In the dental world, the method of manufacturing prosthetics was to take an impression and use the impression to go through several stages and use the completed working model. However, compensating for errors that occurred during the many stages of the production process and problems in terms of time and economics were issues that needed to be resolved.

Recently, in the rapidly developing dental world, the manufacturing process of prosthetics has also changed. Simplification is being achieved by designing using a CAD/CAM computer program and then 3D printing it. These changes in the production process also brought changes to model production. Nowadays, dentistry takes pictures of the patient’s oral cavity using an intraoral scanner and sends the files to a dental laboratory to manufacture prosthetics using CAD/CAM without a working model, or to print the working model using 3D printing resin. An oral scanner used in dentistry is to project a laser or white light onto tissue or hemorrhoids in the oral cavity to acquire information about the condition within the oral cavity, and convert the results into digital information [1]. The process of using this oral scanner was able to increase the patient’s satisfaction with chair time by reducing the work in the oral cavity that occurred as the existing method was popularized as an impression of the patient’s oral condition. In addition, the method of using a working model, which is an indirect manufacturing method, has the disadvantage of taking a long manufacturing period and increasing the possibility of errors in the accuracy of the prosthesis because it is manufactured manually by a dental technician. It will also affect the economic aspect due to the costs incurred during the production process.

If the patient’s oral condition is measured and produced on a computer, it will not be possible to check secondary fit for errors that occur after production. In addition, since the method of using an intraoral scanner still requires skill on the part of the operator and requires secondary confirmation of suitability, the method of printing the working model in resin and manufacturing it is still widely used.

Resin used in 3D printing has various applications in the dental field. It is possible to make temporary dentures, make wax molds, and make working models. Temporary dentures are designed on a computer and then printed using 3D printing. The manufacturing process of making a wax mold with wax and then making a prosthesis with metal can also be done in the same way as the analog manufacturing process of investing, burn out, and casting by designing with CAD/CAM and then printing it with resin. Additionally, a working model using plaster can be produced using 3D printing. The process of producing a working model with resin involves receiving the condition of the oral cavity in the form of a file and using 3D printing to print it with resin corresponding to the working model.

3D printing refers to a manufacturing technology that creates three-dimensional objects by stacking successive layers of raw materials using a 3D printer [1]. In the dental field, shapes were reproduced by cutting zirconia blocks or printing resin, but now more diverse materials are used, and more complex shapes can be produced at a faster rate. With the development of 3D imaging techniques, 3D printing methods are being used in various dental fields, including surgery, prosthetics, and orthodontics [2].

The types of 3D printers currently used in the dental industry are largely divided into four types. First, Fused Deposition Modeling (FDM) is an extrusion lamination molding method that melts thermoplastic resin and then builds it up from the bottom through a nozzle. Second, the Stereolithography apparatus (SLA) method irradiates a laser beam into a water tank containing photocurable resin to harden the parts that the laser touches on the liquefied photocurable plastic. It is a photocuring lamination method that creates a shape by stacking layers by performing these operations repeatedly.

Third, the Selective Laser Sintering (SLS) method is a sintering laser method that forms plastic, metal, or glass chunks into small powders and then melts, solidifies, and stacks them with a laser. Lastly, Digital Light Processing (DLP) is a method that uses photocurable resin to harden liquid resin by irradiating light from a beam projector.

Among the four methods, the SLA and DLP methods use photocurable liquid resin and are based on the principle of layering through a photopolymerization reaction. However, the DLP method is used more often in dental laboratories because the printing area at once is wider and shapes can be produced quickly.

3D printers in the dental industry consist of a three-step process. After modeling work to create a 3D digital source, printing work to manufacture materials by additive manufacturing, and finishing processes include removal of supports and deposition of surface materials. In the case of resin made through this process, its physical properties are expected to be affected by various manufacturing processes. It is thought to be affected by the speed of lamination, polymerization time, and post-processing of the resin. Among them, the curing time may be determined by the material company’s guide, but research on how much that time affects is still insufficient.

