## 4 Results
### Optimal Parameter Settings for the Individual Performance Measurements
#### 4.1.2 Dimensional Accuracy  
Table 8 shows the statistically significant process parameters that affect the dimensional accuracy. It seems that dimensional accuracy is differently affected by the process parameters depending on the printed designs. Although dimensional accuracy in the ASTM D695 and ASTM D3039 cases is associated with a similar parameter effect, the ASTM D638 type has all the main and interaction terms as statistically significant factors on the dimensional accuracy except for the interaction effect between the layer thickness and build orientation and the three-way interaction effect. The statistically significant main effects in Figure 7a show that each of 0.2 mm in layer thickness, 0\({}^{\circ}\) in build orientation, and 1200 mm/min in printing speed for the ASTM D638 case is associated with the lowest mean dimensional error. The ASTM D695 and ASTM D3039 cases, however, only have a layer thickness as a statistically significant factor in which the 0.2 mm layer thickness results in the minimum mean dimensional error (see Figure 7b,c). The interaction effects of the ASTM D638 case in Figure 8 support that the build orientation of the part design can play an important role in dimensional accuracy; the 0\({}^{\circ}\) build orientation significantly decreases the dimensional error at different layer thickness and printing speed levels. Based on the above main interaction effects, it can be inferred that layer thickness is a critical factor that is closely related to dimensional accuracy regardless of the sample designs. The fact that the 0.2 mm layer thickness always offers lower dimensional error values in the experiments supports that the decrease in layer thickness can result in more sophisticated fabrication. In addition, a plausible explanation for the impact of build orientation in the ASTM D638 case can be found in the design characteristic of the fabricated part. If the build orientation becomes 90\({}^{\circ}\), the ASTM D638 design has a bridge form that needs support structures for proper fabrication (see Figure 9). Since support structures were not created for the experiments to eliminate the possible impacts of support structure generation on the performance measurements, poor dimensional accuracy always occurs in the bridge structure of the ASTM D638 design at the 90\({}^{\circ}\) build orientation. However, the default printing speed (1200 mm/min), which is the recommended printing speed for CFR-PEEK from the machine provider, can decrease the negative impact of the 90\({}^{\circ}\) build orientation on dimensional accuracy. This seems to be caused by the deviations from the default printing speed that exacerbate the sagging problem on the bridge part as seen in Figure 9b. \begin{table}
\begin{tabular}{l c c c c c c c c c c c} \hline \hline \multicolumn{13}{c}{**(a) Estimated Printing Time (min)**} \\ \hline \hline
**Sample** & **LIO1P1** & **LIO1P2** & **LIO1P3** & **LIO2P1** & **LIO2P2** & **LIO2P3** & **LIO1P1** & **LIO1P2** & **LIO1P3** & **LIO2P1** & **LIO2P2** & **LIO2P3** \\ \hline ASTM & & & & & & & & & & & \\ D638 & 34.17 & 30.67 & 27.83 & 41.83 & 36.67 & 33.17 & 26.17 & 24.33 & 22.5 \({}^{*}\) & 30.83 & 27.33 & 24.83 \\ ASTM & & & & & & & & & & & \\ D695 & 90.00 & 77.08 & 67.25 & 99.67 & 85.58 & 74.42 & 61.33 & 52.92 & 47.08 \({}^{*}\) & 68.67 & 59.08 & 51.92 \\ ASTM & & & & & & & & & & & & \\ D3039 & 61.33 & 54.00 & 49.00 & 84.67 & 73.00 & 65.67 & 46.67 & 41.33 & 39.00 \({}^{*}\) & 59.00 & 52.00 & 47.00 \\ \hline \multicolumn{13}{c}{**(b) Regression Model**} \\ \hline \hline
**Sample** & & & & & & & & & & & & & & & \\ \hline ASTM & & & & & & & & & & & & & \\ D638 & 0.751-0.2 & -0.721-0.11-0.111-