Influence of Post-Printing Polymerization Time on Flexural Strength and Microhardness of 3D Printed Resin Composite

Abstract

Background: The adoption of 3D printing in restorative dentistry is increasing, with the post-curing duration of a material being identified as a key determinant of its performance. This study evaluated the effect of the post-polymerization time on the flexural strength (FS) and Vickers microhardness (VHN) of a 3D-printed composite. Methods: Specimens of Formlabs Permanent Crown Resin were 3D printed and divided into four groups according to their post-curing time: no post-curing time and 20, 40, and 60 min post-curing time. Flexural strength testing was carried out using a three-point test of 40 bar-shaped specimens (n = 10/group), followed by fractography observations under a scanning electron microscope (SEM). Vicker microhardness testing was also conducted with 40 disk-shaped specimens (n = 10/group). The inorganic filler content was measured using the ash method, and the filler morphology was characterized under an SEM. Statistical analyses were performed using adjusted ANOVA and regression tests. Results: The highest median FS values were observed at 40 min post-curing (133.07 MPa), with significant differences across all groups (p < 0.0001). The highest median VHN values were found at 40 min post-curing (32.09 VHN), with significant differences between groups (p < 0.0001). A significant positive correlation (rho = 0.7488; p < 0.0001) was found between the flexural strength and Vickers hardness with changing post-curing durations. The 3D resin composite had an average filler content of 66.82% based on weight. Conclusions: With the limitation of the current in vitro setup, a post-polymerization time of 40 min was shown to lead to the best material performance. The post-printing polymerization time significantly affects the Vickers hardness and FS of 3D Formlabs Permanent Crown Resin. Further research should explore the effects of different resin compositions, clinical conditions, and curing protocols to enhance the general applicability of these findings. Clinical implications: Extending the post-printing polymerization time does not inherently result in improved material properties. A considered adjustment of the post-curing time can significantly impact the mechanical properties of a 3D-printed composite.

1. Introduction

The rapid advancement of computer-aided design/computer-aided manufacturing (CAD/CAM) has resulted in significant developments in all aspects of dentistry, particularly in the field of restorative dentistry. CAD/CAM systems manufacture restorations using either a subtractive method (milling) or an additive method (3D printing) [1]. In subtractive CAD/CAM, the final restoration is milled out of a block or ingot [2]. While effective, subtractive CAD/CAM processes encounter significant limitations, particularly raw material wastage during the milling process [3]. In contrast, contemporary additive manufacturing methods are poised to address this issue efficiently [4].

The dental 3D printing workflow comprises three main phases: digital design, printing, and post-processing. The design phase involves using CAD software (e.g., Onshape 1.193) to create a precise dental restoration model [5]. The printing phase involves fabricating the model using a 3D printer, typically with photo-curable resin materials. Two common 3D printing technologies in dentistry are DLP (digital light processing) and SLA (stereolithography) [6,7]. A 3D printer with SLA technology uses an ultra-violet laser to solidify liquid photopolymer resin, creating the 3D object layer by layer, while DLP printers use a projected digital light to allow for a complete polymerization of a resin layer at once, making it a faster technology [8,9]. The final phase, post-processing, includes cleaning, post-printing polymerization, and surface finishing procedures [10].

The multifactorial photopolymerization reaction significantly impacts the mechanical properties of a resin polymer [11]. Multiple intrinsic and extrinsic factors influence the efficiency of polymerization reactions in resin composites. Intrinsic factors include the photoinitiator system, monomer composition, and resin viscosity [12]. The formulation of a resin composite has a defining impact on their clinical performance. In addition to organic matrix modifications in 3D composites [13], inorganic fillers have a significant impact on the mechanical properties of 3D resins, including their load, size, and shape (i.e., irregular nanoparticles, rods, or sphere-like shapes) [14,15,16].

Furthermore, the extrinsic factors of the multifactorial photopolymerization reaction involve the curing time, distance between the light and uncured resin, and settings of the light curing unit (i.e., irradiance, radiant energy, and light spectrum) [17]. Some studies have investigated the effect of the post-curing time on the mechanical properties of 3D-printed composites [18,19]. The post-curing duration has been reported to significantly influence the flexural strength (FS) and Vickers hardness (VHN) [20,21]. When 3D composites were post-cured (30, 60, 90, or 120 min), it was observed that, with an increased post-curing time, the FS increased [22]. Moreover, Alkhateeb et al. investigated the effects of the post-curing time on the fracture resistance of 3D-printed interim dental prostheses. Their findings showed that longer post-curing times significantly influenced the fracture load of the temporary prostheses [23].

