Digital Light Processing Stereolithography Research Articles

Elastic and dimensional properties of newly combined 3D-printed multimaterials fabricated by DLP stereolithography

In the field of stereolithography 3D printing, the portfolio of commercially available photopolymers has burgeoned. Each material family possesses its individual properties. However, corresponding products with specific requirements remain a major challenge. This gap could be filled by combining existing materials. This study aimed to predict Young’s modulus of the specimen manufactured by combining multiple materials using digital light processing (DLP), a subtype of stereolithography. It also aimed to investigate the effects of the printing process on the geometry and mechanical properties of such 3D-printed multimaterials. Using a DLP 3D printer, samples were produced from commercially available pure and mixed materials, and half of the samples underwent post-printing curing. Three-point bending tests were performed to determine the elastic modulus of the samples. The elastic properties have been compared to linear interpolation using the properties of the primary materials. The measurements showed that Young’s modulus ranged from 1.6 GPa to 2.2 GPa for the post-cured materials, with the mixed materials fitting well with the linear interpolation approach. For eight out of nine sample sets, the prediction was within the range of the measurements. In the case of as-printed samples, the elasticity of the primary materials ranged from 0.4 GPa to 0.9 GPa, but all of the mixed materials showed a stiffer behavior than the linear interpolation prediction, up to 57% above the prediction. The dimensions of the printed specimen were measured, and groups of different geometrical deviations were identified. These were analyzed with regard to the printer system and material mixture. In conclusion, this study shows and discusses the effects of the printing process on mechanical and dimensional properties of specimens fabricated using a stereolithographic 3D printer from multiple commercially available primary materials. It discusses a process for predicting the elastic properties of these multimaterials and selecting the mixing ratios to achieve specifically desired properties.

3D-Printed Microfluidic One-Way Valves and Pumps.

New microfluidic lab-on-a-chip capabilities are enabled by broadening the toolkit of devices that can be created using microfabrication processes. For example, complex geometries made possible by 3D printing can be used to approach microfluidic design and application in new or enhanced ways. In this paper, we demonstrate three distinct designs for microfluidic one-way (check) valves that can be fabricated using digital light processing stereolithography (DLP-SLA) with a poly(ethylene glycol) diacrylate (PEGDA) resin, each with an internal volume of 5-10 nL. By mapping flow rate to pressure in both the forward and reverse directions, we compare the different designs and their operating characteristics. We also demonstrate pumps for each one-way valve design comprised of two one-way valves with a membrane valve displacement chamber between them. An advantage of such pumps is that they require a single pneumatic input instead of three as for conventional 3D-printed pumps. We also characterize the achievable flow rate as a function of the pneumatic control signal period. We show that such pumps can be used to create a single-stage diffusion mixer with significantly reduced pneumatic drive complexity.

Study of alumina ceramic parts fabricated via DLP stereolithography using powders with different sizes and morphologies

AbstractSelecting suitable ceramic powders for the preparation of UV‐curable ceramic suspensions, which are well suited for printing processes and production of high‐performance ceramic components, is a crucial factor in the practical industrial application of digital light processing (DLP) stereolithography. Therefore, this study aims to provide a comprehensive evaluation of alumina ceramic parts fabricated via DLP stereolithography using a variety of alumina powders with varying sizes and morphologies. Experiments were conducted to examine the rheological response, recoating performance, and curing behavior of UV‐curable alumina suspensions. Additionally, the thermal decomposition behavior of three‐dimensional (3D)‐printed green‐bodies, as well as the physical and mechanical properties of 3D‐printed sintered alumina components were thoroughly investigated. The best physical and mechanical performances were achieved by printing 55 vol% suspensions prepared using near‐spherical AA04 alumina powders (median diameter .4 μm). This study elucidates the effects of ceramic particle size and morphology on the entire technological process of DLP‐based ceramic stereolithography, thereby establishing the guidelines for the fabrication of high‐performance 3D‐printed ceramic objects in industrial and engineering production by selecting appropriate ceramic powders.

