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Comparison of Flexural Strength According to Post-Curing Treatment Time of Cast Resin Printed by 3D Printing Method
Int J Clin Prev Dent 2024;20(4):170-174
Published online December 31, 2024;  https://doi.org/10.15236/ijcpd.2024.20.4.170
© 2024 International Journal of Clinical Preventive Dentistry.

Jihyun Kim1,2

1CNZ Center, Seoul, 2Department of Dental Technology, Shinhan University, Uijeongbu, Korea
Correspondence to: Jihyun Kim
E-mail: sschocos@naver.com
https://orcid.org/0009-0008-0909-875X
Received December 5, 2024; Revised December 12, 2024; Accepted December 20, 2024.
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Abstract
Objective: This study compared and analyzed the effect of post-curing treatment time of cast resin manufactured using DLP type 3D printing technology on the flexural strength.
Methods: The experiment was performed by outputting a specimen designed according to ISO 20795-1:2013 standard using a DentaCAST resin (Asiga, Australia), and then dividing the post-curing treatment time into three groups (A, B, C) by setting the time to 5, 10, and 15 minutes. The flexural strength of each group was measured using a universal tester, and the data were analyzed with an SPSS statistical program and then the significance analysis was performed by one-way ANOVA test. Multiple comparisons after analysis were performed by Tukey HSD Test (p<0.001).
Results: As a result of the experiment, the flexural strength significantly increased as the curing time increased, and the average flexural strength of the 5-minute cured experimental group (A) was 88.51 MPa, the 10-minute cured experimental group (B) was 105.32 MPa, and the 15-minute cured experimental group (C) was 121.89 MPa. This is judged to be the result of improving the mechanical characteristics of the output by increasing the density of the polymer network due to the additional polymerization reaction during the photopolymerization process.
Conclusion: This study confirmed that the curing process after output is an important factor in determining the mechanical properties of 3D printing outputs. In future studies, follow-up studies on various 3D printing methods and materials are needed.
Keywords : 3D printing, DLP method, resin for cast, post-curing treatment, flexural strength
Introduction

The field of dental pores requires precise work to produce a prosthesis suitable for the patient's dental condition and oral structure. The traditional prosthesis manufacturing method relies heavily on the working time and skill of the dental technician, and involves a high cost and complex working process due to the high consumption of manpower and materials [1]. In addition, errors and material deformation that may occur during the working process can negatively affect the quality and accuracy of the prosthesis [2].

In order to solve such a problem, digital technology is being actively introduced and utilized in the dental pore industry. With the recent development of science and technology and the acceleration of digitalization, the dental pore field is changing in the direction of overcoming existing limitations through digital manufacturing processes and simultaneously improving productivity and quality [3]. A representative technology of digital dental pore is the dental CAD/CAM system.

The dental CAD/CAM system consists of data collection using scanning technology, design through CAD (Computer-Aided Design), and processing through CAM (Computer-Aided Manufacturing). The dental CAD/CAM processing system is a precise CNC (Computer Numerical Control)-based digital milling technology. However, in the case of the milling method, it has been reported that productivity is lowered due to limitations in the processing range and material loss [4].

3D printing (Three-dimensional printing) technology is a technology introduced to compensate for the shortcomings of digital cutting methods [5,6]. 3D printing is a technology of a lamination processing method in a two-dimensional shape based on digitized three-dimensional shape information, and various materials such as polymers, ceramics, or metal powder can be used. Depending on the output method, methods of laminating polymers or ceramics include a stereo-lithography application (SLA), a mask projection image hardening (DLP), a polyjet method, a liquid crystal plane solidification (LCPS), and a flexible deposition modeling (FDM) [7] method that melts alloy powder SLM and a metal lamination method such as SLS [8]. Each has advantages and disadvantages and can select a suitable method depending on the characteristics and requirements of the prosthesis. The 3D printing method is differentiated from the existing manufacturing method in that it can produce multiple prosthetics at the same time, reduce time and cost, and produce high-precision design data-based production.