The relationship between the mechanical properties and post-curing durations of 3D-printed permanent resin materials that are intended for durable use in single crowns, onlays, and inlays requires further investigation. Additionally, a deeper understanding of the filler composition and characteristics of the tested material offers a more comprehensive insight into the performance of the 3D-printed material. Therefore, this study aimed to measure the VHN and FS of a 3D-printed composite that was subjected to four post-cure durations and to characterize the morphology and content of inorganic fillers in this 3D-printed resin. The null hypothesis is that the post-curing time will have no significant effect on the FS or VHN of a 3D-printed composite, with no correlation between the FS and VHN due to different post-curing durations.

2. Materials and Methods

2.1. Flexural Strength

For the FS testing, 40 specimens were fabricated using a 3D printing technique (n = 10/group). The FS specimens were designed using Onshape© software 1.193 (Boston, MA, USA; exported in an STL file format) in a bar-like shape (25 mm length, 2 mm thickness, and 2 mm width) (Figure A1 and Figure A2) following ISO 4049:2019 [24]. Subsequently, the specimens were printed using a permanent resin material with a 3D printer (

Table 1. Details of the 3D printing in this study.

Brand Name	Manufacturer	Material Composition	Printer	Shade: Lot Number
Formlabs Permanent Crown	Formlabs Inc., Somerville, MA, USA.	Organic Polymers: BisEMA * Inorganic fillers: Ceramic micro-filler * Photoinitiater system: diphenyl(2,4,6-trimethylbenzoyl) phosphine oxide (TPO, photoinitiater).	Pionext D128, Piocreat 3d, Shenzhen, China.	A3: 600164

* The manufacturer did not report a detailed formulation.

) with a 405 nm laser wavelength and a layer thickness of 50 μm, following the recommended printing parameters by the manufacturer (Figure A3) [25]. The width, thickness, and length of all specimens were confirmed by measuring their dimensions using a high-precision digital caliper (Neiko 01407A Electronic Digital Caliper, Zhejiang Kangle Group, Wenzhou, China) with an accuracy of 0.02 mm (Figure A4) [26]. The specimens were randomly divided—using research randomizer software—into four groups as follows: a negative control without post-curing (green-state group) and 20 min, 40 min, and 60 min of post-curing. Before the post-curing process, the specimens were washed with clean isopropyl alcohol for 3 min to remove any uncured resin from the surface of the printed parts. The specimens were post-cured using a post-curing unit (UV-01 Thermostatic, Shenzhen PioCreat 3D Technology Co., Ltd., Shenzhen, China) with a 60 °C internal temperature and an UV intensity of 220 μW/cm² (140 °F), where only the upper surfaces of the samples were directly exposed to UV light. The samples were placed flat on a tray, with about 5 cm between the light source and the samples. After 24 h, the specimens were subjected to the three-point bending test using a universal testing machine (Instron- model 5965, Instron Corp., Canton, OH, USA) at a 1.0 mm/min crosshead speed until the breaking point (Figure A5). The FSs were measured by using a computer-controlled universal testing machine and were calculated using the following formula [24]:

F = 3LS/(2WH²)

(1)

where F = the flexural strength, L = the maximum load, S = the span, W = the width of the specimen, and H = the height of the specimen.

2.2. Vickers Microhardness

For the Vickers microhardness testing, an additional 40 specimens were fabricated using the same 3D printing technique (n = 10/group) and designed in a disk shape (10 mm length, 2 mm thickness) (Figure A1 and Figure A2) in compliance with ISO 6507-1:2018 [27]. The printing process, dimensional verification, sample washing, and post-curing followed the same protocols as the FS specimens (Figure A3 and Figure A4). After 24 h, the Vickers hardness numbers (VHNs) were determined using the Innova Test machine (Nova 130/240 series, INNOVATEST Europe BV, Borgharenweg, The Netherlands). A triangular configuration of three indentations was created in the center of the top surface with a 15 s dwell time under a 200 g load. Indentations were created halfway between the center and external surface with equal distances about 1 mm from adjacent specimen margins. Three measurements were obtained for each specimen, and the average values were calculated.