Therefore, this study conducted experiments using the resin used in working models among various photocurable resins. At a time when there is a shift from model production using analog methods in Korea to production of working models using 3D printers, we sought to help with manufacturing methods through research on photocurable resins used in manufacturing working models. After lamination using photocurable resin, which is currently used as a working model, we attempted to look at the difference in flexural strength by varying the curing time. If there is a difference in flexural strength depending on the photocuring time, it is believed that when manufacturing a restoration, it can function as a working model to its full potential by polymerizing it with an appropriate curing time after lamination.

1. Specimen production

In this experiment, the design of photocurable 3D printing resin used for model production was completed using the Inventor program. It was designed as a specimen according to ISO 20795-1:2013 with a length of 65 mm, a width of 10 mm, and a thickness of 3.3 mm, and was then saved and manufactured as an STL file. At this time, a V-shaped notch with a depth of 1.0 mm and a width of 1.0 mm was made in the center of the specimen to enable measurement of bending strength at the same location.

2. Design and output placement

Mix the DentaMODEL (Asiga, Austrailia) resin solution on a tube roller (MX-T6-S Tube Roller) for 30 minutes (Figure 1). Supports were attached and placed on the specimen designed using a CAD program (Figure 2).

Figure 1. Resin mixing on tube rollers.

Figure 2. Specimen production.

3. Specimen printing, cleaning and support removal

The specimens were printed in three groups of 12 each using a MAX UV 3D Printer (ASIGA, Australia) with a lamination thickness of 0.050 mm. The 3D printing resin printed at this time was laminated using the DLP method. The printed specimen was washed for 20 minutes using an alcohol-containing washer (Formlabs-Form wash, USA), then the alcohol was removed and dried. Afterwards, the support was removed to prevent damage to the specimen.

4. Light curing treatment

After washing, the dried specimen is placed in Cure M U102H (UV Curing Unit, Korea) and subjected to three types of curing treatment for 5 minutes, 10 minutes, and 15 minutes (Figure 3).

Figure 3. MCT-1150.

5. Measurement and analysis

The flexural strength data of the completed specimen was measured using MCT-1150 (A&D, Japan). The V-shaped notch of the specimen was placed in the center of the upper indenter using the lower support, and the loading speed was set at 10 mm/min. A load of 5N was applied and measured, and the maximum point load data value was obtained.

Flexural strength data from 12 specimens were measured and analyzed using SPSS 22 statistical program (IBM, USA), and Tukey HSD test was used for post-hoc analysis (p<0.05) (Figure 4).

Figure 4. Photopolymerized specimen.

The results of the flexural strength according to the curing time of the photocurable 3D printing model resin used in this experiment are shown in Table 1 below.

Table 1 . Flexural strength test results according to curing time of photocurable resin (n=12, unit: N)

Time (minute)	M±SD	Min	Max	p
5*	85.89±7.20	67.40	94.65	<0.05
10*	90.89±8.48	79.79	105.33
15	103.75±5.47	90.97	110.06

*No statistically significant difference.

p-value by one-way ANOVA.

The flexural strength value was the lowest at 67.40 and the highest at 94.65 at a curing time of 5 minutes. At a curing time of 10 minutes, 79.79 was the lowest value and 105.33 was the highest value. At a curing time of 15 minutes, 90.97 was the lowest value and 110.06 was the highest value. The standard deviation at 5 minutes was 7.20, at 10 minutes it was 8.48, and at 15 minutes it was 5.47, with the lowest value at 15 minutes.

It can be seen that the difference in flexural strength according to the curing time of the resin was small between 5 and 10 minutes, but the difference between 15 minutes was large.

