*2.2. Fabrication of Specimen via Additive Manufacturing*

A100 27.24 0.1 100

Figure 1 shows the specimen for this study was designed in nine hollow structures as a 2 mm square of 20 mm × 20 mm × 1 mm using CAD software (SolidWorks 2016, Dassault Systems Corp., Vélizy-Villacoublay, France) was then converted to a standard tessellation language (STL) file for additive manufacturing. Zirconia specimens were prepared using DLP equipment (Octave Light R1, Octave Light Ltd., Shatin, Hongkong) (wavelength 365–405 nm), setting the thickness of each layer to 50 µm and the exposure time to 4 s (*n* = 5) [31].

#### *2.3. Geometrical Overgrowth Evaluation*

Geometrical overgrowth occurs by curing a specimen into a wider area than the existing area exposed to the light source resulting from the light scattering effect of zirconia particles [31]. This experiment measured the degree of reduction of the nine hollow

square structures. The overgrowth observed in the experiment was compared to the STL file standard in length and area. Figure 2 shows the length was measured using a stereomicroscope (EGVM-452M; EG Tech, Gwangyang, South Korea) by one person at a specific position throughout the study (*n* = 5). prepared using DLP equipment (Octave Light R1, Octave Light Ltd., Shatin, Hongkong) (wavelength 365–405 nm), setting the thickness of each layer to 50 µm and the exposure time to 4 s (n = 5) [31]. (a) (b) (a) (b) Figure 1. (a) Schematization of the STL file of the specimen. (b) The square hole (2 × 2 mm) for geometrical overgrowth measurement.

tessellation language (STL) file for additive manufacturing. Zirconia specimens were

tessellation language (STL) file for additive manufacturing. Zirconia specimens were prepared using DLP equipment (Octave Light R1, Octave Light Ltd., Shatin, Hongkong) (wavelength 365–405 nm), setting the thickness of each layer to 50 µm and the exposure

tessellation language (STL) file for additive manufacturing. Zirconia specimens were prepared using DLP equipment (Octave Light R1, Octave Light Ltd., Shatin, Hongkong) (wavelength 365–405 nm), setting the thickness of each layer to 50 µm and the exposure

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time to 4 s (n = 5) [31].

time to 4 s (n = 5) [31].

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Figure 1. (a) Schematization of the STL file of the specimen. (b) The square hole (2 × 2 mm) for geometrical overgrowth measurement. **Figure 1.** (**a**) Schematization of the STL file of the specimen. (**b**) The square hole (2 × 2 mm) for geometrical overgrowth measurement. stereomicroscope (EGVM-452M; EG Tech, Gwangyang, South Korea) by one person at a specific position throughout the study (n = 5).

Figure 2. Measurement of linear deviation using a stereoscopic microscope. **Figure 2.** Measurement of linear deviation using a stereoscopic microscope. Figure 3 shows the area measured using image analysis software (ImageJ software,

Figure 3 shows the area measured using image analysis software (ImageJ software, National Institutes of Health, Bethesda, MD, USA). Geometrical overgrowth was quantified by calculating the difference between the actual measured and the designed Figure 3 shows the area measured using image analysis software (ImageJ software, National Institutes of Health, Bethesda, MD, USA). Geometrical overgrowth was quantified by calculating the difference between the actual measured and the designed size (2 mm). National Institutes of Health, Bethesda, MD, USA). Geometrical overgrowth was quantified by calculating the difference between the actual measured and the designed size (2 mm).

size (2 mm). Figure 3. Measurement of area deviation using image analysis software. **Figure 3.** Measurement of area deviation using image analysis software.

#### *2.4. Cure Depth*

Figure 3. Measurement of area deviation using image analysis software. Cure depth was measured to evaluate the effect of the UV absorber during lamination. Under the same conditions as the DLP 3D printer, a distance of 20 cm, a light of 100 mW/cm<sup>2</sup> , and a light exposure time of 4 s were maintained. Figure 4 shows the final cured thickness (*n* = 3) based on the volume fraction of the UV absorber, which was measured using digital vernier calipers (*n* = 3).

#### *2.5. Microstructural Analysis*

Figure 3. Measurement of area deviation using image analysis software. The conditions for degreasing and sintering were set based on the results of previous experiments [39]. The microstructure and surface of the specimens were observed before and after sintering, and the defects were observed using a field emission scanning electron microscope (FE-SEM; JEOL) (ISO-13356) based on the UV absorber volume fraction [38].

measured using digital vernier calipers (n = 3).

Figure 4. Schematic representation of curing depth of zirconia suspension by UV exposure. **Figure 4.** Schematic representation of curing depth of zirconia suspension by UV exposure.