Photopolymerization refers to a process in which an initiation reaction begins by irradiating UV to a UV initiator and a monomer, oligomer, and various additives are photopolymerized through a continuous reaction [9,10]. The photopolymerizable 3D printer improves the mechanical strength and physical properties of the output through curing treatment after printing the dental prosthesis designed in 3D [11]. Post-curing treatment is an important step in increasing hardness and strength by placing the output prosthesis in a curing machine and irradiating UV, and the results of physical experiments such as steaming surface hardness, bending strength, and polymerization rate are related to post-curing treatment conditions.

Therefore, this study aims to investigate the effect and importance of post-curing treatment in the dental prosthesis manufacturing process by analyzing the effect of post-curing treatment time of cast resin manufactured using DLP-type 3D printing technology on the flexural strength of the output based on the photopolymerization process.

Materials and Methods

1. Materials

Among the photocurable 3D printed resins, DentaCAST resin (Asiga, Austrailia), which is mainly used to manufacture partial denture frames, was used (Figure 1).

Figure 1. DentaCAST resin (Asiga, Austrailia).

2. Methods

1) Experimental design and specimen preparation

The specimen was designed with a length of 65 mm, a width of 10 mm, and a thickness of 3.3 mm, and was designed according to ISO 20795-1:2013. A V-shaped notch with a depth of 1.0 mm and a width of 1.0 mm was formed in the center of the specimen to measure the flexural strength. The designed specimen was designed using the Inventor program and then stored in the STL file format and used for 3D printing (Figure 2).

Figure 2. Experimental design and specimen preparation.

2) Print out specimen

DentaCAST resins (Asiga, Australia) were uniformly mixed for 30 minutes using an MX-T6-S tube roller. The specimens designed through the CAD program were placed with Generate Support. The specimens were 3D-printed with a stacking thickness of 0.050 mm by using a DLP type MAX UV 3D Printer (Asiga, Australia) (Figure 3). Thirteen each group was 3D-printed, and a total of three groups were 3D-printed.

Figure 3. MAX UV 3D Printer (Asiga, Australia).

3) Wash and removing support

The 3D-printed specimen was washed with alcohol for 20 minutes using a Formlabs-Form Wash (USA) cleaner (Figure 4), and then the alcohol was removed and dried. Thereafter, the support was removed to prevent damage to the 3D-printed specimen.

Figure 4. Formlabs-Form Wash (USA).

4) Post-polymerization conditions

After washing, the dried specimen was placed in a curing device, Cure MU102H (UV Curing Unit, Korea) equipment and cured for 5, 10, and 15 minutes, respectively (Figure 5).

Figure 5. Cure MU102H (UV Curing Unit, Korea).

5) Statistical analysis

The flexural strength of the specimen was measured using MCT-1150 (A&D, Japan) equipment (Figure 6). The specimen was fixed to the lower support, and the V-shaped notch was arranged so that it was located in the center of the upper crush. The load speed was set to 10 mm/min, and a load of 5 N was applied to obtain the maximum point load data value. The flexural strength was measured in 13 specimens per group, and the measured data were analyzed using SPSS statistical program (IBM, USA) and then the significance analysis was performed by one-way ANOVA test. Multiple comparisons after analysis were performed by Tukey HSD Test (p<0.001).

Figure 6. MCT-1150 (A&D, Japan).
Results

In this study, cast resin specimens were fabricated using DLP-type 3D printing at a stacking direction of 90°. Post-curing treatment times were varied across three experimental groups: Group A (5 minutes), Group B (10 minutes), and Group C (15 minutes). A flexural test was performed on the specimens from each group to measure their fracture load. The flexural strength was calculated using the measured fracture load and the cross-sectional area of the specimens (Table 1).

