## 1 Introduction  
The transition of additive manufacturing (AM) from a prototyping tool to a rapid fabrication tool [1, 2] implicitly brought to attention the necessity of ensuring the process repeatability in terms of mechanical properties, surface quality and dimensional accuracy of the printed products. Being one of the most widely used AM technologies [3, 4], the Fused Deposition Modeling (FDM) method presents, among others, the advantage of an impressive number of process variables that can be individually set by the users [5, 6]. But on the other hand, one has to consider that finding the optimal combination of these numerous printing parameters, in order to meet the product requirements, is often a complex challenge. Several studies have been carried out to characterize the influence of the FDM process parameters on the mechanical properties, dimensional accuracy and surface quality of 3D printed parts made of polymers. The most analyzed parameters are [5, 7, 8]:  
- layer thickness;  
- printing head temperature;- build plate temperature;  
- printing speed;  
- build orientation;  
- infill pattern and density;  
- raster angle. The adhesion of successive layers deposited by the FDM method is influenced by the temperature variations of the deposited material [9]. These variations are related with the print head temperature and the build plate temperature, but are also influenced by the print speed, the layer thickness, the infill, the time interval between deposition of material on two successive layers and the air flow in the printing workspace [5]. [PERSON] et al. [10] pointed out that the \"weld\" line between two successive filament depositions (within and between layers) has been identified as one of the major factors influencing the mechanical properties of FDM printed components. [PERSON] et al. [11] showed that the layers closest to the heated build plate have a denser structure with fewer pores (cooling of these layers occurs more slowly). Differences between the periphery and the central part of the pieces are also highlighted, with the central part having a higher level of crystallinity. [PERSON] et al. [12] analyzed the variation of the thermal field generated by the printhead and the cooling cycles, showing its influence on the previously deposited layer. [PERSON] et al. [13] measured the temperature of the deposited filament for 1 min after deposition, using an 80 \(\upmu\)m thermocouple. Three printing cases were analyzed with the build plate temperature set to 50 \({}^{\circ}\)C, 70 \({}^{\circ}\)C and 100 \({}^{\circ}\)C. A cyclic variation of the temperature of the deposited filament was observed, with a tendency to stabilize after 1 min. It should be noted that the monitoring was carried out on a specimens built on the Z-axis with only one line per layer (vertical one-line wall part). The printing speed influences the time interval between deposition of material on two successive layers in the same area of the part. This time interval is also dependent on factors less studied: the size of the printed part and the number of pieces printed simultaneously. To define this issue more clearly, we could imagine the following printing scenarios of cubic specimens with 100% infill, manufactured with the same values of the process parameters:  
1. Individual printing of a cube with base area equal to A;
2. Individual printing of a cube with base area equal to 2A;
3. Simultaneous printing of two cubes, each with base area equal to A. The time interval between the deposition of material in two successive layers in the same area of the part (or the time interval required to make a complete layer) will be double for the cases described in points (b) and (c), compared to the situation described in point (a). Increasing the area of the base of the cube by a factor of two has the same effect on time as a reduction of the printing speed by half.