Enhanced Mechanical Properties of the Additively Manufactured Modified Hybrid Stereolithography (SLA)–Glass Powder

Abstract

This research successfully enhances the mechanical properties of the ready-market resin product additively printed by using stereolithography (SLA) reinforced with glass powder. Using Esun Standard Resin (Shenzhen, China), various proportions of glass powder (0%, 1%, 2%, 3%, 4%, 5%, 10%, 15%, and 25%) were mixed to create test specimens. The results indicated that the incorporation of glass powder markedly enhanced tensile strength and hardness. Specimens containing 25% glass powder exhibited a tensile strength of 37.01 MPa and a hardness of 84.5 HV, in contrast to 24.03 MPa and 73.7 HV for those without glass powder. The density rose with the addition of glass powder, attaining 1.338 g/cm³ at 25% concentration. Nevertheless, heightened brittleness and reduced strain values signified a compromise between strength and ductility. Compression testing revealed increased maximum stress but more brittleness with higher glass powder content, while flexural testing demonstrated diminished flexural strength attributed to inadequate filler adherence and dispersion. This study highlights that the addition of glass powder to SLA resins can improve mechanical strength while reducing flexibility and ductility. Enhancing the concentration and dispersion of glass powder is crucial for attaining a balance in mechanical properties, which enhances SLA 3D printing for dependable, high-performance engineering applications.

1. Introduction

Additive Manufacturing (AM), commonly known as 3D printing, was first proposed in the 1980s [1]. Despite its potential, the manufacturing sector hesitates to adopt AM due to issues like poor mechanical properties, uneven polymerization, weak interlayer adhesion, and porosity, which reduce strength [2]. Metal AM methods exist but are often costly and inefficient [3,4,5], so thermoplastics and photopolymers are commonly used [6,7]. Enhancing the mechanical properties of AM components could expand their use in structural applications [8,9]. AM is well suited for producing complex, multi-material parts at lower costs [10,11]. The AM technique utilizes a computer to generate a three-dimensional design, which then transforms into Gcode files [12]. AM converts the three-dimensional design from the computer into a physical object, allowing direct fabrication into various shapes and structures from the three-dimensional model [13]. Various industries take advantage of the advantages of using AM technology, including jewelry and fashion [14,15], construction engineering [16], and the automobile [17], aerospace [18,19], dental [20], and medical sectors [21], which all utilize 3D printing technology [14,22]. The benefits of 3D printing in these industries include product customization, automation, and reduced waste compared to traditional manufacturing processes [23,24,25,26,27]. Various techniques fall under the umbrella of 3D printing, such as Fused Deposition Modelling (FDM) [28,29], stereolithography (SLA) [30,31], and Selective Laser Sintering (SLS) [32,33], among others [34,35,36].

Stereolithography (SLA) is a technique employed to fabricate three-dimensional printed items, among other approaches [23,37]. SLA manufactures goods by solidifying resin into a tangible object. Charles Hull, a prominent figure in the 3D printing business, patented the technology as a quick prototyping tool in 1986 [38,39,40]. SLA machines utilize a tank in which resin is poured. The SLA system consists of four primary components: a resin-filled tank, a descending platform, an ultraviolet (UV) laser, and a computer for controlling the platform and UV laser [41]. During the initial stage of the SLA process, a slender film of photopolymer, measuring between 0.05 and 0.15 mm in thickness, is illuminated on the platform. The UV laser will strike the platform, generating a pattern of the object being fabricated [42]. When the UV laser comes into contact with the UV-exposed liquid, it will rapidly solidify, serving as the initial layer for the 3D-printed item. After the object’s first layer has solidified, the platform is moved down, exposing a fresh layer of liquid polymer. The laser traces the object’s cross-section undergoing molding, resulting in immediate adhesion to the solidified piece beneath. This procedure is iterated continuously until the entire thing is shaped and immersed in the tank [43].

