Effect of ultraviolet post-curing, laser power, and layer thickness on the mechanical properties of acrylate used in stereolithography 3D printing

Three-dimensional (3D) printing technology has developed rapidly in the past few years. This technology is widely used in houses, workpieces, artworks, organs, and even the food industry. Among the multiple 3D printing methods, the stereolithography appearance (SLA) technique is well known for its high printing speed and considerable accuracy. In this research, the optimization parameters of the laser curing technology were applied by changing laser power and molding thickness. The mechanical properties were tested using a tensile test and a compression test of photosensitive resin materials after ultraviolet (UV) curing. The experimental results revealed that increasing the laser power or reducing the molding thickness increased the ultimate tensile strength, tensile Young’s modulus, and compressive strength of the material. However, when the stretch was decreased after UV curing, the strength of the material effectively improved, but the toughness of the material decreased. The abovementioned experimental results will be helpful to researchers for further studies on SLA.


Introduction
In a highly competitive market, each manufacturer aims to minimize the processing time of products. Stereolithography appearance (SLA) laser light curing technology was developed to shorten the production cycle time while retaining the complexity of the product. Here, a photosensitive resin is cured and irradiated by using laser light as an energy source. During the printing process, the laser light spot is moved constantly on the surface of the photosensitive resin and solidifies the resin layer by layer. The final product is produced after the execution of numerous push-stacking processes. The advantages of SLA are a short production processing cycle, high precision, and good surface quality. This technology can effectively accelerate the prototyping procedure. However, in the case of rapid prototyping, the product's unstable material properties are considered. In recent years, many studies have mainly focused on the influence of manufacturing parameters on mechanical properties, by using diverse methods, including prediction software [1][2][3][4] and various porosity values of the material [5]. Several studies have also changed the properties of materials by adding other materials [6][7][8][9][10][11] to make up for the raw materials' deficiencies in certain physical properties. Recent reports have investigated the effect on materials' mechanical properties, changes in the printing parameters and the most commonly used post-processing methods. Lu et al [12] reported the effects of different laser intensities, layer thicknesses, and ultraviolet (UV) curing on a material's mechanical properties and microstructure. They observed that when the laser intensity increases and the layer thickness decreases, the material's mechanical properties improve. If the intensity is too low, the layer thickness is too large, or the curing is insufficient, the material shrinks and/or deforms. Salmoria et al [13] studied the mechanical properties of the Somos 7110 resin and analyzed the fracture Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. morphology of the specimens after UV curing, heat curing, tensile, and impact tests; their results revealed that after the post-curing, the specimens exhibited brittle fracture behavior, and after UV curing by conventional heating, they exhibit better mechanical performance. Chockalingam et al [14] explored the effect of the layer thickness of the photocurable molding mechanical properties according to the ASTM standards, by using an SL 5000 printer, and analyzed different print layer thicknesses of specimens through tensile, yield, and impact tests; residual stress analysis; and the drilling method. Their results revealed that a smaller thickness confers more strength to the considered part. Adamczak et al [15] study the influence of SOMOS 7110 resin on the material properties by changing the molding thickness and post-curing method of 3D printing SLA technology. The result shows that the shrinkage of the part depends on the printing direction. The post-curing method can improve the curing degree, isotropy and precision of the part. Mendes-Felipe et al [16] used the Clear 02TM resin from Formlabs to build samples and evaluated the effects of the post-curing process on the final properties of the printed pieces. They observed better mechanical properties of the materials post-curing with UV light. Anastasio et al [17] research 3D printing SLA technology was studied. Through the analysis of the influence of UV curing and thermal curing on the mechanical properties of acrylic resin, tensile test and micro-compression nanoindentation test were carried out. The results showed that the overall mechanical properties of the workpiece after thermal curing were better than that of UV curing. Ni et al [18] research 3D printing SLA technology using Hydroxyethyl Methacrylate (HEMA) as 3D printing material, by changing the printing laser intensity, testing its mechanical properties, the results show that the laser power scans at low power and high speed , leading to incomplete curing, low speed and high power may cause excessive polymerization. Ambrosio et al [19] research on 3D printing SLA technology, use resin epoxy acrylate, acrylate, methacrylate (Methacrylic Acid Esters) to print test pieces by changing the printing parameters, the results show that the factor that has the greatest impact on mechanical properties is printing orientation, UV curing time has a high impact on the chemical composition of the material. Saini et al [20] research on 3D printing SLA technology, using transparent photosensitive resin, by changing the printing direction, conducting tensile, compression, bending, impact, fatigue and vibration analysis to test its mechanical properties, the results show that the samples printed at 22.5°a nd 67.5°have the largest Tensile strength, the sample printed at 67.5°has the highest flexural strength, while the sample printed at 0°has the highest impact strength and fatigue resistance.
The abovementioned studies have shown that changing the material properties or manufacturing parameters affects the mechanical properties of the materials. However, although material modifications can effectively improve the material's mechanical properties, they increase the production cost. In this research, by using the SLA technology, we analyzed the influence of different laser powers, molding thicknesses, and UV post-curing conditions on the mechanical properties of photosensitive resins, and attempted to improve the mechanical properties of the considered materials without changing their original chemical properties. The results of this study are expected to contribute to further commercialization in the industry.

