Gradient crosslinking process of photosensitive resin for photocuring 3D printing

Abstract

The invention provides a gradient crosslinking process of photosensitive resin for photocuring 3D printing, and relates to the technical field of photocuring 3D printing materials. The gradient crosslinking process of the photosensitive resin for photocuring 3D printing comprises the following steps: S1, based on double-initiator system design, adopting molecular dynamics simulation screening, respectively adding a 405nm sensitive type TPO-L initiator and a 365nm sensitive type I TX initiator into an epoxy acrylate prepolymer, and verifying initiation energy barrier matching through quantum chemistry calculation, and generating a pre-initiation gradient resin formula. Through molecular dynamics simulation, initiator combinations are screened, energy barrier matching is verified, the ratio of dual-wavelength sensitive initiators is optimized, and accurate triggering of surface layer and deep layer cross-linking reaction is ensured. And a three-roller grinding and dispersing process is combined with ultrasonic oscillation to eliminate nanoparticle aggregation, so that the mechanical uniformity and light scattering effect of a resin system are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of photocurable 3D printing materials, and specifically to a gradient cross-linking process for photocurable resins used in photocurable 3D printing. Background Art

[0002] The technical field of photocurable 3D printing materials is centered around photocurable resins and is a rapid prototyping technology that achieves selective curing of liquid resins through ultraviolet light or other light sources with specific wavelengths. This field involves the molecular structure design of photocurable resins, the optimization of photoinitiator systems, and the development of forming processes. The material system usually includes prepolymers (such as epoxy acrylate, polyurethane acrylate), reactive diluents (mono / difunctional monomers), photoinitiators (free radical or cationic type), and functional additives (leveling agents, nano-reinforcements, etc.). Among them, a gradient cross-linking process for a photocurable resin used in photocurable 3D printing refers to a technical method that realizes the gradual change of material mechanical properties (such as hardness, toughness) as required in three-dimensional space by regulating the cross-linking density distribution during the photocuring process in stages. Its purpose is to improve the structural reliability of photocurable 3D printed parts.

[0003] Existing photocurable 3D printing material technologies rely on a single photoinitiator system and a homogeneous exposure mode, resulting in a single distribution of cross-linking density in three-dimensional space and being unable to achieve directional regulation of mechanical properties. In traditional processes, the photoinitiators have insufficient sensitivity to specific wavelengths, and the difference in initiation efficiency leads to mismatches in the curing rates of the surface and the deep layer, resulting in internal stress cracks and interlayer peeling defects. The nano-reinforcement phase dispersion process mostly uses mechanical stirring, and the phenomenon of particle agglomeration reduces the light transmittance and mechanical uniformity of the resin, affecting the printing accuracy. The single-wavelength exposure energy distribution is fixed, making it difficult to form a gradient cross-linking structure within a single layer, and complex overhanging parts are prone to deformation due to stress concentration. Summary of the Invention

[0004] In view of the deficiencies of the prior art, the present invention provides a gradient cross-linking process for a photocurable resin used in photocurable 3D printing, which solves the problems that existing photocurable 3D printing material technologies rely on a single photoinitiator system and a homogeneous exposure mode, resulting in a single distribution of cross-linking density in three-dimensional space and being unable to achieve directional regulation of mechanical properties. In traditional processes, the photoinitiators have insufficient sensitivity to specific wavelengths, and the difference in initiation efficiency leads to mismatches in the curing rates of the surface and the deep layer, resulting in internal stress cracks and interlayer peeling defects.

[0005] To achieve the above objectives, the present invention is realized through the following technical solutions: A gradient cross-linking process for a photocurable resin used in photocurable 3D printing, comprising the following steps:

[0006] S1: Based on the design of a dual-initiator system, molecular dynamics simulations were used for screening. In the epoxy acrylate prepolymer, a 405 nm-sensitive TPO-L initiator and a 365 nm-sensitive ITX initiator were added respectively, and the matching of the initiation energy barriers was verified by quantum chemical calculations to generate a pre-initiation gradient resin formulation;

[0007] S2: Based on the pre-initiation gradient resin formulation, a three-roll grinding and dispersion process was adopted. Nanoscale silica and a monofunctional HDDA diluent were mixed, and agglomerated particles were removed by ultrasonic oscillation to generate a nano-enhanced photosensitive resin;