Tensile strength, rigidity, and durability among other mechanical characteristics produced with SLA 3D printing have lower values than those of conventional 3D-printed items [38,44]. These constraints might limit the application of SLA technology in major sectors [45]. Consequently, the advancement of resin materials tailored for SLA 3D printing is imperative. Researchers want immediate advancements to improve the mechanical properties of SLA 3D-printed materials, specifically tensile strength, stiffness, and durability [46]. Consequently, the creation of materials tailored for SLA 3D printing is essential. Additional enhancements are required to optimize the specific mechanical qualities, including tensile strength, stiffness, and longevity, of SLA 3D-printed items [34,38]. Enhancing resin materials with superior mechanical qualities can improve the mechanical characteristics of SLA 3D-printed objects.

One feasible method to improve the mechanical properties of SLA products is reinforcement [47]. In SLA 3D printing, reinforcement is the use of additional materials to improve the mechanical properties of the produced goods [34,48]. Usually, this is achieved by including reinforcing ingredients, such as powder or particles, into the resin used for printing [49]. The reinforcement improves the strength, stiffness, and lifetime of the molded product, therefore making it more suitable for uses requiring strong mechanical properties [50]. This approach enables the modification of material properties by precisely controlling the composition and alignment of reinforcements during the SLA 3D printing process [51], hence enabling the development of strong and highly performing components. Usually used as the basic material for SLA products, materials such as carbon fiber, glass fiber, and metal powder help to strengthen the resin [52].

The eSUN resin is a base material commonly used in the SLA 3D printing process due to its good mechanical properties, including tensile and compressive strength [53]. This resin is also suitable for various applications such as prototyping and modeling [54]. Additionally, this material is compatible for reinforcement with other materials such as fillers or fibers [55].

In addition to the previously mentioned types of reinforcement, glass powders have received great attention to be applied as filler materials and as the structural system in GFRP or other materials [56,57]. Glass powder fillers have several advantages such as ease of processing, availability, and low cost [58,59]. A filler with glass powder has the advantage of increasing the strength and stiffness of the material [59]. Glass powder incorporated into the matrix can form a linking and locking mechanism. Such a strengthening mechanism can increase z-axis loading [58]. Cylindrical milled glass loaded into the matrix can increase the modulus by 45% [60]. However, adding reinforcement may affect the quality of SLA 3D-printed products, requiring careful consideration of the manufacturing methods. Overall, glass powder is a suitable reinforcement material for SLA 3D printing.

Although advances in SLA 3D printing have enhanced material quality and printing techniques, the influence of glass powder as a reinforcement filler in SLA-printed resins remains insufficiently explored. Existing studies rarely quantify the mechanical trade-offs—such as between tensile strength and brittleness—across a wide range of filler concentrations. This study addresses that gap by systematically evaluating how varying glass powder content (0–25%) affects tensile strength, compression resistance, flexural behavior, density, and hardness. The novelty of this work lies in using a commercially available eSUN resin matrix modified with controlled amounts of fine fibrous glass powder, offering a reproducible and scalable approach to tuning mechanical performance in SLA products. Our findings establish a pathway to optimize filler dispersion and concentration, enabling tailored mechanical properties. These insights could expand the use of SLA 3D printing for high-stress and structural applications, including customized biomedical tools, lightweight mechanical parts, and load-bearing prototypes in engineering fields.

2. Materials and Methods

2.1. Materials

The resin used in this study is the eSUN Standard Resin (Shenzhen, China). The glass powder used from JUSTUS Kimia Raya Company (Semarang, Jawa Tengah, Indonesia) shown in Figure 1, has an average particle size of 1.1024 μm with a fibrous, elongated shape that increases surface area, enhancing resin bonding. Its high silica content improves stiffness and structural stability, while sodium adds flexibility and calcium enhances adhesion between the resin and glass particles. This combination significantly boosts tensile and compressive strength, making the final SLA-printed product more durable for high-stress applications. The glass powder composition includes 90% silicate, sodium, and calcium [56]. After curing, 96% alcohol is used to clean any remaining resin from the specimen.

2.2. Methods

2.2.1. Raw Materials Mixing Process

In this study, specimens were prepared with varying percentages of glass powder (0, 1, 2, 3, 4, 5, 10, 15, and 25%). The process began by measuring the required resin volume for the SLA 3D printer tank, followed by weighing the glass powder. The resin-to-glass powder ratio affects the properties of the final product, with higher ratios yielding stronger SLA prints. The glass powder was mixed into the resin and stirred for 10 min using a stirrer. After mixing, the resin–glass powder mixture was poured into the SLA 3D printer tank.

