Effects of post-curing duration on the mechanical properties of complex 3D printed geometrical parts.

This study aims to assess the efficacy of post-curing guidance supplied by 3D printing resin manufacturers. Current guidance applies generically to all geometries with the caveat that post-curing should be extended for 'large' or 'complex' geometries but specific guidance is not provided. Two vat-polymerisation 3D printers (Form3B, Figure 4 Standalone) were used to print test models in 6 biocompatible resins (Pro Black, Med White, Med Amber, Biomed Black, Biomed White, Biomed Amber). The test model is of a complex geometry whilst also housing ISO 527 test specimens in concentric layers. Two separate intervals of curing were applied (100%, 500% stated guidance) creating different curing treatments of the specimens throughout the model. Post processed test models were disassembled and pull testing performed on each of the specimens to assess the mechanical properties. The analysis showed that extending the curing duration had significant effects on the mechanical properties of some materials but not all. The layers of the model had a significant effect except for elongation at break for the Med Amber material. This research demonstrates that generic post-curing guidance regarding UV exposures is not sufficient to achieve homogenous material strength properties for complex geometries. Large variations in mechanical properties throughout the models suggest some material was not fully-cured. This raises a query if such materials as originally marketed as biocompatible are fully cured and therefore safe to use for medical applications involving complex geometries.

Micropillar with Radial Gradient Modulus Enables Robust Adhesion and Friction.

The gradient modulus in beetle setae plays a critical role in allowing it to stand and walk on natural surfaces. Mimicking beetle setae to create a modulus gradient in microscale, especially in the direction of setae radius, can achieve reliable contact and thus strong adhesion. However, it remains highly challenging to achieve modulus gradient along radial directions in setae-like structures. Here, polydimethylsiloxane (PDMS) micropillar with radial gradient modulus, (termed GM), is successfully constructed by making use of the polymerization inhibitor in the photosensitive resin template. GM gains adhesion up to 84 kPa, which is 2.3 and 4.7 times of soft homogeneous micropillars (SH) and hard homogeneous micropillars (HH), respectively. The radial gradient modulus facilitates contact formation on various surfaces and shifts stress concentration from contact perimeter to the center, resulting in adhesion enhancement. Meanwhile, GM achieves strong friction of 8.1 mN, which is 1.2 and 2.6 times of SH and HH, respectively. Moreover, GM possesses high robustness, maintaining strong adhesion and friction after 400 cycles of tests. The work here not only provides a robust structure for strong adhesion and friction, but also establishes a strategy to create modulus gradient at micron-scale.

Recent advances in lignin-based 3D printing materials: A mini-review.

With the growing global population and rapid economic development, the demand for energy and raw materials is increasing, and the supply of fossil resources as the main source of energy and raw materials has reached a critical juncture. However, our overexploitation and overconsumption of fossil resources have led to serious problems, including environmental pollution, climate change, and ecosystem destruction. In the face of these challenges, we must recognize the negative impacts of the shortage of fossil resources and actively seek sustainable alternative sources of energy and resources to protect our environment and sustainable development in the future. Three-dimensional (3D) printing, an additive manufacturing technology, has been used in many fields to manufacture complex and high-precision products. While traditional manufacturing processes typically produce large amounts of waste and emissions that are harmful to the environment, 3D printing is much more energy efficient compared to traditional manufacturing methods, which helps to lower energy costs and reduce reliance on non-renewable energy sources. The development of low-carbon and environmentally friendly 3D printing materials can help to reduce carbon emissions and environmental pollution and realize the goal of sustainable development. Lignin, as the second largest renewable green biomass resource after cellulose, has great potential for manufacturing low-carbon and environmentally friendly 3D printing materials. This review presents some recent studies on the applications of lignin and its derivatives in photo-curing 3D printing, including the preparation and performance of lignin-based photosensitive prepolymers, lignin-based reactive diluents, lignin-based photo-initiators, and lignin-based additive. This review also provides recent studies on the preparation and performance of lignin-based thermoplastic polymer for Fused Deposition Modeling (FDM) 3D printing. Finally, the future challenges and industrialization prospects of lignin-based 3D printing materials are discussed.

Progress in Photocurable 3D Printing of Photosensitive Polyurethane: A Review.

