# Investigations on Factors Affecting 3D-Printed Holes Dimensional Accuracy and Repeatability
## 3 Results and Discussion  
### Results of the Experiment 1  
Table 4 presents the mean values of the straight holes' diameters, as well as information on the printing time for each specimen (provided by the Prusa slicer). The mean diameter for each group of diameters (6 mm, 8 mm, and 10 mm) was calculated based on the caliper measurements for each of the twelve test parts. The dimensional errors were calculated as the difference between the nominal values of the holes and the means of their measured values, for each test part. For investigating the statistical significance of the effects of process parameters on each analyzed diameter, balanced analysis of variance (ANOVA) was performed. The results presented in Table 5 showed that for each group of diameters. The most important factor of influence is the layer thickness, followed by the number of shells and the printing speed. The confidence interval was 95%. Regarding the latter parameter, the \(p\)-value for the largest diameter indicated that printing speed does not significantly influence the 10 mm holes accuracy. However, the printing speed does influence the accuracy of the 8 mm and 6 mm diameter holes, despite being less relevant than the layer thickness and number of shells. Figure 4 presents the main effects plots (Minitab, Minitab UK) for the analyzed diameters, while in Figure 5 are represented the interaction plots between the layer thickness, number of shells and printing speed for each diameter value (6 mm, 8 mm, and 10 mm). The lines which connect the points corresponding to each parameter illustrate the magnitude of the effect. A horizontal line or closer to horizontal means that there is no main effect present (or the parameter effect is not statistically significant). It can be noted that for all diameters, the set of parameters providing the best accuracy was 0.2 mm layer thickness (the smallest tested layer thickness), two shells, and 30 mm/s printing speed, which correspond to the specimen 1. However, if this information is corroborated with the results in the Table 4, it can be seen that the dimensional errors for the first two specimens are almost similar while the difference in the manufacturing times \begin{table}
\begin{tabular}{c c c c c c} \hline \multirow{3}{*}{**Specimen**} & **10 mm** & **8 mm** & **6 mm** & \multirow{3}{*}{**Printing Time**} & **Dimensional** \\  & **Holes–Mean** & **Holes–Mean** & **Holes–Mean** & **Holes–Mean** & **Error (mm)** \\  & **Diameter** & **Diameter** & **Diameter** & & \\ \hline
1 & 9.866 & 7.873 & 5.876 & 2 h 44 min & 0.134 \\
2 & 9.864 & 7.868 & 5.844 & 2 h 26 min & 0.136 \\
3 & 9.837 & 7.840 & 5.824 & 2 h 25 min & 0.163 \\
4 & 9.800 & 7.803 & 5.816 & 3 h 7 min & 0.200 \\
5 & 9.812 & 7.809 & 5.785 & 2 h 42 min & 0.188 \\
6 & 9.826 & 7.778 & 5.762 & 2 h 36 min & 0.174 \\
7 & 9.784 & 7.750 & 5.753 & 2 h & 0.216 \\
8 & 9.774 & 7.727 & 5.734 & 1 h 51 min & 0.226 \\
9 & 9.758 & 7.728 & 5.688 & 1 h 48 min & 0.242 \\
10 & 9.734 & 7.698 & 5.681 & 2 h 12 min & 0.266 \\
11 & 9.718 & 7.683 & 5.639 & 1 h 58 min & 0.282 \\
12 & 9.708 & 7.667 & 5.630 & 1 h 54 min & 0.292 \\ \hline \end{tabular}
\end{table} 
Table 4: Results of the experiment 1: diameters’ mean values and printing times. is 18 min (which represent about 11% of the specimen 1 printing time). Therefore, when also considering the printing time as optimization criterion, the best settings correspond to those of the specimen 2. Another observation was that the printing speed was more relevant for the 6 mm diameter hole. As one can observe from Figure 5, no interaction between the studied parameters could be inferred for any value of the diameters. Experiment 1 outcomes confirmed the conclusions of [PERSON] et al. (2018) who studied 0.4 mm and 1.6 mm shell thickness and observed that the larger the thickness, the larger the mean dimensional deviation for a 65 mm diameter hole, with the same observation applying to the printing speed.