2.3. Measurement of Filler Content and Filler Characterization

To measure the inorganic filler mass percentage of the tested resin material—as it was not precisely reported by the manufacturer—the ash method of ASTM D2584, ISO 3451, was followed [28,29]. Four rectangular specimens were 3D printed in dimensions of 2 mm thickness and 4 mm diameter (Figure A1 and Figure A2). After 24 h of storage of the samples at 37 °C, a silica crucible was kept in an electric furnace (Universal Oven U, Memmert GmbH, Schwabach, Germany) for 30 min at a temperature of 630 °C. Subsequently, the crucible was cooled in a silica gel within a desiccator at 37 ± 1 °C to an ambient temperature. The mass of the crucible was then weighed (a₁) using an analytical balance (Explorer™ Analytical, Ohaus Corporation, Parsippany, NJ, USA). Each composite specimen was placed in the crucible, and the same analytical balance was used again to weigh the specimens, including the crucible (a₂). The specimen-containing crucible was then heated in the electric furnace at 600 °C for 30 min to burn out the organic matrix and then re-weighed after being cooled to an ambient temperature in a desiccator. The crucible and residue were weighed (a₃) using the same analytical balance. The latter weight consisted of inorganic fillers only.

The filler weight fraction (wt%) was determined using the following formula:

$$\begin{array}{c}\mathrm{F}\mathrm{i}\mathrm{l}\mathrm{l}\mathrm{e}\mathrm{r} \mathrm{w}\mathrm{t}\backslash \mathrm{\%}={\displaystyle \frac{{\mathrm{a}}_3-{\mathrm{a}}_1}{{\mathrm{a}}_2-{\mathrm{a}}_1}}\times 100\end{array}$$

(2)

where a₁ is the crucible mass, a₂ is the crucible mass with the specimen, and a₃ is the final crucible mass with residue after heating.

2.4. SEM Fractography Analysis

Three specimens from each group were selected (the highest, median, and lowest values of FS) and gold-coated (JEOL ION SPUTTER JFC-1100, JEOL). The gold coating was applied to samples before the SEM to enhance their electrical conductivity, improve the image quality, reduce beam damage, and create a uniform surface for clearer and more accurate imaging [30]. The fracture surfaces of each specimen were viewed using an SEM (JSM-6360LV, JEOL). The fracture surfaces were then analyzed at ×100 to ×200 magnifications, and representative images were taken for each group. Other images were also obtained using an SEM with higher magnification (JSM-7610F Schottky Field Emission, JEOL, Tokyo, Japan) to characterize the filler morphology, followed by the determination of the filler size using MIPAR v5.1 software (MIPAR, Columbus, OH, USA) [31].

2.5. Statistical Analysis

First, we calculated and presented the means with standard deviations and medians with interquartile ranges to describe the quantitative outcome variables. The Shapiro–Wilk test was used to examine the normality of the data. The FS and VHN were compared between the groups by using adjusted ANOVA and regression analyses. The significance level was set at α = 0.05 for all tests. Spearman’s correlation coefficient was used for nonparametric correlations in changes in FS and VHN for the tested 3D resin material.

3. Results

3.1. Flexural Strength

The normality of the distribution for each group was examined using the Shapiro–Wilk test and a visual analysis of the histograms (Figure 1). The no post-cure group exhibited a non-normal distribution due to a significant clustering of its low flexural strength values. This pulled the distribution tail towards the left, rendering the data distribution skewed in this group (p < 0.0001). In contrast, the FS values in the 20, 40, and 60 min post-curing groups did not significantly deviate from normality, with Shapiro–Wilk p-values of 0.48, 0.90, and 0.80, respectively, demonstrating a more uniform distribution of the flexural strength values in these groups. In addition, other ANOVA assumptions were examined. Levene’s test for equal variances indicated that the flexural strength variances across the different post-curing groups were significantly different (p < 0.05), which violates the assumption of homogeneity of variance that is required for a classic ANOVA. This indicated that the flexural strength values showed different levels of variability across the different post-curing groups (

Table 2. Descriptive statistics of the flexural strength values (MPa) across the groups.

