Among various dental materials, resin for temporary crown prosthesis is mainly used during the period required to produce the final prosthesis. At this time, the abutment teeth are safely protected from elements applied to the abutment teeth and damage to periodontal tissue is prevented. It is also used to minimize changes in the spacing and position of teeth. Until recently, there were two main methods for producing temporary prosthetics: first, the direct method of producing a temporary restorative prosthesis within the patient’s mouth, and the indirect method of making a plaster model and manufacturing it from the model [1,2]. However, as 3D printers have recently been used for temporary prosthetics, the production of digital dental prosthetics is increasing.

3D printing refers to a manufacturing technology that creates three-dimensional objects by stacking raw materials in successive layers using a 3D printer [3]. 3D printers have the advantage of making less noise during the manufacturing process, saving time and cost, and because processing is performed by irradiating a laser, it is easy to reproduce complex shapes and have relatively high precision.

Dental 3D printing is mainly based on photocurable liquid resin curing using water bath light curing for the production of prosthetics and restorations or models and guides [4]. Among 3D printers, the SLA and DLP methods are most widely used in the dental field. The SLA and DLP methods have the same principle of using photocurable liquid resin and solidifying it through a photopolymerization reaction, but they print on a surface-by-face basis compared to SLA printers, which print on a dot-by-point basis. The advantage of the DLP printer is that the production speed is fast [5]. For this reason, DLP-type 3D printers have recently become widely used in labs and dental clinics.

In modern society, implants are widely used for prosthetic restoration of long-span patients. The scope and duration of temporary restorations used in implant surgery is gradually increasing [6]. Temporary restorations are restorations that are placed on prepared teeth until the final prosthesis is restored. They maintain aesthetics to some extent and are used to protect the prepared teeth and prevent gingival recession. Additionally, the accuracy of temporary restorations has a significant impact on the condition of the oral cavity when the final prosthesis is installed. Recently, light-polymerizable resin using 3D printers has been widely used to produce temporary restorations, as it is relatively inexpensive to produce and shortens the production time. However, polymeric resin for 3D printing can affect the output even with slight differences in temperature, light, and plate, so it is important to set the brightness and layer thickness when printing. Therefore, various studies on the environment during output by 3D printers are needed.

In this study, we aim to investigate fracture strength among the material properties. One study reported that there was a large difference between several previous reports on ISO 4049 for the same material [7], and non-uniform polymerization that can occur in each specimen may be one of the causes [8]. As an alternative to this problem, some researchers investigated bending specimens with a length smaller than the standard (10 to 15 mm) [9-14].

In this study, we attempted to determine the difference in fracture strength according to the difference in layer thickness when manufacturing a prosthesis using a 3D printer. Accor-dingly, we intend to produce specimens with different layered thicknesses of 0.010 mm, 0.050 mm, and 0.150 mm and compare and analyze the fracture strengths of the specimens to help with clinical application.

In this study, to evaluate the fracture strength of photocurable 3D printing resin, the specimen specifications were 65 mm in length, 10 mm in width, and 3.3 mm in thickness according to ISO 20795-1 (Figure 1). In order to obtain more consistent results in this experiment, a notch of 1.0 mm in length and 1.0 mm in width was formed in the center of the specimen, and an STL file was created using the Inventor Program (Inventor, Autodesk, USA), supports were connected to 15 manufactured specimens. And it was placed on the build plate.

Figure 1. A specimen of length 65 mm, width 10 mm, thickness 3.3 mm.

Prepare for use by mixing the C&B 5.0 Hybrid (Arum, Korea) resin solution using a tube roller (MX-T6-S Tube Roller, Korea) for 30 minutes, and then print it using a MAX UV 3D Printer (MAX UV, ASIGA, Australia). 15 specimens each were produced by printing with three layer thicknesses of 0.010 mm, 0.050 mm, and 0.150 mm (Figure 2). The manufactured specimen was layered and printed using the DLP method, and after printing, it was washed for 20 minutes using a washer containing alcohol (Formlabs-Form wash-USA) and then dried. The dried specimen was cured for 20 minutes using a UV ARUM Curing Machine (ARUM, Korea) (Figure 3). For the specimen after curing was completed, the surface of the support part was trimmed using a denture bur (D079E, Dedeco Long Eddy, USA) to prevent damage to the specimen.

Figure 2. MAX UV 3D printer.