Cure depth was measured to evaluate the effect of the UV absorber during lamination. Under the same conditions as the DLP 3D printer, a distance of 20 cm, a light of 100 mW/cm², and a light exposure time of 4 s were maintained. Figure 4 shows the final cured thickness (n = 3) based on the volume fraction of the UV absorber, which was

#### 2.5. Microstructural Analysis *2.6. Statistical Analysis*

2.4. Cure Depth

The conditions for degreasing and sintering were set based on the results of previous experiments [39]. The microstructure and surface of the specimens were observed before and after sintering, and the defects were observed using a field emission scanning electron microscope (FE-SEM; JEOL) (ISO-13356) based on the UV absorber volume fraction [38]. All statistical analyses were performed using SPSS 21.0 (SPSS Inc.; Chicago, IL, USA). All results were tested for significance at the *p* < 0.0083 level. As normality was not satisfied in the Shapiro–Wilk test, the Kruskal–Wallis, a nonparametric test, was performed. The post hoc test was performed using the Bonferroni correction method.

## **3. Result and Discussion**

#### 2.6. Statistical Analysis *3.1. Geometrical Overgrowth Evaluation*

All statistical analyses were performed using SPSS 21.0 (SPSS Inc.; Chicago, IL, USA). All results were tested for significance at the p < 0.0083 level. As normality was not satisfied in the Shapiro–Wilk test, the Kruskal–Wallis, a nonparametric test, was performed. The post hoc test was performed using the Bonferroni correction method. 3. Result and Discussion 3.1. Geometrical Overgrowth Evaluation It is essential to evaluate the strength and precision of additive manufacturing of zirconia dental prostheses using DLP. Geometric overgrowth, caused by over-curing resulting from the light scattering effect, is a factor that can affect precision. Light It is essential to evaluate the strength and precision of additive manufacturing of zirconia dental prostheses using DLP. Geometric overgrowth, caused by over-curing resulting from the light scattering effect, is a factor that can affect precision. Light scattering is affected by the volume fraction of zirconia, the difference in refractive index between the zirconia powder and the medium, and the curing time [28–30]. Jang et al. [29] reported that zirconia has a refractive index of 2.1, which is 20–27% higher than silica and alumina. Thus, the degree of light scattering is large, and the cure depth is limited. In this study, a UV absorber was added to the 54 vol% zirconia photopolymer suspension to evaluate the degree of overgrowth. The cure depth was measured to ensure sufficient curing for additive manufacturing.

scattering is affected by the volume fraction of zirconia, the difference in refractive index between the zirconia powder and the medium, and the curing time [28–30]. Jang et al. [29] reported that zirconia has a refractive index of 2.1, which is 20–27% higher than silica and alumina. Thus, the degree of light scattering is large, and the cure depth is limited. In this study, a UV absorber was added to the 54 vol% zirconia photopolymer suspension to evaluate the degree of overgrowth. The cure depth was measured to ensure sufficient curing for additive manufacturing. Figures 5 and 6 and Table 3 show the degree of overgrowth by subtracting the length and area of each specimen measured manufactured based on the UV absorber ratio by the designed value. In all the groups, the void structure decreased and showed an overgrowth Figures 5 and 6 and Table 3 show the degree of overgrowth by subtracting the length and area of each specimen measured manufactured based on the UV absorber ratio by the designed value. In all the groups, the void structure decreased and showed an overgrowth pattern. As the UV absorber ratio increased, the degree of overgrowth tended to decline. In terms of length, the average degree of overgrowth was 40 µm in the A100 group and 251 µm in the O group. Regarding the ratio, the length showed an overgrowth rate of 2.1% in the A100 group and 12.5% in the O group, and the area showed an overgrowth rate of 8.5% in the A100 group and 12.25% in the O group. Significant differences were observed in the length and areas of the O group and the A50 and A100 groups without a UV absorber (*p* < 0.0083). No significant difference was observed based on the UV absorber ratio.

pattern. As the UV absorber ratio increased, the degree of overgrowth tended to decline. In terms of length, the average degree of overgrowth was 40 µm in the A100 group and 251 µm in the O group. Regarding the ratio, the length showed an overgrowth rate of 2.1% in the A100 group and 12.5% in the O group, and the area showed an overgrowth rate of 8.5% in the A100 group and 12.25% in the O group. Significant differences were observed in the length and areas of the O group and the A50 and A100 groups without a UV absorber (p < 0.0083). No significant difference was observed based on the UV absorber ratio. The results highlight that the group prepared by adding 0.05% and 0.1% UV absorber to the zirconia suspension (A50 group, A100 group) showed a significantly greater degree of overgrowth in length and area than the group without the addition of UV absorber (Group O). Our previous study [29] measured geometrical overgrowth according to the volume fraction of zirconia, showing an overgrowth rate of 33.52% at 54 vol%, higher than the results obtained in the current study. Despite the differences in the composition of the UV absorber and suspension, a geometrical overgrowth can be considered a factor that influences the larger exposure area and longer cure time than the specimen in this study. Geometrical overgrowth can be affected by the presence or absence of a UV absorber, as shown in this study, in addition to factors such as curing time, light quantity, exposure area, and difference in suspension composition [7,24,29,30].

