Effect of Printing Angle and Resin Type on the Flexural Strength of 3D-Printed Dental Materials

Abstract

Three-dimensional printing is growing rapidly in applied dentistry. To print faster, increase workflow, and minimise resin consumption, it is important to use the right printer and correct printing orientation. This report aims to analyse whether different printing orientations and types of printing materials could affect the flexural strength of a series of photopolymerisable resin samples. Seven different dental light-curing resins (Keyguide, C&B, Ivory, Vertysguide, Bite, Tera, and Nextdent Cast) and a single modern digital light processing (DLP) 3D printer (Moon Night) were used for this purpose. Different printing orientations (0°, 45°, and 90°) were evaluated. The resin specimens were designed using 3D Builder 20.0.4.0, MeshMixer 3.5.0 and Chitubox software 2.0.8. A total of 15 specimens (five for each orientation evaluated) in the shape of a rectangular parallelepiped with dimensions of 2 mm × 2 mm × 25 mm were produced for each of the seven selected resin materials with the Moon Night printer. After printing and post-processing (MoonWash 2 and MoonLight 2), each resin specimen was subjected to a mechanical test with a universal testing machine. After breaking the specimen, the flexural strength values were recorded using Bluehill computer software (Instron Corporation, Canton, MA, USA). According to the obtained results, the build angle does not affect the flexural strength of the printed products, whereas the difference occurs due to the different printing materials used.

1. Introduction

Additive manufacturing (AM), commonly referred to as three-dimensional (3D) printing [1,2,3], has emerged as one of the most transformative technologies in modern dentistry. By converting digital datasets into tangible models or devices, it enables clinicians to fabricate customized components quickly and with high dimensional accuracy. The continuous improvement of printing systems has made this technology accessible beyond industrial laboratories, allowing dental professionals to integrate in-house 3D printing for daily clinical and educational purposes.

Today, 3D printing supports multiple areas of dental practice [4], including restorative, orthodontic, surgical, and endodontic fields, facilitating the production of anatomical replicas, surgical templates, and personalized appliances. This technology enables the creation of anatomically accurate models, prosthetic structures, and patient-specific surgical templates, while allowing multiple components to be fabricated simultaneously in a fraction of the time required by conventional laboratory methods. By streamlining production and minimizing manual intervention, additive manufacturing promotes greater clinical efficiency and supports the shift toward personalized, precision-based treatment approaches.

In the past, the widespread adoption of 3D printers in dental settings was hindered by their high cost and complex operation. Continuous technological improvements, however, have led to compact, user-friendly, and cost-effective systems that are now increasingly incorporated into private practices [5,6,7,8]. Beyond clinical applications, 3D printing has also become an important educational resource, allowing the fabrication of realistic anatomical models that help teach dental morphology and simulate operative procedures such as cavity preparation or endodontic access without the need for extracted teeth [9,10]. Endodontics has particularly benefited from additive manufacturing; the use of CBCT-based 3D-printed guides facilitates access to calcified canals in complex non-surgical treatments and improves the accuracy of apicoectomy procedures in posterior teeth [11].

Similarly, prosthodontics has taken advantage of this technology for the fabrication of removable and fixed frameworks, overdentures, and implant-supported components [12]. The possibility of printing provisional crowns and bridges further expands its restorative potential [13]. In oral and maxillofacial surgery, the combination of CBCT imaging with 3D printing allows the production of customized drilling, resection, and repositioning guides that enhance surgical precision and reduce operative time.

Computer-assisted design and manufacturing (CAD/CAM) workflows enable detailed preoperative planning and improve predictability, while metal printing techniques such as direct metal laser sintering (DMLS) can be used to fabricate titanium miniplates and fixation devices tailored to the patient’s anatomy [14,15]. Orthodontics has also experienced a major evolution through 3D printing. Custom surgical templates assist in the safe and accurate placement of miniscrews even in restricted anatomical sites, minimizing the risk of damaging adjacent structures [16].

The technology is further applied to fabricate indirect bonding trays that shorten chairside time and improve the precision of bracket positioning [17]. The growing demand for nearly invisible appliances has fueled the development of clear aligners produced from digital scans.

As new high-speed printers are introduced, the workflow could eventually include the direct in-office printing of aligners within hours, thus reducing turnaround time and overall cost [18]. Additionally, functional appliances such as occlusal splints [19,20] and vacuum-formed retainers [21] can be designed and printed with dedicated CAD software, providing efficient and customizable options for both clinicians and patients.

Several additive manufacturing methods have been developed, each relying on different mechanisms of material deposition or light activation. Among these, stereolithography (SLA), digital light processing (DLP), PolyJet, fused deposition modeling (FDM), selective laser sintering (SLS), and selective laser melting (SLM) are the most established [22]. Owing to its high resolution and compatibility with light-curable resins, DLP printing has become the preferred approach in dental manufacturing, enabling precise reproduction of fine anatomical details using biocompatible materials.

Compared to other photopolymerization-based systems, DLP offers faster build times and superior reproduction of fine details. In contrast, SLA and FDM differ significantly in terms of operating principles, resolution, and material suitability. SLA is capable of producing smoother surface finishes at the expense of longer printing times; FDM relies on thermoplastic filaments that limit the ability to reproduce delicate dental geometries. These variations in material composition, polymerization mechanisms, and interlayer adhesion can affect important mechanical properties such as flexural strength and durability [23].

Numerous investigations in the dental field have explored the mechanical performance of restorative materials, focusing on parameters such as deflection, maximum load, flexural strength, and elastic modulus. Historically, these analyses have primarily concentrated on conventional restorative materials, encompassing various generations and formulations of dental composites. Additionally, several studies have evaluated the flexural strength and Weibull distribution of milled zirconia [24], while Keerthana et al. assessed the flexural strength of two glass ionomer cements after immersion in fruit juices [25]. However, limited research has addressed the mechanical characterization of 3D-printed materials within dentistry.

A study published in 2020 compared the mechanical behavior (including flexural strength and surface hardness) of denture base materials fabricated using conventional heat-curing, CAD/CAM milling, and 3D printing technologies. The findings indicated that CAD/CAM materials exhibited superior mechanical properties compared to both heat-cured and 3D-printed counterparts [26]. Similarly, a 2022 systematic review and meta-analysis compared temporary crowns and fixed dental prostheses (FDPs) manufactured by 3D printing, CAD/CAM milling, and conventional methods. The authors concluded that 3D-printed materials displayed enhanced mechanical performance but inferior physical characteristics relative to CAD/CAM and traditional resins, suggesting their potential as viable long-term alternatives [27].

More recently, an in vitro investigation published in January 2023 examined the flexural strength of denture base resins (DBRs) fabricated through CAD/CAM milling, 3D printing, and conventional compression molding. The results demonstrated that CAD/CAM-milled DBRs achieved the highest flexural strength values among the tested groups [28].

It is essential to acknowledge that the performance and quality of 3D-printed dental components are determined by a complex interplay of factors, including printing accuracy, processing parameters, and fundamental material characteristics such as ultimate tensile strength, elastic modulus, yield strength, impact resistance, and fatigue resistance [29].

In stereolithography (SLA) and digital light processing (DLP) systems, the build orientation plays a pivotal role, as it dictates the layer-by-layer deposition and the direction in which the object is sectioned during fabrication [30].

