## 2 The Basics of Additive Manufacturing
### Materials Used in the Printing Processes
The selection of these materials varies based on the additive manufacturing method and the core material in use. For instance, in material jetting (MJT), supports are often made from a material that is easy to manually remove or dissolve. When using selective laser sintering (SLS), the powder that surrounds the object acts as a support by nature. In the context of fused filament fabrication (FFF), the process of creating filament from recycled PET serves as a prime example of how base materials are melted, extruded, and transformed into thin threads, which are then used to construct objects layer by layer in 3D printing. The production of filament for 3D printing via the fused filament fabrication (FFF) method involves the precise melting, extrusion, and formation of base materials, such as polymers, into thin threads suitable for additive manufacturing [18]. In the case of recycled PET (rPET), the integration of this material into the material extrusion (MEX) process offers a sustainable and efficient method for producing 3D-printing filament from discarded PET bottles. PET's high recyclability makes it ideal for upcycling into complex, functional parts [19; 20]. The filament creation process begins with extruding PET flakes into pellets, which are subsequently extruded into filament. A crucial aspect of this process is the thorough drying of the material before extrusion, as moisture can lead to degradation and adversely affect the quality of the filament. rPET, in particular, exhibits higher tensile strength compared to commercially available PETG, making it a robust alternative despite its moisture sensitivity [21]. For the FFF method, PET can be processed using either single-screw or twin-screw extruders. While twin-screw extruders are more effective for material mixing, they are less consistent in maintaining tightly controlled filament output. In contrast, single-screw extruders require precise temperature regulation (typically between 250 \({}^{\circ}\)C and 270 \({}^{\circ}\)C) to prevent PET degradation [21]. The production of filament from rPET involves two primary steps: first, recycled PET flakes are processed into pellets using a twin-screw extruder, and then these pellets serve as feedstock for a single-screw extruder to produce the final filament. Once the filament is extruded, it is cooled, cut, and prepared for use in 3D printing. Recycled PET offers numerous advantages, including excellent compatibility with virgin PET and superior mechanical properties, making it a viable and sustainable option for filament production in the MEX process [22]. Support materials in 3D printing are crucial for providing temporary stability during the printing process, particularly for designs with overhangs or complex geometries. Acting as scaffolding, they prevent warping and distortion during the layer-by-layer deposition of filament. In some processes, like powder bed fusion (PBF), the surrounding unbonded powder naturally serves as support, while in fused filament fabrication (FFF), separate, easily removable support materials are used. However, support material can often be minimized or avoided when using filaments reinforced with short fibers. For instance, in polypropylene (PP) and carbon fiber composites, the inclusion of short fibers enhances the rigidity and mechanical properties of the printed parts, reducing the need for external support. This reinforcement allows for better handling of overhangs and complex designs,while also improving overall print quality. Additionally, the use of optimized filament-making processes further strengthens print integrity, offering a sustainable method by repurposing waste materials such as PP and carbon fibers [23]. It is essential to emphasize that the variety of materials utilized in additive manufacturing is continually expanding due to ongoing research and development, which makes it possible to introduce new materials with unique properties and applications.