In recent years, as a class of advanced additive manufacturing (AM) technology, photocurable 3D printing has gained increasing attention. Based on its outstanding printing efficiency and molding accuracy, it is employed in various fields, such as industrial manufacturing, biomedical, soft robotics, electronic sensors. Photocurable 3D printing is a molding technology based on the principle of area-selective curing of photopolymerization reaction. At present, the main printing material suitable for this technology is the photosensitive resin, a composite mixture consisting of a photosensitive prepolymer, reactive monomer, photoinitiator, and other additives. As the technique research deepens and its application gets more developed, the design of printing materials suitable for different applications is becoming the hotspot. Specifically, these materials not only can be photocured but also have excellent properties, such as elasticity, tear resistance, fatigue resistance. Photosensitive polyurethanes can endow photocured resin with desirable performance due to their unique molecular structure including the inherent alternating soft and hard segments, and microphase separation. For this reason, this review summarizes and comments on the research and application progress of photocurable 3D printing of photosensitive polyurethanes, analyzing the advantages and shortcomings of this technology, also offering an outlook on this rapid development direction.

Low Dielectric Constant Photocurable Fluorinated Poly (Phthalazinone Ether) Ink with Excellent Mechanical Properties and Heat Resistance.

The photosensitive resins for 3D printing technology have been widely applied throughout the advanced communication field due to their merits of high molding accuracy and fast processing speed. Regardless, they, in particular, should have better mechanical properties, heat resistance, and dielectric properties. Herein, photocurable fluorinated poly (phthalazinone ether) (FSt-FPPE) was utilized as a prepolymer to improve the performance of photosensitive resin. A series of UV-curable inks named FST/DPGs were prepared with FSt-FPPE and acrylic diluents of different mass fractions. The FST/DPGs were cured into films by UV curing and post-treatment. After curing, their properties were characterized in detail. In terms of heat resistance, glass transition temperature (Tg) could reach 233 °C and the 5% thermal decomposition temperature (Td5%) was 371 °C. The tensile strength surprisingly reached 61.5 MPa, and the dielectric constant (Dk) could be significantly reduced to 2.75. Additionally, FST/DPGs were successfully employed in UV-assisted direct writing (DIW) to print 3D objects that benefited from their commendable fluidity and rapid curing speed. A stiff cylinder sample with a smooth surface and distinct pattern was ultimately obtained, indicating their remarkable 3D printing adaptation. Such photosensitive resin for UV-assisted DIW exhibits tremendous potential in the electronic industry.

Experimental Validation of Diffraction Lithography for Fabrication of Solid Microneedles.

Microneedles are highly sought after for medicinal and cosmetic applications. However, the current manufacturing process for microneedles remains complicated, hindering its applicability to a broader variety of applications. As diffraction lithography has been recently reported as a simple method for fabricating solid microneedles, this paper presents the experimental validation of the use of ultraviolet light diffraction to control the liquid-to-solid transition of photosensitive resin to define the microneedle shape. The shapes of the resultant microneedles were investigated utilizing the primary experimental parameters including the photopattern size, ultraviolet light intensity, and the exposure time. Our fabrication results indicated that the fabricated microneedles became taller and larger in general when the experimental parameters were increased. Additionally, our investigation revealed four unique crosslinked resin morphologies during the first growth of the microneedle: microlens, first harmonic, first bell-tip, and second harmonic shapes. Additionally, by tilting the light exposure direction, a novel inclined microneedle array was fabricated for the first time. The fabricated microneedles were characterized with skin insertion and force-displacement tests. This experimental study enables the shapes and mechanical properties of the microneedles to be predicted in advance for mass production and wide practical use for biomedical or cosmetic applications.

3D Printing of Ultrathin MXene toward Tough and Thermally Resistant Nanocomposites.

Liquid crystal display (LCD)-based 3D printing, a facile and cost-effective manufacturing technique, is often applied when fabricating objects with porcelain structures using photosensitive resins (PSRs). Currently, 3D printed constructions are typically used as models for demonstration purposes rather than industrial applications because of their poor performance. In this study, we prepared nanocomposites by incorporating Ti3C2 MXene nanosheets to enhance the overall characteristics of a PSR, including mechanical properties and thermal resistance. Notably, the designed nanocomposites showed optimum performance at an MXene loading of 0.5% w/w. The mechanical properties of the designed nanocomposites confirmed the enhanced ultimate tensile and flexural strengths (by 32.1% and 42.7%, respectively), at 0.5% w/w MXene loading. Moreover, the incorporated MXene presented no substantial influence on the toughness of the PSR. The glass transition and thermal degradation temperatures at 5% weight loss increased by 7.4 and 10.6 °C, respectively, resulting predominantly from the hydrogen bonding between the PSR and MXene. Together, the experimental results indicate that the designed PSR/MXene nanocomposites are expected to replace pristine resins for LCD printing in various practical applications.

Influence of Exposure Period and Angle Alteration on the Flexural Resilience and Mechanical Attributes of Photosensitive Resin.

