# Investigations on Factors Affecting 3D-Printed Holes Dimensional Accuracy and Repeatability
## 4 Conclusions  
Several practical conclusions can be drawn from this research in which the dimensional accuracy and repeatability of 3D-printed holes were studied in relationship to different factors, such as layer thickness, number of shells around the holes, printing speed, hole axis orientation angles, slicers, 3D printers and materials, as well as post-processing by medical decontamination. The most useful finding was that there was no significant difference in the dimensional accuracy of the straight 3D-printed holes between parts manufactured in different sites. This is a relevant aspect as the production delocalization is an advantage of 3 DP, and the users of 3D-printed products (end-use parts, spare parts, etc.) have to be sure that the technology and equipment deliver reliable outcomes in terms of dimensional accuracy and repeatability regardless of the printer producers or slicing software used. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Hole Diameter** & **Source** & **F-Value** & \(p\)**-Value** \\ \hline \multirow{3}{*}{2.3 mm before} & Layer thickness & 0.12 & 0.741 \\  & Number of shells & 0.12 & 0.741 \\  & Angle & 0 & 1 \\ \hline \multirow{3}{*}{2.3 mm after} & Layer thickness & 0.02 & 0.883 \\  & Number of shells & 0.03 & 0.865 \\  & Angle & 0.01 & 0.993 \\ \hline \multirow{3}{*}{4. 5 mm before} & Layer thickness & 20.05 & 0.03 \\  & Number of shells & 4.88 & 0.063 \\  & Angle & 0.84 & 0.471 \\ \hline \multirow{3}{*}{4. 5 mm after} & Layer thickness & 0.72 & 0.425 \\  & Number of shells & 6.29 & 0.041 \\  & Angle & 3.81 & 0.076 \\ \hline \multirow{3}{*}{6. 5 mm before} & Layer thickness & 0.48 & 0.510 \\  & Number of shells & 0.21 & 0.657 \\ \cline{1-1}  & Angle & 2.66 & 0.138 \\ \hline \multirow{3}{*}{6. 5 mm after} & Layer thickness & 0.27 & 0.619 \\  & Number of shells & 0.21 & 0.659 \\ \cline{1-1}  & Angle & 2.31 & 0.170 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: ANOVA results on parameters effects on holes diameter in test part 2. Experiment 1 showed that the layer thickness is the most relevant process parameter affecting the holes dimensional accuracy, followed by the number of shells around the holes. However, in experiment 2, no correlation was found between the analyzed factors (layer thickness, number of shells, and holes axis angles) and the diameters' accuracy. Moreover, the results indicated a large variability of the diameter dimensions, which is assumed to a mixt of causes: the holes axis inclination, the additive approach to building the part, and medical decontamination. More research is needed and currently underway for explaining the phenomenon, but another important and practical piece of advice can be formulated here, which is the necessity to drill the holes after the 3D printing process. The inclined holes are built undersized and enlarging them by drilling allows obtaining accurate diameters along the entire length of the hole. The medical decontamination process produced micro-deformations of the smallest diameter holes, but not with a critical influence on their dimensional accuracy, having a more negative impact on the holes diameter dimensional repeatability/variability. Another important conclusion is that attention should be paid to establishing a trade-off between the accuracy and the printing time as in many applications very tight tolerances (such as two significant digits) might not be a requirement for 3D prints. Reducing the layer thickness by a factor of 2 (from 0.2 mm to 0.1 mm) proved not to have a large impact on the dimensional accuracy, but it doubled the printing time. The following supporting information can be downloaded at: [[https://data.mendeley.com/datasets/4h6](https://data.mendeley.com/datasets/4h6) ttzh9 bf/1]([https://data.mendeley.com/datasets/4h6](https://data.mendeley.com/datasets/4h6) ttzh9 bf/1) (accessed on 17 August 2022). Conceptualization, D.P. ; data curation, R.M. and N.L.C. ; formal analysis, R.M. and M.C.I. ; funding acquisition, D.P. ; investigation, D.P., R.M. and M.C.I. ; methodology, D.P., C.G.A., M.C.I.