The results show that the geometrical overgrowth was the lowest in the A100 group, containing 0.1% UV absorber, with overgrowth rates of 2.1% and 8.5% in length and area, respectively. When the overgrowth rate of the length was lower, the overgrowth was not constant by region, and the periphery of the rectangle tended to be rounded. Thus, we assumed that there was a difference in the area when the length was measured at a certain intermediate position. Additional studies are needed on the degree of overgrowth and the

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direction and pattern of overgrowth according to the specimen shape for precise additive manufacturing of zirconia. Materials 2022, 15, x FOR PEER REVIEW 6 of 10

Figure 5. Comparison of length (µm) and area (mm2) of the prepared sample. Different letters indicate significant differences and when the same letters included there is no significant differences. (p < 0.0083). **Figure 5.** Comparison of length (µm) and area (mm<sup>2</sup> ) of the prepared sample. Different letters indicate significant differences and when the same letters included there is no significant differences. (*p* < 0.0083). Figure 5. Comparison of length (µm) and area (mm2) of the prepared sample. Different letters indicate significant differences and when the same letters included there is no significant differences. (p < 0.0083).

Figure 6. The bar graph represents the cure depth (µm) of the prepared sample. Figure 6. The bar graph represents the cure depth (µm) of the prepared sample. **Figure 6.** The bar graph represents the cure depth (µm) of the prepared sample.

#### Table 3. The mean and standard deviation of length and area. Table 3. The mean and standard deviation of length and area. *3.2. Cure Depth*

Group Length (μm) Area (mm2) A5 20 ± 657.4 0.426 ± 0.043 A50 122 ± 62.3 0.355 ± 0.051 A100 40 ± 50.4 0.338 ± 0.071 O 251 ± 37.1 0.490 ± 0.050 Group Length (μm) Area (mm2) A5 20 ± 657.4 0.426 ± 0.043 A50 122 ± 62.3 0.355 ± 0.051 A100 40 ± 50.4 0.338 ± 0.071 O 251 ± 37.1 0.490 ± 0.050 The results highlight that the group prepared by adding 0.05% and 0.1% UV absorber Zirconia prostheses are generally manufactured by setting the layer thickness between 25 and 100 µm during DLP additive processing [17]. The pre-sintered body is additively manufactured according to the fixed layer thickness. If the curing depth is lower than the layer thickness, delamination may occur between layers, causing micro-defects and deterioration of properties [7]. Therefore, attempts to suppress light scattering by adding a UV absorber or adjusting the curing time should consider whether the cure depth is sufficient beyond the set layer thickness.

The results highlight that the group prepared by adding 0.05% and 0.1% UV absorber to the zirconia suspension (A50 group, A100 group) showed a significantly greater degree of overgrowth in length and area than the group without the addition of UV absorber (Group O). Our previous study [29] measured geometrical overgrowth according to the volume fraction of zirconia, showing an overgrowth rate of 33.52% at 54 vol%, higher than to the zirconia suspension (A50 group, A100 group) showed a significantly greater degree of overgrowth in length and area than the group without the addition of UV absorber (Group O). Our previous study [29] measured geometrical overgrowth according to the volume fraction of zirconia, showing an overgrowth rate of 33.52% at 54 vol%, higher than In this study, the thickness of the laminated layer was set to 100 µm and measured to check whether adding a UV absorber affects layering. Figure 6 and Table 4 show there was no significant difference in the cure depth based on the UV absorber. All groups showed a cure depth of 100 µm or more. The cure depth was obtained at a uniform intensity of

the results obtained in the current study. Despite the differences in the composition of the UV absorber and suspension, a geometrical overgrowth can be considered a factor that

the results obtained in the current study. Despite the differences in the composition of the UV absorber and suspension, a geometrical overgrowth can be considered a factor that light and distance and measured using digital vernier calipers (*n* = 3). Surface defects and microstructures were analyzed using a field emission scanning electron microscope (FE-SEM; JEOL). The specimens were sintered at 1450 ◦C to observe the surface after sintering.