Despite the large number of studies addressing the effect of acrylic resin polymerization concerning flexural properties, limited research has been conducted on the manufacturing impact on a polymer's mechanical properties. Photosensitive resinous materials are used in various engineering applications where they may be exposed to multiple detrimental environments during their lifetime. Therefore, there is a need to understand the impact of an environment on the service life of resins. Thus, flexural tests were conducted to study the effects of exposure time and angle on the flexural strength of resins. Herein, the main objective was to explore the strength, stability, and flexural durability of photosensitive resin (EPIC-2000ST) fabricated at different exposure times (E) and angle deviation varying from 0° to 85° with a 5° increment. The samples in circular rings were manufactured and divided into five groups according to their exposure time (E): 10 s, 20 s, 30 s, 40 s, and 50 s. In each exposure time, we designed rings via SolidWorks software and experimentally fabricated at different oblique angles (OA) varying from 0° to 85° with a 5° increment during each fabrication, i.e., OA = 0°, 5°, 10°, 15°, 20°, 25°, 30°, 35°, 40°, 45°, 50°, 55°, 60°, 65°, 70°, 75°, 80°, and 85°. Flexural strength was evaluated using a three-point bending test. Optical electron microscopy was used to examines the samples' exterior, interior, and ruptured surfaces. Our experimental analysis shows that flexural strength was significantly enhanced by increasing exposure time and at higher oblique angles. However, at lower angles and less exposure time, mechanical flexural resilience declines.

Synthesis and Optimization of a Free-Radical/Cationic Hybrid Photosensitive UV Curable Resin Using Polyurethane Acrylate and Graphene Oxide.

Cost-effective, practical, and efficiently performing photosensitive resin composite materials are essential, as the current materials are expensive, lack better alternatives, and do not meet 3D printing standards. In this study, based on orthogonal experiments for photosensitive resin curing, we prepared a free-radical/cationic hybrid photosensitive UV cured resin (UVR) using acrylic ester and epoxy resin as the prepolymers, tripropylenediol diacrylate (TPGDA) as the active diluent, and triaryl sulfonium salt (I-160) and 2,2-dimethyl-α-hydroxy acetophenone (1173) as the photoinitiators, in the optimized formula of acrylic-ester:epoxy-resin:TPGDA:I-160:1173 = 37.5:37.5:20:2.5:2.5. Further, we investigated the effects of polyurethane acrylates (PUA) and Graphene oxide (GO) on the surface morphology, chemical structure, hydrophobicity, mechanical strength, and gelation rate of the hybrid resin. We observed that 20% PUA improved tensile strength to the maximum of 36.89 MPa from 16.42 MPa of the unmodified hybrid resin, whereas 1% GO reduced volume shrinkage to the minimum of 2.89% from 3.73% of the unmodified hybrid resin. These photosensitive resins with higher tensile strength and lower volume shrinkage can be used to synthesize high performance functional materials in the future.

Preparation of Bis[(3-ethyl-3-methoxyoxetane)propyl]diphenylsilane and Investigation of Its Cationic UV-Curing Material Properties.

Precusor EHO(3-ethyl-3-hydroxymethyloxetane) was synthesized with diethyl carbonate and trihydroxypropane as the main raw materials. Intermediate AllyEHO(3-ethyl-3-allylmethoxyoxetane) was synthesized with 3-ethyl-3-hydroxymethyloxetane and allyl bromide as the main raw materials. Prepolymer bis[(3-ethyl-3-methoxyoxetane)propyl]diphenylsilane was synthesized with 3-ethyl-3-methoxyoxetane)propyl and diphenylsilane. Photoinitiator triarylsulfonium hexafluoroantimonate of 3% was added to the prepolymer, and a novel kind of the photosensitive resin was prepared. They were analyzed and characterized with FTIR and 1H-NMR. Photo-DSC examination revealed that the bis[(3-ethyl-3-methoxyoxetane)propyl]diphenylsilane has great photosensitivity. The thermal properties and mechanical properties of the photosensitive resin were examined by TGA and a microcomputer-controlled universal material testing machine, with thermal stabilities of up to 446 °C. The tensile strength was 75.5 MPa and the bending strength was 49.5 MPa. The light transmittance remained above 98%.

Synthesis and Characterization of Hyperbranched and Organosilicone Modified Waterborne Polyurethane Acrylates Photosensitive Resin.