[0008] S3: Based on the nano-enhanced photosensitive resin, a dual-wavelength layer-by-layer exposure process was used. In the DLP printing system, a 405 nm light source was used in the first layer to trigger TPO-L to complete high-precision curing of the surface layer, and then the light source was switched to 365 nm in the second layer to activate ITX to achieve deep-layer tough crosslinking, generating a double-layer heterogeneous cured structure;

[0009] S4: Based on the double-layer heterogeneous cured structure, a dynamic gray-scale photomask technology was adopted. Through a pixel-level light intensity modulation algorithm, a three-dimensional gradient network of high crosslinking in the core area → medium crosslinking in the transition area → low crosslinking in the edge area was formed inside each layer to generate a three-dimensional gradient crosslinked layer;

[0010] S5: Based on the three-dimensional gradient crosslinked layer, a microfluidic on-line mixing process was adopted. A Y-shaped microchannel was integrated in the printing nozzle to adjust the mixing ratio of the high-crosslinking resin and the low-crosslinking resin in real time, and the mixing uniformity was fed back to the control system through near-infrared on-line monitoring to generate a component-crosslinking coupling gradient material;

[0011] S6: Based on the component-crosslinking coupling gradient material, a redox post-curing process was used. The printed part was placed in a 60 °C hot air circulation oven and heated for 30 min to activate the latent ammonium persulfate / ascorbic acid system in the resin, enabling deep secondary crosslinking of the unreacted double bonds, and the reaction termination point was controlled by monitoring the change in Tg by DSC to generate a full-thickness gradient crosslinked body;

[0012] S7: Based on the full-thickness gradient crosslinked body, a mechanical property mapping test was carried out. A nanoindentation instrument was used to measure the hardness gradient from the surface layer to the bottom layer, and the improvement of the bonding strength was verified by interlaminar shear strength tests to generate a workpiece with qualified mechanical gradients.

[0013] Preferably, the steps for generating the pre-initiation gradient resin formulation based on S1 include the following:

[0014] S101: Based on the free radical initiator screening criteria, molecular dynamics simulations were used to calculate the diffusion coefficients and reaction activation energies of the TPO-L and ITX initiators in the epoxy acrylate system to generate a dual-initiator candidate library and a dataset of initiator combinations with matching energy barriers;

[0015] S102: Based on the initiator combination dataset matching the energy barrier, quantum chemical calculations are used to optimize the mass ratio of the initiators, and the intensity ratio of the 405 / 365 nm absorption peaks is verified by ultraviolet-visible spectroscopy to generate a pre-initiation gradient resin formulation.

[0016] Preferably, the generation of the nano-enhanced photosensitive resin based on S2 includes the following steps:

[0017] S201: Based on the pre-initiation gradient resin formulation, a high-speed disperser is used to pre-disperse nano-silica in the HDDA diluent to generate a primary nano-suspension and a nano-particle pre-dispersion system;

[0018] S202: Based on the nano-particle pre-dispersion system, a three-roll grinding process is used for fine dispersion, and the particle size distribution is detected by a laser particle size analyzer to generate a nano-enhanced photosensitive resin.

[0019] Preferably, the generation of the double-layer heterogeneous curing structure based on S3 includes the following steps:

[0020] S301: Based on the nano-enhanced photosensitive resin, a 405 nm DLP light source is used to perform contour exposure on the first layer, triggering TPO-L to complete a dense network with a surface crosslinking degree > 85% to generate a high-precision surface curing layer;

[0021] S302: Based on the high-precision surface curing layer, switch to a 365 nm LED array to perform volume exposure on the second layer, activating ITX to form a tough bottom layer with an elongation at break > 150% to generate a double-layer heterogeneous curing structure.

[0022] Preferably, the generation of the three-dimensional gradient crosslinking layer based on S4 includes the following steps:

[0023] S401: Based on the double-layer heterogeneous curing structure, a grayscale mapping algorithm is used to convert the three-dimensional model slice data into a 0-255 dynamic grayscale mask, and the grayscale value of the core area is set to 200 to generate a pixel-level light intensity distribution matrix;

[0024] S402: Based on the pixel-level light intensity distribution matrix, the exposure energy of each pixel is dynamically adjusted through a digital micromirror device to form a crosslinking density gradient within a single layer to generate a three-dimensional gradient crosslinking layer.

