# Aging of PA12 Powder in Powder Bed Fusion
## 1 Introduction  
The first layer-by-layer additive manufacturing technique dates from 1986 [1; 2]. Generally, traditional technologies do not allow obtaining the same level of geometry that is generally manufactured in a very short time through the additive approach. Layered additive manufacturing systems include powder bed fusion (PBF), where the designed geometry is produced using a laser source that fuses a thermoplastic powder. In this way, the non-fused powder acts as support, reducing process time and raw material consumption. Moreover, PBF technology allows the adoption of a broad range of metals and plastics [3]. There are several studies focusing on the design and optimization of support structures in AM processes. Considering the design, there are three aspects in PBF that are of crucial importance for strategic decisions in order to adapt AM: firstly, due to the layer-by-layer fabrication of products, design complexities are no longer an obstacle at the production level, and thus the functionality of products is not sacrificed by processing and production limitations. Furthermore, designers would be able to consolidate the product into a single object, free from the multiple assemblies that must be present in conventional technologies. This would directly translate into a reduction in overall weight, waste, resources, and energy, while increasing product functionality. Secondly, PBF's high level of design flexibility would offer manufacturers a valuable window of opportunity to instantly change product design. This is a valuable opportunity for manufacturers whose product designs need to be revised or completely redesigned in the final stage, right before market launch. Thirdly, the PBF design features allow for high levels of customization according to customer needs and requirements. The ability to reduce the overall number of parts made with complicated geometric designs, which is the ultimate goal in some sensitive industries such as aerospace, the reduction of the need for large supplies of raw materials, high material efficiency compared to traditional processes, on-demand production of spare parts, andenvironmental benefits such as a low carbon footprint are some of the most important features of PBF in terms of sustainability and advantages over traditional manufacturing [4]. Using this technology involves a reduction in production costs by reusing the powder; a ratio between used and virgin powder of about 70/30 is commonly adopted [5]. Nevertheless, thermal cycles, which affect reused powder, influence negatively the chemical and physical properties of the fused part [6]. Polyamide-12 represents one of the most used thermoplastic materials for PBF. In particular, the broad range between the temperature at which melting starts and the temperature at which crystallization begins, associated with the melting point at about 180 \({}^{\circ}\)C, makes PA12 a material very suitable for PBF manufacturing. The powder is melted by a laser beam, after which a cooling phase begins, leading to minimal deformation [7]. Mechanical properties of built products vary greatly in the PBF building process and are affected by a wide range of process variables. The implementation of technology for structural part production is complicated. It was recently demonstrated that the orientation of the fabrication within the build volume, as well as its thickness, have a significant influence on the mechanical properties of PA12 fused parts. [PERSON] et al. investigated why workpieces oriented differently within the build volume exhibit different mechanical performances [8]. However, the applicability of 3D building technology remains relatively limited due to its low cost-effectiveness and high variability of part sizes and mechanical properties. Among the various 3D building technologies, PBF has proven to be superior in the manufacture of automotive parts with material flexibility and without support. A limitation of the PBF building process is dimensional errors due to thermomechanical deformation.