Post-Curing Time	Flexural Strength Values (MPa)
	Mean ± SD	Median	Interquartile Range (IQR)	Minimum–Maximum
No post-curing	11.33 ± 0.17	11.25	0.15	11.24–11.78
20 min	72.21 ± 1.7	72.47	1.87	70.12–75.17
40 min	133.07 ± 1.19	132.39	1.26	131.26–135.38
60 min	105.53 ± 8.55	106.63	9.74	91.49–117.76

). The highest variability, as indicated by the standard deviation, was found when samples were cured for 60 min post-printing at 105.53 ± 8.55 MPa, which was different to the smaller spread of the data around the FS means in the 20- (72.21 ± 1.7) and 40 min post-curing groups (133.07 ± 1.19) and the no post-curing group (7.26 ± 0.16 MPa). This means that the flexural strength values were less consistent in group four, indicating that the 3D specimens experienced a wider range of flexural strength values when post-cured for 60 min.

Therefore, Welch’s ANOVA was conducted to examine the effect of the post-curing time on the flexural strength among the four post-curing groups. The analysis indicated a significant effect of the post-curing time on the flexural strength, with F(3, 15.30) = 35,271.57, and p < 0.001. To explore these differences further, pairwise comparisons were made between each group (

Table 3. ANOVA comparisons of the flexural strength between the groups.

Comparison	Mean Difference (M)	95% CI Lower	95% CI Upper	p-Value
20 min vs. no post-curing	60.88	56.88	64.88	<0.001
40 min vs. no post-curing	121.73	117.74	125.73	<0.001
60 min vs. no post-curing	94.19	90.2	98.19	<0.001
40 min vs. 20 min	60.86	56.86	64.85	<0.001
60 min vs. 20 min	33.32	29.32	37.31	<0.001
60 min vs. 40 min	−27.54	23.54	31.54	<0.001

). The results revealed significant differences between all groups. Post-curing for 20, 40, and 60 min significantly increased the flexural strength compared with no post-curing, with mean differences of 60.88, 121.73, and 94.19, respectively (all p < 0.001). The 40 min group had a significantly higher flexural strength than the 20 min group (p < 0.001). The 60 min group also exhibited a significantly higher flexural strength than the 20 min group (p < 0.001). In contrast, the 60 min group showed a significantly lower flexural strength than the 40 min group (p < 0.001). These results suggest that increasing the post-curing time significantly enhances the flexural strength, with each additional interval leading to an increase in strength up to 40 min. However, extending the post-curing time from 40 to 60 min did not improve flexural strength values (Figure A6, Figure A7, Figure A8 and Figure A9).

3.2. Vickers Microhardness

A Shapiro–Wilk test was conducted to assess the normality of the VHN data across the four post-curing groups: no post-curing, 20 min, 40 min, and 60 min. For all groups, the p-values were greater than 0.05, indicating that the null hypothesis of normality could not be rejected. Therefore, the VHN data for each post-curing group appear to be normally distributed (Shapiro–Wilk p-values: 0.42, 0.80, 0.27, 0.62, respectively). Levene’s test was conducted to evaluate the assumption of equal variances, and the test results indicated significant differences in the levels of VHN among the four post-curing groups (p < 0.001). This implied that a robust regression analysis would be appropriate for comparing these groups.

The overall model was statistically significant (F(3, 36) = 19,064.15; p < 0.001), with an R-squared value of 0.9939, indicating that the model explained approximately 99.39% of the variance in VHN values. This suggests a very high level of explanatory power for the effect of the post-curing time on hardness. A normal quantile–quantile (Q-Q) plot of the residuals from the regression analysis was generated to evaluate the normality assumption of the residuals (Figure 2). The linear alignment of most points along the reference line suggested that, for the most part, the residuals were normally distributed, which meant that the regression model fit the majority of data points well.

The regression results indicated that all the post-curing times had a statistically significant effect on the VHN when compared with the reference group (0 min post-curing time) (

Table 4. Vickers’ hardness numbers across the groups.

