EFFECT OF PRINTING PARAMETERS AND POST-CURING ON MECHANICAL PROPERTIES OF PHOTOPOLYMER PARTS FABRICATED VIA 3D STEREOLITHOGRAPHY PRINTING

: Three-dimensional printing has recently come into the spotlight due to its promising potential to create physically three-dimensional parts or structures through computer-aided design. While there are many options for 3D printing methods, photopolymerization 3D printing has garnered much attention because of its high resolution. However, the mechanical properties of photopolymerized 3D printed parts can vary widely depending on the manufacturing parameters and post-processing settings used. This research focuses on studying the effect of printing variables on the mechanical properties of samples printed using a Stereolithography machine (Formlabs, Form+3). Three variables are used: layer thickness (25 and 50 μ m), part orientation (X and Z directions), and post-curing. Also, eight groups of 3D-printed photopolymer specimens for twenty-four specimens are used for the tensile test results. The results showed the printing variables affected the mechanical properties of samples, which were proven by Young's modulus, ultimate stress, and ultimate strain.


INTRODUCTION
3D printing is one of the modern manufacturing methods that are gaining popularity because of the possibility of its use in various fields of engineering, medicine, and more. Compared to other manufacturing methods, simple and complex parts can be easily https://doi.org/10.31436/iiumej.v24i2.2778 manufactured in record time using 3D printing. Also, the manufacturing process' waste and its cost are very limited in the printing process, which reduces the manufacturing cost [1]. According to the annual growth rate, 3D printing industry sales in 2020 reached more than 8 billion dollars in sales, equivalent to 14%. In 2013, the worldwide demand for 3D printing materials reached about 2 tons, which is anticipated to grow due to the increased use of printed products [2]. Operating 3D printers and manufacturing is simple; anyone can efficiently handle the machines and manufacture parts. The manufacturing process begins with drawing the 3D part with one of the engineering drawing programs, such as SolidWorks. Then the drawing is saved in STL file format. After that, the file is sent to the 3D printer to start the manufacturing process layer-by-layer after the required manufacturing process parameters are determined [3][4][5][6][7]. As shown in Fig. 1, stereolithography is one of the most important methods of producing 3D parts with good quality. SLA system uses a laser to polymerize a liquid resin and transform it into a solid part by a process called photopolymerization [8,9]. Printing thinner layers results in more cohesion and higher mechanical properties, but it does so at the expense of increased construction time [10]. Layers in Stereolithography are kept in a semi-reacted "green state" with polymerizable groups between them because the polymerization reaction is incomplete, and that helps with layer-to-layer bonding by supplying layers for subsequent polymerization. At the same time, post-curing procedures are used to finish the reaction and covalently bind successive layers. After curing, UV is typically employed to complete polymerization by activating photoinitiators [11]. Researchers used an SLA printer in previous work to produce samples with a wide range of print orientations and layer thicknesses [12]. Aznarte et al. [13]  hardness for 0° direction were greater than 45° direction by 106%, and greater also for 90° orientation by 92% for 50 μm print resolution. The findings demonstrated that mechanical properties strongly depended on resolution, print direction, and UV curing time. Agrawal [15] attempted to determine which orientation angle is best for which kinds of loads, specifically the fracture test and the dynamic mechanical analysis (DMA) test. Various mechanical property values were obtained during the inspection process. Results showed the orientation angle had a significant impact on the examination process. An average of three samples were taken for each test to reduce the error. After considering the stress-strain and load-extension graphs, the researcher concluded that the orientation angle should be 0°.
The parts manufactured using the SLA system change their mechanical properties according to the selected variables of the printing process. Therefore, some printed samples have poor mechanical properties due to the values of the used printing process variables, as the printing process variables have a significant impact on the mechanical properties of the produced samples. Therefore, the work aim is to determine the effect of part orientation in the X and Z axes, layer thicknesses of 25 and 50 μm, and post-curing on the mechanical properties of printed samples that are fabricated using the SLA system. The specimens' elastic modulus, ultimate stress, and ultimate strain are evaluated and analyzed depending on tensile test results to recognize the variables' values that affect printed specimens' mechanical properties.

MATERIAL AND EXPERIMENTAL DETAILS
A detailed description presents the manufacturing process and sample preparation, where the material used in the printing process is explained, as well as the variables of the printing process, the preparation of the number of experiments that are completed, the preparation of samples, and the examination process for samples that are produced through the printing process by a tensile test device.

Material
Clear resin is ideal for fluidics and mold making, optics, lighting, and any component needing translucency or displaying internal characteristics. It possesses several crucial characteristics, including quality. Formlabs' precisely crafted clear resin captures a model's finest features. Formlabs clear resin is excellent for quick prototyping and product development because it produces accurate, durable pieces, a glossy appearance, and the surface finish of the printed parts is smooth [15]. In the present investigation, specimens are printed with photocurable acrylic-based resin FLGPCL4 (Formlabs, MA, USA).

