Simulation and Process Analysis of DLP 3D Printing with High-strength Resin

By utilizing digital light processing (DLP) printing equipment and technology, this study investigates the temperature field, stress changes, and the impact of various process parameters on the formation and strength characteristics of high-strength resin during the printing process. Abaqus birth-death element method simulation and related process tests are employed for this purpose. The simulation results demonstrate a gradual decrease in the temperature field through wavy diffusion from the center to the boundary during printing. The node temperature, displacement, and stress curves are observed to fluctuate frequently due to the subsequent printing layers, with the maximum stress point located close to the printing platform. The experimental results reveal that the test parameters possess different effects on the surface quality and tensile strength. Inappropriate parameters tend to result in surface defects. The influencing factors on the tensile strength of the sample are ranked in the following order: layer thickness, exposure time, and exposure light intensity. The molded resin sample achieves a tensile strength of 58.5MPa, which is comparable to the tensile properties of traditionally injection-molded parts.


Introduction
3D printing technology offers notable advantages, including cost-effectiveness and shorter production cycles.It enables the creation of complex products that are difficult to manufacture using traditional processing methods, thus offering broad application prospects [1].Depending on the specific processes used, 3D printing can be categorized into methods such as fused deposition modeling (FDM), laminated object manufacturing (LOM), and stereo lithography appearance (SLA) [2].In recent years, there has been increasing interest in faster digital light processing printing (DLP), which shares similarities with the light-curing process of SLA printing [3][4].
DLP printing technology, developed based on digital projectors, is a second-generation light-curing technology that enables layer-by-layer curing of photosensitive polymer liquid [5].In the study conducted by Qi Jianfeng et al from Zhongyuan University of Technology, the influence of single-layer exposure time on the forming accuracy of DLP photosensitive resin 3D printing was investigated.The researchers identified the impact of exposure time on product shrinkage and dimensional accuracy [6]. 2 Additionally, the strength of the printing material plays a crucial role in its application across various fields.A higher strength expands the range of potential application scenarios.Currently, there are no existing reports on the simulation and process research of DLP printing high-strength resin.
Based on the development of existing DLP printing technology, this article conducts an analysis of simulation and related processes for high-strength resin DLP 3D printing.The study focuses on the temperature field during the printing process, changes in stress, and the impact of various process parameters on its strength indicators.This research provides a fundamental basis for the future industrial application of high-strength resin DLP printing.

Test equipment, materials, and methods
For this study, the selected test equipment is the Rayshape Shape 1 professional DLP 3D printer.The printer has a total power of 450W, operates with a 405 nm light source band, and has a pixel size of 100 microns.Before printing, it is important to ensure that the surface of the printing platform and the optical system are clean.The resin in the resin tank should also be stirred.After printing, the sample should be removed gradually as the temperature decreases.The parts' surfaces should be wiped with 95% alcohol, followed by placing them in an ultrasonic cleaning machine for 3 minutes.Subsequently, curing should be carried out for 40 minutes.A schematic of the DLP printing principle and equipment can be found in Figure 1.The test employs high-strength photosensitive resin obtained from Suzhou Rayshape Smart Technology Co., Ltd.This material has a density of 1.112 g/cm 3 and a viscosity of 1100 mPa• s (25°C).The tensile properties of the specimens were evaluated using an INSTRON 5967 universal testing machine.Each set of parameters was tested with three samples, and the results were averaged.

Simulation principle and method
The forming process of the 3D printed part was simulated in this study using the Abaqus birth-death element method [7].This model is extensively utilized for simulation and modeling of additive manufacturing and welding processes.Regarding coupling, sequential coupling was employed for analysis, with thermal analysis conducted first, followed by thermal stress analysis [8].The established physical model is shown in Figure 2. The dimensions of the formed part were 10×10×15mm.The material properties were defined using linear elastic material properties in the SI-mm unit system, with a density of 0.9 ton/mm ^3.Since the 3D printing process involves layer-by-layer material addition using birth-death elements, a Python script was used for model slicing.The model was sliced along the Y-axis (height of 15mm) into 150 sections.In the created analysis steps, the elements were removed in the initial step, and in the subsequent steps, the elements were incrementally activated based on the printing layer thickness.The convective heat transfer coefficient was set to 2000, and the radiative heat transfer emissivity was set to 0.85.Default field variables were outputted, including temperature, heat flux, and displacement.A cooling step was then created to calculate the heat dissipation.Hexahedral meshing was performed, with a global size approximation of 0.5, and an ambient temperature of 20°C was set.
Finally, the results from the thermal conduction analysis were applied to the stress analysis, obtaining corresponding temperature and thermal stress field results.