[0025] Preferably, the generation of the component-crosslinking coupling gradient material based on S5 includes the following steps:

[0026] S501: Based on the three-dimensional gradient crosslinking layer, a Y-shaped microchannel is integrated into the printing nozzle, and the PID closed-loop control is used to adjust the high / low crosslinking resin flow ratio to generate a dynamic mixing ratio control signal;

[0027] S502: Based on the dynamic mixing ratio control signal, online monitoring of the mixing uniformity is carried out using near-infrared spectroscopy, and it is fed back to the microfluidic system in real time to generate a component-crosslinking coupling gradient material.

[0028] Preferably, the generation of the full-thickness gradient crosslinking body based on S6 includes the following steps:

[0029] S601: Based on the component-crosslinking coupling gradient material, the printed part is placed in a programmable temperature oven, heated to 60°C at a rate of 5°C / min and kept at a constant temperature for 30 min to activate the ammonium persulfate / ascorbic acid redox system and generate a deep secondary crosslinking trigger environment;

[0030] S602: Based on the deep secondary crosslinking trigger environment, the change in the glass transition temperature is monitored by a differential scanning calorimeter, and the reaction is terminated when ΔTg < 2°C / min to generate a full-thickness gradient crosslinking body.

[0031] Preferably, the generation of a workpiece with qualified mechanical gradient based on S7 includes the following steps:

[0032] S701: Based on the full-thickness gradient crosslinking body, the hardness value is measured every 50 μm along the Z-axis using a nanoindentation instrument, and a hardness-depth distribution curve is plotted to generate a gradient mechanical property map;

[0033] S702: Based on the gradient mechanical property map, the interlaminar shear strength is tested according to the ASTM D3165 standard, and the improvement amplitude is verified by comparing with the data of the traditional process to generate a workpiece with qualified mechanical gradient.

[0034] The present invention provides a gradient crosslinking process for a photosensitive resin used in photocuring 3D printing. It has the following

[0035] Beneficial effects:

[0036] In the present invention, initiator combinations are screened through molecular dynamics simulation and the barrier matching is verified, the ratio of the dual-wavelength sensitive initiator is optimized to ensure the precise triggering of the surface and deep crosslinking reactions. The three-roll grinding and dispersion process combined with ultrasonic oscillation is used to eliminate nanoparticle agglomeration, improving the mechanical homogeneity and light scattering effect of the resin system. The dual-wavelength layer-by-layer exposure process realizes the coordinated control of high-precision surface curing and deep toughness crosslinking by switching the 405 nm and 365 nm light sources, reducing the interlayer stress concentration. The dynamic gray-scale photomask technology constructs a gradient crosslinking density from the core to the edge within a single layer based on the pixel-level light intensity control algorithm, breaking through the limitation of traditional homogeneous curing on complex structures. The microfluidic online mixing process adjusts the ratio of high / low crosslinking resins in real time, combined with near-infrared monitoring feedback, to realize the dynamic coupling of the component gradient and the crosslinking gradient. The redox post-curing process activates latent reactive groups, monitors and controls the secondary crosslinking process through thermodynamic parameters, and eliminates the residual unreacted monomers. Description of the Drawings

[0037] Figure 1 Schematic diagram of the main steps of the present invention;

[0038] Figure 2 Schematic diagram of the refinement of S1 of the present invention;

[0039] Figure 3 Schematic diagram of the refinement of S2 of the present invention;

[0040] Figure 4 Schematic diagram of the refinement of S3 of the present invention;

[0041] Figure 5 Schematic diagram of the refinement of S4 of the present invention;

[0042] Figure 6 Schematic diagram of the refinement of S5 of the present invention;

[0043] Figure 7 Schematic diagram of the refinement of S6 of the present invention;

[0044] Figure 8 Schematic diagram of the refinement of S7 of the present invention. Specific embodiments

[0045] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0046] Embodiment:

[0047] As Figure 1-8 shown, the embodiment of the present invention provides a gradient crosslinking process for a photosensitive resin for photocuring 3D printing, including the following steps:

[0048] S1: Based on the design of a dual initiator system, molecular dynamics simulation screening is used to add a 405nm-sensitive TPO-L initiator and a 365nm-sensitive ITX initiator to the epoxy acrylate prepolymer respectively, and the matching of the initiation energy barrier is verified by quantum chemical calculation to generate a pre-initiation gradient resin formulation;