Defocus digital light processing stereolithography for rapid manufacture of microlens arrays

This study applied defocusing to digital light processing (DLP) stereolithography for the fabrication of microlens arrays (MLAs) within 1 s. We studied the effects of exposure energy, defocus distance, and a diameter adjustment algorithm to ensure printed MLAs with designed dimensions. Experiment results demonstrate that our simple fabrication method is applicable to the manufacture of various MLA designs, while achieving nanoscale surface roughness and good image quality. We determined that the height of the MLAs is proportional to the exposure energy. The highest height-to-diameter ratio achieved in this study was 0.63 (diameter = 200 µm; height = 126 µm). Our use of defocusing significantly reduced the average surface roughness (Ra) of the printed MLAs to 129 nm with a corresponding improvement in image resolution 45.3 lp/mm. Our use of a diameter adjustment algorithm in designing the digital mask helped preserve the dimensionality of the printed MLAs. The proposed manufacturing process was used to create a variety of MLAs with numerical apertures as high as 0.980, multiple focal lengths on a single substrate (from 21 µm to 199 µm), and fill factors as high as 77.98 %.

Exploring the limits of toughness enhancers for 3D printed photopolymers as bone replacement materials

AbstractVinyl esters (VEs) are well‐examined monomers for additive manufacturing technology in tissue engineering. They display benefits like low cytotoxicity, good biocompatibility and favorable degradation behavior. Nevertheless, mechanical properties of crosslinked VEs are poor, as they show high brittleness. A previous study showed, that covalent incorporation of high molecular weight additive into an established vinyl ester‐thiol network circumvents these problems. In this work, additives based on poly(ε‐caprolactone) are modified with photopolymerizable end groups to improve the mechanical properties focusing on the toughness and to allow the application in digital light processing stereolithography. The different toughness enhancing additives and their influence on the photochemical and (thermo)mechanical characteristics are studied. As well, the impact of a commercial, flexible and a synthesized rigid thiol on these properties are investigated. It was shown that without forfeiting photoreactivity, the covalently incorporated toughness enhancers led to materials with excellent mechanical properties, above all the tensile toughness, which could be tripled.

High resolution DLP stereolithography to fabricate biocompatible hydroxyapatite structures that support osteogenesis.

Lithography based additive manufacturing techniques, specifically digital light processing (DLP), are considered innovative manufacturing techniques for orthopaedic implants because of their potential for construction of complex geometries using polymers, metals, and ceramics. Hydroxyapatite (HA) coupons, printed using DLP, were evaluated for biological performance in supporting viability, proliferation, and osteogenic differentiation of the human cell line U2OS and human mesenchymal stem cells (MSCs) up to 35 days in culture to determine feasibility for future use in development of complex scaffold geometries. Contact angle, profilometry, and scanning electron microscopy (SEM) measurements showed the HA coupons to be hydrophilic, porous, and having micro size surface roughness, all within favourable cell culture ranges. The study found no impact of leachable and extractables form the DLP printing process. Cells seeded on coupons exhibited morphologies comparable to conventional tissue culture polystyrene plates. Cell proliferation rates, as determined by direct cell count and the RealTime-GloTM MT Cell Viability Assay, were similar on HA coupons and standard tissue culture polystyrene plates). Osteogenic differentiation of human MSCs on HA coupons was confirmed using alkaline phosphatase, Alizarin Red S and von Kossa staining. The morphology of MSCs cultured in osteogenic medium for 14 to 35 days was similar on HA coupons and tissue culture polystyrene plates, with osteogenic (geometric, cuboidal morphology with dark nodules) and adipogenic (lipid vesicles and deposits) features. We conclude that the DLP process and LithaBone HA400 slurry are biocompatible and are suitable for osteogenic applications. Coupons served as an effective evaluation design in the characterization and visualization of cell responses on DLP printed HA material. Results support the feasibility of future technical development for 3D printing of sophisticated scaffold designs, which can be constructed to meet the mechanical, chemical, and porosity requirements of an artificial bone scaffold.

Digital light processing stereolithography of zirconia ceramics: Slurry elaboration and orientation-reliant mechanical properties

Digital Light Processing (DLP) is a promising technique for the preparation of ceramic parts with complex shapes and high accuracy. In this study, 3 mol% yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) UV-curable slurries were prepared and printed via DLP. Two different solid loadings (40.5 and 43.6 vol%, respectively) and printing directions were investigated to assess the influence of these parameters on physical and mechanical properties of the sintered parts. Zirconia samples were sintered at 1550 °C for 1 h, achieving a very high relative density (99.2%TD), regardless of solid loading and printing direction. FE-SEM micrographs shown a homogeneous and defect-free cross section with an average grains size of 0.56 ± 0.19 µm. Finally, mechanical properties were influenced by printing direction and zirconia vol%. Indeed, the composition with the higher solid loading (i.e. 43.6 vol%) had the highest three-point flexural strength (751 ± 83 MPa) when tested perpendicular to the printing plane.