Selecting an appropriate orientation can promote self-supporting geometries and reduce the dependence on additional support structures. Previous investigations have shown that the build angle in DLP printing markedly influences the flexural strength of dental resins, with the magnitude of the effect varying according to the specific material and printing orientation. These observations underscore the importance of systematically assessing build orientation to enhance the mechanical reliability and clinical applicability of 3D-printed dental polymers.

Conversely, research addressing the impact of build angle on dimensional accuracy has yielded inconsistent or negligible results, particularly when optimal calibration and printer settings are maintained. Therefore, although build orientation may exert only a minor effect on geometric precision, its influence on mechanical performance remains substantial due to the critical contribution of interlayer adhesion and polymerization kinetics to the final structural integrity of printed dental materials.

To date, the literature has not yet established conclusive evidence regarding the optimal build angles for specific dental applications. Multiple investigations have approached this topic from different methodological perspectives. Quintana et al. reported that, while build axis and positioning exerted no significant effect on tensile strength or elastic modulus, the overall layout configuration substantially influenced both properties, with corner-oriented specimens demonstrating superior mechanical performance compared to other orientations [31]. In a similar context, another study found that samples with printed layers arranged perpendicular to the loading direction exhibited higher compressive strength than those printed in parallel alignment [32]. Alharbi and co-workers further explored the effects of build angle and substrate configuration on the dimensional accuracy of full-coverage dental restorations fabricated using SLA technology. Their results indicated that both parameters can markedly influence the final geometry and dimensional fidelity of printed restorations [33].

Moreover, Unkovskiy et al. evaluated the influence of printing parameters on the flexural behavior and dimensional precision of standardized prismatic specimens fabricated with SLA systems. Their findings confirmed that print orientation has a measurable and statistically significant impact on both accuracy and mechanical response [34].

In their investigation, Sfondrini et al. assessed the flexural strength of specimens fabricated from two light-curing dental resins (Keyguide and C&B) using two different Digital Light Processing (DLP) 3D printers. Rectangular samples were produced at three distinct build orientations (0°, 45°, and 90°) following rigorous calibration procedures to ensure dimensional precision.

After printing, the specimens underwent standardized post-processing and were subsequently subjected to mechanical testing with a universal testing machine. The results revealed that printing orientation did not significantly influence the flexural strength of either resin. Nevertheless, specimens fabricated using the newer Moon Night printer demonstrated higher maximum load resistance, which the authors attributed to improved device design and enhanced calibration accuracy [35].

Based on these findings, the present in vitro study was designed to eliminate the printer-dependent variable by employing a single DLP system. This approach aimed to isolate the effect of build angle and determine whether variations in printing orientation could meaningfully influence the mechanical performance of the evaluated resins [36,37,38].

Following the printing stage, post-processing steps are commonly required to enhance the mechanical and aesthetic performance of printed specimens. However, these additional treatments inevitably increase both manufacturing time and overall cost. Techniques such as ultraviolet (UV) and microwave post-curing have been reported to enhance the modulus of elasticity and ultimate tensile strength, while optimising laser power can further improve material resistance.

Although a substantial body of literature has explored the dimensional accuracy and biocompatibility of dental resins, comparatively fewer investigations have focused on their mechanical properties, particularly flexural strength, under varying build orientations. This parameter is of particular importance in DLP-based additive manufacturing, where the build angle critically affects polymerisation dynamics and interlayer bonding.

To bridge this gap in current knowledge, the present study evaluates the influence of build orientation on the flexural strength of widely used dental resins. Standardised rectangular specimens (2 mm × 2 mm × 25 mm) were fabricated from seven distinct light-curing resin formulations using the same DLP 3D printer.

Each material was printed at three orientations (0°, 45°, and 90°), allowing for a controlled comparison of mechanical performance across different build configurations. By systematically analysing flexural strength across multiple materials and orientations, this investigation aims to clarify the role of printing orientation in determining the mechanical reliability of 3D-printed dental resins and to provide evidence supporting material selection and clinical application.

Based on the current literature on 3D printing in dentistry, we hypothesized that the printing orientation (0°, 45°, and 90°) may influence the flexural strength of the printed specimens. Moreover, we hypothesized that the mechanical performance of 3D-printed photopolymerizable materials may vary depending on the resin formulation, resulting in significant differences in flexural strength values when using the same DLP 3D printer.

Although several studies have investigated the mechanical properties of 3D-printed dental resins, few have conducted a systematic and standardized comparison of multiple commercially available materials under identical DLP printing and post-processing conditions. This study evaluates seven widely used light-cured resins printed at three distinct build orientations (0°, 45°, and 90°) using a consistent manufacturing protocol (Moon Night printer, MoonWash 2, MoonLight 2).

By simultaneously assessing the influence of printing angle and resin formulation on flexural strength, this work provides novel insights into the interactive effects of material composition and printing parameters on mechanical performance. These findings offer valuable, practical guidance for optimizing additive manufacturing processes in dentistry and contribute to advancing the field towards more reliable and durable 3D-printed dental devices.

The first null hypothesis was that there is no significant difference in flexural properties among different printing orientations; the second null hypothesis was that there is no significant difference among the tested materials.

2. Materials and Methods

2.1. Sample Preparation

The materials and tools used in this study are summarized in

Table 1. Summary of materials and software used in this study.

Product/Software	Manufacturer	Description
3D Builder 20.0.4.0	Microsoft, Redmond, WA, USA	Software for transforming 2D images into 3D objects
MeshMixer 3.5.0	Autodesk, Inc., San Rafael, CA, USA	3D design manipulation software
Chitubox 2.0.8	CTB Systems, Shenzhen, China	Software for finalizing specimens printed with the Moon Night printer and adding print supports
Moon Night	Vertysystem, Altavilla Vicentina (VI), Italy	3D Printer
MoonWash 2	Vertysystem, Altavilla Vicentina (VI), Italy	Cleaner for post-production
MoonLight 2	Vertysystem, Altavilla Vicentina (VI), Italy	UV photopolymerizer for post-production using a universal testing machine
Instron 3343	Instron Corporation, Canton, MA, USA	Universal testing machine
Keyguide	Keystone Industries GmbH, Singen, Germany	Resin material
C&B	NextDent, Soesterberg, The Netherlands	Resin material
Ivory	Keystone Industries GmbH, Singen, Germany	Resin material
Vertysguide	Keystone Industries GmbH, Singen, Germany	Resin material
Bite	Keystone Industries GmbH, Singen, Germany	Resin material
Tera	Graphy Inc., Geumcheon-gu, Seoul, Republic of Korea	Resin material
Nextdent Cast	NextDent, Soesterberg, The Netherlands	Resin material

.

The seven resin materials chosen and used for printing the specimens in this phase of the experiment were Keyguide (Keystone Industries GmbH Stockholzstr., Singen, Germany), C&B (NextDent, Centurionbaan 190, Soesterberg, The Netherlands), Ivory (Keystone Industries GmbH Stockholzstr., Singen, Germany), Vertysguide (Keystone Industries GmbH Stockholzstr, Singen, Germany), Bite (Keystone Industries GmbH Stockholzstr., Singen, Germany), Tera (Graphy Inc., Geumcheon-gu, Seoul, Republic of Korea), Nextdent Cast (NextDent, Centurionbaan 190, Soesterberg, The Netherlands).

