Process Analysis of DLP Photocuring 3D Printing with Modified High-strength Resin

The excellent properties of graphene oxide, along with its dispersibility and hydrophilicity, make it suitable for blending with other substrates to obtain reinforcements. In this paper, high-strength photosensitive resin is mixed with graphene oxide to obtain reinforced photosensitive resin. The impact of different mass fractions of graphene oxide on the appearance, dimensional accuracy, bending strength, tensile strength, and fracture morphology of printed samples is investigated using DLP surface molding 3D printing technology. As the mass fraction of graphene oxide increases, the size error of the printed sample decreases, while the bending strength and tensile strength increase, indicating a favorable bonding effect. However, when the mass fraction of graphene oxide exceeds 0.6%, the bonding between the two materials becomes poorer, leading to the opposite outcome. The hierarchical influence of different parameters on the tensile strength of mixed-material DLP printed samples can be summarized as follows: graphene oxide content exhibits the strongest influence, followed by layer thickness, exposure time, and exposure intensity.


Introduction
Compared with other processing methods, 3D printing technology offers advantages such as low cost, complex personalized modeling, and processing cycles [1][2][3] .It can be categorized into different methods based on the processes involved, including fused deposition modeling (FDM), selective laser sintering (SLS), laminated object manufacturing (LOM), stereo lithography appearance (SLA), and others [4][5][6] .With the development of technology, digital light processing (DLP) printing technology, which has a similar photocuring process to SLA printing, is getting more and more attention [7][8] .
The properties of 3D printing materials have a direct impact on the accuracy and mechanical properties of photocured parts [9] .In the conventional DLP printing process, pure photosensitive resin is generally used as the raw material [10] .This material is inferior to polymer material in terms of strength, rigidity, heat resistance, and other aspects [11] .Graphene, with its high specific surface area, strength, and modulus, exhibits favorable electrical and thermal properties [12] .Therefore, graphene and its derivatives are ideal candidates for enhancing the function of materials.In recent years, people have carried out relevant research on enhanced 3D printing.Che Jiangning et al. studied the process and properties of carbon fiber-reinforced DLP printing photosensitive resin and determined the best parameters suitable for printing and the strength variation rule [13] .Additionally, Zong Xuewen et al. carried out the process research on ceramic-reinforced photosensitive resin parts, studying the influence of various parameters on the shrinkage and hardness of the parts, and optimizing the process parameters

Test equipment, materials and methods
In this research, the Rayshape Shape 1 printer was employed for DLP printing of the samples.The cleaning and post-curing processes were conducted using the Shape Wash 020S ultrasonic cleaner and the ShapeCure curing drying oven, respectively.Figure 1 illustrates the principle and device utilized for DLP printing.The high-strength photosensitive resin served as the material for this experiment.Graphene oxide was prepared by improved Hummers, and then graphene oxide was prepared by ultrasonic stripping method [15] .The resin and graphene oxide were mixed, mechanically stirred for 1h, and then ultrasonically dispersed for 1h to obtain a uniform graphene-resin dispersion solution.
Unless specified, the experimental parameters adopted in this study were as follows: a layer thickness of 0.05 mm, an exposure light intensity of 90%, and an exposure time of 4 seconds.After completion of the printing process, the specimen was separated from the substrate, subjected to ultrasonic cleaning to remove excess resin on the surface, and subsequently dried.The dimensions of width (X direction) and thickness (Y direction) within the marking distance of the printed specimen were measured using a microscope.Each specimen was measured 10 times in the X direction or Y direction, and the average value was taken as the actual measurement value, compared with the set value, and the forming error of the printed specimen was calculated.After the test, the tensile and bending properties of the specimens were tested with a universal testing machine (INSTRON 5967).Three samples were tested for each set of parameters, and the results were averaged.The tensile fracture specimens were observed by scanning electron microscopy (ZEISS MERLIN Compact).

Morphology of the mixed resin and graphene oxide
The morphology of photosensitive resin mixed with different contents of graphene oxide is shown in Figure 2. As can be seen from the figure, when the content of graphene oxide is 0.2%, the entire color appears lighter, and the graphene oxide is sporadically distributed in the photosensitive resin.Upon increasing the graphene oxide content to 0.6%, the presence of graphene oxide in the photosensitive resin becomes more pronounced, resulting in a deeper color and a more uniform distribution.However, when the content of graphene oxide reaches 1%, the color turns grey-black, accompanied by agglomeration of graphene oxide.