A Comprehensive Study of Alumina Ceramic Parts Fabricated Via Digital Light Processing Stereolithography Using Alumina Powders with Different Sizes and Morphologies

A Comprehensive Study of Alumina Ceramic Parts Fabricated Via Digital Light Processing Stereolithography Using Alumina Powders with Different Sizes and Morphologies

Digital Light Processing Stereolithography of Zta Composite Ceramics: Synergistic Anisotropy Manipulating on Mechanical Property and Surface Roughness

Digital Light Processing Stereolithography of Zta Composite Ceramics: Synergistic Anisotropy Manipulating on Mechanical Property and Surface Roughness

Spatially and optically tailored 3D printing for highly miniaturized and integrated microfluidics

Traditional 3D printing based on Digital Light Processing Stereolithography (DLP-SL) is unnecessarily limiting as applied to microfluidic device fabrication, especially for high-resolution features. This limitation is due primarily to inherent tradeoffs between layer thickness, exposure time, material strength, and optical penetration that can be impossible to satisfy for microfluidic features. We introduce a generalized 3D printing process that significantly expands the accessible spatially distributed optical dose parameter space to enable the fabrication of much higher resolution 3D components without increasing the resolution of the 3D printer. Here we demonstrate component miniaturization in conjunction with a high degree of integration, including 15 μm × 15 μm valves and a 2.2 mm × 1.1 mm 10-stage 2-fold serial diluter. These results illustrate our approach’s promise to enable highly functional and compact microfluidic devices for a wide variety of biomolecular applications.

Development of a design and characterization framework for fabrication of functionally graded materials using magnetic field-assisted digital light processing stereolithography

• A design, manufacturing, and characterization framework is established to fabricate functionally graded materials. • The correlation between the local magnetic particle contents and Young. • The correlation between the magnetic flux density and resulting particle contents is established to facilitate the design and manufacturing of FGMs. Functionally graded materials (FGMs) offer several advantages over traditional materials. Fabrication of FGMs via additive manufacturing (AM) processes would allow creating a part with a complex geometry while controlling the graded structure and has gained significant interest in the recent years. Magnetic field-assisted digital light processing (DLP) stereolithography is a promising technique to fabricate FGMs. However, it is still challenging to be able to fabricate FGMs with precisely controlled structures and properties. In this study, a design, manufacturing, and characterization framework is developed for using DLP to fabricate FGMs. In this work, a customized DLP printer with assisitve magnetic field is developed and used to fabricate polymeric composites with embedded magnetic particles. By varying the magnetic field, different gradients of particle concentration can be achieved. The particle distribution within the sample is characterized by nano-computed tomography and the greyscale analysis using optical microscopy. Nanoindentation is performed to measure the local material properties in the graded area. Standard tensile tests are also conducted on samples with preset particle contents, and the results are cross compared with nanoindentation results. A finite element model is developed to analyze the effect of magnetic flux density on the resulting particle content. Correlations between the magnetic flux density and particle content, and between the particle content and the local material behavior are shown, which form the foundation of the proposed framework. This framework provides a guided method for designers and manufacturers to create FGMs with more predictable results.

Digital light processing stereolithography of hydroxyapatite scaffolds with bone‐like architecture, permeability, and mechanical properties

AbstractThis work deals with the additive manufacturing and characterization of hydroxyapatite scaffolds mimicking the trabecular architecture of cancellous bone. A novel approach was proposed relying on stereolithographic technology, which builds foam‐like ceramic scaffolds by using three‐dimensional (3D) micro‐tomographic reconstructions of polymeric sponges as virtual templates for the manufacturing process. The layer‐by‐layer fabrication process involves the selective polymerization of a photocurable resin in which hydroxyapatite particles are homogeneously dispersed. Irradiation is performed by a dynamic mask that projects blue light onto the slurry. After sintering, highly‐porous hydroxyapatite scaffolds (total porosity ~0.80, pore size 100‐800 µm) replicating the 3D open‐cell architecture of the polymeric template as well as spongy bone were obtained. Intrinsic permeability of scaffolds was determined by measuring laminar airflow alternating pressure wave drops and was found to be within 0.75‐1.74 × 10−9 m2, which is comparable to the range of human cancellous bone. Compressive tests were also carried out in order to determine the strength (~1.60 MPa), elastic modulus (~513 MPa) and Weibull modulus (m = 2.2) of the scaffolds. Overall, the fabrication strategy used to print hydroxyapatite scaffolds (tomographic imaging combined with digital mirror device [DMD]‐based stereolithography) shows great promise for the development of porous bioceramics with bone‐like architecture and mass transport properties.