DLP printing temperature field simulation
Figure 3 presents the temperature distribution and curve for the central section and node of the model.The figure displays the temperature of each node as well as the heat-affected zone.In the heating layer, the temperature swiftly rises to the resin curing temperature of 80°C.The formed part undergoes heat transfer via conduction, convection, and radiation.The heat-affected zone expands, and the temperature decreases through diffusion.From the figure, we observe that due to external heat exchange with the air, the heat near the edges dissipates rapidly and quickly returns to room temperature.However, the central area gradually decreases in temperature layer by layer, reaching 60°C, then 40°C, and finally 25°C.At this point, the heat-affected zone exhibits an approximately half-ellipsoid shape.The node reaches the resin curing temperature of 80°C upon activation and maintains this temperature during the exposure time.Subsequently, the temperature sharply drops after curing.As the printing process progresses, the node's temperature periodically fluctuates between 20°C and 40°C, following a zigzag temperature curve.These fluctuations arise from the periodic temperature loads imposed by the subsequent printing layers.Eventually, the node's temperature gradually decreases to reach room temperature.

DLP Print Stress Field Simulation
Figure 4 illustrates the displacement distribution of the printed part.As depicted in Figure 4a), the deformation displacement of the entire printed model is minimal at both ends, larger in the middle, and especially pronounced near the internal small boundary.The upper part, being in close proximity to the metal printing table, exhibits fast heat dissipation, effectively limiting deformation.Conversely, the lower part benefits from heat dissipation on five sides, resulting in a large heat dissipation area and minimal deformation.The slight deformation observed in the middle section is attributed to the accumulation of heat.Furthermore, Figure 4b) demonstrates that in the measured central section, which corresponds to the maximum deformation area, the vertical distance between the two sides after deformation is 9.96mm.This translates to a deformation of 0.04mm or 0.4% compared to the set size of 10mm.Considering the small deviation from the actual measurement results (9.90mm, with a deformation of 1%), it can be inferred that the simulation results exhibit high accuracy.Figure 5 illustrates the stress distribution in the printed object.As depicted in the figure, the stress attains its peak near the boundary of the printing platform and gradually diminishes towards the bottom and inward direction.Since the upper printing platform serves as a fixed end, the subsequent printing layers solidify and expand under the influence of UV light during the printing process.Due to the constrictions imposed by adjacent materials, these layers are unable to deform freely.Consequently, compressive stress is repeatedly applied to this specific node, with each subsequent printing layer exerting additional stress.As a result, the stress near the boundary of the initial printing layer accumulates to its maximum value.Therefore, to ensure optimal printing results, the exposure time of the initial printing layer should be prolonged several times compared to the subsequent layers' exposure time.This ensures a secure connection between the printed sample and the printing platform, mitigating the risk of printing failure even under stressful conditions.As the number of printing layers increases, the rear printing layer intermittently exerts stress on the node, resulting in periodic changes in node displacement.Consequently, the curve displays zigzag fluctuations.Once these fluctuations reach a certain extent, they have minimal impact on the node due to the distance between the printing layers.As a result, the accumulated displacement remains relatively constant and gradually approaches a gentle and stable state.

Sample surface and section status
After the completion of printing, both the external appearance and cross-sectional state of the tensile specimen were assessed.Figure 7 showcases the sample fabricated using a layer thickness of 0.05mm, and an exposure light intensity of 90% (this parameter corresponds to a light intensity of 3.33mw/cm2, reflecting the light produced by a 150uw light machine power).The subsequent exposure light intensity can be converted based on this proportional parameter.As depicted in the figure below, the external surface of the sample appears to be in excellent condition, devoid of any cracks or defects.
The cross-section is displayed in Figure 7b).As elucidated in the earlier stress field analysis, stress accumulates, resulting in deformation.Given the sample's lengthy nature, a slight warping is observed on the surface.However, the cross-section exhibits fine quality, entirely free from any undesirable defects.The appearance and cross-section of the majority of the tested parameters resemble this particular set.Thus, it can be inferred that most of the tested parameters fall within a reasonable range.When the parameters are set to a layer thickness of 0.1mm, an exposure light intensity of 80%, and an exposure time of 2s, the surface quality is satisfactory, as depicted in Figure 8a).However, as shown in Figures 8b) and 8c) below, insufficient energy reception leads to increased deformation and pronounced warping, with the presence of unconsolidated interlayers in the cross-section.Upon analysis, it is determined that the bottom layer exhibits good curing due to the extended curing time.However, during the transition to normal printing, the subsequent layers fail to adhere closely to the properly cured lower layer due to the shorter curing time, thicker layer thickness, and inadequate light intensity.As a result, printing defects arise.It is worth noting that the subsequent printing layers do not replicate this defect when consistent parameters are maintained.Therefore, attention should be paid during printing to prevent such occurrences.