Post-Curing Time	Coefficient	Robust Std. Error	t-Statistic	p-Value	95% Confidence Interval
20 min of post-curing	21.6867	0.152994	141.75	<0.001	[21.37638, 21.99695]
40 min of post-curing	24.8367	0.171707	144.82	<0.001	[24.59771, 25.07563]
60 min of post-curing	16.6542	0.417085	137.38	<0.001	[16.65426, 18.56538]
Intercept	7.26	0.052749	137.57	<0.001	[7.152968, 7.367032]

). The coefficient for 40 min of post-curing was 24.84 (SE = 0.1717; t = 144.82; p < 0.001). This means that post-curing for 40 min led to an average increase in hardness values of approximately 24.84 VHN compared with no post-curing. Overall, these results suggested that the post-curing times significantly influenced the Vickers hardness of the tested composite resin, with longer post-curing times generally leading to higher hardness levels, although the effect diminished after 40 min (Figure 3). The increase in hardness was statistically significant for all post-curing groups compared with the control (no post-curing), highlighting the importance of post-curing in improving the mechanical properties of 3D-printed dental materials.

The correlation between the FS and VHN values across the different post-curing times was examined. Spearman’s rank correlation coefficient (rho) was 0.7488, indicating a statistically significant strong positive relationship between the FS and Vickers hardness. This implied that, as one variable increased, the other was likely to also increase, and this relationship was statistically significant (p < 0.001).

3.3. Filler Content and Characterization

The ashing of the 3D samples of the Formlabs Permanent Crown Resin indicated a filler content of 66.82% in terms of weight (Table 2). An SEM analysis at magnifications of 3000 to 30,000× reflected that, in a 40 min post-cured specimen (Figure 4), the fillers exhibited a variety of shapes, including spherical, elongated, and irregular forms; some images showed fillers with sharp edges, while others appeared more rounded. The fillers did not show a specific orientation and appeared to be randomly distributed across the images. In some regions, the fillers appeared to be densely packed together, while, in others, they were more dispersed. A variation in filler size with both small and large fillers was noted. The MIPAR software reported a mean filler size of 0.61 μm ± 0.42, with the maximum filler size being 5 μm, while the minimum filler size was 10 nm.

3.4. SEM Fractography Observations

Figure 5 shows various fractographic features of a sample material from group one with no post-printing polymerization. When analyzing these fractographic features of the negative control group, we found that the green-state material tended to exhibit a brittle behavior and was prone to sudden catastrophic failure, with minimal plastic deformation and low toughness. When the post-printing polymerization was set to 20 min, the fractographic features (Figure 6) of the resin polymer leaned towards showing a predominantly ductile behavior in their smooth zones and micro-voids across the fracture surface, indicating possible significant plastic deformation and suggesting a higher toughness overall with more energy absorption and resistance to catastrophic failures. As the post-printing polymerization was increased to 40 min, the fractographic analysis under an SEM (Figure 7) showed a predominantly ductile mechanism, with significant plastic deformation and more controlled crack propagation at localized areas of cleavage features and river patterns. At this level, the material appeared to have a favorable toughness level, with relatively good resistance to fracturing. Further, as the post-printing polymerization was increased to 60 min, the observed features in the SEM images (Figure 8) indicated that the composite polymer experienced a predominantly brittle fracture. The presence of river patterns and Wallner lines suggested rapid crack propagation through the material at this point of post-printing polymerization.

4. Discussion

This study investigated the impact of different post-printing polymerization times on the flexural strength and Vickers microhardness of a 3D-printed resin composite. The findings revealed that the post-curing duration can significantly influence the mechanical properties of 3D dental resin, with a significant correlation between the FS and VHN. Hence, the null hypotheses was rejected. The 40 min post-curing time resulted in the highest median flexural strength and VHN, suggesting optimal mechanical properties at this duration given the present in vitro setup. However, extending the post-curing time to 60 min did not enhance the tested mechanical properties. The findings from this experiment highlight the importance of optimizing the post-curing duration to enhance material properties without causing deterioration, as variable post-curing durations may introduce inconsistencies in the mechanical properties of 3D dental resin.

For the present study groups, several considerations informed the chosen durations of 20, 40, and 60 min. The manufacturer’s recommendation of 20 min was used as a positive control. According to Formlabs, the recommended post-curing duration for Permanent Crown Resin is best optimized in 20 min, assuming the use of the same manufacturer’s printer and curing unit. However, it is common practice to interchange printers and curing protocols in clinical and laboratory settings, raising a practical concern about whether the manufacturer’s post-curing time recommendations remain effective under different clinical workflows. The inclusion of a negative control with no post-curing was to assess the material’s baseline properties without additional polymerization. The evaluation of extended post-curing durations, including 40 min (double the manufacturer time) and 60 min (triple the manufacturer time) to examine the effects of prolonged post-curing on material properties.