High‐Quality Translucent Alumina Ceramic Through Digital Light Processing Stereolithography Method

Herein, translucent alumina ceramics with fine complex geometric shapes are successfully fabricated by a digital light processing (DLP) process followed by vacuum sintering. The interlayer bonding condition is the key for preparing high‐quality translucent alumina for this layer‐by‐layer additive manufacturing (AM) method. The curing characteristics of alumina suspensions are carefully investigated by curing depth and polymerization conversion behavior. It is clear that the residual stress resulted from the polymerization process is more critical than the curing depth for selecting optimum curing time. Finally, high‐quality translucent alumina ceramic and complex structures are realized using this novel AM technique. It reveals that DLP 3D printing method can be a promising technology for the fabrication of transparent ceramics.

Fabrication and Compressive Behavior of a Micro-Lattice Composite by High Resolution DLP Stereolithography.

Lattice structures are superior to stochastic foams in mechanical properties and are finding increasing applications. Their properties can be tailored in a wide range through adjusting the design and dimensions of the unit cell, changing the constituent materials as well as forming into hierarchical structures. In order to achieve more levels of hierarchy, the dimensions of the fundamental lattice have to be small enough. Although lattice size of several microns can be fabricated using the two-photon polymerization technique, sophisticated and costly equipment is required. To balance cost and performance, a low-cost high resolution micro-stereolithographic system has been developed in this work based on a commercial digital light processing (DLP) projector. Unit cell lengths as small as 100 μm have been successfully fabricated. Decreasing the unit cell size from 150 to 100 μm increased the compressive stiffness by 26%. Different pretreatments to facilitate the electroless plating of nickel on the lattice structure have been attempted. A pretreatment of dip coating in a graphene suspension is the most successful and increased the strength and stiffness by 5.3 and 3.6 times, respectively. Even a very light and incomplete nickel plating in the interior has increase the structural stiffness and strength by more than twofold.

Study of mixing process of low temperature co-fired ceramics photocurable suspension for digital light processing stereolithography

The Low Temperature Co-fired Ceramic (LTCC) materials are highly used for high frequency devices required for high-speed data communications, representing an attractive material for electronic applications with a direct industrial applicability. The development of a photocurable LTCC suspension for Digital Light Processing Stereolithography (DLP-SLA) technology is presented in this work. The LTCC suspension, with an optimal solid load of 40.4 vol%, was characterized along its preparation regarding the rheological behaviour, dispersant content, particle size distribution in function of milling time and photocuring properties in a visible light range. The effect of the particle size change, through ball milling, on viscosity and photocuring behaviour was studied, achieving an optimal mixing range time, which highlights the importance of the manufacturing standardization of the photocurable suspensions. The optimized suspension presents a viscosity of 3.6 Pa s at shear rate of 2 s−1, a sensitivity of 41 μm and a critical energy dose of 15 mJ cm−2. The printing process was successfully achieved, demonstrated by some defect-free printed pieces.

Inter-particle interactions of alumina powders in UV-curable suspensions for DLP stereolithography and its effect on rheology, solid loading, and self-leveling behavior

For the UV-curable alumina suspensions used in digital light processing (DLP) stereolithography, optimizing the dispersant type is important for achieving low viscosity, high solid loading, and remarkable self-leveling behavior. However, the inter-particle interactions in UV-curable alumina suspensions dispersed using different dispersants are overlooked. Herein, the effect of inter-particle interactions on rheology, solid loading, and self-leveling behavior of UV-curable alumina suspensions was systematically investigated. Three different commercial dispersants were used: oleic acid (OA), alkane-acrylic phosphate ester (PM1590), and copolymer dispersant (BYK111). After dispersing, BYK111 endowed alumina powders with thicker adsorption polymer layer to provide stronger steric repulsion force and facilitated better wetting of alumina powers in the photocurable resin, resulting in a reduced network structure degree, which decreased the viscosity (1.04 Pa s at 30 s−1); homogeneous packing of alumina powders, which enhanced the maximum solid loading (55 vol%); and inhibition of particle flocculation, which facilitated the spontaneous spreading of suspension.