Table 1 . Flexural strength of cast resin specimens according to post-curing treatment time (unit: MPa)

NO.5 minute10 minute15 minute
170.55109.59122.51
287.89101.65108.7
391.57110.19139.99
484.0492.68129.06
574.64101.92135.2
6101.23112.1137.83
795.7999.57118.01
8104.9198.28113.43
983.6116.56120.54
1096.71102.02122.08
1188.3114.1105.61
1298.33100.82106.73
1373.08109.74124.86
Mean±SD88.51±11.00a105.32±7.12b121.89±11.46c
F=35.80, p<0.001

p-value by one-way ANOVA.

a,b,cSame letters indicate statistically indifferent by Tukey HSD Test multiple comparison.



The flexural strength of the cast resin specimens varied according to the post-curing treatment time. The results are summarized as follows:

∙ Experimental group A (5 minutes of post-curing) exhibited the lowest average flexural strength of 88.51 MPa.

∙ Experimental group B (10 minutes of post-curing) showed an intermediate average flexural strength of 105.32 MPa.

∙ Experimental group C (15 minutes of post-curing) demonstrated the highest average flexural strength of 121.88 MPa.

The experimental results revealed a significant increase in flexural strength with longer post-curing times. Statistical analysis confirmed that the differences in flexural strength among the three experimental groups (A, B, and C) were statistically significant (p<0.001).

Discussion

In this study, the effect of the post-curing treatment time of the cast resin manufactured using a DLP type 3D printer on the flexural strength was analyzed. As a result of the study, as the post-curing time increased, the flexural strength of the specimen was significantly improved (p<0.001), which is believed to be due to the fact that the unreacted photopolymerizer in the resin formed the polymer network more densely through additional polymerization during the photopolymerization process. In particular, an increase in flexural strength of about 37.7% was observed when the curing time was increased from 5 minutes to 15 minutes, and it was confirmed that sufficient post-curing treatment was a key factor in determining the mechanical properties of the resin.

Previous studies have also emphasized the importance of post-curing treatment time. Cho et al. [12] said that the polymerization efficiency of the photopolymerization resin is greatly affected by UV irradiation intensity as well as irradiation time, and reported that the polymerization depth and polymerization rate increase as the irradiation time increases. This is consistent with the results of this study and suggests that the mechanical properties of the resin can be enhanced through sufficient UV irradiation during the post-curing process. Conversely, if the UV irradiation time is insufficient, it could be confirmed once again that there is a possibility that the polymerization reaction proceeds incompletely and the strength and durability of the resin may decrease.

However, contrary to the general tendency that the strength increases as the post-curing treatment time increases, Kim [9] reported that photopolymerization at 5, 15, and 30 minutes resulted in a slight contraction rate as the time increased, but there was no statistically significant difference. These conflicting results can be attributed to differences in the properties, output methods, or photopolymerization conditions of the resin used, and it needs to be clearly identified through subsequent studies.

In addition, this study targeted a DLP type 3D printer and a specific cast resin, and there is a limit to generalization in that the experiment was carried out under limited conditions such as the stacking direction (90°). A broader understanding is needed through research including various 3D-print device methods, resin materials, and stacking directions. In addition, it is important to propose optimal 3D-printed and post-curing conditions for dental prosthetics by comprehensively analyzing the effects of the resin composition before 3D-printed, the intensity and spectrum of UV light sources, and the type of post-curing equipment on mechanical properties.

In conclusion, this study emphasized the importance of post-curing treatment in the dental prosthesis manufacturing process by examining the effect of the post-curing process of the cast resin using a DLP type 3D printer on the mechanical properties of the printed matter. However, an in-depth follow-up study is required to set the optimal post-curing treatment conditions when manufacturing a dental prosthesis. Through this, it is expected that the efficiency and quality of dental prosthesis manufacturing using 3D printer technology can be further improved.

Conclusion

This study investigated the effect of post-curing treatment time on the flexural strength of cast resin printed with DLP-type 3D printing. The results showed a significant improvement in flexural strength with increased curing time.

These findings underscore the importance of the post-curing process in enhancing the mechanical properties of 3D-printed cast resins, highlighting the need for optimal curing conditions in dental prosthesis production.

Conflict of Interest

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

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