The FS is a crucial mechanical property when testing 3D-printed dental composites, as it indicates the material’s ability to withstand bending forces without fracturing [32]. This property is vital for the structural integrity of indirect restorations under functional stress and, hence, the material’s performance in terms of longevity, durability, and suitability for clinical use [33,34]. The use of clinical trials to evaluate the clinical performance of 3D composite formulations is expensive and time-demanding, so it would be practical to be able to predict a material’s performance—to an extent—based on single or multiple in vitro tests [35,36]. Also, there is evidence that FS data correlate moderately with clinical wear [37]. This explains the rationale behind investigating the FS in the present study rather than using other methodologies of mechanical testing of deformation resistance. This study confirmed that the FS of the 3D-printed samples increased significantly when post-polymerization was performed for 40 min. No improvements in flexural strength were observed when the post-curing time was longer, at 60 min. Other studies assessing the FS of 3D-printed resins based on the post-curing time have only used post-curing durations of up to 40 min [11,38]. Discrepancies in alternate changes in the FS values according to post-curing durations were also observed in some in vitro studies. For instance, Soto-Montero et al.’s results showed lower mean FS values when the post-curing duration was increased to 20 min, with 96.7 ± 4.5 MPa compared with 98.5 ± 4.8 MPa for 15 min of post-curing of the material Resilab 3D Temp. While this difference was not statistically significant, it underscores the potential variability in FS values following different durations of post-curing [39]. Another example of irregularity in FS readings can be seen in Kim D et al.’s study [40]. They evaluated the FS of a 3D resin material (NextDent C&B) according to the post-curing time. A flexural strength of 106.37 MPa was reported for their specimens without post-curing, while it was 107.40 MPa for their group after 15 min of post-curing. The group that underwent 30 min of post-curing showed 111.79 MPa, followed by a reduction in FS to 103.9 MPa at 60 min. A gradual increase in FS was observed thereafter in the 90 and 120 min groups [40]. Although the decrease in FS at 60 min was non-significant in Kim et al.’s study, its alignment with the similar trend that is reported by the current study might suggest a potential mechanism that is worth investigating further. This observation is thought to be due to the complex interactions between multiple factors, such as the post-polymerization process, printing parameters, and the resin composition [41,42,43]. While the 60 min group’s data maintained normality, the greater spread in the FS values in this group reflects the complexity and variability that are introduced by extended post-curing times. This observed high variability could result from natural differences in the curing efficiency across different specimens. It might also reflect that longer curing times can lead to more heterogeneous material properties, and some specimens may have been more affected by the extended curing than others.

The FS of 3D-printed dental composites is influenced by several factors relating to the resin composition. The type and ratio of the fillers that are used in the 3D-printed resin are shown to influence the material’s mechanical performance significantly [44,45]. Inorganic fillers reinforce composite resins and help minimize polymerization shrinkage, increase radiopacity, and reduce polymer degradation over time [16]. Hence, the most notable advancements in resin formulations were focused on enhancing the characteristics of inorganic fillers [46]. Therefore, measuring the inorganic filler content was considered in the current setup for the tested 3D resin, which showed a relatively high filler weight (66.82% wt.). To the best of the authors’ knowledge, the fillers of the tested Formlabs Permanent Crown Resin have not been previously criticized in the published literature. This observation contributes meaningfully to the existing body of knowledge. Evidence suggests that higher filler contents in 3D resin polymers can enhance their FS, hardness, and wear resistance [47]. However, mechanical properties are negatively affected if the filler content exceeds an optimal threshold [47,48]. The filler size, shape, and distribution within the resin formulation can also influence the 3D composite’s performance [49,50]. Previous studies have indicated that the surface morphology of porous fillers determines the filler–resin matrix interfacial interaction and the properties of composites [51]. Large filler particles or excessively high filler concentrations can cause issues with fabrication, making the 3D composite ink unsuitable for consistent and reliable 3D printing [52]. The characterization of the filler morphology in the present study revealed an average of 0.61 μm irregular fillers in the 3D Formlabs Permanent Crown Resin. However, the manufacturer did not provide specific data on the filler characteristics. In another study, Bora et al. reported 0.2–1.5 μm spherical particles in an OnX Tough 3D-printed composite and 1–5 μm irregular particles in Ceramic Crown 3D resin [53].

