## 1 Introduction  
Aligers, particularly those made from polyurethane, are popular in orthodontics for their aesthetic appeal and comfort compared to traditional fixed orthodontic treatment, which includes brackets and wires [1]. These clear, removable devices gradually move teeth into their desired positions using a series of custom-made aligners, each slightly different from the previous one [2]. The effectiveness of aligners depends significantly on their material properties, which must balance rigidity to exert adequate force and flexibility for patient comfort [3]. The production of clear aligners generally falls into two categories: thermorforming and 3D printing. Thermorforming involves heating a polymer sheet to make it flexible, then shaping it over a dental model to create the aligner. This technique utilizes thermoplastic resin polymers, including materials like poly(vinyl chloride) (PVC), poly(ethylene terephthalate) (PET), poly(ethylene terephthalate glycol) (PETG), and thermoplastic polyurethane (TPU). In contrast, 3D printing, also known as additive manufacturing, constructs the aligner by adding material layer by layer from a digital model. These aligners are typically made from polyurethane-based materials, selected for their combination of clarity, durability, and flexibility--key factors needed for applying consistent force to reposition teeth. Polyurethane, in particular, stands out for its excellent mechanical properties and the comfort it provides to patients [3; 4; 5]. Three-dimensional printing has revolutionized the production of aligners, allowing for precise customization and rapid production [6]. Photocurable resins are categorized into two primary types based on their polymerization mechanisms: photoinitiated free-radical polymerizations and photoinitiated cationic polymerizations [7]. Free-radical polymerizations include the polymerization of acrylates, while cationic polymerizations encompass the polymerization of epoxides, lactones, and vinyl ethers, which are not affected by radicals. A significant advantage of cationic polymerizations is their resistance to atmospheric oxygen, unlike free-radical polymerizations, which suffer from oxygen inhibition where radicals are lost to oxygen [7]. Polyurethane-based aligners, made by photoinitiated free-radical polymerization, have been the subject of numerous studies focusing on their mechanical and thermal properties, which are crucial for their efficacy in orthodontic applications [8]. Previous in vitro research has shown that manufacturing and post-processing conditions, such as UV light exposure during manufacturing, rinsing, and polymerization in a different environment, as well as the use of different 3D printers, affect the mechanical properties of 3D-printed aligners [9; 10; 11; 12; 13; 14; 15]. These investigations aim to optimize the aligners' performance by understanding the impact of factors like UV curing during post-processing on their structural integrity and longevity. However, the mechanical properties of the aligner material, influenced by factors such as UV curing during 3D printing, play a critical role in their overall performance and durability [16]. It is important to note that while UV light exposure in the post-processing phase has been studied, the effect of UV exposure on the layers during the 3D printing process has not yet been investigated thoroughly. The mechanical properties of polyurethane-based aligners are critical for their performance and longevity in orthodontic treatments, as they determine the aligners' ability to maintain shape, resist deformation, and perform under stress [17; 18]. Several 3D printing parameters, including the type and intensity of UV light used during curing, the duration of UV exposure, the printing temperature, and post-processing conditions, significantly impact the mechanical and thermal properties of the aligners [19; 20; 21].