Experimental section 2.1. Materials and machine
The Moai Laser SLA 3D printer manufactured by Peopoly Co., Ltd. (Hong Kong) was used in this research. The detailed operation conditions of the 3D printing machine are presented in table 1 and figure 1. This experimental material adopted to the test is the photosensitive resin (gray) of Peopoly Co., Ltd. (Hong Kong). According to its datasheet, the resin consists of a methacrylate oligomer diluted with the corresponding methacrylate monomer and an unknown photoinitiator. The printing material properties are illustrated in table 2.

Print parameters and methods
First, use Solidworks software to draw a 3D picture of the test piece. The slicing software used in this study is Cura. There are two different experimental slicing software factors, namely control factors and variable factors. Control factors include 130 mm s −1 print speed and 100% fill density. Variable factors include layer thicknesses

Tensile and compression tests
The tensile test was carried out according to the ASTM D638 test standard, and the compression test was carried out according to the ASTM D3410 test standard, as shown in figures 2 and 3 respectively. The mechanical properties of the materials were measured using an electromechanical testing machine (model: YM-H5101) manufactured by Yang Yi Technology Co., Ltd. (Tainan, Taiwan). The test loaded along the longitudinal axis with a speed of 5 mm min −1 , the maximum load is 100 kN, and the sampling rate of the signal 5 Hz. Elongation was measured using an extensometer. Data obtained from the tests were calculated using Microsoft EXCEL In this study, the change rate of mechanical properties is illustrated in formula (1).
where CR represents the rate of change. UV and Green indicate the Young's modulus, yield strength, elongation, and ultimate strength of the 3d printed material after UV curing and before post-curing, respectively.