Microhardness is a surface property of a 3D resin that reflects its surface strength and which can also indicate abrasion resistance [38,54]. At low hardness values, more damage to the resin surface or scratches may occur during use [55]. Therefore, the current study measured the hardness values using Vickers’ hardness number. The pattern of change in VHN values was correlated with the change in FS. The hardness measurements that were carried out in this study demonstrated an increase with an extended post-curing duration up to 40 min, followed by a noticeable decline at 60 min post curing. In contrast, the hardness values reported by Siqueira et al. for Resilab Temp 3D showed a steady rise in Knoop microhardness, increasing from 24.7 ± 5.2 (baseline) to 28.4 ± 6.2 at 60 min, with no decline being observed after a prolonged post-curing process [56]. Nevertheless, Al-Dulaijan et al. investigated the microhardness of the 3D-printed resin NextDent under different post-curing timings [22]. Using the same printing angle, they reported mean hardness values of 23.0 ± 1.3 VHN at 30 min and 23.1 ± 1.4 VHN at 60 min post-cure, with insignificant differences between these two durations of polymerization. In their case, extending the post-curing time to double did not significantly enhance the hardness numbers. A subsequent increase in hardness, up to 25.2 ± 2.0 VHN, was observed when the post-curing time was extended to 120 min. They also reported hardness values of 26.4 ± 3.4 VHN being observed at 30 min post cure for ASIGA 3D resin, which they found to be statistically insignificantly lower than the 25.3 ± 3.0 VHN at 60 min. These different results demonstrate the reported variability in hardness values, which might be attributed to differences in study setups, the evaluated materials, and variations in other post-curing parameters or printing settings.

In the current study, the observed remarkable reduction in the tested properties within 60 min post curing was explored in depth. The initial reason for this observation was attributed to a possible technical flaw. Hence, the detailed methodological steps were revised, and the FS and VHN tests were repeated. The results, however, showed comparable readings for the second test as well. A key factor influencing this behavior might be attributed to the glass transition temperature (Tg) of the material, as prolonged curing times and higher exposure doses have been shown to cause a significant shift towards a higher Tg in 3D-printed resins [57,58,59]. The Tg represents the temperature at which the polymer transitions from a flexible, rubbery state to a more rigid, glassy state [60]. As the curing process progresses and the material approaches its Tg, the mobility of the unreacted monomers becomes increasingly restricted. This decreased monomer mobility directly impacts the polymerization process [61,62,63]. In materials with a higher Tg, higher energy or longer post-curing times are necessary to achieve full polymerization, because the material needs to overcome this transition to allow for proper cross-linking [64,65,66].

For the material tested in this study, at 40 min, the energy that was provided during curing appeared to be sufficient to overcome the limitations imposed by Tg, allowing the material to reach its optimal degree of polymerization. This appeared to lead to the highest cross-linking density and maximum hardness, as indicated by the peak VHN value. The material had sufficient time and energy input to reach a fully polymerized state, where most reactive monomers had converted to stable cross-linked polymers. The significant decrease in hardness after 60 min of post-curing can be explained by the over-curing and the consequences of exceeding the optimal curing spectrum. The Tg is dynamic, changeable, and controlled by the material composition, curing conditions, and cross-link density [67,68]. Once the material has fully transitioned through the Tg and most of the polymerization is complete, further energy can cause the polymer chains to become overly rigid or even degrade, resulting in reduced mechanical properties [69]. Other possible reasons include inconsistent polymerization or degradation of the material. Uneven post-curing can result in areas within the material that are either under-cured or over-cured. Under-cured regions may retain unreacted monomers, leading to softer zones with inferior mechanical properties. Over-cured areas, on the other hand, can become excessively cross-linked, making them more brittle [70,71].

