###### Abstract  
Three-dimensional printing technology is widely being adopted in the manufacturing of oral appliances. The purpose of this study was to determine the most suitable method of manufacturing oral appliances by comparing the physical and mechanical properties of various 3D printing methods with the conventional method. Experimental groups consisted of six 3D-printed specimens via FDM, two polyjets, SLS, SLA, and DLP, and the milling methods. The control group consisted of an acrylic resin specimen made by the conventional manual method. The water absorption and solubility, color stability, flexural strength, and surface hardness were tested and statistically analyzed. The FDM, SLS, and DLP methods exhibited comparable water absorption and solubility with the control group, and only the SLA method exhibited significantly higher water solubility than the control group. In terms of the color stability, only the milling method met the requirements of the allowable clinical range. The FDM, SLA, and DLP methods exhibited comparable flexural strength with the control group. The surface hardness of the PJ-2, DLP, and milling methods was acceptable for replacing conventional manual method. Therefore, the most suitable method of manufacturing oral appliances among the experimental groups was the DLP method in terms of its water absorption and solubility, flexural strength, and surface hardness. 3D printing technology; water absorption; water solubility; color stability; flexural strength; surface hardness +
Footnote †: journal: Applied Sciences  
their effectiveness, compromised accuracy and reliability due to the conventional multi-step laboratory procedure often make them difficult to apply in everyday clinics. Various industries have already been using additive manufacturing profitably for many years. Many additive manufacturing produced parts are already incorporated into aerospace engines and vehicles [12]. Medical companies value the ability of additive manufacturing to convert patient-specific data for customized products and medical interventions [13; 14]. Accordingly, digital technology was introduced in dentistry, and new changes have been made in the manufacturing methods of oral appliances, which can largely be divided into subtractive milling methods and additive 3D printing methods. A computerized milling system is involved with the procedures of selective reduction of pre-manufactured blocks to the final shape with a cutting tool. On the contrary, three-dimensional (3D) printing technology implements laminar manufacturing of the final structure; hence, not only this method is free of restricted movement of cutting tools, the prices can also be more economic than the subtractive milling method [15; 16; 17; 18]. Therefore, the application of 3D printing technology has rapidly increased recently in dentistry [18]. The method of 3D printing can be divided into various methods according to the technology and material used. The fused deposition modeling (FDM) method uses extruded thermoplastic materials and was commercialized in the industry [19]. Powder-based 3D printers such as selective laser sintering (SLS) use nylon or a similar thermoplastic powder that is locally melted with a laser beam. Recently, liquid-based 3D printing technologies such as stereolithography apparatus (SLA) and digital light processing (DLP) and polyjet (PJ) have been adopted. Ultraviolet-curable resin is polymerized to form the desired shape by light sources in these technologies. These various 3D printing technologies would be applied in oral appliance manufacturing in the near future. Prototypes of the occlusal splint fabricated with various fabrication methods are shown as Figure 1. However, several important physical and mechanical properties need to be evaluated for the clinical application of 3D-printed devices.