Figure 3. Photopolymerized specimen.

The fracture strength of the completed test specimen was measured using a universal testing machine (Instron 5942, Instron co. USA). At this time, the speed of the crosshead was set to 1.0 mm/min.

Statistical analysis was performed using SPSS 22.0 Ko (SPSS, USA). One-way ANOVA was performed to compare the average results of the fracture strength test results of resin according to the layer thickness. As a post hoc test, multiple comparisons were made using Tukey HSD, and the significance level was set at 0.05.

The results of fracture strength according to the layer thickness of photopolymerizable 3D printing resin are shown in Table 1.

Table 1 . Fracture strength test results (n=15)

Thickness	M±SD	Min	Max	p
0.010*	0.088±.012	0.075	0.117	p<0.05**
0.050*	0.087±.010	0.065	0.099
0.150	0.066±.009	0.047	0.080

*Only 0.010 and 0.050 have no statistically significant difference.

**p-value by one-way ANOVA.

The average value of fracture strength according to 3-layer thickness ranged from 0.010 to 0.088, with 0.050 being 0.087 and 0.150 being 0.066, with 0.010 being the largest. The minimum value was 0.075 for 0.010, 0.065 for 0.050, and 0.047 for 0.150, with 0.150 being the lowest minimum value. The maximum value was 0.117 for 0.010, 0.099 for 0.050, and 0.080 for 0.150, with the value of 0.010 being the largest.

In these results, there was no statistically significant difference between 0.010 and 0.050 (p<0.05).

In this study, in order to determine the difference in fracture strength according to the 3D printer’s layered thickness, specimens were manufactured with layered thicknesses of 0.0.1, 0.05, and 0.15, and the fracture strength was identified and analyzed through experiments to help clinical application. Dental resin is a material that is consistently used in the production of various prosthetics due to its cost, time, and simplicity of use. Because it has excellent accessibility compared to alternative materials, various resins are still being developed for the production of prosthetics.

Recently, digital prosthesis production has already become common in the dental industry. Equipment with increasingly diverse technologies is also being distributed, and the proportion of 3D printers among them is steadily increasing. There are many different materials for prosthetics produced using 3D printers, but the main one is resin. In this situation, many dental labs and dental clinics that have 3D printers also use various types of resin. Accordingly, using a commonly used DLP-type 3D printer, we compared and analyzed the fracture strength according to the layered thickness and the completeness and strength of the output according to the position of the printing plate.

Alharbi et al. [15] showed that when printing a specimen using AM technology and measuring the compressive fracture strength, the compressive fracture strength was high when the output angle was perpendicular to the compressive force. This is because the bonding force between layers during printing is weaker than the bonding force within a layer. In clinically used forms that are not ISO standards, it is possible that various other factors may be involved in flexural strength. In the clinically used form, it is shown that crack propagation may occur in a different way from ISO standard specimens due to non-uniform shape or form [16].

The results of this experiment showed that it was strongest when layered with a thickness of 0.010, and the strength became weaker as the thickness increased to 0.050 and 0.150. Consistent with what was predicted, the strength of the specimen printed with a thin layered thickness was shown to be strong. However, since thinning the layered thickness takes a lot of production time, it is not possible to make the layered thickness unconditionally thin to improve strength, so an appropriate printing thickness and time must be used. It is believed that the ideal manufacturing method is to confirm and manufacture.

This study looked at the fracture strength according to the printing thickness using a 3D printer, and it is believed that various follow-up studies will be needed in the future, such as the fracture strength according to the printing material or printing method. In addition, research on the support method should also be conducted.

In this study, we conducted a study on the fracture strength of resin, the most widely used 3D printer material in the dental industry, according to the layer thickness. In the fracture strength test, the specimen was divided into three kind with differences of 0.010, 0.050, and 0.150, respectively, and the specimens were manufactured based on ISO standards.

Looking at the average value according to the layered thickness, the thicker the thickness, the lower the average value. No significant difference was found between the 0.010 specimen and the 0.050 specimen, with an average difference of 0.001. It was found that the layered thickness had a significant effect on the fracture strength, with a difference of 0.022 between the 0.010 and 0.150 specimens.

When 3D printing, the fracture strength of the specimen varies depending on the layer thickness.

Therefore, it is believed that printing by setting an appropriate layer thickness considering production time will be helpful in producing ideal output.

No potential conflict of interest relevant to this article was reported.