## 3 Results and Discussion  
### Case Study from the Automotive Sector  
The selected case study was part of the Horizon 2020 European project FiberEUse, focused on the recycling and reusing of glass and carbon fiber composites from products at their End-of-Life. This project aimed to develop new solutions integrated into real contexts through practical case studies in collaboration with industrial partners. Among these, the automotive sector was selected to demonstrate the feasibility of using CFRPs for reusable structures and components in a vehicle frame [28; 29]. As a matter of fact, technical lightweight and high-performance application fields such as the automotive sector may significantly take advantage of the reuse and recycling of GFs and CFs [30; 31], i.e., reducing the manufacturing costs and decreasing the environmental impact during the production and use of components [6; 32].  
Focusing on recycling, the automotive sector offers a wide range of possible areas for using recycled fibers, both rGFs and rCFs [6; 33]. A case study of a structural part in the battery housing was selected because this part of the car structure is seen as critical in crash incidents, representing an interesting use case for new composite polymers reinforced with rGFs or rCFs. The structural part under examination, visible in Figure 2a, is a connecting element between the rocker panel and a reinforcing aluminum rib inside the battery housing. The main load case of the part is the provision of stiffness, defining it as a structural part of this sub-assembly. In battery housing, six of these elements are needed. In a crash incident, in particular a side pole crash, it takes up the loads from the deformation of the rocker panel and supports it by leading the loads into the reinforcing profiles. This fact leads to a safe battery compartment that is not prone to intrusions leading to damage to the batteries.  
The selected part, as visible in Figure 2b, consists of a wide connection area that fits closely to the sill and absorbs the forces, as well as to the so-called battery floor pan. Ribs are used to collect these forces and lead them into the aluminum profile. For that purpose, the shape of the profile, including the webs, is mapped, creating a connection by form. The part is then fixed by an epoxy adhesive to the aluminum profile and the battery floor pan. A material bonding to the sill is not applicable since the battery compartment must stay detachable for repair and remanufacturing. A feature of the part is a central guidance hole for a cable. The part is designed in a moldable shape because, considering the batches of mass production, it is expected to achieve higher production rates of about 10,000 pcs./year.  
Focusing on AM processes, this case study, as well as the part, was chosen to demonstrate the possible use of 3D printing processes in the automotive sector for reprocessing rGFs and rCFs. Despite its quite recent adoption in this sector, AM processes have already been demonstrated to positively affect the supply chain performances of automotive industries, as well as fostering customization and complex freeform structures in medium and small production batches [34; 35; 36]. Furthermore, AM has already been exploited for manufacturing fiber-reinforced polymer materials, including recycled rGFs and rCFs [37; 38]. As visible in Figure 3, two different workflows were followed to exploit different advantages of AM technologies, especially focusing on Material Extrusion processes [39]. The firstdeals with Rapid Tooling and uses FFF to manufacture 3D-printed molds to cast rGFRPs or rCFRPs (indirect FFF 3D printing), whereas the second directly reprocesses rGFRPs by means of UV-DIW (Figure 3). The main phases of the two processes are summarized in Figure 4.