## 2 Materials and Methods  
The experimental investigations were carried out on ISO 527-2 [22] tensile specimens, type 1A, 3 mm thick. The test specimens were printed with polylactic acid, using four types of filaments with a diameter of 2.85 mm: PLA Natural (transparent), PLA Black, PLA Red and PLA Grey (manufacturer: Verbatim GmbH, Eschborn, Germany). The specimens were produced on an Ultimaker 2+ Connect equipment (Ultimaker B.V., Utrecht, Netherlands) with air filter and closed workspace (the printer was equipped with top cover and front door). A glass build plate with 223 mm on X-axis and 220 mm on Y-axis was used. Three printing layouts were defined:  
* individual printing of a single tensile test specimen (codified 1.1), placed in the center of the build plate, with YXZ orientation [23] (Figure 1a);
* simultaneous printing of three tensile test specimens (codified: 3.1, 3.2 and 3.3), placed in the center of the build plate, with YXZ orientation and 30 mm distance between the specimens' symmetry axes (Figure 1b). The order of deposition was: first layer of specimen 3.1, first layer of specimen 3.2, first layer of specimen 3.3, second layer of specimen 3.1, second layer of specimen 3.2, second layer of specimen 3.3 (similar for all layers); simultaneous printing of five tensile test specimens (codified: 5.1, 5.2, 5.3, 5.4 and 5.5), placed in the center of the build plate, with YXZ orientation and 30 mm distance between the specimens' symmetry axes (Figure 1c). The order of deposition was: first layer of specimen 5.1, first layer of specimen 5.2, first layer of specimen 5.3, first layer of specimen 5.4, first layer of specimen 5.5, second layer of specimen 5.1, second layer of specimen 5.2, second layer of specimen 5.3, second layer of specimen 5.4, second layer of specimen 5.5 (similar for all layers). Using the layouts described above, specimens 1.1, 3.2 and 5.3 have the same position relative to the build plate origin. Similarly, specimen 3.1 has the same position as specimen 5.2, and specimen 3.3 has the same position as specimen 5.4. Each of the configurations presented above was printed five times, with a total of 180 tensile test specimens (45 specimens per color: 5 \(\times\) 1.1, 5 \(\times\) 3.1, 5 \(\times\) 3.2, 5 \(\times\) 3.3, 5 \(\times\) 5.1, 5 \(\times\) 5.2, 5 \(\times\) 5.3, 5 \(\times\) 5.4, 5 \(\times\) 5.5). The printing parameters were maintained constant for all specimens (Table 1) and were set at values resulting from previous research [5; 9; 16], considering also the recommendations of the filament manufacturer (Verbatim, [24]). Fur  
Figure 1: The layouts for 3D printing: (**a**) individual printing, (**b**) three specimens printed simultaneous, (**c**) five specimens printed simultaneous. thermore, all specimens of the same color were printed from the same filament spool. No pre-process or post-process treatments were applied. For the ISO 527-2 1A tensile test specimens with 3 mm thickness, using 0.2 mm layer thickness, the total number of layers is 15 (YXZ build orientation). The time interval between the deposition of material in two successive layers, in the same area of the part (or the time interval to generate a full layer) was measured for each of the three layouts described in Figure 1. The following values were obtained:  
* 5 min and 36 s per layer for individual printing of a test specimen. * 16 min and 52 s per layer for simultaneous printing of 3 specimens. * 25 min and 10 s per layer for simultaneous printing of 5 specimens. Previous research has shown that black PLA filaments are more sensitive to thermal variations [16; 21]. For this reason, in order to correlate the results of the experiments with the thermal printing conditions, temperature measurements were carried out on the upper surface of the black PLA specimens, using an Optris Laser Sight infrared thermometer (Optris GmbH, Berlin, Germany) with a resolution of 0.1 \({}^{\circ}\)C. The equipment was placed above the specimen (approx. 200 mm), with the printer cover removed.