*3.3. Shape Accuracy and Replicability*

The three replications of the same prosthesis manufactured by both the investigated processes were analysed one to each other and with respect to the initial CAD surface of the skull. Tables 2 and 3 show the comparison of the scanned data concerning the three replications obtained by SPIF and SPF in terms of average alignment error.

‐ ‐ ‐ ‐ It can be seen that the investigated forming processes are characterized by a low variability, since in both the cases the higher value of the average error between the replications is below 0.01 mm.

‐ ‐ ‐ ‐ At the same time, the SPF process reveals to be able to produce prostheses characterised by higher accuracy (the average error between the deformed geometry and the CAD model is close to zero). However, it should be noted that such a result is determined by the adoption of a female die which, on the contrary, was not necessary for the SPIF process.

 It should be also noted that, even without a die, the SPIF process allowed us to obtain a prosthesis characterised by an average error between the deformed geometry and the CAD model very small (about 0.12 mm). The proposed results confirm the effectiveness of the investigated manufacturing processes in ensuring a high dimensional accuracy of the formed blanks: SPIF prostheses' deviations are comparable to those achieved after the manufacturing of a TA1 cranial implant [22]. Both the investigated processes were accurate, ensuring lower deviations if compared to the accuracy achievable using the same alloy but more conventional [23] or, alternatively, an additive manufacturing approach [24]. The level of correspondence between the formed and the designed geometry is comparable, in the case of the SPIF, or even lower, in the case of the SPF, to the one reported in the

literature regarding the manufacturing of PEEK [25] and PMMA [26] customized implants by 3D printing.


**Table 2.** Average error of the three replications obtained by SPIF.

 **Table 3.** Average error of the three SPF replications.