2.2.2. Specimen Manufacturing Process

In this study, the stereolithography process began with designing test specimens using Autodesk Inventor 2022, saving the 3D model in “.stl” format, and processing it with the Photon Workshop Software V2.1.29.

Table 1. SLA 3D printing parameters (personal data).

Machine and Resin Settings	Value	Unit
Layer Thickness	0.050	mm
Normal Exposure Time	0.8	s
Off Time	0.500	s
Bottom Exposure Time	60.000	s
Bottom Layers	6
Anti-alias	2
Gray Level	0	mm/s
Image Blur	0
Basic Control	Value	Unit
Z Lift Distance	6.00	m
Z Lift Speed	2.0	mm/s
Z Retract Speed	6.00	Mm/s

shows the ideal parameters used in this study for SLA printing reinforced with glass powder. Key 3D printing parameters included a layer thickness of 0.050 mm for high resolution, a standard exposure time of 0.8 s for balanced curing, a 0.5 s off time to ensure layer adhesion, and a bottom exposure time of 60 s with six bottom layers for a strong base. The Photon Mono SE 3D printer (Anycubic, China) was used to ensure file compatibility. The resin was mixed with glass powder at predetermined ratios, stirred for 10 min, and poured into the printer’s tank after levelling. The prepared design file was uploaded via flash drive, and the printing process was initiated from the 3D printer menu. The printing parameters are provided in Table 1.

Figure 2a–c illustrate the manufacturing process for creating specimens, with at least three specimens produced for each test combination. The process began with material preparation (Figure 2a), followed by designing standard specimens (ASTM D638 (ASTM, 2016), D2240 (ASTM, 2015), D792 (ASTM, 2008), D790 (ASTM, 2002)) using the Inventor CAD 2024 for student software, and transferring the design to the Photon Workshop slicer software V3.1.4 (Figure 2b). After printing, specimens were detached from the platform using a spatula, washed with alcohol for 5 min, and cured under UV light for 5 min using an Anycubic wash and cure machine (Shenzhen, China). The washing and curing process is shown in Figure 2c.

2.2.3. Testing Method

This research utilizes several tests (tensile, hardness, density, compression, and flexural), as seen in Figure 2d, to identify the properties of the specimen. In each test, there will be 3 specimens that will be tested for each percentage of glass powder, namely, 0, 1, 2, 3, 4, 5, 10, 15, and 25%. The test methods carried out in this study can be seen in the subchapters below.

• Tensile Test

The tensile strength test, conducted using a Universal Testing Machine (UTM) (Carson, Taipei, Taiwan), evaluates material strength and resistance to tensile loads. Specimens, measuring 63.5 mm × 9.53 mm × 3 mm, were tested following ASTM D638 standards (ASTM, 2016) with a speed of 5 mm/min. Figure 3a shows the test configuration, with a Dino-Lite microscope capturing real-time sample conditions (Figure 3b). Before and after images of the sample, highlighting crack propagation, are shown in Figure 3c. Tensile force was applied until fracture, with the UTM automatically recording failure force and key data, including time, load, displacement, stress, and strain.

B.

Hardness Test

Hardness measurements followed ASTM D2240(ASTM, 2015) using a Shore D HTTK-37D (Hong Kong, China) machine. Specimens measured 10 mm × 10 mm × 10 mm, with three specimens tested per mixing ratio. Figure 4a shows the specimen position during testing, while indentation traces are illustrated in Figure 4b,c. The test involved applying a load on the specimen’s plane surface and measuring indentation depth at five points on the flattest side. Testing was conducted at 18 °C ambient conditions. Hardness levels were recorded, and surface conditions post-testing were analyzed using a Dino-Lite microscope (Taipei, Taiwan).

C.

