## 1 Introduction  
3D printing technologies are currently increasing their importance in dentistry, both for clinical and research purposes [1, 2]. In the field of orthodontics, 3D printers play an essential role in the manufacturing of aligners, which are becoming increasingly popular as the answer to aesthetic demand and the need for comfort [3]. Since thermoplastic aligners can be accurately 3D printed directly from a digital image of teeth, the extra steps of creating a physical model may be eliminated. Furthermore, 3D printing may help decrease the cost as well as increase treatment performance and improve patient comfort [4, 5]. Several 3D printing processes may be used for the direct printing of clear aligners, such as fused deposition modelling, selective laser sintering, selective laser melting, direct pellets fused deposition, stereolithography, multi-jet photo-cured polymer process, or continuous liquid interface production technology. Photo-polymerization from clear resin is nowadays the most suitable 3D printing method [6]. To further minimize these errors, a clear biocompatible aligner can be 3D printed for direct patient usage [7, 8]. During this manufacturing process not only is the chosen printer influencing the fitting and aesthetics of the aligner, but also the thermoforming disks [9] [PERSON] et al. demonstrated that thermoforming increases water absorption and solubility and reduces the transparency levels of thicker materials and, together with the surface hardness of tested materials [10]. The optical properties of the materials are determined by the way they react to light in terms of absorbance and transmittance. The greater the transmittance the more transparent the material; Conversely, the greater the absorbance, the less transparent the material. Transparency is the key to the aligner's success. Transmittance is the fraction of incident light at a settled wavelength, that passes through the material [11]. 3D print quality has influenced transparency by two parameters: resolution and accuracy. The former is the minimum size that the device manages to print onto a specific spatial plane. The latter is the ability of a printer to produce items as identical as possible to their 3D original files. The higher the resolution of the printer, the thinner the individual layers that make up the cast on which the aligner will then be thermoprinted on. A recent study compared different 3D technologies, considering several variables such as precision, authenticity, and accuracy, and concluded that there were statistically significant differences between printers using different technologies. Thanks to the evolution of digital technology, new methods of 3d printing have been developed. The photopolymer printer (PPP) works similarly to an inkjet printer, with the exception of a component in which UV light cures the material, while it is dispensed from the nozzles. The object is cured layer by layer, starting from bottom to top, which determines a better quality of printing. Once the liquid resin is sprayed onto the platform, the UV light connected to the printer head simultaneously cures the material as it is printed. One technology is represented by Stereolithography (SLA), which consists of a photosensitive resin and an ultraviolet (UV) light acting on the resin positioned on the platform. The laser keeps on tracing and forming each layer of the previous one, starting from the button until the final result is achieved. The UV light cures every layer; as the layer is processed, the tray deforms to add more liquid resin. Instead, digital light processing (DLP) has a projector that cures the entire layer each time. Another approach to the DLP printer is represented by a continuous digital light printer (cDLP), which allows the light to cure the polymers with no interruption. Previous investigations on the precision of 3D printed models indicated significant differences among 3D printing technologies [12].