Currently, the dental industry is rapidly transitioning from the traditional manufacturing process of producing prostheses by indirect and direct methods to 3D printing by using an oral scanner to photograph the condition of the oral cavity and then using a CAD/CAM program to create a 2D cross-section of the condition. This is because the traditional method of manufacturing prosthetics involves many steps and is not as cost-effective or time-efficient as printing using CAD/CAM.

Among prostheses, temporary prostheses are used to prevent damage to the missing tooth until the final prosthesis is fitted [1]. Temporary restorations using resin can be made by both direct and indirect methods, and in the past, the direct method, which involves manipulating the resin directly in the patient's mouth, was commonly used, but nowadays, rather than making restorations using resin directly in the mouth, as is the case with other prosthetics, the method is changing to using an oral scanner to data the condition of the mouth, storing it in a program, and printing it through CAD/CAM. This method of designing and outputting in a program using oral scan data is becoming more preferred because it reduces the time spent in the patient's mouth. As mentioned in the traditional method, there is an indirect method besides the direct method, but the indirect method is made by hand with a dental technician making a plaster model of the restoration, so the production period is long and there is a high probability of error in the accuracy of the prosthesis [2]. In addition, the problem of manufacturing error, caused by many steps and variables of the method, affects the prosthesis and cases of the indirect method are now gradually being reduced.

In the case of 3D printing using a scanner in the oral cavity, a 3D scanner is a manufacturing technology that acquires information about the shape of an object by projecting a laser or white light onto tissues or hemorrhoids in the oral cavity, and converts the resulting value into digital information by using a 3D printer to create a three-dimensional object by stacking successive layers of raw materials [3]. The process of creating the shape of an object in three dimensions on a computer, i.e., designing the shape and creating a three-dimensionally designed item. These 3D printers are used in many different fields such as aviation, aerospace, medical, and so many other fields, so the market is getting bigger and bigger. Among the healthcare sectors, dentistry is one of them. Up until now, computer-aided manufacturing (CAM) in dentistry was a subtractive technique, using blocks of zirconia or resin [4]. Today, a wider variety of materials are used and more complex shapes can be created at a faster rate. With the advancement of 3D imaging, 3D printing is being utilized in many areas of dentistry, including surgery, prosthodontics, and orthodontics [5].

The types of 3D printers currently in use are Fused Deposition Modeling (FDM), Stereolithography apparatus (SLA), Selective Laser Sintering (SLS), and Digital Light Processing (DLP). First, FDM is an extrusion additive manufacturing method in which a thermoplastic resin is melted and deposited from the bottom up through a nozzle. SLA is a light-curing additive manufacturing method that uses a laser to irradiate a water bath, causing the parts it touches to harden and build up to form a shape. In the case of SLS, it is a sintering laser method that melts plastic, metal, glass, etc. in the form of small powders with a laser and then solidifies them to create a layer. Finally, DLP is a method that utilizes photocurable resins by irradiating the liquid resin with light using a beam projector to cure it. The LA and DLP methods are based on the principle of laminating a photopolymerization reaction to a photocurable liquid resin, but the DLP method is more widely used in dental laboratories because it can be produced more quickly with a wider surface area.

In the general framework of subtractive and additive manufacturing, printing is carried out by various types of technologies such as photopolymerization, material spraying, and powder additive fusion. Among them, photopolymerization printing has the advantage of being able to produce multiple prints at once with high molding speed and precision by repeatedly curing the liquid photocurable plastic with a laser beam.

The physical properties of these 3D printed resins are expected to be different from those produced using traditional methods. It is also possible to control the thickness of the layers when using 3D printing, and it is thought that different thicknesses of layers will have different physical properties. Research is being done on restorations made with traditional methods and 3D printed resins. However, there hasn't been much experimentation with resin lamination thicknesses. Therefore, this study aimed to compare the difference in microhardness by different thicknesses of laminations of 3D printing, using a light-curable resin that is used to make temporary dental restorations in Korea. If there is a difference in microhardness based on the thickness of the lamination, then it is likely that maintaining an appropriate lamination thickness when fabricating restorations may be a better way to create a temporary restoration.

1 Specimen design

In this experiment, to measure the microhardness of photocurable 3D printing resin, a specimen with a length of 30 mm, a width of 10 mm, and a thickness of 3 mm was designed using the Inventor program and saved as an STL file to complete the design.

2. Placing printouts

Mix the C&B 5.0 Hybrid (Arum, Korea) resin solution with the printing resin using a tube roller (MX-T6-S Tube Roller) for 30 minutes. The Cadcam program was used to attach supports to the designed specimens and automatically place them in batches of 15.