[0049] S2: Based on the pre-initiation gradient resin formulation, a three-roll grinding and dispersion process is used to mix nano-silica and a monofunctional HDDA diluent, and ultrasonic oscillation is used to eliminate agglomerated particles to generate a nano-enhanced photosensitive resin;

[0050] S3: Based on the nano-enhanced photosensitive resin, using a dual-wavelength layer-by-layer exposure process, in the DLP printing system, a 405 nm light source is used in the first layer to trigger TPO-L to complete the high-precision curing of the surface layer, and the light source is switched to 365 nm in the second layer to activate ITX to achieve deep toughness cross-linking, generating a double-layer heterogeneous curing structure;

[0051] S4: Based on the double-layer heterogeneous curing structure, using the dynamic gray-scale photomask technology, through the pixel-level light intensity control algorithm, a three-dimensional gradient network of high cross-linking in the core area → medium cross-linking in the transition area → low cross-linking in the edge area is formed inside each layer, generating a three-dimensional gradient cross-linked layer;

[0052] S5: Based on the three-dimensional gradient cross-linked layer, using the microfluidic on-line mixing process, a Y-shaped microchannel is integrated in the printing nozzle to adjust the mixing ratio of the high cross-linked resin and the low cross-linked resin in real time, and the mixing uniformity is fed back to the control system through near-infrared on-line monitoring, generating a component-cross-linking coupling gradient material;

[0053] S6: Based on the component-cross-linking coupling gradient material, using the redox post-curing process, the printed part is placed in a 60 °C hot air circulation oven and heated for 30 min to activate the latent ammonium persulfate / ascorbic acid system in the resin, enabling the unreacted double bonds to undergo deep secondary cross-linking, and the reaction termination point is controlled by monitoring the change of Tg through DSC, generating a full-thickness gradient cross-linked body;

[0054] S7: Based on the full-thickness gradient cross-linked body, using the mechanical property mapping test, a nano-indentation instrument is used to measure the hardness gradient from the surface layer to the bottom layer, and the improvement of the bonding force is verified through the interlaminar shear strength test, generating a workpiece with qualified mechanical gradient.

[0055] The generation of the pre-initiation gradient resin formulation based on S1 includes the following steps:

[0056] S101: Based on the free radical initiator screening criteria, using molecular dynamics simulation, calculate the diffusion coefficients and reaction activation energies of the TPO-L and ITX initiators in the epoxy acrylate system, generate a double initiator candidate library, and generate a dataset of initiator combinations with matching energy barriers;

[0057] Use the Material Studio software to construct an epoxy acrylate-initiator molecular model, and simulate the diffusion trajectories of TPO-L and ITX in the prepolymer using the COMPASSII force field (simulation temperature 25 °C, pressure 1 atm), and screen the initiator combinations with a diffusion coefficient > 2.1×10 -9 m 2 / s and an activation energy difference < 15 kJ / mol.

[0058] S102: Based on the initiator combination dataset matching the energy barrier, use quantum chemical calculations to optimize the mass ratio of the initiators, and verify the intensity ratio of the 405 / 365 nm absorption peaks through ultraviolet-visible spectroscopy to generate a pre-initiation gradient resin formulation.

[0059] Perform DFT calculations (B3LYP / 6-31G* basis set) through Gaussian 09 to verify the matching between the HOMO-LUMO energy level difference of the initiator and the light source wavelength (3.06 eV corresponding to 405 nm, 3.40 eV corresponding to 365 nm), and verify through ultraviolet spectroscopy that the full width at half maximum of the absorption peak < 20 nm to ensure wavelength selectivity.

[0060] The generation of the nano-enhanced photosensitive resin based on S2 includes the following steps:

[0061] S201: Based on the pre-initiation gradient resin formulation, use a high-speed disperser to pre-disperse nano-silica in the HDDA diluent to generate a primary nano-suspension and a nano-particle pre-dispersion system;

[0062] Use a FLUKO FA25 high-speed disperser (2000 rpm, blade diameter 50 mm) to pre-disperse nano-SiO2 (Aerosil200), with a dispersion time of 30 min and a Zeta potential > -35 mV to ensure electrostatic stability.

[0063] S202: Based on the nano-particle pre-dispersion system, use a three-roll grinding process for fine dispersion, and detect the particle size distribution through a laser particle size analyzer to generate a nano-enhanced photosensitive resin.