Keyguide is a Class I, rigid, and biocompatible photopolymer resin specifically engineered for Digital Light Processing (DLP) 3D printers operating within the 385–405 nm wavelength range. Its chemical formulation primarily consists of urethane acrylate oligomers combined with dimethacrylate monomers, including 2-hydroxyethyl methacrylate (HEMA) and hexanediol diacrylate. In addition, the resin contains phosphine oxide-based photoinitiators (TPOs) and stabilizing agents such as 4-methoxyphenol (MEHQ) to ensure efficient polymerization and long-term storage stability.

According to the manufacturer’s technical specifications, Keyguide exhibits a flexural strength ranging from approximately 106 to 140 MPa, a flexural modulus between 2400 and 2500 MPa, and a Shore D hardness value of 95. These properties confirm its mechanical rigidity and clinical suitability, particularly for the fabrication of intraoral surgical guides. For safe handling and manipulation, the use of protective nitrile or latex gloves is recommended [39].

The C&B resin used in this study is a light-curable photopolymer specifically designed for the additive manufacturing of temporary crowns and bridges. Its chemical formulation comprises a methacrylic oligomer, glycol methacrylate, and phosphine oxide as the primary photoinitiator. Classified as a high-molecular-weight acrylic resin, C&B combines ease of handling with excellent esthetic and mechanical performance.

It offers notable advantages such as enhanced resistance to abrasion and fracture, low bacterial plaque accumulation, and high biocompatibility, even when fabricated at reduced thicknesses. Due to its superior mechanical stability, this resin exhibits minimal polymerization shrinkage and deformation during the printing process, ensuring consistent dimensional accuracy compared to similar materials.

According to the manufacturer’s technical documentation, C&B resin demonstrates a flexural strength of approximately 107 MPa, low water sorption (54 µg/mm³), and minimal solubility (5.9 µg/mm³) [40].

Ivory (KeyModel Ultra™) is a photopolymer resin specifically developed for the high-resolution 3D printing of dental models, optimized for use with 4K DLP printers. Its low-shrinkage formulation makes it suitable for the fabrication of highly accurate bridge and crown models, implant models, aligner models, and diagnostic casts.

It can be employed for rapid prototyping, enabling the production of physical prototypes directly from CAD data with minimal dimensional deviation and reduced resin odor. This resin is composed of urethane oligomers, acrylate monomers, photoinitiators, and titanium dioxide. Ivory exhibits a maximum tensile strength greater than 50 MPa and a tensile modulus above 1700 MPa.

It shows an elongation at break of 5%, while the maximum flexural strength exceeds 70 MPa, with a flexural modulus above 1940 MPa. Its viscosity at 25 °C ranges between 500 and 600 cP. These characteristics make Ivory a rigid, dimensionally stable resin with sufficient resistance for dental model applications. The material provides ultra-fast printing, detail reproduction, availability in multiple colors, and the presence of a thermoforming quick-release agent, which facilitates the fabrication of orthodontic aligner models; printed parts can be trimmed without chipping.

Although not indicated for intraoral use, Ivory is suitable for the production of accurate, durable, and reliable dental models and prototypes [41].

Vertysguide is a transparent, pressure-thermopolymerizable resin designed for low-temperature processing, primarily used in implantology and orthodontics. It is a Class I biocompatible material developed for the 3D printing of surgical guides for implant placement and occlusal splints. The resin provides dimensional stability, compactness, mechanical resistance, and high polishability, making it suitable for clinical use.

Vertysguide is composed of acetone, toluene, 2-hydroxy-2-methylpropiophenone, diphenylphosphine oxide, isobutanol, isobutyl acetate, acrylate oligomers, diacrylates, trimethylolpropane and additional photoinitiators. Vertysguide exhibits an ultimate flexural strength of 85 MPa (ISO 20795-1) and a flexural modulus of approximately 2118 MPa [42]. The residual monomer content is <0.1%, well below the limit of 2.2% required by ISO standards, ensuring safety and material stability.

Furthermore, Vertysguide complies with ISO 10993-1 biocompatibility standards, being classified as non-cytotoxic, non-mutagenic, and non-sensitizing [43]. These properties highlight Vertysguide as a stable and reliable resin for the fabrication of precise surgical guides and orthodontic devices, ensuring both mechanical strength and high clinical safety [44].

The Bite resin is a transparent, pressure-thermopolymerizable, low-temperature 3D printing material designed for implantology and orthodontics, ideal for the fabrication of bite splints, flexible dental splints, and night guards. It is composed of methacrylate monomers and a photoinitiator. This resin is characterized by dimensional stability, compactness, high resistance, and excellent polishability.

Optimized for DLP technology, it combines the mechanical strength required to protect teeth from disorders such as bruxism with enhanced flexibility, ensuring greater patient comfort. Among its main advantages are biocompatibility (ISO 10993-1; USP Class VI compliance), easy cleaning and polishing, resistance, and flexibility.

The resin demonstrates a flexural strength of 2.6–4.4 MPa (ISO 20795-2) [45], a flexural modulus of 1356 MPa (ASTM D790) [46], a tensile strength of 52 MPa (ASTM D638), and a tensile Young’s modulus of 1790 MPa (ASTM D638) [47].

It exhibits elongation at break of 110% (ASTM D638), which highlights its flexibility, and water sorption below 18 µg/mm³ (ISO 20795-2), ensuring durability and dimensional stability in the oral environment [48].

Tera Harz TC-80DP is a Class IIa photopolymer resin designed for 3D printing with DLP/LCD 405 nm UV technology, certified for the additive manufacturing of long-term dental restorations. With a flexural strength of ≥220 MPa and a flexural modulus ≥ 4500 MPa, this material is suited for the fabrication of permanent and provisional restorations on natural teeth and implants, including bridges, crowns, implant-supported superstructures, long-term substructures, inlays, onlays, and overlays.

It represents the first 3D printing resin worldwide approved for the production of long-term bridges, crowns, Toronto prostheses, and substructures, overcoming the limitations of conventional resins typically restricted to temporary applications. Its mechanical properties, such as a Shore D hardness ≥ 90 and a biaxial flexural strength ≥ 350 MPa, are comparable to zirconia, allowing the creation of highly durable and functional restorations with an additive process that maximizes efficiency while minimizing waste.

Tera Harz TC-80DP ensures high aesthetics along with excellent dimensional stability, durability, and precision. This combination of strength, hardness, and rigidity makes it a groundbreaking material for permanent 3D-printed dental restorations [49].

NextDent Cast is a castable, violet-colored resin specifically developed for 3D printing with DLP/LCD 405 nm UV technology. It is optimized to ensure complete and residue-free burnout, making it suitable for the lost-wax casting process.

This material can be employed for the fabrication of supporting structures, removable partial denture frameworks, orthodontic appliances, and study models, including both full arches and removable dies. Its ease of use and predictable burnout behavior allow for the production of high-definition objects that can be subsequently cast in the desired metal alloy.

NextDent Cast exhibits a flexural strength of 85 MPa, a flexural modulus of 2193 MPa, and a Shore D hardness of 83, all in compliance with ISO 178 standards [50]. These properties provide dimensional stability and adequate handling performance during the design and processing phases, without compromising its complete combustibility [51].