Orthogonal test analysis of the influence of each factor on tensile capacity.
The influence of these factors on tensile strength was studied using orthogonal analysis.The orthogonal test scheme and the analysis of the results are presented in Table 1 below.The tensile strength value represents the average of measurements taken from three samples.Each factor in the table has three levels.The three factors are put into the L9 (3 3 ) orthogonal table, and the results are analyzed by intuitive analysis.Kij (i=1~3, j=1~3) is the sum of all experimental results of the jth factor with level number i, and ij K is the average of the three values.The range indicates the difference between the maximum and minimum values of ij K , which can directly reflect the influence of the jth factor on the tensile strength value.A larger range value corresponds to a stronger influence.Based on the range values, the factors can be ranked in terms of their influence on tensile strength as follows: layer thickness > exposure time > exposure light intensity.This suggests that the test parameters should be arranged in this order for the subsequent printing process.It is also observed that the material's tensile strength ranges from 40 to 60, which is comparable to the strength of engineering plastics like ABS and represents a higher level among lightcuring resins.

Conclusions
This paper investigates the temperature field, stress field, sample appearance, and tensile strength of DLP printing using high-strength resin.The results indicate that the shape of the heat-affected zone is approximately half-ellipsoid, with rapid heat dissipation and lower temperatures at the edges.The stress contour plot reveals that the maximum stress occurs near the printing platform.Moreover, the temperature, displacement, and stress curves of the nodes exhibit a zigzag pattern and are heavily influenced by the subsequent printing layers.Poor parameter matching can lead to surface defects.The exposure light intensity, layer thickness, and exposure time all impact the tensile properties of the samples to varying degrees.Among these factors, layer thickness has the strongest influence, followed by exposure time and exposure light intensity.

3 .
distribution of the central section b) Temperature curve of the midpoint node Figure Temperature distribution and curve

Figure 4 .
Figure 4. Distribution of displacement.Figure5illustrates the stress distribution in the printed object.As depicted in the figure, the stress attains its peak near the boundary of the printing platform and gradually diminishes towards the bottom and inward direction.Since the upper printing platform serves as a fixed end, the subsequent printing layers solidify and expand under the influence of UV light during the printing process.Due to the constrictions imposed by adjacent materials, these layers are unable to deform freely.Consequently, compressive stress is repeatedly applied to this specific node, with each subsequent printing layer exerting additional stress.As a result, the stress near the boundary of the initial printing layer accumulates to its maximum value.Therefore, to ensure optimal printing results, the exposure time of the initial printing layer should be prolonged several times compared to the subsequent layers' exposure time.This ensures a secure connection between the printed sample and the printing platform, mitigating the risk of printing failure even under stressful conditions.

Figure 5 .
Figure 5. Stress distribution diagram Figure 6 depicts the displacement and stress curves during the printing process of the boundary point of the central profile, which represents the point of maximum deformation.As observed in the figure, the two trends align closely due to the displacement induced by stress.Upon the emergence of a node, it undergoes deformation and displacement following the influence of stress post-exposure.As the number of printing layers increases, the rear printing layer intermittently exerts stress on the node, resulting in periodic changes in node displacement.Consequently, the curve displays zigzag fluctuations.Once these fluctuations reach a certain extent, they have minimal impact on the node

Figure 6 .
Figure 6.Displacement and stress curve at maximum point of boundary deformation in central section.

Figure 8 .Figure 9 .
Figure 8.Samples in parameter layer thickness 0.1mm, exposure intensity 80% and exposure time 2s.3.4Analysis of the influence of various parameters on tensile properties3.4.1 Influence of single factor on tensile properties of the sample.Figure9illustrates the impact of various factors, including exposure intensities, layer thicknesses, and exposure times, on the tensile strength of the sample[9].As observed in the figure, the tensile strength initially increases as these parameters are raised, reaching a peak value, after which it plateaus or declines.The maximum tensile strength across all the parameters is approximately 58.5 MPa.When other parameters remain constant, a low exposure light intensity, excessive layer thickness, or short exposure time results in weak light penetration.As indicated by the previously simulated temperature field results, the temperature diffuses layer by layer, with less energy being delivered by subsequent solidified layers.Consequently, the curing ability is diminished, leading to insufficient gel formation, weak resin bonding, and inadequate sample connection, ultimately resulting in reduced tensile strength.On the other hand, when the parameters are reversed, the resin's inherent reaction requires a certain amount of time, impeding the complete absorption of light energy.This incomplete absorption allows light to pass through, hindering the improvement of the gelation rate.Moreover, excessive exposure can interfere with the cured layer, leading to over-curing phenomena, which can slow down or diminish the overall strength of the sample.

Table 1 .
Orthogonal test scheme and results Number Layer thickness/mm Exposure intensity Exposure time/s Tensile strength/MPa