Density Test

Density testing was carried out using the OHAUS PX (Parsippany, NJ, USA) series machine from each specimen tested and was calibrated before the test was started. The specimen’s dimensions conform to the ASTM D 792 standard(ASTM, 2008), measuring 1 cm × 1 cm × 1 cm. The surface of the specimen was ensured to be pristine and devoid of any contaminants. As shown in Figure 5, the specimen’s mass was determined by using a precise balance while it was in air. Distilled water was used in this study to determine the density of the specimens in water. After all the data were obtained, the calculation of the density of the specimens was performed with Equations (1) and (2) based on ASTM D792:

$$SpecificGravity={\displaystyle \frac{a}{\left[\left(a+W\right)-b\right]}}$$

(1)

where:

$a$ = mass of specimen in air.

b = mass of specimen and sinker (if used) in water.

W = mass of totally immersed sinker (if used) and partially immersed wire.

$$density=\left(specificgravity\right).\left(997.5\right)$$

(2)

where 997.5 kg/cm³ is the density of water at 23 °C.

D.

Compression Test

The compression test involved placing the specimen on the bottom platen and applying an increasing load using a Universal Testing Machine (UTM) (Carson, Taipei, Taiwan) until deformation occurred. Figure 6a shows the test setup, while Figure 6b illustrates the load applied from above, with the specimen centered on the bottom platen for accuracy. The test cylinder was consistently positioned for each iteration. Three specimens were tested per glass powder ratio, with the UTM automatically recording load and displacement data, which were processed for analysis.

E.

Flexural Test

The flexural test, conducted using a Universal Testing Machine (UTM) (Carson, Taipei, Taiwan) shown in Figure 7 and following ASTM D790 standards(ASTM, 2002), utilized specimens measuring 80 mm × 13 mm × 4 mm. Tests were performed at a crosshead speed of 2 mm/min, with three repetitions for each glass powder percentage. Loading was applied from the top, with support provided at both ends (Figure 7a), causing specimen deflection. The UTM automatically recorded time, displacement, strain, and stress data, with the maximum force used to calculate flexural strength.

3. Results

3.1. Tensile Test

Figure 8 shows that increasing glass powder content reduces strain and enhances tensile strength in SLA 3D-printed specimens. Glass powder resists the forces experienced by the resin because its cylindrical shape optimizes force resistance. However, due to a lack of adhesiveness, its rigidity decreases [61]. Strain values consistently decreased with 2–15% glass powder, dropping sharply to 0.0267 at 25%. This stiffness change is attributed to disrupted resin molecular connectivity caused by the glass powder. Conversely, tensile strength increased from 25.42 MPa at 1% glass powder to a peak of 37.01 MPa at 25%. These results highlight how glass powder enhances tensile strength while reducing strain by efficiently distributing tensile loads across the composite. This approach prioritizes strength over deformation capability, offering a strategic method for engineering brittle materials like resin. It demonstrates that targeted glass powder reinforcement can customize 3D-printed materials for technical applications requiring higher strength.

3.2. Hardness Test

The experimental findings illustrated in Figure 9 confirm that adding glass powder to SLA 3D-printed specimens significantly improves hardness. Baseline specimens without glass powder had a hardness of 73.7 HV, which increased to 75.5 HV with 1% glass powder and reached 84.5 HV with 25% glass powder. This improvement is driven by effective interaction between the glass powder and resin matrix, creating a tougher outer layer and enhancing surface resistance to deformation. The glass powder evenly disperses within the resin, strengthening connections between particles, reducing micro-gaps, and minimizing surface flaws [62]. This uniform distribution improves load distribution, structural integrity, and resistance to indentation. Additionally, the glass powder forms a protective layer on the specimen’s surface, acting as a barrier against external forces and wear. These findings demonstrate that the strategic inclusion of glass powder enhances both surface quality and internal reinforcement, tailoring 3D-printed materials for specific technical requirements.

