# Aging of PA12 Powder in Powder Bed Fusion
## 1 Introduction
These dimensional errors result from numerous factors, such as material properties, temperature control, and the inherent PBF technology, which affect the shrinkage of horizontal layers and vertical bending deformation. In addition, layer height and powder size are also dimensional error factors in PBF. Although builder manufacturers suggest that the quality of 3D-built samples can be better controlled by placing them in the center of the builder within two-thirds of the entire space, these recommendations limit operational management plans by increasing processing time and costs. In addition, practitioners are recommended to use a range of constant scaling factors (SF) during the three-dimensional building (3 DP) process to compensate for dimensional inaccuracy and shrinkage [9]. It appears in the literature that the deviations from the nominal were lower in the samples built at 0\({}^{\circ}\) than in the samples built at 90\({}^{\circ}\); those deviations need to be minimized in order to reduce their transfer to assembly products [10; 11; 12; 13]. As a result, it is estimated that only 10% of the powder used in a build cycle is fused. To reduce building costs, it is recommended that powder be reused for subsequent build cycles. [PERSON] et al. conducted extensive research on the reuse of the powder, its modifications, and performance changes [14]. The aging of PA12 powders is due to thermal oxidation reactions accompanied by chain cross-linking, chain scission, and post-condensation. Studies show that the surface morphology, microstructure, thermochemical properties, and fluidity of aged powders vary compared to virgin powders, while the size distribution remains unchanged. The microstructure of components made from aged powders shows coarse spherulites and a degraded surface. Higher energy density in the building process for aged powders can improve the mechanics of molded parts [15]. The degradation of the material depends on the applied laser energy. High values increase deterioration while low values prevent the particles from melting, thus improving the mechanical strength of the molded components; therefore, unused PA12 becomes less effective after several cycles of use [16]. The aging of the powder is mainly due to two causes. The first is related to the crystals that in the virgin powder have a denser and more homogenous shape, while in the aged powder they have a greater spacing. The second is related to the polycondensation in the solid state; the polymer chains grow, leading to an increase in the amorphous phase and therefore to a lower crystallinity of the aged powder. These variations are reflected in a macro-performance analysis of molded components [17; 18]; the next step will be focused on the relationships between aging status and the reflected mechanical properties [19; 20]. Further research revealed that increasing the molecular weight also increases the powder viscosity [21; 22], resulting in an involved effect on the surface finish of the built part, whose skin resembles an orange peel [23]. The goal of this work is to mechanically characterize PA12 molded specimens using virgin or aged powder and then to analyze the PA12 powder to determine the physical and chemical parameter values of the powder in order to use those values inside simulation software to optimize the process. To accomplish this goal, a set of specimens was designed, built, measured, and mechanically tested; the results were then compared to the values of the process parameters used to build them. The building area in this work was located on the x-y plane, the powder layers were deposited along the \(z\)-axis, and three orientations of the built specimens were considered: horizontal, vertical, and tilted. The specimens' dimensions were measured with a coordinate measuring machine and a digital micrometer. Three-point bending and short-beam tests were performed on the manufactured specimens.