBM are different. The main differences between them are the following [9]:  
* EBM technology has usually a higher building rate in respect to the SLM, therefore it is more suitable for the production of complex shapes, for example lattice structures; EBM technology has usually a higher building rate compared to SLM. This is due to two main factors: first, layer thickness has higher values (50-100 \(\upmu\)m in respect to the 20-100 \(\upmu\)m of the SLM process); secondly, the immediate electron beam motion from one location to another, thanks to the instantaneous response of magnetic coils, can considerably speed up component fabrication (for example, electron beam speed can reach 8000 m/s in ARCAM machines [11], with the laser reaching 7 m/s in EOS machines [12]). For instance, in the production of truss-like structures (lattice structures), EBM is to be chosen;
* The surface finishing of a component produced with EBM is much lower in respect to an SLM product. Is it sufficient to consider that the roughness parameters of EBM \"as built\" specimens are about twice those of SLM \"as built\" ones: in fact the roughness of EBM is 30-40 \(\upmu\)m while for SLM specimens is 11-18 um. This is mainly due to the fact that the layer thickness, powder size, and melting pool size in the case of SLM are half the one used in EBM specimens [13];
* The components produced with SLM undergo strong thermic gradients during the 3D printing process, since the preheating temperature of the powders is generally low, since the temperature of the chamber in SLM process is the environment temperature, assumed to be 293 K [14]. That is why they need some thermic post treatments in order to reduce the residual stresses that take origin inside the material because of the rapid and iterative phase change of the metal (solid-liquid-solid). On the contrary, in the EBM technology the temperature of the powder bed is higher and ensures the absence of thermal stresses inside the melted material, around 870 K during the melting process [15];
* On the contrary, maintaining the powder bed at high temperature has a bad influence on the quality of the microstructure of the melted metal that results coarse with large grains. This fact has a direct influence on the mechanical characteristics of the material: an SLM component, in fact, has higher resistance to traction while being less ductile in respect to an EBM product [16; 17].
* In the EBM, the pre-heating of the powder allows the unmelted particles to bind together and to act as a support for the overhanging geometrical features. Generally, EBM products need fewer physical supports in respect to the SLM ones.  
The correct positioning of the supports, especially when using the SLM, accounts for the good quality of the final piece in terms of dimensional precision, surface finishing, and defects. The main functions of support structures are [18]:  
* Withstand deformation or even collapse of processed material caused by gravity during the manufacturing process;
* Mitigate the effects of thermal gradients generated during production, since thermal distortions may lead to cracks, curling, sag, delamination, and shrinkage;
* Anchor the part to the build platform;
* In PBF, they stop any layer shifting during the re-coating phase.