3.3. Density Test

Density testing is carried out on each percentage of specimens that have been made. The density of each percentage of the specimen is ascertained by measuring the mass of the cube in both air and water using the OHAUS PX (Parsippany, NJ, USA) series tool, as shown in Figure 10. The 3D-printed SLA specimen reinforced with glass powder has a higher density value than the specimen without glass powder, according to the results of the density test. When material properties change, the original specimen which was made without the use of glass powder becomes standard. Figure 10 shows that the density value of the specimen with 0% glass powder is 1.201 g/cm³, while the density value of the specimen with 1% glass powder is 1.203 g/cm³. The data show an increase in density in the specimens with glass powder added. This increase in density is caused by the addition of glass powder into the resin. This can be proven by the gradual increase along with the addition of glass powder percentage. Glass powder has a higher density than resin, which is around 2.5 g/cm³. The addition of glass powder into the resin will increase the density value of the specimen as a whole. This is evidenced by the specimen with the highest percentage of 25% glass powder having the greatest density among all specimens, which is 1.338 g/cm³. This improvement can occur because glass powder has a higher density than resin, so the addition of glass powder can increase the material’s density [63]. Based on the results of density testing in this study, the conclusion that can be raised is that 3D-printed SLA specimens with reinforced glass powder have a higher density value than specimens without glass powder. The increase in density is due to the addition of glass powder into the resin.

3.4. Compression Test

Compression tests in this study followed ASTM D695 standards, using specimens with 0–25% glass powder reinforcement. Figure 11 shows that glass powder significantly increased the maximum stress of SLA 3D-printed specimens. The baseline specimen without glass powder had a stress value of 399.38 MPa, while the specimen with 25% glass powder achieved the highest stress of 434.39 MPa. This improvement is attributed to the glass powder’s ability to distribute compression loads evenly and prevent plastic deformation. However, the addition of glass powder also increased brittleness, as indicated by a decrease in strain. The baseline strain value was 0.595, which dropped consistently to 0.526 in specimens with 25% glass powder. This brittleness is likely due to the smooth surface of the glass powder, which hinders bonding with the resin. In conclusion, glass powder reinforcement enhances maximum stress but increases brittleness in SLA 3D-printed specimens.

3.5. Flexural Test

The mechanical characteristics of flexural stress in resin specimens produced using SLA 3D printing technology are investigated in this work in relation to glass powder inclusion. The analysis of the data shows a clear decreasing trend in flexural stress as glass powder concentration rises. With a peak of 32.13 MPa, specimens free of glass powder showed the highest flexural stress(Figure 12). By contrast, specimens with a 25% concentration of glass powder showed the lowest stress levels, with a value of 27.93 MPa. This reduction suggests that adding glass powder could negatively influence the flexural strength of the resin generated with the SLA 3D printing technique.

The observed decrease in flexural stress is most probably caused by poor adhesion between the glass powder and resin matrix and unequal dispersion of glass powder particles inside the resin matrix. While insufficient adhesion reduces the material’s capacity to efficiently transmit load, non-uniform particle dispersion might cause uneven stress concentrations inside the material [64]. These results imply that there is a barrier to the quantity of filler that may be added without compromising the mechanical integrity of the material rather than that a higher glass powder concentration always improves the mechanical qualities of the 3D-printed resin. Therefore, balancing strength and flexibility in the ultimate product depends on maximizing the concentration and distribution of glass powder inside the resin.

4. Discussion

The addition of glass powder increases the tensile strength, compressive strength, and density of stereolithography (SLA) resins. The addition of 25% glass powder proves to be the most optimal percentage. Glass powder enhances tensile and compressive strength by helping to resist the forces applied to the SLA resin when it is subjected to static loads [63,64]. The addition of glass powder also increases the density of the SLA resin because the glass powder has a higher weight than the SLA resin. In addition to providing improvements, glass powder also reduces the flexural strength of the SLA resin. This can occur due to the lack of adhesion between the glass powder and the SLA resin. Weak particle dispersion causes stress concentrations that reduce the flexibility of the SLA resin.

The incorporation of 25% glass powder into the resin for SLA 3D printing represents the optimal proportion, as established through experimental outcomes. At higher concentrations, such as 30%, the printing process encounters significant disruptions, as evidenced by Figure 13. At this concentration, the manufactured models exhibit multiple defects, including bending, imprecise edge formation, and overall incomplete structures. These defects are primarily due to the excessive glass powder interfering with the ultraviolet (UV) light necessary for curing the resin, thereby compromising the integrity of each printed layer.

Further examination of the mechanical characteristics, as shown in Figure 14a–i, reveals the evolution of material behavior under tensile stress.