Sample surface and section state
After the printing process, the appearance and cross-section of the tensile specimens were evaluated.Figure 3 illustrates the surface of the samples with increasing mass fractions of graphene oxide.It is evident from the below figure that all the samples exhibit a smooth external surface without any observed defects such as cracks.The surface of the samples is uniform and devoid of any uneven or excessive local roughness.These observations indicate that when the mass fraction of graphene oxide is equal to or less than 1% in the mixture with high-strength photosensitive resin, the samples exhibit a desirable surface appearance.Due to the brown-yellow color of graphene oxide, the color of the samples gradually transitions from light gray to dark gray as the mass fraction of graphene oxide increases.The color deepens consistently across all the samples, and there is no presence of impurities or defects, indicating an even distribution and successful fusion between graphene oxide and high-strength photosensitive resin.Figure 4 shows the cross-section state of the sample with an increasing mass fraction of graphene oxide.As observed in the figure below, all the samples exhibit satisfactory cross-sectional integrity without any flaws such as poor bonding or interlayer separation.This indicates that when the mass fraction of graphene oxide is equal to or less than 1% in the mixture with high-intensity photosensitive resin, the samples present a favorable cross-sectional state.Since the surface forming printing requires adhesion to the printing table and the samples are long, warping is inevitably observed.However, it can be noted from the figure that the degree of warping decreases with the increase in graphene oxide content.Analysis suggests that the photosensitive resin undergoes three-dimensional interweaving and experiences shrinkage during the curing process.The addition of graphene oxide serves to fill the pores resulting from the shrinkage of the photosensitive resin.Through the incorporation of high-strength graphene oxide, bonding crosslinking is formed with the surrounding photosensitive resin, which exerts a greater force of crosslinking compared to the shrinkage force during the resin's curing process.
Consequently, the presence of graphene oxide reduces warping caused by the curing shrinkage of the photosensitive resin.

Influence on the molding accuracy
By measuring and calculating the printed test piece, the forming error in the X direction and Y direction are obtained, as shown in the following Table 1.It can be seen from the table that with the increase of the content of graphene oxide, the error of the printed graphene oxide-photosensitive resin sample gradually decreases at first.When the content of graphene oxide is 0.6%, the error reaches the minimum, signifying the maximum level of accuracy.As the content of graphene oxide continues to increase, the error begins to increase.This phenomenon can be attributed to the formation of a densely intertwined three-dimensional structure by the photosensitive resin during the curing process, resulting in volume shrinkage and planar error.Graphene oxide, acting as a stable nanoparticle, fills the voids created by the shrinkage, thereby reducing shrinkage and error [16] .Nevertheless, beyond a graphene oxide content of 0.6%, the plane orientation error increases.This can primarily be attributed to the deterioration in the transmittance of the graphene oxide-photosensitive resin solution, leading to a reduced curing rate and subsequent increase in planar error.

Influence on the bending properties
Bending strength refers to the maximum stress that the material can bear when breaking or reaching the specified bending moment under the action of bending load [17] .This stress represents the maximum normal stress experienced during bending and is used as a measure of the material's bending performance [18] .Figure 5 illustrates the influence of different graphene oxide content on the bending strength of samples.It can be seen from the figure that the addition of graphene oxide improves the bending properties of the samples to varying degrees.When the content of graphene oxide is 0.2%, the bending strength of the sample increases by about 11.8%.Continue to increase the graphene oxide, between 0 and 0.6%.With the increase of graphene oxide content, the bending strength increases.However, beyond 0.6% graphene oxide content, the bending property of the sample starts to decline.Thus, it can be inferred that when the graphene oxide content is below 0.6%, increasing its content improves the degree of bonding with the photosensitive resin, and the enhanced stiffness of graphene increases the stiffness of the composite.However, when the content exceeds 0.6%, difficulties arise in dispersing graphene oxide in the photosensitive resin, resulting in agglomeration and weakened bonding between graphene oxide and the photosensitive resin, ultimately leading to a decrease in bending strength.These results significantly surpass those obtained by Wang Lufang, who used 2-hydroxyethyl methacrylate functionalized graphene oxide (HFGN)/free radical-initiated and cationic-initiated resin (FRCI), wherein the maximum bending strength achieved was 87.3 MPa, falling short of the strength achieved without the addition of graphene oxide in this study.The variation is primarily attributed to the different photosensitive resin materials utilized [19] .

Influence on the tensile properties
Tensile strength, defined as the maximum stress value a material can withstand before fracturing, serves as an indicator of the material's fracture resistance [20] .Figure 6 exhibits the impact of varying graphene oxide content on the tensile strength of the specimen.As observed in the figure, the tensile properties display a similar trend to bending, initially increasing and then decreasing with increasing graphene oxide content.The maximum tensile strength is achieved when the graphene oxide content reaches 0.6%.Given that tensile strength plays a crucial role in preventing material failure, and considering the superior strength of graphene oxide compared to photosensitive resin, the addition of graphene oxide significantly enhances the material's resistance to damage when combined with photosensitive resin.However, it should be noted that graphene oxide does not disperse infinitely in the photosensitive resin but instead aggregates after reaching a certain concentration.As a result, the increasing inability of graphene oxide to disperse and combine leads to a reduction in material transmittance and a decrease in tensile strength for the specimen.