UV curing test
In this study, the Multicure 180 light curing machine manufactured by XYZ Printing Company (New Taipei, Taiwan) was used. The UV light with the wavelength of 405 nm was used for secondary curing. The energy intensity, the curing time, and the rotary speed were maintained at 60%, 60 min, and 1 rpm, respectively. The technical parameters of the post curing machine used in this experiment are presented in table 4.      . The results revealed that the different levels of laser power were not critical to the print time, while the larger the layer thickness was, the shorter was the printing time. To summarize, the factor affecting the printing time of SLA was determined to be the layer thickness. Figure 4 shows the tensile Young's modulus values and the change rate before and after UV curing. Figure 4(a) shows that Young's modulus increased with increasing laser power. Low laser power resulted in looser polymerization and larger resin voids, leading to an inevitable deformation of the material [12]. Furthermore, thinner layer thicknesses led to higher Young's modulus because of the overlap between the layer thicknesses, which made the deformation of the material more difficult [14]. After UV curing, in addition to the main network (the network created during printing), a sub-network (network generated after UV light irradiation) was generated inside the sample [17]. The closer cross-linking of the molecules and complete cure of the resin allowed the material to be more resistant to deformation. Figure 4(b) shows that the rate of change was all positive, which implied that the material's rigidity improved effectively post UV curing. As can be seen from the figure, the mass of the test specimen with the largest change rate was at the place where the laser power was 60 mW and 0.15 mm. This was attributed to the fact that when the laser power is small and the layer thickness is large, the resin content in the specimens that has not been fully cured is high, so the overall change rate after UV light irradiation is large. In conclusion, the UV curing process had the most significant effect on the material's mechanical properties. Figure 5 illustrates the elongation and the change rate before and after stretching UV curing. Figure 5(a) shows that as the laser power increased, the elongation gradually decreased because of UV irradiation. Therefore, the material showed poor ductility. In addition, when the molding thickness was enhanced, the gaps between the layers became bigger and easier to fracture [12], which led to lower ductility. Figure 5(b) shows that after UV curing, the rate of change became all negative, indicating that the ductility of any laser power and molding thickness after UV curing was lower than that of the standard specimen (Green). This showed an improvement in the degree of curing of the material after UV curing, which reduced the ductility and made the specimens considerably difficult to deform [13].   Figure 6 presents the yield strength values and the rate of change before and after UV curing. With or without UV curing, the yield strength of the material decreased with decreasing laser power at each layer thickness. Under the same laser power condition, the yield strength of the UV-cured material decreased with an increase in the layer thickness; i.e., under the same laser power condition, the greater the layer thickness of the cured material was, the higher was the rate of change of the yield strength. However, the yield strength of the material with a smaller layer thickness was shown to be lower than that of the green after UV curing. This was because the smaller layer thickness produced a denser main network and sub-network, for which the discontinuity of the material structure easily resulted in some defects, hence affecting the continuity of the overall structure [18]. Therefore, the figure shows a lower yield strength, but the main network and sub-network had a lower density in a single unit of the specimens with a larger layer thickness, which caused fewer defects in the material and resulted in higher yield strength. Figure 7 presents the ultimate strength value and the rate of change before and after UV curing. With an increase in the laser power, the ultimate strength increased. This was because the higher laser power caused the stronger polymerization of the resin. The higher structural density and smaller layer thickness caused better adhesion between the layers, which improved the strength of the material. When the laser power was lower, a larger rate of change could be observed because the lower laser power and the stronger polymerization of the resin made it easy for the voids to appear inside the specimens, which contained a large amount of uncured resin. After UV curing, the uncured resin in the voids could be cured completely [12]. Therefore, the structural strength of the material could be improved significantly. Figure 8 shows the fracture diagram of the tensile sample, which (from left to right) shows the material samples printed with the laser power of 55 mW, 60 mW, 65mW, 70 mW, and 75 mW, respectively, for each layer  thickness. The figure also shows that the fracture plane perpendicular to the tensile direction section was quite straight at the fracture site. Moreover, no necking phenomenon appeared on the specimens. The images also show that the shape of the fracture surface was not affected by the different laser power values. Figure 9 illustrated the CCD image of a cross-section of the tensile test specimen post-curing and after UV curing. When the layer thickness was thinner, the fracture surface was denser. As the layer thickness increased, the print path became clearer, and the gaps in the dark part between layers could be observed. After UV curing, the gap between the layers of the specimens became less noticeable, and tighter layers were observed on the cross-section, indicating that UV curing could promote the complete curing of the resin and effectively improve the strength of the material. When the laser power was 75 mW, the resin was completely solidified to form a dense structure. After ultraviolet irradiation, the resin components were gasification and over-cured was induced, leading to the formation of bubbles as observed [18]. Figure 10 shows the Young's modulus and the change rate before and after UV curing. This result indicates that the smaller layer thickness decreased the voids and enhanced the material's rigidity, resulting in a higher Young's modulus value and strength. This result also indicated that a stronger laser power could improve the curing efficiency of the material. Thus, the strength and rigidity of the material also increased. In a comparison of the Young' modulus in the tensile and compression tests, a strong tension and compression asymmetry were observed [21]. After UV curing, the rate of change was mostly down, because the specimens were thin and the resin was almost completely cured to cause a dense structure inside these specimens after the printing process. The resin molecules vaporized to form bubbles because of the excessive curing, which caused the material to  break easily [18], resulting in poor rigidity. The rate of change increased at 60 mW because of the lower structural density, indicating that UV curing could improve the compressive Young's modulus. Figure 11 shows the compression yield strength and the rate of change before and after UV curing. It can be seen from the figure that the yield strength of the UV-cured material increased with an increase in the laser power under any molding thickness condition. However, under the same laser power conditions, the yield strength of the material with larger layer thickness after UV curing was higher than that of the green material, but the yield strength of the material with smaller layer thickness after UV curing was lower than that of the green material. This was because the number of main networks and secondary networks in materials with a small layer thickness was large, which affected the continuity of the overall structure. Therefore, the yield strength was poor, but the specimen with a large layer thickness had a less dense main network and secondary network printed per unit volume, resulting in fewer material defects [17]; thus, the yield strength increased more. Figure 12 shows the ultimate compressive strength and the change rate before and after UV curing. It can be seen from the figure that the ultimate compressive strength increased with an increase in the laser power, and  increased with a decrease in the layer thickness. The change rate of UV curing was similar to the change rate of yield strength. When the laser power was small and the layer thickness was large, the ultimate strength tended to increase after UV curing. This was because when the laser power was small and the layer thickness was large, the curing degree of the specimen was low and the UV curing was insufficient [14], resulting in excessive defects and affecting the final strength.

Conclusions
In this paper, the effects of printing parameters, including laser power and layer thickness, on the mechanical response of materials produced using SLA were studied. With an increase in the laser power, the Young's modulus, yield strength, and ultimate tensile and compressive strength increased; only the tensile elongation decreased with an increase in the laser power. The increase in the layer thickness reduced the Young's modulus, yield strength, elongation, and ultimate tensile and compressive strength. Tension, compression asymmetry, and the compressive Young's modulus, yield strength, and ultimate strength were greater than the tensile strength. After UV curing, the tensile Young's modulus, yield strength, and ultimate strength improved; only the tensile elongation was reduced. However, the compressive Young's modulus, yield strength, and ultimate strength decreased. After UV curing, all mechanical properties of the tensile and compression tests faced a situation where the values of mechanical properties were close, the trend appeared to be centralized, and the amount of change in the compression was greater than that in the tension, and the amount of change in the mechanical properties was relatively large under the conditions of low laser power and large layer thickness.