## *3.3. Microstructural Analysis*

Figure 7 shows the cross-sectional microstructure of the zirconia specimen before and after sintering between the final selected A100 group specimen and the control group O group specimen. It confirmed that a 100 µm layer and slight microcracks were visible before sintering in both groups (Figure 7a). It is judged that the microcracks are generated during the cleaning process after printing is completed, suggesting caution is needed even during the post-treatment process. In Figure 7b, the cross-section of each specimen after sintering was observed. After sintering, it was confirmed that each printing layer confirmed before sintering had healing and simultaneous defects. It is judged that defects occurred due to the characteristic of zirconia shrinking during heat treatment. Compared to the control group, the lower cure depths of A100 did not provide adhesion to cause perfect healing between each layer. Figure 7c shows the high magnification of each sintered specimen. The grain sizes of 0.46 (±0.06) and 0.58 (±0.083) nm of the A100 group and O group can be confirmed, and also, it is confirmed that group O shows a difference of about 100 nm. However, there was no significant difference in each group. It is due to similar grain growth and powder characteristics since the experiment was performed under the same degreasing and sintering conditions. Materials 2022, 15, x FOR PEER REVIEW 8 of 10 similar grain growth and powder characteristics since the experiment was performed under the same degreasing and sintering conditions.

Figure 7. FE-SEM surface image at magnifications of (a) green bodies, (b) sintered specimens and (c) sintered specimens of high magnification. **Figure 7.** FE-SEM surface image at magnifications of (**a**) green bodies, (**b**) sintered specimens and (**c**) sintered specimens of high magnification.

overgrowth between the groups without the UV absorber (Group O) and the group with 0.05% and 0.1% (A50 group and A100 group). The degree of overgrowth was the smallest in the A100 group and the highest in the O group. Length measurement revealed an average value of 40 µm in the A100 group and 251 µm in the O group. The average area

groups with and without the UV absorber. Cure depths of 100 µm or more were observed in all groups. No surface defects or laminated zirconia layers were observed in the specimen after sintering in all groups. Thus, the geometrical overgrowth of pre-sintered bodies decreased when the UV absorber was added to the DLP-based additive manufacturing of zirconia, suggesting the possibility of increased accuracy of additive manufacturing of zirconia. Thus, sintering shrinkage compensation, reflecting geometrical overgrowth, is necessary to increase the accuracy in the additive

Author Contributions: Conceptualization and writing—original draft preparation, S.-W.P., J.-H.K. and K.S.; writing—review and editing, S.-W.P., J.-H.K., K.S., C.P. and K.-D.Y.; investigation and data

This study investigated the degree of geometrical overgrowth when a UV absorber was added to the additive manufacturing of zirconia using DLP. When evaluating the overgrowth rate, the length measurement ranged from 2.1% to 12.5%, and the area

in group O. There was no significant difference in the cure depth in the

4. Conclusions

was 0.490 mm<sup>2</sup>

manufacturing of zirconia.


**Table 3.** The mean and standard deviation of length and area.

**Table 4.** The mean and standard deviation of cure depth.


#### **4. Conclusions**

This study investigated the degree of geometrical overgrowth when a UV absorber was added to the additive manufacturing of zirconia using DLP. When evaluating the overgrowth rate, the length measurement ranged from 2.1% to 12.5%, and the area measurement ranged from 8.5% to 12.25%. Results showed a significant difference in the overgrowth between the groups without the UV absorber (Group O) and the group with 0.05% and 0.1% (A50 group and A100 group). The degree of overgrowth was the smallest in the A100 group and the highest in the O group. Length measurement revealed an average value of 40 µm in the A100 group and 251 µm in the O group. The average area was 0.490 mm<sup>2</sup> in group O. There was no significant difference in the cure depth in the groups with and without the UV absorber. Cure depths of 100 µm or more were observed in all groups. No surface defects or laminated zirconia layers were observed in the specimen after sintering in all groups. Thus, the geometrical overgrowth of pre-sintered bodies decreased when the UV absorber was added to the DLP-based additive manufacturing of zirconia, suggesting the possibility of increased accuracy of additive manufacturing of zirconia. Thus, sintering shrinkage compensation, reflecting geometrical overgrowth, is necessary to increase the accuracy in the additive manufacturing of zirconia.

**Author Contributions:** Conceptualization and writing—original draft preparation, S.-W.P., J.-H.K. and K.S.; writing—review and editing, S.-W.P., J.-H.K., K.S., C.P. and K.-D.Y.; investigation and data curation, H.-A.K., M.-J.J. and H.-P.L.; methodology, H.-A.K. and S.A.H.T.; visualization, S.A.H.T. and C.P.; supervision, S.-W.P. and K.-D.Y.; funding acquisition, J.-H.K. and S.-W.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by a National Research Foundation of Korea Grant funded by the Korean government (MSIP) (Grant no. 2022R1A2C20096331140982119420101 and 2022R1A6A3A01087 0061140982119420101).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** All the data have been illustrated in the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