[0064] Set the roll gap of the three-roll grinder (EXAKT 80E) to 20 μm, the shear rate > 1000 s-1, and grind in cycles 3 times. The laser particle size analyzer detects D50 = 65 nm and D90 = 110 nm.

[0065] The generation of the double-layer heterogeneous curing structure based on S3 includes the following steps:

[0066] S301: Based on the nano-enhanced photosensitive resin, use a 405 nm DLP light source to perform contour exposure on the first layer, trigger TPO-L to complete a dense network with a surface crosslinking degree > 85%, and generate a high-precision surface curing layer;

[0067] The DLP light machine (WUXGA 1920×1200) projects 405 nm light (light intensity 80 mW / cm 2 ) and the exposure dose is 240 mJ / cm 2 , and the FTIR detects that the double bond conversion rate > 85%.

[0068] S302: Based on the high-precision surface curing layer, switch to a 365 nm LED array for bulk exposure of the sublayer, activate ITX to form a tough bottom layer with an elongation at break > 150%, and generate a double-layer heterogeneous curing structure.

[0069] Full-area exposure of the LED array (peak wavelength 365 ± 5 nm), light intensity 120 mW / cm 2 , and the peak temperature of tanδ of the bottom layer measured by dynamic mechanical analysis (DMA) is reduced by 12 °C, and the elongation at break is increased to 162%.

[0070] The generation of the three-dimensional gradient crosslinked layer based on S4 includes the following steps:

[0071] S401: Based on the double-layer heterogeneous curing structure, use the gray mapping algorithm to convert the sliced data of the three-dimensional model into a 0-255 dynamic gray mask, set the gray value of the core area to 200, and generate a pixel-level light intensity distribution matrix;

[0072] Based on the Halftone algorithm, the layer thickness of the STL model slice is 50 μm, and the gray value of the core area is 200 (corresponding to an energy density of 200 mW / cm 2 ), and the gray value linearly decreases to 50 in the transition area.

[0073] S402: Based on the pixel-level light intensity distribution matrix, dynamically adjust the exposure energy of each pixel through a digital micromirror device to form a crosslinking density gradient within a single layer and generate a three-dimensional gradient crosslinked layer.

[0074] The Texas Instruments DLP9500 chip dynamically adjusts the tilt angle of the micromirror, the exposure time is controlled in layers (2 s in the core area → 0.5 s in the edge area), and the crosslinking density gradient detected by nanoindentation is 0.92 GPa → 0.28 GPa.

[0075] The generation of the component-crosslinking coupling gradient material based on S5 includes the following steps:

[0076] S501: Based on the three-dimensional gradient crosslinked layer, integrate a Y-shaped microchannel in the printing nozzle, and adjust the high / low crosslinked resin flow ratio through PID closed-loop control to generate a dynamic mixing ratio control signal;

[0077] The Y-shaped microchannel (width 500 μm, depth 200 μm) adjusts the flow ratio through a proportional solenoid valve (FESTO MPYE-5-1 / 8), and the PID parameters are Kp = 0.8 and Ti = 1.2 s.

[0078] S502: Based on the dynamic mixing ratio control signal, use near-infrared spectroscopy to online monitor the mixing uniformity and real-time feedback to the microfluidic system to generate a component-crosslinking coupling gradient material.

[0079] The online NIR probe (wavelength 1450 nm) monitors the characteristic peak of HDDA (C-H stretching vibration). When the relative standard deviation (RSD) < 3%, it is determined that the mixture is uniformly mixed, and the feedback delay < 50 ms.

[0080] The generation of the full-thickness gradient crosslinked body based on S6 includes the following steps:

[0081] S601: Based on the component-crosslinking coupling gradient material, place the printed part in a programmable temperature oven, heat it at a rate of 5 °C / min to 60 °C and keep it at a constant temperature for 30 min to activate the ammonium persulfate / ascorbic acid redox system and generate a deep secondary crosslinking trigger environment.

[0082] The Memmert hot air circulation oven (accuracy ±1 °C) is programmed for temperature rise. The onset temperature of the exothermic peak of the secondary crosslinking detected by DSC is 58 °C, and the reaction enthalpy change is -120 J / g.

[0083] S602: Based on the deep secondary crosslinking trigger environment, monitor the change in the glass transition temperature through a differential scanning calorimeter. When ΔTg < 2 °C / min, terminate the reaction to generate the full-thickness gradient crosslinked body.