A new type of waterborne polyurethane acrylate was synthesized for use as a UV curing coating. The N,N-dihydroxy methyl ethyl-3-Methyl aminopropanoate monomer was first prepared via adding reactions of methyl acrylate and diethanol amine with methyl alcohol as the solvent. Then, the hyperbranched prepolymer was obtained by addition of trimethylolpropane with toluenesulfonic acid as catalyst and N,N-dimethyl formamide as solvent. The resulting hyperbranched and organosilicone modified waterborne polyurethane acrylates was synthesized through the mixed reaction of prepolymer and Hydroxy silicone oil, polyethylene glycol-1000, toluene diisocynate, dimethylolpropionic acid, 1,2-propylene glycol, hydroxyethyl acrylate, and triethylamine with dibutyltin dilaurate as the catalyst. The molecular structures were characterized by FT-IR and 1H NMR spectroscopy and GPC analysis and the thermal stability was studied by using TGA. Moreover, the influence of contemodnt of hydroxyl silicone oil, dimethylolpropionic acid, polyethylene glycol-1000, and prepolymer to various of properties such as glossiness, hardness, adhesive force, abrasion resistance, water absorption, elongation at break and tensile strength of films were analyzed. The temperature and catalyst dosage impact on percent conversion of isocyanate group (-NCO) were also studied. It was proven that the best dosage of hydroxyl silicone oil and dimethylolpropionic acid were 4.6%, the dosage of polyethylene glycol-1000 was 50%, and the amount of hyperbranched prepolymer was 0.5%, which could make the film achieve the optimum properties. The percent conversion of isocyanate group (-NCO) was maximum when reacting two hours at 80 °C with 0.2% catalyst.

Design and Synthesis of Free-Radical/Cationic Photosensitive Resin Applied for 3D Printer with Liquid Crystal Display (LCD) Irradiation.

In this work, aiming at a UV-curing 3D printing process with liquid crystal display (LCD) irradiation, a novel free-radical/cationic hybrid photosensitive resin was designed and prepared. After testing, the results showed that the acrylate monomers could be polymerized through a free-radical mechanism, while the epoxides were polymerized by a cationic curing mechanism. During the process of UV-curing, the acrylate and epoxide polymers were crosslinked and further locked together by non-covalent bonds. Therefore, an interpenetrating polymer network (IPN) structure could be formed through light-curing 3D-printing processes. Fourier transform infrared spectroscopy (FT-IR) revealed that the 3,4-epoxy cyclohexyl methyl-3,4-epoxy cyclohexyl formate and acrylic resin were both successfully involved in the UV-curing process. Furthermore, in order to make the 3D-printed objects cured completely, post-processing was of great importance. The results from the systematic study of the dynamic mechanical properties of the printed objects showed that the heating treatment process after UV irradiation was very necessary and favorable for the complete cationic polymerization of UV-6110 induced by Irgacure 261. The optimum heating treatment conditions were achieved at a temperature of 70 °C for 3 h.

Layered Manufacturing of Dental Ceramics: Fracture Mechanics, Microstructure, and Elemental Composition of Lithography-Sintered Ceramic.

PURPOSE: To compare the fracture mechanics, microstructure, and elemental composition of lithography-based ceramic manufacturing with pressing and CAD/CAM. MATERIALS AND METHODS: Disc-shaped specimens (16 mm diameter, 1.2 mm thick) were used for mechanical testing (n = 10/group). Biaxial flexural strength of three groups (In-Ceram alumina [ICA], lithography-based alumina, ZirkonZahn) were determined using the "piston on 3-ball" technique as suggested in test Standard ISO-6872. Vickers hardness test was performed. Fracture toughness was calculated using fractography. Results were statistically analyzed using Kruskal-Wallis test followed by Dunnett T3 (α = 0.05). Weibull analysis was conducted. Polished and fracture surface characterization was made using scanning electron microscope (SEM). Energy dispersive spectroscopy (EDS) was used for elemental analysis. RESULTS: Biaxial flexural strength of ICA, LCM alumina (LCMA), and ZirkonZahn were 147 ± 43 MPa, 490 ± 44 MPa, and 709 ± 94 MPa, respectively, and were statistically different (P ≤ 0.05). The Vickers hardness number of ICA was 850 ± 41, whereas hardness values for LCMA and ZirkonZahn were 1581 ± 144 and 1249 ± 57, respectively, and were statistically different (P ≤ 0.05). A statistically significant difference was found between fracture toughness of ICA (2 ± 0.4 MPa⋅m1/2 ), LCMA (6.5 ± 1.5 MPa⋅m1/2 ), and ZirkonZahn (7.7 ± 1 MPa⋅m1/2 ) (P ≤ 0.05). Weibull modulus was highest for LCMA (m = 11.43) followed by ZirkonZahn (m = 8.16) and ICA (m = 5.21). Unlike LCMA and ZirkonZahn groups, a homogeneous microstructure was not observed for ICA. EDS results supported the SEM images. CONCLUSIONS: Within the limitations of this in vitro study, it can be concluded that LCM seems to be a promising technique for final ceramic object manufacturing in dental applications. Both the manufacturing method and the material used should be improved.