A 2D square was selected with 3D Builder software version 20.0.4.0. (Microsoft, Redmond, WA, USA) and transformed into a 3D cube, which, in turn, was modified with MeshMixer 3.5.0 (Autodesk, Inc., San Rafael, CA, USA) software to make it a rectangular parallelepiped with the dimensions of 2 mm × 2 mm × 25 mm. The software was used to create .stl files of 15 Keyguide material samples, 15 C&B material samples, 15 Ivory material samples, 15 Vertysguide material samples, 15 Bite material samples, 15 Tera material samples, and 15 Nextdent Cast material samples.

Finally, Chitubox 2.0.8 (CTB Systems, Shenzhen, China) (Figure 1) is the software that was used to finalize and add printing supports to the Keyguide, C&B, Bite, Vertysguide, Tera, Ivory, and Nextdent Cast samples before sending them to print with the Moon Night 3D printer by Vertysystem (Vertysystem, Altavilla Vicentina (VI)-Italy).

The sample size was calculated using the following equation:

$$n={\displaystyle \frac{2{\left(Z_{\raisebox{1ex}{$\alpha $}\!\left/ \!\raisebox{-1ex}{$2$}\right.}+{{\rm Z}}_{\beta }\right)}^2{\sigma }^2}{{\Delta }^2}}$$

where Z_α/2 = 1.96 for a confidence interval of 5% (α = 0.05), Z_β = 0.84 for a power of 80% (1 − β = 0.80), σ = 14.1 MPa (standard deviation), and ∆ = 25 MPa (expected difference), considering the results from previous studies [26].

Based on preliminary data and expected effect sizes, an a priori power analysis determined that five specimens per group were sufficient to detect significant differences in flexural strength with appropriate statistical power. Specimen geometries were designed in MeshMixer and subsequently imported into Chitubox 2.0.8 (CTB Systems, Zhongcheng Future Industrial Park, Shenzhen, China) for print preparation. Within the software, groups of five specimens at 0°, 45°, and 90° orientations were positioned on the reference build plane [38,52,53,54].

Automatic support generation was employed using the software’s intelligent algorithm to ensure specimen stability while minimizing the number of supports. Additional manual supports were added as needed to optimize printing accuracy and prevent potential distortions: four supports for the 45° specimens, one for the 90° specimens, and four for the 0° specimens. Although flat models typically do not require supports, they were deliberately included to standardize experimental conditions across all groups.

Supports played a critical role in maintaining print success, particularly for specimens oriented at positive or negative angles up to 45°, and facilitated detachment from the aluminum build plate.

At a layer resolution of 20 µm, supports preserved surface integrity and minimized the risk of collapse in unsupported resin layers. In some cases, intermediate supports were added between existing structures to further enhance stability and maintain dimensional accuracy of the printed specimens (Figure 2).

This procedure was repeated for the 5 samples with a print orientation of 0° and then replicated for the other 5 with orientations of 45° and 90°, respectively, for all resin materials used in this study: Keyguide, C&B, Ivory, Bite, Tera, Vertysguide, and NextDent Cast.

2.2. Three-Dimensional Printing

Specimens were fabricated using an inverted 3D printing approach, in which the build platform is positioned upside down within the resin vat. Among the tested orientations, 90° specimens were the easiest to print, requiring only a single support structure. In this configuration, layers were sequentially deposited without issues, minimizing dimensional deviations or sagging [34,55].

Specimens printed at 45° were also successfully produced without significant dimensional alterations, as the resin was incrementally added layer by layer, allowing a stable and controlled build [34,55]. In contrast, the 0° orientation presented the greatest challenge. The layered resin between supports lacked sufficient structural reinforcement, potentially creating weak points (locus minoris resistentiae) and increasing the risk of collapse during fabrication.

To address this, additional supports were incorporated; however, this intervention inevitably introduced minor changes to specimen dimensions and thickness.

In this study, support structures were allocated as follows: four supports for specimens printed at 45°, one support for those at 90°, and four supports for the 0° specimens. The layer thickness was set at 0.050 mm. Printing was performed using the Moon Night 3D printer (Vertysystem, Altavilla Vicentina, Italy), applying the manufacturer-recommended resin profile and validated settings.

The first six layers were printed to form the support base and ensure strong adhesion to the build platform, with a base exposure time of 35,000 s and a standard layer exposure of 1600 s for subsequent layers. These parameters were maintained unchanged throughout the process.

Once the supports were established, the printer proceeded with fabrication at a 5 μm layer resolution. Chitubox 2.0.8 software (CTB Systems, Shenzhen, China) generated 600 sliced images, which were sequentially projected onto the LCD display. White areas on a black background corresponded to polymerized regions of resin. Progressive alignment of these white stripes across layers indicated correct polymerization, whereas isolated white dots or discontinuous stripes signaled potential printing errors. Upon completion, the build platform and resin vat were removed. Specimens were detached from the platform using a metal-bladed scraper, carefully inserted beneath the support edges to lift and separate the printed objects without compromising their structural integrity.

2.3. Post-Processing

Post-processing represents a fundamental phase in the additive manufacturing workflow, as it determines the final precision, mechanical reliability, and long-term stability of printed parts. Immediately after fabrication, each specimen was subjected to a cleaning step to eliminate residual, non-polymerized resin from the surface. In this study, cleaning was carried out with the MoonWash 2 device (Vertysystem, Altavilla Vicentina, Italy), a unit specifically developed for the treatment of 3D-printed dental resins. The process involved the use of isopropyl alcohol (IPA) as the primary solvent, which efficiently dissolves remaining resin residues.

To enhance the removal of uncured components, auxiliary cleaning products, Vertys Splash, Vertys Spray Dry, and Vertys Flusher (Vertysystem, Altavilla Vicentina, Italy), were applied in sequence as part of a multi-stage washing protocol. This approach ensured the complete elimination of superficial contaminants, thereby improving the consistency of subsequent polymerization and reducing potential cytotoxic effects associated with unreacted monomers.

After cleaning, specimens underwent a secondary curing process to achieve full polymer cross-linking and optimal material performance. The MoonLight 2 polymerization chamber (Vertysystem, Altavilla Vicentina, Italy) was used for this purpose. Controlled UV exposure promoted completion of the curing reaction, enhancing the flexural strength, dimensional accuracy, and wear resistance of the printed resins. Insufficient or irregular post-curing, conversely, could negatively affect both the mechanical integrity and the biological safety of the material.

After post-processing, the temporary support structures generated during printing were carefully detached using precision cutters. This operation required particular attention to avoid deformation or surface defects that could compromise the dimensional accuracy of the specimens. The fabrication protocol was reproduced for each selected build orientation, 0° (horizontal), 45° (oblique), and 90° (vertical), which were chosen following the reference framework proposed by Unkovskiy et al. [34].

These orientations represent typical manufacturing configurations, allowing evaluation of how the printing angle influences the mechanical response of photopolymer resins. Specifically, the 0° configuration corresponds to layers stacked parallel to the build platform, the 45° orientation represents an intermediate inclination, and the 90° setup involves layers deposited perpendicular to the platform.

