Lithography-based Additive Manufacturing Research Articles

Machine learning assisted material development for lithography-based additive manufacturing of porous alumina ceramics

The transition of applying ceramic additive manufacturing (AM) from prototyping to mass production and from monolithic to multi-material (MM) components can be supported by continual development in materials and processes. Lithography-based ceramic manufacturing (LCM) can be used for MM printing of ceramics with high accuracy by introducing different approaches that enable discrete/smooth multidirectional material transitions. Adaptation of ceramic slurries plays important role for the successful printing and co-sintering. Especially for co-sintering, the shrinkage of the materials must be adapted so that no internal residual stresses occur. Machine learning (ML) offers promising opportunities for development of new materials and optimization of processes in AM by introducing relations between input features and output responses. In this article, ML algorithms were used for the prediction of shrinkage and porosity of alumina samples dependent on input features including material, printing, and thermal processing parameters so that defect-free porous/dense alumina combination can be achieved.

Mechanical and tribological performances of epoxy-acrylate resin nanocomposites production via stereolithography

In order to overcome the problems of high brittleness, poor mechanical properties and the tribological performances of photocurable resin, nan-silica (SiO2) particles were modified by coupling agent, and then added into liquid photocurable resin, to synthesize epoxy-acrylate resin nanocomposites. The mechanical properties of the composites have been greatly improved due to the good strengthening and toughening effects of the nano-silica particles. The tribological results showed that the frictional coefficients and the wear mass losses reduced significantly when the nano-silica particles content is 5 wt%. The results highlight the potential of inorganic ceramics particles filled photocurable polymer toward the fabrication of high performance epoxy-acrylate resin based composites by lithography-based additive manufacturing.

Effects of light exposure intensity and time on printing quality and compressive strength of β-TCP scaffolds fabricated with digital light processing

Digital light processing (DLP), derived from stereolithography (SL), is a lithography-based additive manufacturing (AM) process that offers the ability to tailor scaffold shape, size, material, and biometrics of fabricated scaffolds to patient requirements. The additive manufacturing of tricalcium phosphate (TCP) bioceramic scaffolds via DLP is explored in the proceeding research paper. Particularly, the role of exposure intensity and exposure time on resultant scaffold quality and mechanical properties is studied. Increased exposure intensity results in an increased deviation from the designed lattice structure and thus an increase in compressive strength. Scaffolds produced with the highest exposure intensity, resulted in a 27.6 % error increase in mean beam diameter, resulting from an increased scattering effect, penetration depth, and over-curing of peripheral slurry. TCP scaffolds with an exposure time of 3.5 s and exposure intensity of 40 mW/cm2 possessed the highest print accuracy, resulting in the lowest average compressive strength – 82 MPa for the scaffolds of cube structure and 48 MPa for the ones of I-WP structure. Statement of significanceDigital light processing (DLP) is an effective additive manufacturing (AM) method applicable for bioceramics. This study focuses on the influence of light exposure intensity on scaffold quality and compressive strength. The increase of exposure intensity can improve the compressive strength with the sacrifice of structure accuracy. The conclusions of this study can provide insights into the relationship of DLP printing parameters and scaffolds quality.

Effect of slight short fiber aluminosilicate proportion on alumina slurry devoted to additive manufacturing: Rheological behaviour and final properties of sintered materials

Lithography based additive manufacturing (L-AMT) is a widely used additive manufacturing technique for the fabrication of ceramics like Al2O3. Al2O3 shows many favorable properties e.g. high temperature resistance, high hardness, and good corrosion and erosion resistance As a drawback, ceramics are generally brittle, limiting the achievable strength and toughness of the material. One approach to increase fracture toughness is by short fiber reinforcements. The scope of this work focuses on the processing of short aluminosilicate fiber reinforced alumina by lithography based ceramic manufacturing (LCM). As fibers tend to agglomerate when incorporating them into a ceramic slurry, different dispersing additives and amounts were investigated. Using 5 wt% Disperbyk 2155, a slurry containing 1 wt% of aluminosilicate fibers could be processed, showing a viscosity of 11 Pa s at 10 s−1 and agglomerates of maximal (45 ± 17) μm. This viscosity makes the material processable in L-AMT.

