Designing for strength: enhancing mechanical performance through structured patterns in 3D printed elastomer

The mechanical performance of 3D printed elastomers is a crucial factor for their successful utilization in various applications, including soft robotics, wearable devices, and biomedical engineering. This study focuses on investigating the influence of different structured patterns, namely vertical and crosswise vertical SC, on the strength and mechanical performance of 3D printed elastomers. Through a series of experimental tests and numerical simulations, it was found that the cross-shaped structure exhibited the best strength among the tested patterns. This enhanced performance can be attributed to the unique arrangement of the crosswise structure, which effectively distributes stress and reduces strain concentration. This study provide valuable insights into the design and fabrication of high-performance 3D printed elastomers, paving the way for the development of advanced materials and devices with enhanced mechanical properties.


Introduction
The mechanical performance of 3D printed elastomers plays a pivotal role in various applications across a diverse range of soft robotics and wearable devices to biomedical engineering [1][2][3]. In the field of soft robotics, the elastomers employed must possess elastomers that can withstand repetitive deformations and exhibit high tensile strength [4]. Similarly, wearable devices require materials that can withstand external forces while retaining their flexibility and providing comfort [5]. Moreover, in biomedical engineering, elastomers with the mechanical properties of elastomers hold great importance for applications such as tissue scaffolds and implantable devices [6]. To address these requirements, the incorporation of structured patterns has emerged as a promising avenue [7,8].
Recent advancements in 3D printing technology have enabled the fabrication of complex structures with precise control over geometry and internal architecture, providing precise control over geometry and internal architecture [9][10][11]. However, there is still room for improvement in special structural designs, such as lattice structure, minimal surface auextical structure, as well as mechanical properties, including toughness and compressive strength [12][13][14]. Researchers have also explored the potential of incorporating structured patterns into the design of 3D printed elastomers to improve their mechanical properties. Various studies have investigated the influence of pattern parameters, such as pattern type, density, and arrangement, on the mechanical behavior of printed elastomers using different 3D printing techniques.
Leoa et al [15] studied the effect of infill density (10, 20, 50, 80%, and 100%) and infill patterns (gyroid, honeycomb, and grid) on the tensile strength of 3D printed elastomers, and found that a honeycomb pattern with a 50% infill density exhibited improved mechanical performance. Kim et al [16] investigated the performance of various 3D printed piezoresistive structures subjected to cyclic tensile loads, and found that the curved re-entrance (CRE) structure demonstrated the highest sensitivity. Based on hybrid additive manufacturing combining fused deposition modeling (FDM) 3D printing and ultrasonic treatment, Li et al [17] fabricated TPU flexible auxetic structure strain sensors with embedded CNTs to achieve stability and high sensitivity. Inspired by hierarchical structures, it has been proven that the toughness of 3D printed elastomeric lattice structures can be enhanced [18]. Nevertheless, the toughness and stretchability of UV-curable 3D printed elastomer in-plane structure patterns with LCD 3D printing have not yet been studied.
The aim of this study was to investigate the effects of different structured patterns on the strength and mechanical performance of 3D printed elastomers. Specifically, we examined the impact of four distinct patterns: across, vertical, cylindrical, and square columns. Through systematic variation of the pattern design, we seek to uncover valuable insights into how these patterns influence the mechanical behavior of elastomers. The outcomes of this study hold significant implications for the design and fabrication of high-performance 3D printed elastomers. By understanding how structured patterns influence mechanical behavior, researchers and engineers can optimize design parameters to achieve superior strength and performance. This knowledge opens new possibilities for the development of advanced materials and devices, leading to innovations in various fields where flexible and robust elastomers are essential.

Materials
The elastomers were synthesized by carefully mixing cyclic trimethylolpropane formal acrylate (CTFA), hydroxypropyl acrylate (HPA), hydroxyethyl acrylate (HEA), and photoinitiator-819 in specified ratios. Three different mixtures, designated as CTHP1, CTHP2, and CTHP3, were used. The weight content of the photoinitiator-819 was 2% of the total weight of the liquid resin, as shown in table 1.

3D printing processes
The 3D printed structure was first sliced into a series of images using Chitubox slicer software. It was then transferred to a USB drive, which was linked to a Liquid Crystal Display (LCD) 3D printer ( figure 1(a)). Next, the  mixture was poured into a liquid tank and printed layer-by-layer using an LCD 3D printer. The exposure times for the initial three layers and other layers were set to 30 s and 15 s, respectively. The thickness of the printer layer was set at 50 μm. After printing, the printed structure was washed with absolute ethanol (99.5%) and exposed to a 405 nm light source for 10 min to facilitate post-curing.

Tensile testing
The elastomers and printed structures were evaluated using a digital tensile machine (TSE504C Wance, Shenzhen, China). As shown in figure 1(b), dog bone-shaped samples with lengths of 14 mm, gauge lengths of 4 mm, widths of 4 mm, and thicknesses of 0.8 mm, were tested to determine stress, strain, modulus, and  toughness. The loading rate was set to 0.1 s −1 . The elastic modulus was obtained from the slope of the stressstrain curve, and the toughness was calculated as the area under the stress-strain curve.
Thermal properties testing A differential scanning calorimetry (DSC) machine-TA Q600 instrument was implement to measue the thermal properties of 3D printing elastomer. The instrument measures the heat flow into or out of the sample, generating a thermogram that represents the heat flow as a function of temperature. Various thermal transitions and associated properties can be determined from the thermogram. In this work, we meausre the glass transition temperature (Tg), melting temperature (Tm), and crystallization temperature (Tc) of the 3D printing elastomer with DSC instrument.