Together, these positions provide a comprehensive overview of the role of layer orientation on structural performance. At the completion of all fabrication and curing steps, fifteen specimens were produced for each of the seven resin types investigated (Keyguide, Crown & Bridge, Bite, Vertysguide, Tera Harz TC-80DP, Ivory, and NextDent Cast). All samples exhibited standardized rectangular bar dimensions of 2 mm × 2 mm × 25 mm (height × width × length). Printing was performed with the Moon Night DLP system (Vertysystem, Altavilla Vicentina, Italy), ensuring consistent manufacturing parameters and reproducibility across all experimental groups.

2.4. Mechanical Testing

The printed samples were organized into three experimental groups, each containing five bar-shaped specimens, to guarantee consistent subgroup comparison and reproducibility of the mechanical data. Following fabrication and post-processing, flexural performance was tested using a universal mechanical testing apparatus. Each specimen was positioned on a precision-engineered aluminum fixture designed to maintain correct alignment throughout the procedure and to prevent undesired displacement during loading. This standardized setup enabled the application of force under tightly controlled conditions, ensuring that the recorded flexural response accurately reflected the intrinsic mechanical behavior of the tested resins (Figure 3).

Flexural strength testing was performed using a universal testing machine (model 3343, Instron Corporation, Canton, MA, USA) configured to apply a compressive load on the printed resin specimens. The instrument is a single-column electromechanical system designed for tensile and compressive measurements. It operates within a force range of 1–200 N and features a maximum load capacity of 1 kN.

The vertical test space available for sample placement is 1067 mm, while the overall dimensions of the apparatus are 32.0 cm in height, 20.0 cm in width, and 18.0 cm in depth, with a total weight of approximately 94 lb. The crosshead speed can be adjusted between 0.05 mm/min and 1000 mm/min. The machine is equipped with automatic transducer recognition for both load cells and strain gauges and is interfaced with Bluehill Software 2 (Instron Corporation) for automated calibration, data acquisition, and subsequent analysis. During each test, load and displacement data were continuously recorded at a sampling frequency of 500 Hz, enabling precise construction of load–deflection curves.

Following the specifications of ISO 178:2019, “Plastics: Determination of flexural properties”, the three-point bending configuration was adopted to evaluate flexural behavior [56,57,58,59,60,61,62]. The span length between supports was fixed at 21 mm, and the crosshead speed was set to 1 mm/min. The loading process continued until each specimen fractured or detached from the supports (Figure 4 and Figure 5).

All tests were carried out using a 1 kN load cell. It is worth noting that, at very small deflection values (approximately 0.1 mm), a few recorded forces were below 5 N, corresponding to 1/200 of the total load cell range. This condition represents the lower recommended detection limit for optimal accuracy and therefore constitutes a minor methodological limitation of the present work.

After specimen failure, the flexural strength data were automatically recorded through the dedicated control software (Bluehill software 2, Instron Corporation, Canton, MA, USA). Once the raw load–deflection data were obtained, both the flexural strength (σ) and the elastic modulus (E) were computed. The flexural strength, expressed in megapascals (MPa), was determined from the maximum load recorded (F) using the following equation:

$$\sigma ={\displaystyle \frac{3FL}{2b{d}^2}}$$

where L = 21 mm represents the support span, and b = d = 2 mm correspond to the cross-sectional width and thickness of the specimens, respectively. By substituting these values into the general equation, the expression for flexural strength can be simplified as follows:

$$\sigma ={\displaystyle \frac{63F}{16}}$$

2.5. Statistical Analysis

Statistical analyses were carried out using the R statistical environment (version 3.1.3; R Development Core Team, R Foundation for Statistical Computing, Vienna, Austria). For each experimental condition, descriptive statistics were computed, including mean, standard deviation, median, and range (minimum–maximum) in accordance with standard reporting conventions for mechanical characterization of biomaterials. The normality of the datasets was verified through the Kolmogorov–Smirnov test.

Since the results indicated that the data followed a normal distribution, parametric methods were employed for further analysis. A one-way analysis of variance (ANOVA) was used to determine significant differences among the tested groups. When the ANOVA revealed significance, Tukey’s post hoc comparisons were performed to examine pairwise differences between materials and between the three build orientations (0°, 45°, and 90°). To complement group comparisons, linear regression models were developed to assess the individual and combined effects of resin type and printing angle on two primary mechanical outcomes, maximum load at fracture and deflection force.

This analysis made it possible to detect potential interaction effects and to provide a deeper interpretation of how these parameters jointly influence flexural performance. For all analyses, the threshold for statistical significance was established at p < 0.05, consistent with widely accepted standards in biomaterials and dental materials research.

3. Results

In this study, deflection thresholds of 0.1 mm and 0.2 mm were selected in accordance with practical testing considerations and international reference standards. These values correspond to small deformations that lie well within the elastic domain of the evaluated materials, ensuring that the recorded responses primarily represent linear–elastic behavior rather than the onset of plastic or irreversible deformation.

Evaluating the resins at such low deflection levels enables a more accurate and reproducible comparison of their intrinsic mechanical responses across different compositions [63]. To achieve a complete mechanical characterization, two key parameters were examined: flexural strength and elastic modulus. Flexural strength indicates the maximum bending stress sustained by a specimen prior to structural failure, thereby providing a measure of its ultimate fracture resistance.

Conversely, the elastic modulus reflects the stiffness of the material, quantifying its resistance to elastic deformation under applied load [64]. The joint assessment of these parameters offers a comprehensive view of the interplay between rigidity and strength, allowing a deeper understanding of the mechanical performance of the investigated 3D-printed resins.

3.1. Deflection at 0.1 mm

Regarding 0.1 mm deflection values (

Table 2. Descriptive statistics of the values of forces measured for a deflection of 0.1 mm. The unit of measurement of the results obtained is in newtons. In the intra-group column, identical letters correspond to averages that are not significantly different for bars printed at different angles but made of the same material, while in the inter-group column, identical letters correspond to averages that are not significantly different for the various materials but printed at the same angle [65].

Resin	Printing Angle	Mean	St Dev	Min	Mdn	Max	Intragroup	Intergroup
Keyguide	0°	0.58	0.46	0.04	0.54	1.22	A	A
Keyguide	45°	0.55	0.33	0.11	0.78	0.81	A	A
Keyguide	90°	0.46	0.12	0.30	0.44	0.61	A	A
C&B	0°	0.61	0.15	0.49	0.55	0.88	A	A
C&B	45°	0.60	0.23	0.32	0.58	0.96	A	A
C&B	90°	0.59	0.16	0.35	0.57	0.80	A	A
Ivory	0°	0.55	0.08	0.43	0.55	0.66	A	A
Ivory	45°	0.86	0.52	0.16	0.75	1.58	A	A
Ivory	90°	0.62	0.31	0.33	0.51	1.15	A	A
Vertysguide	0°	0.28	0.08	0.20	0.25	0.42	A	A
Vertysguide	45°	0.73	0.48	0.34	0.50	1.54	A	A
Vertysguide	90°	0.69	0.12	0.55	0.64	0.88	A	A
Bite	0°	1.13	0.50	0.37	1.07	1.64	A	A
Bite	45°	1.00	0.28	0.67	1.00	1.42	A	A
Bite	90°	0.75	0.17	0.60	0.74	0.94	A	A
Castable	0°	0.65	0.19	0.48	0.60	0.86	A	A
Castable	45°	0.89	0.25	0.61	1.00	1.19	A	A
Castable	90°	0.64	0.19	0.46	0.61	0.85	A	A
Tera	0°	0.53	0.24	0.32	0.52	0.94	A	A
Tera	45°	0.71	0.29	0.35	0.67	1.09	A	A
Tera	90°	0.59	0.44	0.27	0.59	0.91	A	A

and Figure 6), the ANOVA test revealed no significant differences for the variables tested in relation to the deflection at 0.1 mm.

