# Surface Durability of 3D-Printed Polymer Gears
## 1 Introduction  
3D printing, also known as additive manufacturing (AM) or rapid prototyping (RP), provides several advantages over traditional energy-intensive methods. These include the ability to produce complex geometries as single units without joints, reduced material and labor costs, lower energy requirements, and simplified processing. However, a prominent limitation of this technology manifests in the precision and surface texture of the produced components. Gears made using 3D printing technology represent a good and efficient alternative for mechanisms and machine elements intended for specific mechatronics applications, like industrial robots, robotic surgery systems and diagnostic equipment, printers, scanners, digital cameras and optical equipment, unmanned aerial and ground vehicles, etc. Studies and research in the field are at the beginning, which is why there are no regulations to establish the precision and wear limits of gears obtained through 3D printing technologies. Current studies generally present experimental attempts with good results for determining the mechanical properties of parts made by such technologies [1; 2; 3]. The studies evaluate, in addition to the 3D printing technology, the printing parameters and the type of material used [4; 5; 6]. The thickness of the printing layer influences the mechanical parameters of the printed parts. A greater thickness of the printing layer generates a better overall strength of the structure [7; 8; 9; 10]. The size of the printing layer is inversely proportional to the friction force in the case of FDM-ABS/PLA 3D-printed parts [11] (fused deposition modeling-acrylonitrile butadiene styrene/polylactic acid). If we consider the lower friction surface in the case of a lower printing resolution, this conclusion is normal, although there are studies with contradictory results. The friction force in the samples made of PLA is higher compared to those made of ABS [11]. As a general conclusion regarding the influence ofthe thickness of the printing layer, we can state that the lower the printing resolution, the better the mechanical and tribological performance. Fewer bonds between layers generate superior mechanical properties [12; 13]. However, we must consider that the thickness of the layer directly proportionally influences the printing precision, and therefore, the precision of the parts. Moreover, in the case of gears with toothed wheels, the influence of the printing resolution on the degree of wear must be determined. For FDM technology, a minimum coefficient of friction correlated with a degree of wear has been identified. This shows a linear character for a filling percentage with material of 50% and a layer thickness of 0.1 mm in the case of ABS parts, and a percentage of filling of 50% correlated with the thickness of the printing layer of 0.15 mm for those made of PLA [14]. In the general formulation, through a multi-objective optimization process, the optimal structure specific to the application can be identified from the printing parameters. Another important aspect is represented by the influence of the melting temperature of the material on the tribological properties of the samples. In the case of FDM-PLA printing technology, the most widespread method-material combination currently used, according to the coefficient of friction, the printing temperature has a non-linear influence, obtaining an optimal value for temperatures between 210 and 220 \({}^{\circ}\)C depending on the type of material [15]. This value can be correlated with an optimal level of finishing for the surface [16; 17]. Another particularly important and not sufficiently well-documented aspect up to this point is represented by wear resistance. As mentioned before, the studies show better results in the case of samples with a greater thickness of the printing layer subjected to wear tests. Based on the material, in general terms, parts made of ABS have less wear compared to PLA.