Numerical simulation
The tensile behavior of various structures was simulated using the finite element software ANSYS Workbench 2020. The Static module was employed to simulate tensile testing, and the 2nd Yeoh model was utilized for modeling the elastomeric materials. The material parameters were determined by fitting the stress-strain curve obtained from experimental testing of dog bone samples. The element size for the meshes was set to 0.25 mm. Furthermore, an effective strain load of 0.2 was implemented.

Results and discussion
Tensile strength Figure 2 illustrates the tensile stress-strain curves and mechanical attributes of the 3D printed elastomer. As depicted in figure 2(a), CTHP1 exhibited the greatest extension, whereas CTHP3 demonstrated the least extension. This is attributed to the fact that an increase in CTFA leads to a decrease in the break strain. In addition, the printed elastomer exhibits a hysteresis effect, which contributes to a gradual failure after the stress reaches the peak point.Conversely, CTHP2 displayed the highest tensile strength and toughness, as shown in figures 2(b) and (c). Hence, CTHP2 was selected for further investigation into the mechanical performance of various elastomer structures. Figure 3 illustrates the heat flow-temperature curve, as well as the values of T g , T cc , and T m for three distinct samples: CTHP1, CTHP2, and CTHP3. It is evident from the graph that all three samples exhibit a relatively low T g. Specifically, CTHP1 and CTHP3 demonstrate a T g below 40°C, whereas CTHP2 possesses a slightly higher T g of 40.73°C.In terms of T cc , CTHP1 displays a lower value compared to both CTHP2 and CTHP3. This suggests that CTHP1 has a lower onset temperature for crystallization, indicating its potential for faster solidification compared to the other two samples. On the other hand, CTHP2 and CTHP3 exhibit similar T cc values.

Mechanical behavior of 3D printing elastomer structures
In this study, three distinct 3D printing structural types were chosen for examination: vertical, vertical S C, and crosswise (as show in figure 4). Four unit cell patterns were selected for each structure, which included isosceles triangle rod element with angles of α = 30°, 40°, and 50°, as well as an equilateral triangle with an angle of α = 60°. Four unique vertical structures can be created by arranging four distinct unit cell patterns vertically. In  the vertical S C structure, a vertical rod element is incorporated into the unit cell pattern, setting it apart from the other vertical structures. For the crosswise structure, the unit cells were assembled with a 90°rotation along the vertical axis. The geometric dimensions of the structures are listed in table 2, and the stress is calculated using the effective cross-sectional area.The D ij and L ij means the width and length of 3D printing structures, repsetively. This diverse selection of structures and unit cell patterns allows for comprehensive analysis of their respective properties and performance. Figure 5 illustrates the effective stress-strain curves for the three different structures. When α was set to 30°a nd 50°, the crosswise structure exhibited the highest elongation, whereas the vertical structure demonstrated the greatest elongation at α values of 40°and 60°. At α = 30°, the crosswise structure exhibited the highest effective tensile strength, whereas the Vertical SC structure displayed the highest effective tensile strength in the range of α = 40°-60°. Furthermore, it is evident that the tensile strength of the crosswise structure decreased with an increase in α.
In numerical simulation, the fitting material parameters for CTHP2 are given by the following values: C10 = 569 kPa, C20 = 7.816 kPa, D1 = 0, and D2 = 0. As depicted in figure 6, both the experimental and simulation data indicate that an increase in α leads to a decrease in local deformation, leading to a reduction in strength. This observation is supported by an analysis of the deformation and experimental results. The increase in α correlates with a decrease in local deformation, resulting in a subsequent decline in strength. Figure 7 illustrates the variation in the tensile toughness among different unit cell patterns. When α is 30°or 40°, the tensile toughness increases from the vertical pattern to the Vertical S C pattern, and then to the crosswise structure. However, for α values of 50°or 60°, the tensile toughness increased from the vertical pattern to the Vertical S C pattern, but then decreased for the crosswise structure. Figure 8 shows the tensile toughness of different structures with various unit cell patterns. For the Vertical structure, the trend of the tensile toughness with increasing α is not clear, but the lowest tensile toughness occurs at α = 50°. As α increased, the tensile toughness of the vertical S-C structure improved. In the case of the crosswise structure, the tensile toughness decreased as α increased from 30°to 50°but then increased as α reached 60°. Comparing the three different structures, the crosswise structure exhibits the highest toughness at α = 30°. This is because the Crosswise structure demonstrated significant local deformation (as shown in figure 9) under load, resulting in a higher effective tensile strength.

Conclusion
This study investigated the tensile behavior of 3D printed elastomers, focusing on three different structures with four distinct unit cell patterns. The key findings are summarized as follows: As CTFA increased, there was a corresponding decrease in the elongation of the elastomer materials. Among the materials tested, CTHP2 exhibited the highest toughness and was subsequently used for the 3D printing of the structures.
This study also examined the deformation patterns in both experimental and simulation settings. As α increased, the local deformation decreased, leading to a reduction in both strength and toughness. Interestingly, the toughness of the Vertical SC structure increased as α increases from 30°to 50°, but then decreased when α increased from 50°to 60°. Among the three tested structures, the vertical structure exhibited the lowest toughness.
This research provides valuable insights into the behavior of 3D printed elastomers under tensile stress, and the findings have significant implications for the design and manufacture of 3D printed structures.