3.2. Deflection at 0.2 mm

Regarding 0.2 mm deflection values (

Table 3. Descriptive statistics of the values of forces measured for a deflection of 0.2 mm. The unit of measurement of the results obtained is in newtons. In the intra-group column, identical letters correspond to averages that are not significantly different for bars printed at different angles but made of the same material, while in the inter-group column, identical letters correspond to averages that are not significantly different for the various materials but printed at the same angle [65].

Resin	Printing Angle	Mean	St Dev	Min	Mdn	Max	Intragroup	Intergroup
Keyguide	0°	0.68	0.46	0.14	0.64	1.32	A	A,B
Keyguide	45°	0.64	0.32	0.21	0.81	0.91	A	A
Keyguide	90°	0.56	0.12	0.40	0.54	0.71	A	A
C&B	0°	0.71	0.15	0.59	0.65	0.98	A	A,B
C&B	45°	0.70	0.23	0.42	0.68	1.06	A	A
C&B	90°	0.69	0.16	0.45	0.67	0.90	A	A
Ivory	0°	0.65	0.08	0.53	0.65	0.76	A	A,B
Ivory	45°	0.96	0.52	0.26	0.85	1.68	A	A
Ivory	90°	0.72	0.31	0.43	0.61	1.25	A	A
Vertysguide	0°	0.38	0.08	0.30	0.35	0.52	A	A
Vertysguide	45°	0.83	0.48	0.44	0.60	1.64	A	A
Vertysguide	90°	0.79	0.12	0.65	0.74	0.98	A	A
Bite	0°	1.23	0.50	0.47	1.17	1.74	A	B
Bite	45°	1.10	0.28	0.77	1.10	1.52	A	A
Bite	90°	0.85	0.17	0.70	0.84	1.04	A	A
Castable	0°	0.75	0.19	0.58	0.70	0.96	A	A,B
Castable	45°	0.99	0.25	0.71	1.10	1.29	A	A
Castable	90°	0.74	0.19	0.56	0.71	0.95	A	A
Tera	0°	0.63	0.24	0.42	0.62	1.04	A	A,B
Tera	45°	0.81	0.29	0.45	0.77	1.19	A	A
Tera	90°	0.69	0.44	0.37	0.69	1.01	A	A

and Figure 7), the ANOVA test revealed no significant differences for the variables tested in relation to the deflection at 0.2 mm.

3.3. Maximum Load

Regarding maximum load values (

Table 4. Descriptive statistics inherent to the maximum load. The unit of measurement of the results obtained is in newtons. In the intra-group column, identical letters correspond to averages that are not significantly different for bars printed at different angles but made of the same material, while in the inter-group column, identical letters correspond to averages that are not significantly different for the various materials but printed at the same angle [65].

Resin	Printing Angle	Mean	St Dev	Min	Mdn	Max	Intragroup	Intergroup
Keyguide	0°	28.21	4.75	24.49	27.36	36.17	A,B	A
Keyguide	45°	29.95	0.94	28.66	30.27	31.12	A	A
Keyguide	90°	20.33	0.72	19.27	20.54	21.13	B	A
C&B	0°	34.56	3.57	30.85	34.03	39.28	A	A
C&B	45°	27.36	1.66	24.63	27.6	28.8	A	A
C&B	90°	28.47	4.29	23.99	28.37	32.97	A	B
Ivory	0°	17.22	0.85	16.47	16.86	18.67	A	B
Ivory	45°	15.55	0.84	14.07	15.95	16.1	A	B
Ivory	90°	15.82	1.39	13.6	15.89	17.39	A	A
Vertysguide	0°	34.23	11.51	23.46	35.62	51.25	A,B	A
Vertysguide	45°	26.53	2.35	22.5	26.96	28.57	B	A
Vertysguide	90°	16.75	2.07	14.7	16.73	19.5	C	A
Bite	0°	1.39	0.85	0.91	0.99	2.91	A	C
Bite	45°	2.11	0.88	1.16	2.33	3	A	C
Bite	90°	1.46	0.19	1.32	1.42	1.81	A	C
Castable	0°	5.08	1.04	3.99	5.19	6.08	A	C
Castable	45°	4.60	0.13	4.41	4.59	4.8	A	C
Castable	90°	3.31	0.25	3.14	3.2	3.6	A	D
Tera	0°	28.00	3.01	25.17	26.46	31.72	A	A
Tera	45°	16.77	3.12	12.99	15.54	20.68	B	B
Tera	90°	14.54	0.65	14.08	14.54	15.01	C	A,C

and Figure 8), the ANOVA test revealed significant differences for some variables tested in relation to maximum load.

Tukey’s post hoc test highlighted some significant differences between the various groups in relation to maximum load.

3.4. Flexural Strength

In addition to the maximum load values (N), the corresponding flexural stress values (MPa) were calculated and are shown in

Table 5. Descriptive statistics inherent to flexural strength. The unit of measurement of the results obtained is in megapascals (MPa). In the intra-group column, identical letters correspond to averages that are not significantly different for bars printed at different angles but made of the same material, while in the inter-group column, identical letters correspond to averages that are not significantly different for the various materials but printed at the same angle [65].

Resin	Printing Angle	Mean	St Dev	Min	Mdn	Max	Intragroup	Intergroup
Keyguide	0°	111.09	18.72	96.42	107.73	142.41	A,B	A
Keyguide	45°	117.93	3.73	112.84	119.18	122.53	A	A
Keyguide	90°	80.06	2.84	75.87	80.87	83.19	B	A
C&B	0°	136.08	14.06	121.47	133.99	154.66	A	A
C&B	45°	107.75	6.54	96.98	108.67	113.4	A	A
C&B	90°	112.11	16.89	94.46	111.70	129.81	A	B
Ivory	0°	67.81	3.37	64.85	66.38	73.51	A	B
Ivory	45°	61.23	3.33	55.40	62.80	63.39	A	B
Ivory	90°	62.29	5.47	53.55	62.56	68.47	A	A
Vertysguide	0°	134.78	45.32	92.37	140.25	201.79	A	A
Vertysguide	45°	104.47	9.28	88.59	106.15	112.49	A	A
Vertysguide	90°	65.98	8.15	57.88	65.87	76.78	B	A
Bite	0°	5.50	3.36	3.58	3.89	11.45	A	C
Bite	45°	8.30	3.48	4.56	9.17	11.81	A	C
Bite	90°	5.78	0.77	5.19	5.59	7.12	A	C
Castable	0°	20.02	4.12	15.71	20.43	23.94	A	C
Castable	45°	18.12	0.55	17.36	18.07	18.9	A	C
Castable	90°	13.04	0.98	12.36	12.6	14.17	A	C
Tera	0°	110.27	11.87	99.10	104.18	124.89	A	A
Tera	45°	66.05	12.29	51.14	61.18	81.42	B	B
Tera	90°	57.18	1.83	55.44	57.0	59.10	C	A

. These allow for a standardized comparison with other studies and manufacturer-reported material properties. Regarding flexural strength values, the ANOVA test revealed some significant differences for the variables tested (Figure 9).

