*2.3. SPIF Process Design and Prosthesis Manufacturing*

‐ During the design of the SPIF process, attention was paid not only to the final geometry of the prosthesis but also to the best positioning of the 3D profile on the flat surface, in order to reduce the blank size and to optimize the final accuracy. The first constraint is represented by the feasibility of the process, then ensuring the forming of the part without excessive thinning while minimizing the material's waste.

‐ The possibility to obtain the prosthesis geometry by SPIF requires the construction of a sidewall with a shape generally conical: to deform the sheet incrementally without any fracture occurrence, a vertical wall should be avoided. On the contrary, a small inclination of the sidewall would generate a lot of waste. Starting from the prosthesis surface reported in Figure 7a, the sidewall inclination and the best positioning of the prosthesis in the forming area were designed with the aid of numerical simulations. In detail, FE simulations were performed modeling the sheet as a deformable body with S4R elements and 5 integration points along the thickness, whereas the tool was modeled as a rigid body.

**Figure 7.** (**a**) The reconstructed analytical surface of the ROI and the original portion of the skull (red mesh) near the ROI, (**b**) the designed final geometry for the SPIF.

‐ The tool path was generated with the CAD/CAM software Creo (v7, PTC, Boston, MA, USA). The periphery of the blank was pinned. The contact was modeled as surface-tosurface contact, setting a Coulomb friction coefficient equal to 0.1 between the tool and the sheet. Finally, the numerical problem was solved with the implicit integration scheme. The final geometry of the part produced by SPIF, characterized by an inclined sidewall of 30◦ , is reported in Figure 7b.

‐ ‐ ‐ The above-described geometry was obtained using a hemispherical head tool (diameter: 6 mm) and setting a step depth of 0.05 mm in the CAD/ CAM software used for generating the toolpath for the CNC machine. Figure 8a shows the CAM simulation results of the SPIF geometry after the forming phase. Once completed the SPIF operation, the

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drilling of the four holes for the prosthesis anchoring, as well as the cutting of the outer edge, was simulated (as reported in Figure 8b). 

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‐ **Figure 8.** (**a**) CAM simulation results of the SPIF geometry after the forming phase; (**b**) final prosthesis after the CNC milling simulation. ‐

‐ To manufacture the prosthesis by SPIF, a Mazak Nexus 410 milling machine (Yamazaki Mazak Corporation, Oguchi, Japan) was equipped with a heating chamber able to heat the blank up to 420 ◦C during the forming process and to keep it constant for the whole manufacturing phase. A feed rate of 2 m/min and a spindle speed of 600 RPM were used. The final geometry of the prostheses was obtained with the same milling machine by using a milling cutter with a diameter of 4 mm and a drill bit with a diameter of 2 mm to perform the anchoring holes. ‐ ‐ ‐ ‐