Experimental investigation on bending behaviour of ZrO2 honeycomb sandwich structures prepared by DLP stereolithography

The ceramic honeycomb sandwich structures of 3 mol% yttria-stabilized tetragonal ZrO2 (3Y-TZP) material were fabricated by an additive manufacturing technique based on the digital light processing (DLP) system. The bending behaviour of hexagonal and square honeycomb sandwich structures was investigated with three-point bending test. The effects of geometric parameters, including the thickness of the face-sheet, the height of the cell, the thickness of the cell wall and the size of the cell, were studied through experiments carefully. The results illustrated that the bending strength of the ceramic honeycomb sandwich structure can reach 170 MPa at a relative density of 41.72%. The geometric parameters of the honeycomb core were more critical than the sandwich face-sheet. The cell size, cell wall thickness and cell height were found to be the preferred geometric parameter for justifying the bending resistance performance of the ceramic sandwich panel. Square honeycomb demonstrated better bending performance than the hexagonal honeycomb. This study provides a new basis for further studies on the ceramic lightweight structures.

Prolonged recovery of 3D printed, photo-cured polylactide shape memory polymer networks.

Shape memory polymers are materials that are able to retain a deformed state until an external stimulus, most typically heat, triggers recovery to the original geometry. Whereas typically, shape memory polymers are required to recover fast (seconds to minutes), many applications, particularly in the medical field, would benefit from a slow recovery (days to weeks). In this work, we exploit the broad glass transition range of photo-cured poly(D,L-lactide) dimethacrylate networks to obtain recovery times of up to 2 weeks, at 11 °C below the peak glass transition temperature of 58 °C. Recovery times decreased considerably for higher recovery temperatures, down to ∼10 min at 55 °C. A large spread in glass transition values (53.3–61.0 °C) was observed between samples, indicating poor reproducibility in sample preparation and, hence, in predicting shape recovery kinetics for individual samples. Furthermore, a staged recovery was observed with different parts of the samples recovering at different times. The ability to prepare complex structures using digital light processing stereolithography 3D printing from these polymers was confirmed. To the best of our knowledge, this work provides the first experimental evidence of prolonged recovery of shape memory polymers.

Effects of slurry mixing methods and solid loading on 3D printed silica glass parts based on DLP stereolithography

In this work, digital light processing (DLP) 3D printing technology was used to prepare silica glass parts. The effects of slurry mixing methods and solid loading on the rheological properties of the silica slurry were investigated. The viscosity of the multi-step mixing method was less than the one-step mixing method at different solid loadings. When the solid loading of the slurry was 60 wt%, the viscosity by the multi-step mixing method and the one-step mixing method was 2059.7 mPa s and 5461.2 mPa s respectively at a shear rate of 0.1 sec−1, with a difference of 3401.5 mPa s. The results of heat treatment experiments showed that transparent glass samples could be obtained when the solid loading was ≥40 wt%. The density of the sintered sample remained substantially constant as the solid loading increased, but the linear shrinkage gradually decreased. The heat treatment time could be reduced to less than 16 h by optimizing the debinding and sintering process using a simultaneous thermal analyzer. Glass samples with the relative density of more than 99% and a transmittance of more than 90% in the range of 300–1800 nm were acquired, and the microstructure and physical properties of the sintered glass were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and digital micro-Vickers hardness tester.

A Benchmark Artifact to Evaluate the Manufacturing of Microfeatures by Digital Light Processing Stereolithography

Abstract Suitable benchmark artifacts are needed for assessing the technological capabilities and limitations of a specific process or for comparing the performances of different processes. Only a few benchmark artifacts have been specifically designed for features with microscale dimensions, even if their manufacturing is becoming very common due to the increasing demand for miniaturized parts or objects with microscale features. In this study, a suitable benchmark part is designed to evaluate the geometrical performance of a digital light processing (DLP) stereolithography (SLA) system for manufacturing microfeatures. The effect of the main process parameters (i.e., layer thickness and exposure time) and the feature position within the building platform on the process performance was assessed by a specifically studied experimental campaign. The results show that both the analyzed process parameters influence the minimum feasible size of protruding features and that the feature position influences the dimensional accuracy.