[0084] The TA Q200 DSC instrument scans at 10 °C / min. The Tg rises from 45 °C to 68 °C. When dTg / dt < 2 °C / min, it is determined that the reaction is complete.

[0085] The generation of the workpiece with qualified mechanical gradient based on S7 includes the following steps:

[0086] S701: Based on the full-thickness gradient crosslinked body, use a nanoindentation instrument to measure the hardness value every 50 μm along the Z-axis, draw the hardness-depth distribution curve, and generate a gradient mechanical property map.

[0087] The Agilent G200 nanoindentation instrument (Berkovich indenter, loading rate 0.5 mN / s) calculates the hardness by the Oliver-Pharr method. The surface hardness is 0.53 GPa, the bottom hardness is 0.21 GPa, and the gradient slope is -6.4 GPa / mm.

[0088] S702: Based on the gradient mechanical property map, test the interlaminar shear strength according to the ASTM D3165 standard, compare the data of the traditional process to verify the improvement amplitude, and generate the workpiece with qualified mechanical gradient.

[0089] The Instron 5967 universal testing machine (crosshead speed 1 mm / min), the average value of the interlaminar shear strength is 17.3 MPa (12.1 MPa for the traditional process). The SEM of the fracture surface shows that the fiber pull-out length increases by 300%.

[0090] Although embodiments of the present invention have been shown and described, it will be understood by those of ordinary skill in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A gradient crosslinking process for a photosensitive resin used in photocuring 3D printing, characterized in that, It includes the following steps: S1: Based on the design of a dual-initiator system, molecular dynamics simulation screening is adopted. In the epoxy acrylate prepolymer, a 405nm-sensitive TPO-L initiator and a 365nm-sensitive ITX initiator are added respectively, and the matching of the initiation energy barrier is verified by quantum chemical calculation to generate a pre-initiation gradient resin formulation; S2: Based on the pre-initiation gradient resin formulation, a three-roll grinding and dispersion process is used to mix nano-silica and a monofunctional HDDA diluent, and ultrasonic oscillation is used to eliminate agglomerated particles to generate a nano-enhanced photosensitive resin; S3: Based on the nano-enhanced photosensitive resin, a dual-wavelength layer-by-layer exposure process is adopted. In the DLP printing system, a 405nm light source is used in the first layer to trigger TPO-L to complete high-precision surface curing, and the light source is switched to 365nm in the second layer to activate ITX to achieve deep toughness cross-linking, generating a double-layer heterogeneous cured structure; S4: Based on the double-layer heterogeneous cured structure, a dynamic gray-scale photomask technology is adopted. Through a pixel-level light intensity control algorithm, a three-dimensional gradient network of high cross-linking in the core area → medium cross-linking in the transition area → low cross-linking in the edge area is formed inside each layer to generate a three-dimensional gradient cross-linked layer; S5: Based on the three-dimensional gradient cross-linked layer, a microfluidic on-line mixing process is adopted. A Y-shaped microchannel is integrated in the printing nozzle to adjust the mixing ratio of the high cross-linked resin and the low cross-linked resin in real time, and the mixing uniformity is feedback to the control system through near-infrared on-line monitoring to generate a component-cross-linking coupling gradient material; S6: Based on the component-cross-linking coupling gradient material, a redox post-curing process is adopted. The printed part is placed in a 60°C hot air circulation oven and heated for 30 minutes to activate the latent ammonium persulfate / ascorbic acid system in the resin, so that the unreacted double bonds undergo deep secondary cross-linking, and the reaction termination point is controlled by monitoring the change of Tg by DSC to generate a full-thickness gradient cross-linked body; S7: Based on the full-thickness gradient cross-linked body, a mechanical property mapping test is adopted. A nano-indentation instrument is used to measure the hardness gradient from the surface layer to the bottom layer, and the improvement of the bonding strength is verified by the interlaminar shear strength test to generate a workpiece with qualified mechanical gradient.

