This study focuses on the characterization of 3D printed parts by fused deposition modelling (FDM) technique made from a composite filament, highly loaded of stainless-steel microparticles, prepared at different infill density (0, 50, 100%). Thermo-mechanical properties, morphological aspects and heat transport behaviour of the developed specimens have been investigated by dynamic-mechanical analysis (DMA), thermal diffusivity measurements and scanning electron microscopy (SEM). Experimental results allowed to attest a drastic reduction of storage modulus in the range of testing temperatures by reducing the infill density. In the meantime, an increment of dissipation factor was shown in lesser stiff samples at temperatures near to the ambient. The same increasing trend did not appear in the case of thermal diffusion that showed closer values for samples at 0 and 50% of infill, and an augment in the case of infilling level of 100%. This outcome, explained through SEM pictures, was attributed to the difficulty in realization of perfect empty internal structures within 3D parts. A supporting analysis by IR spectroscopy was conducted on the composite surface to gain qualitative information about constituting polymer filament. Further considerations on the porosity of systems have been obtained elaborating SEM micrographs with ImageJ software.