Process Parameters
3D printing is one of the basic methods for producing prototype parts. Still, 3D printing is not considered one of the mass production methods due to the long production time, anisotropy, etc. In addition, some critical issues face the 3D printing process, including accuracy, curvature, anisotropy, and the formation of voids inside the manufactured parts. The properties of 3D printed parts are dependent on printing parameters such as temperature, 3D printer machine resolution, layer thickness, geometries, and printing orientations. Therefore, one of the important points before the printing process is to focus on choosing the best process variables for printing to avoid defects in the manufactured parts [7,[16][17][18][19][20].
In the current experimental study, there are three process parameters used. Two printing parameters include layer thickness and part orientation, and the third parameter is postcuring. The layer thickness is one of the most critical variables of the printing process, which affects the quality of the produced surface and the mechanical properties of the https://doi.org/10.31436/iiumej.v24i2.2778 manufactured parts. Increasing or decreasing the layer thickness affects the sample's strength. There are many directions for printing, and researchers focus on changing the direction of printing during the sample preparation process to obtain the best quality of the manufactured parts. Part orientation is one of the most studied manufacturing characteristics. Therefore, printing direction statistically impacts the mechanical properties of SLA 3D-printed parts [21][22][23]. Figure 2 shows the part's orientation for both the X-axis and Z-axis. Finishing, including washing and post-curing, are necessary when using SLA printing because areas of uncured resin, whether between layers or on the surface, are considered weak points and damage the material's mechanical properties. The UV post-curing of SLA printed resin can significantly improve the mechanical strength due to the complete curing of any leftover resin.
In addition, the most significant improvement in properties occurs when the UV light is at the same wavelength that the SLA printer uses to cure the resin, as each resin type has a specific wavelength for the curing process. Therefore, the appropriate wavelength must be chosen to cure the resin for the best results [24]. Immediately following the completion of the printing process, the supporting material is removed from the printed part and the part is soaked in isopropyl alcohol for 15 minutes. Alcohol liquefies any uncured resin and cleans the surface of the components. Before testing, materials were allowed to dry for 24 hours on a clean surface. Post-curing is carried out for 50 minutes in a UV chamber, previously heated to 60 °C with a light source of 405 nm and 1.25 mW/cm, see Fig. 3.

Experimental Design
SLA technology generates 3D printed parts from a liquid (photopolymer) resin by employing a UV-light source to solidify the liquid substance (resin). To construct a 3Dprinted object, a build platform is submerged in a tank of photosensitive thermoset polymeric resin. Once the build platform is submerged, a UV light within the machine solidifies the material by mapping each layer of the object through the tank's bottom. After the light source has printed the layer, the platform rises to allow the swiping blade to apply a fresh coating of resin to the surface; this is continued layer-by-layer until the desired object is created [25]. Table 1 represents all experimental variables and groups that will be used to print the specimens, to ensure that the results of the studies can be reliably replicated; each sample is printed three times.

Specimen Preparation
SolidWorks is used to make the 3D model of the specimens following ASTM D638 type IV. Fig. 4 displays the ASTM-required dimensions of the specimen. Slicing can build the model using any CAD software and export it in a 3D printable file format (STL). Each SLA printer includes software to configure printing settings and split the digital model into layers for printing. Once the part design is completed, the print preparation software transmits the instructions to the printer over a wireless or wired connection. For slicing, STL file software named Formlabs preform (Version 3.27.1) is used to slice the specimen into some layers. In addition, the factors are fed to printer software based on the process parameters used in this research. An SLA machine (Formlabs, Form+3) is used in this research to produce the specimens. Form+3 has a 50 μm resolution in the plane parallel to the printing surface (XY resolution) and a 10 μm resolution perpendicular to the printing surface (Z resolution). Figure 5 depicts the machine and the necessary support structure for building the specimens in all directions. It was reported that the printer's maximum build dimensions were 145 x 145 x 185 mm.

Tensile Testing
A Gester universal tensile testing machine is used to test the properties of the printed specimens, with a load cell capacity of 5kN with a crosshead speed of 1mm/min; see Fig. 6.

RESULTS AND DISCUSSION
In this section, the tensile test results are shown and discussed. The section is separated into three subsections that discuss the effect of several elements on Young's modulus, ultimate stress, and ultimate strain, respectively. Table 2 shows eight specimens for twentyfour groups representing a variety of process parameters. Figure 7 shows the relationship between displacement (mm) and force (N) for green and cured samples. The curing process and layer thickness increased the tensile strength of the samples, taking into account the printing orientation. The cured samples had greater tensile strength than the green samples with decreased displacement because the material's behavior tends towards the sample's fragility. However, the green samples had more significant displacement than the cured samples. The tensile strength and displacement values change according to the printing variables, which indicate that the variables significantly impact tensile strength.

