## 1 Introduction  
In contrast to subtractive manufacturing methods, additive manufacturing (AM, also known as 3D printing) produces components and parts by adding material layer-upon-layer of a predefined thickness. AM offers increased design freedom, lead-time reductions, and the possibility of functional integration, with AM technologies being increasingly adopted and developed by industry, resulting in more varied and affordable processes, accompanied by greater market acceptance and penetration [1; 2; 3]. In particular, 3D printing has become a viable alternative to conventional manufacturing processes in numerous applications, i.e., in the automobile, aerospace, medical, and construction industries and widely in furniture production [4; 5; 6; 7]. Especially in the aerospace sector, parts previously manufactured by traditional means are being replaced by additively manufactured low-weight polymer components. Although AM was previously primarily used to produce prototypes, it has now developed sufficiently for customized mass production of functional parts [8; 9]. AM technologies can be classified into seven main categories [10; 11] under which polymers can be 3D printed via polymerization, powder-bed-based processes, and extrusion-based processes [12]. In the polymerization process, an energy source such as a laser beam or UV radiation solidifies/cures a liquid photopolymer (monomer) layer by layer following a slicing strategy. The second method (powder-bed-based process) uses a bonding agent, such as an adhesive applied onto sequential layers of powder which then stick together and in aggregate form the part [13]. The third, extrusion-based process, is where an induction heater is applied to a wire-shaped filament to form the component layer by layer by depositing the molten filament onto the printing platform in a G-code predefined path. An example of the third process is fused filament fabrication (FFF) in which a continuous filament of a thermoplastic material is extruded through a heated \"printer head\" controlled via G-code, which is deposited, layer-by-layer, on a platform to form the component, see Figure 1. The movement of the extruder is invariably in the X-Y directions only, with the build platform moving in the Z direction [14]. By using printers with dual or multiple extrusion nozzles, multicolored or multiple material parts can be produced. Simultaneous 3D printing of multiple materials enables integration of different properties (e.g., flexibility, folding, deployment, shape memory, and electric conductivity) into intricate products in a single manufacturing process [15; 16; 17; 18]. Additionally, a wide range of polymers used for FFF nowadays can be recycled, and the nonmelted material (metal and polymer) from different AM processes can be reused [3; 19; 20]. AM technologies are considered key enablers for modern industry due to their potential to revolutionize manufacturing, by introducing new processes, materials, and applications [21]. Given the present rate of growth, there exists a real need to ensure sustainable development for these technologies [22]. In the context of climate change, production processes need to be environmentally friendly, but they also have to be competitive regarding time, price, and quality. In particular, enhancing sustainability by consuming less energy and reducing material wastage will help unleash the full potential of these technologies, and offer enormous opportunities to realize smart resource-orientated manufacturing in the context of Industry 4.0. However, with FFF printing, defects, inaccuracies, or other problems regarding the quality often arise [23]. In particular, the surface roughness of 3D-printed  
Figure 1: A schematic of the fused filament fabrication (FFF) process. parts is not able to match the quality of those being manufactured using conventional subtractive techniques or injection molding.