Additive manufacturing , a.k.a. 3D printing is one of the most rapidly developing and attractive processes in the engineering world as an enabling technology for rapid manufacturing. Products with complex design and multifunctional use may benefit from the low costs and design flexibility, when compared to classic processes. Among the different 3D printing methods FDM (Fused Deposition Modelling) is widely used for thermoplastics attributable to its simplicity. In the FDM process a thermoplastic melt extrudate is deposited layer by layer to produce the designed product. In most cases the deposited melt has a circular cross–section. When depositing a round extrudate voids are formed between adjacent strands leading to inferior mechanical properties. Furthermore, with the need to enhance the mechanical properties of thermoplastics short and long fibers have been incorporated into the resins. In this case the void content of the deposited fiber reinforced thermoplastics has been shown to increase. Hence, a main challenge of the FDM process is related to reduction of the void content in the as-printed products. The effect of changing the shape of the printer’s die (from circular to rectangular) on the printed part properties (rheological, thermal, structural and mechanical) will be shown, while printing carbon fiber reinforced polyamide. A square/rectangular die delivers a constant flow rate per unit width, resulting in better packing of the melt, thus ensuring a decrease in void content. Experimental results indicate that a rectangular die improved the mechanical properties of printed specimens as the void content was reduced from 15 to 4%, compared to a circular die. A 30% increase in Young's modulus was measured, without any detrimental effect on tensile strength. Scanning electron microscopy (SEM) supported the results displaying an almost void free microstructure.