Lithography-based additive manufacturing of porosity graded alumina

• In recent years, alumina ceramics exhibiting gradient in porosity, i.e., porosity graded alumina ceramics (PGACs), with a controlled amount of both open/closed or macro/micro pores is of high interest for several applications such as thick barrier coating, cores for precision casting, biocompatible implants, membranes/filters, and fabrication of functionally graded metals through infiltration. • Lithography-based Ceramic Manufacturing (LCM) offers high quality and near-net manufacturing of complex ceramic components with high resolution. • Two-vat system in the newly developed multi-material LCM printer setup enables not only assigning one material to any desired layer but also use of different materials in each layer by assigning the respective materials to selected pixels of the layer image. • This article introduces methodologies for the fabrication of porosity gradient alumina with continuous and discrete porosity transitions zones by combining sacrificial templating and LCM. With the developments in manufacturing technologies, interest on porosity graded alumina ceramics has increased for applications such as thick barrier coating, cores for precision casting and biocompatible implants. Lithography-based Ceramic Manufacturing (LCM) offers high quality and near-net manufacturing of complex ceramic components with high resolution. Multi-material LCM printing setup can overcome the main problems of conventional manufacturing techniques for fabrication of porosity graded alumina with desired microstructure and gradient architecture (i.e., gradient direction, fraction, type). Two-vat system in the newly developed multi-material LCM printer setup enables not only assigning one material to any desired layer but also use of different materials in each layer by assigning the respective materials to selected pixels of the layer image. This article introduces methodologies for the fabrication of porosity graded alumina ceramics (PGACs) with continuous and discrete porosity transitions zones by combining sacrificial templating and LCM. It is illustrated how the properties of the slurry and the desired gradient architecture play an important role for the selection of suitable method.

High-strength lithography-based additive manufacturing of ceramic components with rapid sintering

Additive manufacturing technology enables the fabrication of technical ceramics and multi-materials with unprecedented geometrical accuracy and complexity, opening the path to new functionalities for engineering applications. A crucial step to consolidate 3D-printed ceramic parts is “sintering”, a time and energy (temperature) intensive densification process. Here we present a strategy for rapid sintering (∼ 300–450 °C/min) of lithography-based additively manufactured alumina ceramics enabling consolidation of ceramic components of complex shapes within minutes. Highly dense, fine-grained microstructures were achieved by controlling densification and limiting grain growth through rapid radiation heat transfer. The high mechanical strength and toughness measured in additively manufactured alumina (∼ 810 MPa and ∼ 4.3 MPa m 1/2 ) sintered at 1600 °C within 2 min was superior to that of conventionally sintered reference parts. This study opens the path for rapid sintering of complex shaped ceramic architectures of high density with tailored microstructure and properties. • 3D-printed ceramics with complex geometry can be sintered within minutes. • Rapid radiation using a customized Spark Plasma Sintering system promotes densification. • Controlling the dwell sintering time allows tailoring of grain size in refined microstructures. • Dense and exceptionally high strength 3D-printed alumina were achieved. • Our sintering protocol may be applied to other 3D printed ceramic materials.

Stereolithography-based additive manufacturing of polymer-derived SiOC/SiC ceramic composites

In order to overcome challenges typically encountered during additive manufacturing of ceramics via the polymer precursor route, a novel polymer-derived SiOC/SiC composite system suitable for advanced geometric designs achievable by lithography-based ceramic manufacturing was established. The photoreactive resin system filled with 20 wt% SiC exhibits suitable viscosity characteristics, adequate stability against sedimentation, and a fast photocuring behavior. After printing and pyrolytic conversion, SiC particulates were well-dispersed within the polymer-derived SiOC matrix. A direct comparison with the unfilled polysiloxane-based resin system showed that the addition of particulate SiC increases handleability, reduces shrinkage, and significantly increases critical wall thicknesses up to 5 mm. The biaxial Ball-on-Three-Balls testing methodology yielded a characteristic strength of 325 MPa for SiOC/SiC composites. The results highlight the high potential of particle-filled preceramic polymer systems toward the fabrication of high-performance SiC-based materials by lithography-based additive manufacturing.

Effect of milled and lithography-based additively manufactured zirconia (3Y-TZP) on the biological properties of human osteoblasts

Statement of problemLithography-based additively manufactured (AM) zirconia has been used to fabricate dental implants and custom barriers for guided bone regeneration procedures. However, studies on the effect of AM zirconia on the biological properties of human osteoblasts are lacking. PurposeThe purpose of this in vitro study was to compare the effect of milled and lithography-based AM zirconia on the biological properties of normal human osteoblasts (NHOsts), as well as to compare the chemical composition between the milled and lithography-based AM 3 mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP) zirconia. Material and methodsThree mol percentage yttria-stabilized tetragonal zirconia polycrystal disks (Ø6×2-mm) were fabricated using subtractive milling (Priti multidisc ZrO2 monochrome) (M group) and lithography-based additive manufacturing methods (LithaCon 3Y 210) (AM group) (n=10). NHOsts were exposed to different material extracts (1:1, 1:2, and 1:4) and cytotoxicity, cell migration, cell morphology, and cell attachment biological tests were completed. Additionally, for morphological and chemical analysis, a scanning electronic microscopy with energy-dispersive X-ray analysis (SEM/EDX) evaluation was completed. Data were analyzed by using 1-way ANOVA and the Dunnett test (α=.05). ResultsSubstantial cell attachment and spreading were observed in both material surfaces. The presence of zirconium was evident in both groups, although the percentage of zirconium in the AM group (64.7%) was slightly higher than in the M group (52.6%). When NHOsts were cultured in the presence of the different material eluates, the M and AM groups exhibited similar NHOst viability and migration rates when compared with untreated cells; no significant differences were found (P>.05). ConclusionsThe lithography-based AM zirconia tested showed adequate cytocompatibility without differences when compared with the milled zirconia (M group) specimens. Slight chemical element composition differences were found between milled and lithography-based AM zirconia.