3. Specimen output and post-processing

The specimens were printed using a MAX UV 3D Printer (ASIGA, Australia) with laminate thicknesses of 0.010 mm, 0.050 mm, 0.075 mm, and 0.150 mm. Specimens printed by the DLP method were washed for 20 minutes using a washer with alcohol (Formlabs-Form wash, USA) and then dried. After cleaning, the dried specimens are placed in a UV (ARUM, Curing Machine, Korea) for curing for 20 minutes.

4. Support removal and surface preparation

The hardened specimens were removed from the surface of the support using a denture burr (D079E; Dedeco, Long Eddy, NY, US) without damaging the specimen, and sandpaper was used to smooth the specimen. The specimen was finished with 20 grit sandpaper.

5. Measurement and analytics

The microhardness data of the finished specimens were measured using a micro Vickers hardness tester (Mitutoyo HM, Japan). The specimen was placed on a micro Vickers hardness tester, loaded with 0.5 kg for 10 seconds, and measured at M50 to obtain the Vickers Hardness Number (VHN) value.

Statistical analysis was performed using the SPSS 22.0 en (IBM SPSS, USA) program, and a one-way ANOVA was used to compare the means of the experimental results, and the significance level was set at 0.05.

The results of the microhardness experiment as a function of the deposition thickness of the photocurable 3D printing resin are shown in Table 1.

Table 1 . Microhardness test results according to the layer thickness of 3D printing resin

Additional Thickness	N	Mean±SD	Min	Max	p**
0.010	15	293.087±11.191	226.90	382.60	<0.05
0.050*	15	237.307±4.883	200.00	269.50
0.075*	15	214.240±4.807	187.40	261.30
0.150	15	180.867±4.319	146.00	204.40

*Only 0.050 and 0.075 are not statistically significant.

**p-value by one-way ANOVA (Dunnett T3).

The microhardness results show that the highest lamination thickness was 293.087 at 0.010, and the lowest was 180.867 at 0.150. Also, 0.050 is 237.307 and 0.075 is 214.240, but only 0.050 and 0.075 are not statistically significant.

The highest value was 0.010 to 382.60, and the lowest was 0.150 to 204.40, a difference of almost two orders of magnitude. The significance level for the microhardness values was 0.05.

In this study, we measured and compared the microhardness of specimens printed with different lamination thicknesses of 3D prints to understand the impact on actual clinical applications. Composite resins have been shown to be affected by light intensity [6-8], polymerization time [9-12], light transmission of the resin [13], filler size and content [14,15], and color [16]. There are various studies on how the properties of these variously affected resins change when they are printed, and studies on the lamination thickness are increasingly required. The physical properties of outputs with different lamination thicknesses, such as strength, precision, and size, are also being studied. The layer thickness of 3D printing also affects the speed, which may have a greater impact on real-world clinical applications. If the microhardness is adequate for use in the oral cavity, it should not be a problem to manipulate the lamination thickness to a greater extent, but it is recommended that the lamination thickness should be adequate for the time required.

We believe that digital printing is already commonplace in the dental industry. And I think it's only going to get faster with more and more different and advanced technologies and methods. In particular, there are materials such as wax, resin, or metal that can be printed, and resin is a material that is widely used in dental clinics and dental laboratories. The use of in-office dental CAD/CAM systems is becoming increasingly popular due to patient and clinician demand for tooth-colored restorations and same-day restorations [17,18]. As a result, 3D printed dental resins are also being researched.

The dental industry is also increasingly interested in the production of dentures using resin, as well as the production of prostheses using CAD/CAM systems and CAD-printers. Unlike other prosthetics, dentures made of resin and artificial teeth can be easily incorporated into CAD printers, so there will be a need for continued material research in the future. The penetration of CAD/CAM systems in the dental industry is estimated at 60%. We believe that the dental community should continue to research resins that can be used with CAD printers more easily than other prosthetic materials.

In this study, the microhardness of 3D printing resins used to fabricate temporary restorations was measured by fabricating specimens with four different lamination thicknesses. As a result, the following conclusions were obtained.

1. the microhardness of 3D printing resin varies depending on the deposition thickness.

2. the microhardness was highest at 0.010 to 293.087, and lowest at 0.150 to 180.867.

3. the microhardness was 237.307 at 0.050 build thickness and 214.240 at 0.075 build thickness, but only at 0.050 and 0.075 was the difference statistically significant.

Based on the above results, it was confirmed that the deposition thickness of 3D printing resin affects the microhardness of the resin. However, for the production of temporary restorations for use in the oral cavity, the microhardness of the resin is considered to be sufficient.

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