## 1 Introduction  
Additive manufacturing (or 3D printing) technology has recently become a trending technology for manufacturing parts from 3D digital models, due to its remarkable benefits, such as the potential to form complex geometric parts, reduced manufacturing time, low cost, flexible design, print-on-demand, reduced waste, and high-quality products. Various popular 3D printing technologies have been developed, including fused deposition modeling (FDM), stereolithography (SLA), powder bed fusion (PBF), multi-photon lithography (MPL), sintering laser melting (SLM), digital light processing, and so on [1; 2; 3; 4; 5; 6; 7]. Due to its technology- and cost-related advantages, the FDM process has become a practical 3D printing technology for various applications. Moreover, the process operations are straightforward and tenable, in which thermoplastic-based materials, such as acrylonitrile butadiene styrene (ABS), polyamide, polycarbonate, polyethylene, and polypropylene, are typically used. However, their mechanical properties are usually lower in comparison with those of conventional products, and they usually present anisotropic behavior [8; 9]. Thus, the demand for new high-quality printable materials for the FDMprocess has significantly grown recently, due to its various applications in the rapid tooling and manufacturing areas [10; 11; 12]. Motivated by this fact, composite materials have emerged as a promising alternative candidate to expand the 3D printable materials spectrum, in terms of physicochemical properties. Polymer matrix composites reinforced by carbonaceous materials, such as graphene and carbon nanotubes (CNT), have gained attention in previous reports [13; 14; 15; 16]. These studies have demonstrated that FDM 3D printing products fabricated using carbon fiber or carbon nanotube fillers in the polymer matrix had remarkably enhanced electrical/heat conduction, mechanical strength, modulus of elasticity, toughness, and/or durability, revealing the great potential of such materials in various practical applications. By introducing fibers or particles into a polymer matrix, 3D-printed objects in the biomedical, mechanical, and electrical areas, among others, have been widely studied and applied [17; 18]. Many researchers have recently made efforts towards investigating the effects of process and infill parameters on the mechanical properties of 3D-printed specimens fabricated by FDM [19; 20; 21], and have developed predictive models using analytical, numerical, and experimental techniques [22; 23].  
In the present study, a MWCNT filler is used as the reinforcement material, which is added to the ABS matrix--a strong, durable production-grade thermoplastic--in order to create a unique ABS-MWCNT composite, which can be used in the FDM process to improve the desired properties of the printed parts. It is worth noting that the conventional approaches to ABS-CNT composites have utilized a relatively high weight percentage of filler, ranging from 5-10 wt% [24; 25; 26; 27], while few studies using low loading CNT content (\(<\)5 wt%) in ABS matrix have been carried out. In addition, standard solution mixing or thermal blending techniques used in the fabrication of ABS-CNT composite require complicated and high melting temperature treatments, limiting the quality of the obtained products due to a low homogeneous distribution between ingredients. Herein, we propose the possibility of directly synthesizing ABS-MWCNT composite filaments with MWCNT content up to 4 wt% for commercial FDM 3D printing with a standard filament diameter of about 1.75 mm. An extrusion FDM machine is then used to produce various small-scale objects using the ABS-MWCNT filaments. Moreover, the thermal-mechanical characteristics of the composite filaments and the 3D printed parts are further analyzed.