## 1 Introduction  
According to the ISO/ASTM 52900, additive manufacturing (AM) is defined as the joining materials process to make parts from three-dimensional model data, usually layer by layer, as opposed to subtractive manufacturing methodologies. It is also known as additive fabrication, used for direct digital manufacturing, rapid prototyping, etc. Additive manufacturing has many advantages over the conventional manufacturing method. AM enables the production of products with very complex details without any special planning of the manufacturing process, preparation of devices, complex manufacturing operations or any additional tools and accessories. In addition, a significant advantage of these processes is the production of parts without any loss of material, or this loss of material is minimized, thus reducing the production cost. Many researchers have shown the experimental application and the advantages of this process over conventional manufacturing [1, 2, 3]. Depending on the type of materials used and the process physics, many AM systems are accessible in the market, such as fused deposition modeling (FDM), stereolithography (SLA), selective laser sintering (SLS), ink jet modeling, etc. FDM is a widely used additive manufacturing technology for the fabrication of complex geometric parts in various engineering applications. In the FDM technique, printers use a thermoplastic type of filament, which is melted and extruded through the nozzle,which travels on a prescribed path to fabricate a certain 3D structure. Materials such as acrylonitrile butadiene styrene (ABS), polyamide (PA), polylactic acid (PLA), poly carbonate (PC), thermoplastic polyurethane (TPU), and polyethylene terephthalate (PET), are used commonly as filaments in the FDM process [4; 5; 6; 7]. Although a relatively wide range of polymers are in use in FDM, a common disadvantage of all of them is the limited functionality of the manufactured parts due to the inadequate mechanical properties of these materials. A possible solution to this problem is the use of composite materials, comprised of a polymer base, with particles made of metal, ceramics, carbon, glass and wood [8; 9]. Recently, research has focused on the use of different composite materials and optimizing their process parameters for FDM, to improve different mechanical properties and other desired properties of the FDM components. Parts made using these composites have higher mechanical, thermal and electrical properties, compared to the unreinforced polymer printed parts [10; 11; 12]. [PERSON] et al. [13] investigated the effect of process parameters on interlayer adhesion and mechanical properties of the FDM CF/PA6 fabricated parts. CF/PA6 printed along the tensile load direction has been shown to exhibit higher tensile properties than the other two printing directions, indicating a strong anisotropy of mechanical properties. In ref. [14], the authors investigated the effect of short carbon fiber on the mechanical, electrical and piezoresistivity properties of 3D-printed polyamide composite parts. It was shown that the reinforcing of neat polyamide with short carbon fibers improved the mechanical properties of the material. In the XY build orientation, 3D-printed composites exhibited significantly increased tensile strength and tensile modulus in compared to the neat PA. [PERSON] et al. [15] concluded that the layer height and printing bed temperatures are the most influential parameters, while the extrusion temperature and printing speed had a lower influence on the performance of FFF (fused-filament fabrication) printed short-fiber-reinforced polyamide parts under uniaxial tensile loading. [PERSON] et al. [16] found that the reinforcing of polyamide PA12 with 10% short carbon improved the tensile and flexural strength of printed parts in compared with the pure PA12. [PERSON] and [PERSON]. [17] analyzed the influence of the short carbon fibers in nylon PA6 polymer used in the FDM process for two build directions, horizontally and vertically printed specimens.