3.5. Modulus of Elasticity (E)

The modulus of elasticity values (MPa) were calculated and are shown in

Table 6. Descriptive statistics inherent to the modulus of elasticity. The unit of measurement of the results obtained is in megapascals (MPa). In the intra-group column, identical letters correspond to averages that are not significantly different for bars printed at different angles but made of the same material, while in the inter-group column, identical letters correspond to averages that are not significantly different for the various materials but printed at the same angle [65].

Resin	Printing Angle	Mean	St Dev	Min	Mdn	Max	Intragroup	Intergroup
Keyguide	0°	1.29	0.17	1.10	1.27	1.57	A	A
Keyguide	45°	1.32	0.06	1.21	1.32	1.37	A	A,D
Keyguide	90°	0.98	0.05	0.92	1.01	1.03	A	A
C&B	0°	1.67	0.25	1.40	1.78	1.96	A	A,B
C&B	45°	2.35	0.30	2.00	2.46	2.74	A	B
C&B	90°	1.87	0.42	1.19	1.95	2.35	A	C
Ivory	0°	0.61	0.16	0.45	0.54	0.86	A	B
Ivory	45°	0.75	0.18	0.64	0.66	1.07	A	D
Ivory	90°	0.57	0.08	0.47	0.60	0.65	A	A,B,D
Vertysguide	0°	1.90	1.24	1.17	1.42	4.12	A	A
Vertysguide	45°	1.62	0.42	1.08	1.55	2.23	A,B	A,B
Vertysguide	90°	0.91	0.24	0.67	0.88	1.32	B	A
Bite	0°	0.04	0.02	0.03	0.04	0.09	A	B
Bite	45°	0.07	0.03	0.03	0.07	0.11	A	C,D
Bite	90°	0.05	0.06	0.04	0.04	0.06	A	D
Castable	0°	0.12	0.04	0.08	0.14	0.15	A	B
Castable	45°	0.12	0.007	0.11	0.12	0.13	A	C,D
Castable	90°	0.09	0.005	0.09	0.09	0.10	A	A,D
Tera	0°	1.81	0.37	1.43	1.74	2.27	A	A
Tera	45°	0.85	0.18	0.58	0.82	1.06	B	A,D
Tera	90°	0.71	0.13	0.62	0.71	0.81	B	A,D

. These allow for a standardized comparison with other studies and manufacturer-reported material properties. Regarding the modulus of elasticity values, the ANOVA test revealed some significant differences for the variables tested (Figure 10).

The print orientation does not affect the maximum load in most materials. This can be better understood from the overall (Figure 11), which shows that increasing the print angle does not significantly change the influence on the maximum load: it is true that if one had to choose which printing angle to set for the construction of the test specimens, 0° would be preferable, but it is also true that there are no significant differences between the three different angles (0°, 45°, and 90°) in terms of maximum load.

Considering the type of material subjected to mechanical testing, it would appear that, at the same printing angle, the maximum load is influenced by the different materials examined. The overall results (Figure 12) show that the printed samples behave differently depending on the printing material used in relation to the maximum load parameter considered. Five of the seven materials (Keyguide, C&B, Vertysguide, Tera, and Ivory) behave more or less the same, requiring a higher maximum load than the other two materials tested (Bite and Nextdent Cast) in order to break the sample.

3.6. Linear Regressions

From the linear regressions (

Table 7. Results of linear regression analyses evaluating the effect of printing orientation (degree) and material type on the flexural performance of 3D-printed resins. Asterisks indicate statistically significant effects (* p < 0.05).

Dependent Variable	Independent Variable	p-Value
Deflection at 0.1 mm	Material	0.0005 *
	Degree	0.935
Deflection at 0.2 mm	Material	0.0005 *
	Degree	0.935
Maximum Load	Material	0.0001 *
	Degree	0.0129 *

), with regard to deflection at 0.1 mm, the following can be inferred:

-

At the same printing angle, there was no significant influence of the material on deflection at 0.1 mm (p > 0.05).

-

At the same printing material, there was a significant influence of the printing angle on deflection at 0.1 mm (p < 0.05).

With regard to deflection at 0.2 mm, the following can be deduced:

-

At the same printing angle, there was no significant influence of the material on deflection at 0.2 mm (p > 0.05).

-

At the same printing material, there was a significant influence of the printing angle on deflection at 0.2 mm (p < 0.05).

With regard to maximum load, the following observations were made:

-

At the same printing angle, there was a significant influence of the material on Maximum Load (p < 0.05).

-

At the same printing material, there was a significant influence of the printing angle on Maximum Load (p < 0.05).

4. Discussion

4.1. Overview of 3D Printing in Dentistry and Study Rationale

Three-dimensional (3D) printing refers to an advanced manufacturing process that fabricates physical objects from digital models created using computer-aided design (CAD) software. This technology is closely aligned with the foundational principles of Industry 5.0, which emphasizes the integration of automation and digitalization with human-centered and sustainable production paradigms [66]. Within this framework, 3D printing represents a pivotal technological component due to its environmental sustainability and production adaptability.

Its capability to utilize recycled materials, such as powdered raw inputs, further enhances its relevance within the context of the circular economy, thereby contributing significantly to environmental protection and resource efficiency.

This in vitro study focuses on the innovative topic of investigating whether print orientation can influence the mechanical properties of resin materials printed with DLP 3D printing technology, with a special focus on the parameter of flexural strength.

Flexural testing was selected as it closely replicates the combination of tensile and compressive stresses that dental restorations are subjected to under functional intraoral conditions. This loading mode is particularly relevant for temporary crowns, bridges, and surgical guides, which must endure masticatory forces and off-axis stresses within the oral environment. Compared to uniaxial tensile or shear testing, flexural testing offers a more representative evaluation of a material’s resistance to functional stresses encountered during clinical use [56].

This study was conducted by printing seven different materials (Keyguide, C&B, Ivory, Vertysguide, Bite, Tera, Nextdent Cast) with a single printer (Moon Night) at three different angles (0°, 45°, and 90°).

4.2. Effect of Printing Orientation on Flexural Strength

In light of the linear regressions obtained from the study, it would appear that, for the same material examined, the print orientation of the samples only influences the Maximum Load, without affecting the deflection parameters at 0.1 mm and 0.2 mm.

Tukey’s post hoc analysis revealed that printing orientation had no significant effect on maximum load for most materials. Only minor differences were observed for Tera and Vertysguide, which showed statistically significant variations between certain build angles. There may be an inconsistency between regression and inferential analysis: inferential analysis shows that, with the same printing material, the printing angle has no influence on deflection at 0.1 mm and 0.2 mm, but looking at the overall regression, we can see that there is actually a minimal effect of printing angle.

Overall, these results indicate that printing orientation exerts a limited influence on mechanical performance compared with resin type.

The results found in the literature regarding the mechanical characteristics of objects printed with different print orientations are inconsistent.

For SLA or DLP production processes, the build angle indicates the direction in which the object is cut during the accumulation process. Some recent publications argue that build angles affect the accuracy of 3D-printed designs [30], but there are still no conclusive results on the optimal build angles in different dental applications. It should also be noted that the materials tested in previous studies had different shapes and sizes and were printed with different materials than those used in our thesis. According to some studies, the 3D printing orientation setting affects material properties, product accuracy, and biocompatibility [67].