This study printed the photocurable 3D printing resin used in model making in a size that complies with the ISO 20795-1 standard, measured the flexural strength by varying the curing time, and investigated how the curing time affects it. It is known that the factors that affect composite resins, which are currently widely used in the dental field, include light intensity [3-5], polymerization time [6-12], and light transparency [10] of the resin. We wanted to know how much the flexural strength of the resin, which is affected by these various aspects, is affected by the photocuring time. The mechanical properties of dental materials are one of the most important factors in the success of prosthetics in the dental field. In comparing the flexural strength of resins with different polymerization methods, there is also a previous study that reported that Tescera showed higher flexural strength values ​​than Sinfony [11,12]. In addition, the SI (Sinfony; 3M-ESPE, USA), TS (Tescera; BISCO, USA), and TW (Twiny; Yamakin, Japan) groups of composite resins for indirect restoration all have a minimum bending strength of 50 MPa or more as specified in ISO 10477 regulations. Since it was satisfactory, it was said to be clinically applicable [13]. As can be seen from these studies, the flexural strength of the resin used in this study is considered sufficient. Of course, the material of the working model has different material properties from the prosthesis actually used in the oral cavity, but it is an important material in manufacturing prosthesis. The time it takes to produce a working model using photocurable resin, which requires a certain amount of curing time, will also play an important role in processing the work.

In addition, various studies are being conducted on how physical properties change when resins affected by various types of influences are printed, and research on lamination thickness is also increasingly required. It is thought that the layer thickness of 3D printing can also affect production speed and have a greater impact on actual clinical practice. If the micro-hardness and bending strength appropriate for use in the oral cavity are achieved, it is not thought to be a problem to manipulate the lamination thickness to be larger. However, it would be better to obtain an appropriate hardness value compared to the time required depending on the lamination thickness.

There is time required in the process of producing a working model using 3D printing, and there is also time required for light curing. It was thought that if flexural strength above a certain level was maintained, there would be no need to spend a long time on light curing. ISO 20795-1:2013 specifies the minimum flexural strength of denture base resin as 65 MPa. It can be seen that the flexural strengths of the working model resin for 5, 10, and 15 minutes of light curing were all higher than the minimum flexural strength of the denture base resin. As a result of this experiment, it was found that the longer the light curing time, the more significant the fracture strength was. It can be seen that the difference between the light curing times of 10 minutes and 15 minutes is greater than the difference between the light curing times of 5 minutes and 10 minutes.

In general, the resin used in working models can have sufficient flexural strength even with light curing for 5 or 10 minutes, but it is considered preferable to light curing for 15 minutes or more.

Comparison is difficult because there are no studies reported regarding the flexural strength of DentaMODEL, the resin used in the working model in this study. In addition, it is believed that follow-up research should be conducted on the differences in bending strength depending on the location during light curing.

In addition, the current dental community is showing increasing interest in the production of dentures using resin and prosthetics using CAD/CAM systems and CAD-printers. Unlike other prosthetics, dentures, which are made of resin and have denture bases and artificial teeth, can be easily incorporated into CAD printers, so continued research on materials will be needed in the future. The penetration rate of CAD/CAM systems in the dental laboratory is estimated to be 60%. It is believed that research into resins that can make CAD printers easier than other prosthetic materials should be actively conducted in the dental field.

In this study, the bending strength of 12 specimens was measured at different light curing times of 5 minutes, 10 minutes, and 15 minutes, respectively, using model resin that is used to receive files of oral conditions from a dental office and make models using a 3D printer. By measuring the difference, the following conclusions were obtained.

1.Comparing the average value of flexural strength according to curing time, 85.89 for 5 minutes, 90.89 for 10 minutes, and 103.75 for 15 minutes, the flexural strength increased as the curing time was longer.

2.There appears to be a correlation between the bending strengths and the differences in photocuring times. There was a statistically significant difference in all cases, but there was no statistically significant difference only at 5 and 10 minutes.

From the above results, it was confirmed that light curing time affects the flexural strength of the resin. It is believed that no clinical problems will arise if the flexural strength at a level that does not cause problems during work is secured by making good use of the photocuring time of the resin. However, it is believed that there will be differences in bending strength depending on the location of light irradiation during photopolymerization, so additional research is needed.

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