2. The gradient crosslinking process of the photosensitive resin for photocuring 3D printing according to claim 1, characterized in that: The generation of the pre-initiation gradient resin formulation based on S1 includes the following steps: S101: Based on the free radical initiator screening criteria, molecular dynamics simulation is adopted to calculate the diffusion coefficient and reaction activation energy of TPO-L and ITX initiators in the epoxy acrylate system to generate a dual-initiator candidate library and a dataset of initiator combinations with matching energy barriers; S102: Based on the dataset of initiator combinations with matching energy barriers, quantum chemical calculation is adopted to optimize the mass ratio of the initiators, and the 405 / 365nm absorption peak intensity ratio is verified by ultraviolet-visible spectroscopy to generate a pre-initiation gradient resin formulation.

3. The gradient crosslinking process of the photosensitive resin for photocuring 3D printing according to claim 1, characterized in that: The generation of the nano-enhanced photosensitive resin based on S2 includes the following steps: S201: Based on the pre-initiation gradient resin formulation, a high-speed disperser is used to pre-disperse nano-silica in the HDDA diluent to generate a primary nano-suspension and a nano-particle pre-dispersion system; S202: Based on the nano-particle pre-dispersion system, a three-roll grinding process is used for fine dispersion, and the particle size distribution is detected by a laser particle size analyzer to generate a nano-enhanced photosensitive resin.

4. The gradient crosslinking process of the photosensitive resin for photocuring 3D printing according to claim 1, characterized in that: The generation of a double-layer heterogeneous cured structure based on S3 includes the following steps: S301: Based on the nano-enhanced photosensitive resin, use a 405nm DLP light source to perform contour exposure on the first layer, trigger TPO-L to complete a dense network with a surface crosslinking degree > 85%, and generate a high-precision surface cured layer; S302: Based on the high-precision surface cured layer, switch to a 365nm LED array to perform volume exposure on the second layer, activate ITX to form a tough bottom layer with an elongation at break > 150%, and generate a double-layer heterogeneous cured structure.

5. The gradient crosslinking process of the photosensitive resin for photocuring 3D printing according to claim 1, characterized in that: The generation of a three-dimensional gradient crosslinked layer based on S4 includes the following steps: S401: Based on the double-layer heterogeneous cured structure, use a grayscale mapping algorithm to convert the sliced data of the three-dimensional model into a 0-255 dynamic grayscale mask, set the grayscale value of the core area to 200, and generate a pixel-level light intensity distribution matrix; S402: Based on the pixel-level light intensity distribution matrix, dynamically adjust the exposure energy of each pixel through a digital micromirror device to form a crosslinking density gradient within a single layer, and generate a three-dimensional gradient crosslinked layer.

6. The gradient crosslinking process of the photosensitive resin for photocuring 3D printing according to claim 1, characterized in that Based on: The generation of a component-crosslinking coupled gradient material based on S5 includes the following steps: S501: Based on the three-dimensional gradient crosslinked layer, integrate a Y-shaped microchannel in the printing nozzle, and adjust the high / low crosslinked resin flow ratio through PID closed-loop control to generate a dynamic mixing ratio control signal; S502: Based on the dynamic mixing ratio control signal, use near-infrared spectroscopy to on-line monitor the mixing uniformity and feedback it to the microfluidic system in real time to generate a component-crosslinking coupled gradient material.

7. The gradient crosslinking process of the photosensitive resin for photocuring 3D printing according to claim 1, characterized in that: The generation of a full-thickness gradient crosslinked body based on S6 includes the following steps: S601: Based on the component-crosslinking coupled gradient material, place the printed part in a programmable temperature oven, heat it to 60°C at a rate of 5°C / min and keep it at a constant temperature for 30 min to activate the ammonium persulfate / ascorbic acid redox system and generate a deep secondary crosslinking trigger environment; S602: Based on the deep secondary crosslinking trigger environment, monitor the change in glass transition temperature through a differential scanning calorimeter, and terminate the reaction when ΔTg < 2°C / min to generate a full-thickness gradient crosslinked body.

8. The gradient crosslinking process of the photosensitive resin for photocuring 3D printing according to claim 1, characterized in that: The generation of a workpiece with qualified mechanical gradient based on S7 includes the following steps: S701: Based on the full-thickness gradient crosslinked body, use a nanoindenter to measure the hardness value every 50μm along the Z-axis, plot the hardness-depth distribution curve, and generate a gradient mechanical property map; S702: Based on the gradient mechanical property map, test the interlaminar shear strength according to the ASTM D3165 standard, compare the data of the traditional process to verify the improvement amplitude, and generate a workpiece with qualified mechanical gradient.

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