The study by Quintana et al. showed that tensile stress and elastic modulus are not significantly affected by the axis and position, but layout settings have a significant effect on both properties. Samples constructed at an angle performed better (3.53% vs. 4.59%) than those with other placements [31].

Another study showed that the direction of the printed layer perpendicular to the direction of the load was better than the parallel direction in terms of material compressive strength [32].

A study by Alharbi evaluated the influence of the build angle and support configuration on the dimensional accuracy of full-coverage dental restorations printed using SLA technology, and the results of this study revealed that both factors influence the size and accuracy of printed parts [33].

In their study, Unkovskiy et al. evaluated the influence of printing parameters on the flexural properties and accuracy of prismatic material samples with dimensions similar to those in the present study, printed with an SLA 3D printer. Thirty prismatic samples were printed according to three different orientations: 10 at 0°, 10 at 45°, and 10 at 90°.

All samples were then subjected to a three-point bending test to evaluate their flexural properties. This study showed that different printing orientations affect printing accuracy: samples printed along the Z-axis are particularly prone to inaccuracies; samples with a 45° printing orientation were found to be the most accurate.

Samples printed with layer orientation parallel and inclined at 45° to the load direction had a higher ability to withstand axial load than samples with layer orientation perpendicular to the load direction. The anisotropic behavior of the printed samples with respect to orientation and build positioning was revealed [34].

4.3. Influence of Resin Type on Mechanical Properties

To date, very few studies have evaluated the effect of different 3D-printed materials on the mechanical properties and flexural strength of the samples produced.

In their study, Atria PJ et al. evaluated the mechanical and biological properties of three 3D-printed resins currently used for temporary restorations and compared them with an experimental resin intended for permanent fixed dental prostheses.

The results of this study indicate that commercially available resins could be used in clinical settings under certain conditions and for limited periods of time. Following the manufacturers’ protocols is of paramount importance in order not to compromise these properties [68].

Another study aimed to compare the mechanical properties of two 3D-printed resins chosen and used in dentistry (BioMed Amber and IBT). Based on the properties of both materials, IBT resin could be better used as a tray for placing orthodontic brackets using an indirect bonding technique, while BioMed Amber resin would be more useful as a surgical guide for placing dental implants and mini-implants [44].

In this study, considering the type of material subjected to mechanical testing, it would appear that, at the same printing angle, the maximum load is influenced by the different materials examined.

In particular, Tukey’s post hoc test highlights statistically significant differences in maximum load for different materials printed at the same construction angle.

4.4. Clinical Implications

Based on the inferential statistical results, the first null hypothesis, that no significant differences exist among printing orientations, was partially accepted, as post hoc comparisons revealed only limited differences for a few materials (mainly Tera and Vertysguide). Conversely, the second null hypothesis, that no significant differences exist among the tested resins, was rejected, since post hoc tests showed that material type had a statistically significant effect on both maximum load and flexural strength. These findings confirm that resin composition plays a predominant role in determining mechanical behavior, whereas printing orientation exerts only a minor influence within the tested range of parameters.

This result is consistent with the fact that Bite and Nextdent Cast are less resistant and structural materials than the other five materials tested and therefore have lower resistance: Bite is used in dentistry to create gnathological bite guards, while Nextdent Cast is a material composed of 50% resin and 50% wax that can be used to create study models of both entire arches and removable stumps. The other five materials have more or less similar characteristics and, therefore, similar uses: having a higher flexural strength, they are used for the construction of temporary restorations and structures that must withstand a greater load.

In conclusion, using a printer with the same angle but changing the type of material, having expanded the range of materials and uses, we can see that there is a difference in the maximum load. It is true that increasing the angle, i.e., from 0° to 45° to 90°, decreases the force, but basically, the significant variable is the material. This emerged because the sample size and the variability of the materials were increased.

4.5. Limitations and Future Perspectives

Regarding the limitations of this study, print orientation is a factor that may have affected print accuracy: for example, the first layers of the 0° samples require additional exposure time to cure in the light and ensure secure adhesion to the substrates; the same applies to the 90° samples, which need to be cured longer to reach the specified length. Only three printing angles (0°, 45°, and 90°) were tested. Although these represent standard reference orientations, future research will aim to evaluate additional build orientations, such as 30° and 60°, to provide a more detailed understanding of how intermediate angles may affect the mechanical performance of 3D-printed dental resins.

Changes in sample dimensions could also result from the removal of support structures, particularly for the 0° and 45° groups, a procedure that inevitably damages samples of these dimensions. With regard to alterations in sample dimensions, it should be noted that the layer height was set at 0.05 mm, which means that variations of less than 0.05 mm are not controllable by either printer. All these variables may have affected the final measurements of the samples, causing relatively large deviations that may have resulted in overlapping effects with those of the print.

When comparing the experimental flexural strength values obtained in this study with the reference data reported by the manufacturers (

Table 8. Comparison between experimental flexural strength and values declared by the manufacturer.

Material	Experimental Flexural Strength (Mean of 0°, 45°, 90°)—MPa (Mean ± SD Across Orientations)	Manufacturer-Declared Flexural Strength (MPa)	Source (Manufacturer)
Keyguide	103.03 ± 16.48	~106 MPa	[39]
C&B	118.65 ± 12.46	~107 MPa	[40]
Ivory	63.78 ± 2.88	>50 MPa	[41]
Vertysguide	101.74 ± 28.15	~85 MPa	[44]
Bite	6.53 ± 1.26	2.6–4.4 MPa	[48]
Tera	77.83 ± 23.22	≥220 MPa	[49]
NextDent Cast	17.06 ± 2.95	~85 MPa	[51]

), slight variations were observed. In most cases, the measured values were lower than those declared in the technical data sheets, which can be attributed to differences in specimen geometry, post-curing conditions, and printing orientation.

Manufacturer-reported values are typically based on idealized testing environments and do not account for the influence of build direction or support removal. Therefore, the results presented here provide a more realistic representation of material performance under standardized yet clinically relevant conditions.

This study exclusively evaluated flexural strength and maximum load, key indicators of mechanical performance. However, other critical mechanical properties, such as surface hardness, tensile strength, and fatigue resistance, which are particularly relevant in dental applications, were not investigated.

Future research should incorporate these parameters to enable a more comprehensive mechanical characterization of 3D-printed dental materials. Additionally, the use of a limited sample size, while common in preliminary in vitro investigations, may have reduced the statistical power of the analysis and limited the generalizability of the findings, especially given the inherent variability associated with the 3D printing process.

To validate and expand upon these results, further studies employing larger sample sizes and involving a broader range of printers and materials are strongly recommended.

5. Conclusions

This in vitro study evaluated the flexural strength of seven photopolymerizable dental resin materials (Keyguide, C&B, Ivory, Vertysguide, Bite, Tera, Nextdent Cast) printed with a single DLP 3D printer (Moon Night) using three different build orientations. The findings indicate that the type of resin material has a more significant effect on flexural strength than the printing orientation. Specifically, Keyguide, C&B, Vertysguide, Ivory, and Tera demonstrated significantly higher flexural strength compared to Bite and Nextdent Cast, consistent with the manufacturers’ specifications and intended clinical applications.