Effect of Layer Thickness, Part Orientation, and Post-curing on Young's Modulus
In this part, a detailed explanation, supported by values and figures, is given of the relationship between the variables of the specimens manufacturing process with Young's modulus and manufacturing time. Where it will be explained:  16.13% when layer thickness was increased from 25 μm to 50 μm). In addition, the results imply that the elastic modulus increased with a thin layer, and that happens because of the resin's exponential decay in the amount of light it transmits, increased curing speeds along the layer, and increased adhesion between layers [26]. The data also showed that curing significantly increased elastic modulus, as the elastic modulus of a green specimen that was printed with a layer thickness of 25 μm grew to 1648.417 (34.6456 %) MPa when cured. Additionally, after curing, the elastic modulus of the green specimen, which was printed with a layer thickness of 50 μm (796.141 MPa), increased to 1487.119 MPa (69.09%). The elastic modulus of 25 μm samples was higher than that of 50 μm samples because they have a lower fraction of semi-reacted resin due to superior laser beam penetration through a thinner layer, see Fig. 8    Young's Modulus vs. layer thickness for samples printed with Z, 90° orientation & 25 μm, 50 μm layer thickness: for green samples, the average elastic modulus for specimens produced with layer thicknesses of 25 μm and 50 μm was 1326.479 MPa and 1228.141 MPa, respectively. As can be seen, the modulus decreased by 9.8338 % when layer thickness was increased from 25 μm to 50 μm. For cured samples, the average elastic modulus of the specimens printed with layer thicknesses of 25 μm and 50 μm was 1767.274 MPa and 1699.033 MPa, respectively. As can be seen, the modulus dropped by 6.8241% when the layer thickness was increased from 25 μm to 50 μm. The elastic modulus of 25 μm samples was higher because they have a lower fraction of semi-reacted resin due to superior laser beam penetration through a thinner layer, see Fig. 8 (c) &(d). For green samples, the modulus of elasticity with X-direction and layer thickness varying from 50 and 25 μm increased from 796.141 to 1301.961 (increased 63.533%), and for the Z-direction increased from 1228.141 to 1326.479 MPa (increased 8%). For the cured sample, the modulus of elasticity with X-direction and layer thickness varying from 50 and 25 μm increased from 1487.119 to 1648.417 (increased 10.846%), and for Z-direction increased from 1699.033 to 1767.274 MPa (increased 4.016%). Therefore, the results showed that the modulus of elasticity with the X-direction is better than the Z-direction in both green and cured samples (see Fig. 9). Results showed that the thinner layer had higher elastic modulus, and that happened due to the exponential decay of light intensity transmission of the resin and getting higher curing rates along the layers and higher adhesion between layers.

Effect of Layer Thickness, Part Orientation, and Post-cure on Ultimate Stress
This part will explain the effect of the manufacturing process variables on the ultimate stress and printing time. Where it will be explained:  ). Therefore, the results showed that the ultimate stress with the X-direction is better than the Z-direction in both green and cured samples; see Fig. 10 (e) & (f).
The specimens with thin layers withstand greater forces than those with thicker layers, resulting from laser transmittance and providing a higher degree of curing to a thin layer than a thicker layer.

Effect of Layer Thickness, Part Orientation, and Post-cure on Ultimate Strain
The effect of manufacturing process variables on ultimate strain will be explained in this part. Where it will be explained:

CONCLUSIONS
In this study, the mechanical properties of 3D-printed photopolymers are examined and analyzed according to layer thickness, printing orientation, and post-curing. Based on the analyzed properties of elastic modulus, ultimate stress, and ultimate strain used to evaluate the printed samples, the results demonstrated that printing parameters significantly impacted mechanical properties. The results show that mechanical properties increased in Xorientation when the layer thickness varied from 50 to 25 μm in green printed samples. Therefore, the X-axis samples exhibit improvement in tensile strength and elastic modulus and have more elongation to failure when printed layers change to be thinner compared to printed samples in the Z-axis. This could be due to the nature of the 3D printing procedure, which constructs a desired part layer-by-layer. When printing a new layer on the specimens, the additional UV-light exposure to previously printed layers will increase the polymerization of leftover unreacted monomers. The interlayer fracture happens between the printed layers. In a thicker layer, the strength degraded faster in the specimen due to separation in the printed layers and increased interlayer stress. In contrast, the strength of specimens that are printed with a thin layer degraded slowly. Furthermore, in the case of vertical layer printing, the number of layers was large and thin. The laser-exposed surface area was large, enhancing the mechanical performance, which is distinct from horizontal printing.
The post-curing conditions had apparent effectiveness. UV curing under high temperatures and curing time improved the mechanical properties in both the X-axis and Zaxis and with various layer thicknesses. In the X-direction with 25 μm thickness, the elastic modulus increased by 26.61 % compared to the green samples. Also, the elastic modulus of the cured samples printed with 25 μm thickness and in the Z-direction increased by 33.23% compared to the green samples. The elastic modulus printed with 50μm thickness and in the X-axis increased by 86.791% compared to the green samples. Also, the elastic modulus printed with 50 μm thickness and in the vertical direction increased by 38.341% compared to the green samples. In summary, there was an increase in ultimate stress values of the samples. For the ultimate strain, the green samples were generally higher than the cured samples, as the post-curing made the material behavior more brittle. According to the results obtained, the printing orientation, layer thickness, and post-curing of the build-direction of 3D printed samples play a role in improving and controlling the anisotropy of mechanical properties of the printed samples, which is considered a challenge that is faced in the additive manufacturing process.