## 3 Results and Discussion
### Effect of Filler Concentration and Combination of Nanofilers on the THERMAL Conductivity
The results showed that the thermal diffusivity (the rate of transfer of heat) increases with increasing of the filler content and this effect is more pronounced for the mono-filler composites based on PLA/GNP, where the maximum value is 0.485 (mm\({}^{2}\)/s) at PLA/6 wt.% GNP compared with the 0.154 (mm\({}^{2}\)/s) for the pure PLA. The improvement of thermal diffusivity for these systems was about 214%, compared with the neat matrix due to the sheet structure of GNP filler. The effect of increasing of the filler content on the thermal diffusivity of the mono-filler composites based on PLA/MWCNT was less pronounced and at a maximum MWCNT filler content of 6 wt.% the improvement is of about 43%. Thus, obtained results open potential applications for graphene-enabled thermal management, including electronics, which could greatly benefit from graphene's ability to dissipate heat and optimize electronic devices. Figure 6: **(a,b)**Thermal conductivity and thermal diffusivity vs. GNP and MWCNT filler content for two (PLA/GNP and PLA/MWCNT) and bi-filler (PLA/GNP/MWCNT) composites. In micro-and nano-electronics, very often the heat is a limiting factor for smaller and more efficient components. Therefore, graphene or its composites with exceptional thermal conductivity hold enormous potential for this kind of applications. The existence of synergy between different types of carbon nanofillers, shows great potential and could significantly increase applications of carbon-based nanomaterials. This bi-filler approach can be suitable also for stereo-lithography 3-D printing and this will be a topic of our interest for further study. Thus, obtained composite materials were extruded into 1.75 mm diameter filaments, which were applicable to commercial FDM 3D printers. In our previous work [49], on the base of rheological characteristics as percolation threshold and the flow index, as obtained nanocomposites were classified into three groups: Newtonian, percolated composites and elastic solids. As a result, these two characteristics were used to select the printing parameters for the three groups of nanocomposites, suitable for FDM. In order to achieve a good accuracy for the printing objects, the printing of the neat PLA and the nanocomposite systems with GNP and MWCNTs was accomplished at appropriate conditions as constant volumetric flow rate for the neat polymer and the composites, and the printing speeds were increased in correlation with the flow index of the printed material. All these characteristics were determined for the fixed melt temperature and nozzle diameter. This concept was validated by direct printing of samples and, using the filament from the neat PLA, the MWCNT/PLA and GNP/PLA nanocomposite formulations studied. Figure 7 summarizes a scheme for obtaining mono-and bi-filler filaments for a FDM 3D printer on the base of PLA, GNP and MWCNTs. In our previous studies [50; 51; 52] it was found that the PLA is still biodegradable and, only if there is a burning of composites (T \(<\) 850 \({}^{\circ}\)C), residual ash contains an amount of graphite and carbon nanotubes, creating concerns about environmental pollution and damage. A quantitative and qualitative risk assessment has been carried out in two cases--so-called \"bad practices\" (not in compliance with safety measures) and \"good practices\" (subject to modern safety measures). It has been shown that the implementation of specific safety measures can significantly reduce the degree of risk.