The red circles in the illustration show how subtly and irregularly the filler is visible in specimens with 1% and 2% glass powder in the figure (panels b–c). This aligns with the tensile test results, which indicate that the addition of 1% and 2% glass powder does not significantly impact the properties. However, with a notable increase in the surface roughness of the fractured surfaces compared to specimens with lower filler content, the identification of glass powder becomes quite apparent when the filler concentration rises to 3% and beyond. Specimens without filler did not exhibit a completely brittle fracture mode. As seen in Figure 14d–i, the glass powder mostly covers the surface areas in specimens with higher filler content, thus affecting the structural integrity. The glass powder contributes strength to withstand static loads during tensile testing, resulting in an increase in tensile strength.

Figure 15 illustrates the resin unreinforced and reinforced with glass powder. The improvement in tensile testing is due to the cylindrical structure of the glass powder, which helps to withstand and provide strength to the specimen when subjected to static loads. This further clarifies this phenomenon by showing that the internal structure, including the filler, enhances fracture resistance. The excellent bonding between the glass powder and the resin matrix is responsible for this improvement, which significantly increases the structural strength compared to conventional resin formulations.

5. Conclusions

Based on the research that has been performed, the following conclusions can be drawn:

• The tensile test results showed a decrease in elasticity with the addition of glass powder. Baseline specimens without glass powder had a strain value of 0.0363, which dropped to 0.0295 with 20% glass powder, indicating reduced deformation capacity. However, tensile strength improved, increasing from 24.03 MPa in the baseline specimen to 37.01 MPa at 25% glass powder. These findings highlight the positive impact of glass powder in enhancing stiffness and tensile strength in SLA 3D-printed specimens.
• Glass powder significantly enhances the hardness of SLA 3D-printed specimens, as shown by hardness testing. Baseline specimens without glass powder had a hardness of 73.7 HV, which increased to 75.5 HV with just 1% glass powder and peaked at 84.5 HV with 25% glass powder. This improvement is attributed to the effective integration of glass powder into the resin matrix, which acts as a reinforcing agent and evenly distributes applied loads, improving structural integrity and resistance to indentation. These findings demonstrate that adding glass powder can significantly improve mechanical properties, particularly hardness, making it suitable for engineering applications requiring higher material resistance.
• Density testing data show that adding glass powder significantly increases the density of SLA 3D-printed specimens. The baseline specimen without glass powder had a density of 1.201 g/cm³, which rose slightly to 1.203 g/cm³ with 1% glass powder. This trend continued, reaching a peak density of 1.338 g/cm³ with 25% glass powder. The consistent rise is attributed to the higher intrinsic density of the glass powder (about 2.5 g/cm³) compared to the resin. Incorporating glass powder into the resin matrix enhances overall density, improving structural properties. These findings demonstrate that the glass powder not only alters mechanical characteristics but also effectively increases density, making the material suitable for applications requiring higher density.
• Compression testing of the SLA 3D-printed specimens, following ASTM D695, showed that adding glass powder significantly enhances compression resistance. Specimens with 25% glass powder achieved the highest maximum stress of 434.39 MPa, compared to 399.38 MPa in the control specimen (0% glass powder). This improvement is due to efficient load distribution by the glass powder. However, brittleness increased as strain decreased from 0.595 in the control specimen to 0.526 with 25% glass powder, likely due to the smooth surface of the glass powder hindering bonding with the resin. These findings demonstrate that while glass powder improves compression strength, it also reduces ductility, requiring a balance for engineering applications needing both strength and flexibility.
• This study on the flexural properties of SLA 3D-printed resin specimens reinforced with glass powder shows that higher glass powder concentrations reduce flexural stress. Specimens without glass powder had the highest flexural stress at 32.13 MPa, while those with 25% glass powder dropped to 27.93 MPa. This reduction is due to uneven glass powder distribution and poor adhesion with the resin, causing irregular stress concentrations. These findings emphasize the importance of optimizing filler concentration and ensuring uniform distribution to maintain the mechanical properties of 3D-printed materials.
• Mechanical tests on SLA 3D-printed resin specimens reinforced with glass powder show significant improvements in hardness and tensile strength but increased brittleness and density. Higher glass powder concentrations reduced ductility, strain, and flexural stress while increasing compression resistance. These findings highlight glass powder’s dual impact: enhancing strength and hardness while reducing flexibility and increasing brittleness, affecting the material’s ability to handle flexural loads efficiently.