Baytur et al. investigated the flexural strength of 3D resins, including the same 3D composite resin that was examined in the present study, exploring various post-polymerization temperatures and durations. They emphasized the need for further research to evaluate the clinical suitability of these materials [72]. Complementing their work, the current study isolated the effect of the polymerization time while maintaining a constant temperature. The current approach addressed a common clinical assumption that extending the polymerization time leads to improved material properties. By focusing on a single tested material, we aimed to conduct a detailed and controlled investigation and draw qualitative conclusions about the material’s behavior. The present study provides practical insights by utilizing a combination of different 3D printers and curing units, reflecting the variability that is commonly encountered in clinical workflows and enhancing the real-world applicability of our findings. Additionally, we provided a detailed characterization of the filler morphology using SEM and fractographic analysis to qualitatively assess the change in the material’s behavior under mechanical stress. This level of material characterization, which has not been extensively explored in previous studies, might offer a deeper understanding of the structural performance of 3D-printed composites.

In addition to the earlier discussed effects of the fillers and resin composition on the mechanical properties of the 3D-printed composite, printing parameters such as the printing speed, build orientation or printing angle, and layer thickness can be determining factors in the mechanical performance of 3D resin polymers [73,74,75]. While several studies found a significant effect of the build orientation on the mechanical properties of resin-based 3D composites [23,76,77], some indicated that this influence is material-dependent [78,79]. On the other hand, an in vitro study reported that the build orientation had no influence on the microhardness [60] or FS of these materials [78]. The current study samples were printed at a 0° angle with a layer thickness of only 50 μm following the manufacturer’s recommendations [25]. This might have contributed to the currently reported FS and VHN values. Alageel et al. evaluated interim 3D resins at two printing angulations; they found that 0° led to a lower strength than those printed at 90° [80]. Furthermore, Alshamrani et al. found that a layer thickness of 100 μm led to the highest FS values compared with 25 μm and 50 μm [81].

Despite the various important curing parameters, this study only evaluated the effect of the post-curing time on the mechanical properties of a 3D-printed composite. It would also be worthwhile exploring the impact of the light cure unit’s intensity, irradiance, and temperature on a 3D resin’s performance. The current specimens were printed using a third-party printer, meaning that the resin was printed with printers other than those that are recommended by the manufacturer. This raises concerns about any possible effects on the investigated measures of mechanical properties. However, switching 3D resins and printers, or post-curing units, is a practice that has been reported in several methodologies [61,62,63,64]. Chen et al. suggested that switching printers with the same resin type did not affect their strength if the post-curing time and temperature were adequate [65]. This study’s findings should be interpreted with caution due to material dependency and technique sensitivity, which might limit the generalizability of the results.

In the current analysis, the flexural modulus was not measured during testing with the universal testing machine, as this was not within the primary scope of the study. Consequently, the necessary stress–strain data for calculating the elastic modulus were not collected while flexing the samples, which restricted our ability to analyze this key mechanical property in conjunction with the flexural strength. Given the importance of the flexural modulus in evaluating the mechanical performance of resin composites, particularly regarding the glass transition temperature and potential over-curing effects, the inclusion of these data would have offered valuable insights. Therefore, we recommend that future investigations incorporate flexural modulus measurements alongside flexural strength testing to provide a more comprehensive evaluation of the 3D material’s behavior and performance.

5. Conclusions

This study investigated the influence of the post-polymerization time on the mechanical properties—specifically the flexural strength (FS) and Vickers hardness (VHN)—of a 3D-printed resin composite. Within the limitation of this study, the following findings can be concluded:

• The post-curing time significantly impacts the FS and VHN of 3D composite resin, highlighting the critical role of post-polymerization settings in optimizing material performance.
• Increasing the post-curing duration led to improvements in the FS and VHN up to a certain limit, after which a decline in mechanical properties was observed. Therefore, extending the post-printing polymerization time does not inherently result in improved material properties.
• The optimal post-curing time for the current setup was determined at 40 min. At this duration, the 3D composite exhibited a mean flexural strength of 133.07 MP and a mean Vickers hardness number of 32.09 VHN, achieving the best balance of mechanical performance among the tested groups. Followed by a decline in mean FS at 105.53 MP with 60 min of post-curing, while the lowest FS values were observed in the 20 min post-curing, with a mean FS of 72.21 MPa.

These findings underscore the importance of refining post-curing protocols to achieve the desired mechanical properties of 3D-printed composites. Future studies should explore the interplay between the post-curing duration, other post-curing settings (e.g., the light intensity and temperature), and printing parameters to gain further comprehensive insights for optimizing material properties.