Tensile fracture analysis
The analysis of fracture morphology plays a crucial role in elucidating the causes, processes, and mechanisms of fractures [21] .Figure 7 illustrates the fracture surfaces of tensile samples consisting of pure photosensitive resin, as well as those with graphene oxide mass fractions of 0.6% and 1% respectively.In Figure 7(a), the fracture surface of the pure photosensitive resin exhibits a flat and smooth overall appearance, with a relatively simple morphology and no noticeable plastic deformation.This characteristic indicates a typical brittle fracture behavior.Conversely, Figure 7(b) reveals significant differences compared to Figure 7(a).The fracture surface exhibits prominent tearing edges, signifying a brittle fracture pattern.Additionally, wide and shallow dimples are observed at localized positions, suggesting characteristics of ductile fracture.Thus, a composite fracture mode combining both brittle and ductile features is observed when the graphene oxide content is 0.6%.The tear edges and dimples exhibit white and bright graphene oxide components, forming multiple folds and step-like tears.The uniform and irregular distribution of graphene oxide within the resin, alongside its anisotropic nature, facilitates strong contact and bonding with the resin matrix.During the tensile fracture process of the composite material, the graphene oxide exerts a pulling effect on the photosensitive resin, resulting in a coarser cross-sectional appearance and enhanced mechanical properties.Figure 7(c) also displays multiple tear edges and a flat surface, which corresponds to typical brittle fracture characteristics.However, since the content of graphene oxide has exceeded the dispersible limit within the photosensitive resin, the bonding between the two materials is weakened, affecting the curing transmittance.Consequently, there is a reduced presence of attached graphene oxide and weaker bonding, as indicated in Figure 7(c).

Orthogonal test analysis of the influence of each factor on tensile capacity
Given that tensile strength is the preeminent factor in the application of high-strength resin, orthogonal analysis is conducted to investigate the impact of DLP printing process parameters and graphene oxide content on tensile strength.The orthogonal test scheme and result analysis are shown in Table 2 below.The layer thickness signifies the thickness of each deposited layer during the printing process, while the exposure intensity and exposure time refer to the duration and intensity of light exposure for solidifying each layer.The tensile strength values were determined by averaging ten measurements.As shown in the table, the four factors selected are layer thickness, exposure light intensity, exposure time, and graphene oxide content, with three levels for each factor.The layer thickness options are 0.025mm, 0.05mm, and 0.1mm, the exposure light intensity options are 70%, 80%, and 90%, the exposure time options are 2s, 4s, and 6s, and the graphene oxide content options are 0%, 0.5%, and 1%.The four factors are put into the L9 (3 4 ) orthogonal table, and the results are analyzed by intuitive analysis.Kij (i=1~3, j=1~4) 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 [22] .A larger range value corresponds to a stronger influence.As presented in the table, the order of influence of the factors on tensile strength is graphene oxide content > layer thickness > exposure time > exposure light intensity.This implies that the impact of graphene oxide content on tensile strength outweighs that of the DLP printing process factors, necessitating attention in subsequent printing processes.It can also be seen from the table that the tensile strength of this material ranges from 40-65 MPa.From the perspective of tensile strength, it is equivalent to the strength of engineering plastics such as ABS, which is at a higher level in the light-curing resin.

Conclusions
In the study, we investigate the relevant processes involved in DLP printing of high-strength photosensitive resin reinforced with graphene oxide.Our findings reveal that a uniform distribution of graphene oxide within the resin matrix enhances the interfacial bonding strength.Furthermore, as the mass fraction of graphene oxide increases, the cross-section warpage of the printed mixed material sample decreases, resulting in reduced size error and improved bending strength and tensile strength.Specifically, when the content of graphene oxide reaches 0.6%, the tensile strength and bending strength of the mixed sample exhibit an increase of approximately 16.7% and 21.9%, respectively, compared to samples without graphene oxide reinforcement.However, when the mass fraction of graphene oxide exceeds 0.6%, the bonding state and light transmittance deteriorate, leading to a decline in both size and strength, thereby exhibiting an inverse correlation.The order of influence on the tensile strength of mixed material DLP printed mixed material samples is as follows: graphene oxide content > layer thickness > exposure time > exposure intensity.

Figure 3 .
Figure 3.Effect of graphene oxide content on sample appearance

Figure 4 .
Figure 4. Effect of graphene oxide content on sample cross-section

Figure 5 .
Figure 5.Effect of graphene oxide content on bending strength

Figure 6 .
Figure 6.Effect of graphene oxide content on tensile strength

Table 1 .
Error table in X and Y directions

Table 2 .
Orthogonal test scheme and results