## 5 Discussion  
The stability of the aesthetic properties of clear aligner materials has not been widely investigated. Indeed, the color stability of dental materials is influenced by various factors, such as ultraviolet irradiation, staining beverages, and mouthwashes. To satisfy the patient's aesthetic request, the color stability and transparency of clear aligners should be stable for the two week orthodontic period of wearing. The current study compared the transparency of aligners both before and after aging, while trying to analyze how changes in optical properties can be affected by both the thermoforming disk and the printer used. In literature, it has been shown that 3D printed models accurately reproduce the occlusion and are now a very important tool for both diagnosis and treatment planning. However, the manufacturing process of aligners directly from their scanning is still in its initial phase, together with any information related to accuracy and aesthetic properties [20,21]. The printers used in this study carry the most innovative technologies: Rapidshape D100+ and Prodways LD20 use Digital Light Processing (DLP), a technology formed by a platform for building the object, and a projector to cure the resin on the platform and a photosensitive resin. The digital projector has the objective to make a single image of each layer of the 3D object. The light emitted by this type can be UV, if created specifically for the 3D printing process. DLP printer works through an additive process, a projector works on a single layer, and for each layer that is cured, the tray descends to add a layer to the previous one, in this way the aligner is printed from start to finish. The Carbon L1 printer instead uses the cDLP approach, which stands for Continuous Digital Light Processing. This new technology works through vertical movement without interruption of the building plate, allowing light a continuous polymerization. Since the projector is a digital screen, the image of each layer is composed of square pixels. This implies the formation of a 3D layer consisting of small rectangular cubes called voxels. In the case of these printers, what determines the resolution of the XY axis is the size of the square pixels. Consequently, the higher the native resolution of the projector, the higher the resolution of each layer. The focal distance between the projector lamp and the printing area is crucial. This is the reason why DLP printers can have different resolutions corresponding to inversely proportional print areas. The pixels in this case range from 35 to 100 microns. DLP and cDLP technologies have a higher production speed than SLA, as the projector exposes all the dots belonging to the same layer to photopolymerization at the same time. The literature reports significant differences between 3D technologies in the accuracy and precision of 3D printed models. [PERSON] et al. concluded that these two technologies are the most accurate in making the print. These two technologies were studied in their mechanical aspects, such as accuracy and fidelity to the model. On the other hand, in this study, we investigated their influence on aesthetic appearance [12,22,23,24]. The three printers selected were used to produce three identical aligners with two different types of materials: Zendura FLX and Scheu CA Pro+. The aligners currently present on the market differ in the material and the most used is polyethylene terephthalate glycol (PETG), followed by polycarbonate (PC), polypropylene (PP), thermoplastic polyurethane, copolymer, and many others are used to manufacture aligner. Aligner thickness has a value in the range of 0.50 and 1.5 mm [13]. The Zendura FLX is a polyurethane. This material is characterized by the presence of two materials: an elastomeric core that gives flexibility, and a hard coating that characterizes elasticity and firmly grips the teeth [25].