Lithography-Based Additive Manufacturing of Porosity Graded Alumina

Lithography-Based Additive Manufacturing of Porosity Graded Alumina

Slurry development for lithography-based additive manufacturing of cemented carbide components

This article describes slurry development for lithography-based additive manufacturing (AM) of cemented carbide components. Due to their superior material properties compared to steels, tungsten carbide‑cobalt hardmetals are an exciting material for manufacturing tools. AM technologies' geometric freedom makes it possible to design more efficient tools compared to other manufacturing techniques. These technologies are known as resource-efficient and sustainable processes. The material that is not required for manufacturing is re-used for the production of additional components. The high energy input in laser powder bed-based AM processes leads to strong grain growth and brittle phases, so the achievable mechanical properties are significantly limited. Lithography-based additive manufacturing shows high potential due to the manufacturing of a polymer-bound green body and a subsequent heat treatment. Tungsten carbide‑cobalt (88–12 wt%) slurries were developed with a solid content of 40 vol%, based on the resin's reactive components. Dispersants made it possible to increase the maximum processable volume fraction of solid particles. A thixotropic agent helped stabilize the slurry. Rheological investigations characterized these effects. The influence of the photoinitiator concentration and the volume fraction of tungsten carbide and cobalt particles on the curing depth was analyzed. Prepared slurries and manufactured multi-layered green bodies were evaluated for homogeneity using quantitative microstructure analysis.

Transparent laser ceramics by stereolithography

A lithography-based additive manufacturing technique was used for the first time to produce laser-emitting transparent 10 at.% Yb-doped yttrium aluminum garnet (Yb:YAG) ceramics from a mixture of oxide powders as starting materials. The ceramics were densified by vacuum sintering. Output power of more than 2.16 W and a slope efficiency of 17.6% were achieved under quasi-continuous-wave laser pumping.

Effects of the Layer Height and Exposure Energy on the Lateral Resolution of Zirconia Parts Printed by Lithography-Based Additive Manufacturing.

Lithography-based ceramics manufacturing (LCM) processes enable the sophisticated 3 dimensional (3D) shaping of ceramics by additive manufacturing (AM). The build-up occurs, like many other AM processes, layer by layer, and is initiated by light. The built-in digital mirror device (DMD) enables the specific exposure of desired pixels for every layer, giving as a consequence a first estimation of the printing resolution in the x and y axes. In this work, a commercial zirconia slurry and the CeraFab 7500, both from Lithoz GmbH (Vienna, Austria), were used to investigate the potential of reaching this resolution. The results showed that the precision of a part is strongly dependent on the applied exposure energy. Higher exposure energies resulted in oversized dimensions of a part, whereas too low energy was not able to guarantee the formation of a stable part. Furthermore, the investigation of the layer thickness showed that the applied exposure energy (mJ/cm2) was acting in a volume, and the impact is visible in x, y, and z dimensions. The lowest applied exposure energy was 83 mJ/cm2 and showed the most accurate results for a layer thickness of 25 μm. With this energy, holes and gaps smaller than 500 μm could be printed; however, the measurements differed significantly from the dimensions defined in the design. Holes and gaps larger than 500 μm showed deviations smaller than 50 μm from the design and could be printed reliably. The thinnest printable gaps were between 100 and 200 μm. Concerning the wall thickness, the experiments were conducted to a height of 1 cm. Taking into account the stability and deformation of the walls as well, the best results after sintering were achieved with thicknesses of 200–300 μm.

Combining cure depth and cure degree, a new way to fully characterize novel photopolymers

Bottom-up stereolithography has become a common lithography-based additive manufacturing technology (L-AMT) to fabricate parts with high feature resolution for biomedical applications. Novel vinyl ester based photopolymers, with their good biocompatibility and biodegradation behavior, showed a promising capacity as bone replacement materials. Due to further tuning of the mechanical properties, those biophotopolymers exhibit reduced curing speed in comparison to highly crosslinked resins e.g. acrylates. The slow structuring of the polymer network results in difficulties at the printing process. The Jacobs working curve characterizes the cure- and penetration depth of resins, but gives no information about the mechanical properties of the cured layer. The information of cure depth and the mechanical properties of the cured layer (cure degree) is desired. In this work, the conditions at L‑AMT during the structuring process were simulated with a real-time near-infrared photorheometer to evaluate the cure degree of a cured layer at constant cure depth. Therefore, we investigated the curing behavior of mixtures with variable amount of photoinitiator (PI) and light absorber (LA) of vinyl ester based biophotopolymers. We found, that a high amount of LA is crucial for good mechanical properties at constant cure depth. Moreover, we present a technique how to optimize a resin formulation regarding the content of PI and LA.

Fabrication of Dense Silica Ceramics through a Stereo Lithography-Based Additive Manufacturing

Silica ceramics were fabricated via an additive manufacturing process based on stereolithography. Ceramic suspension with low viscosity and high solid loading is of importance to stereolithography based UV-curable. In this work, to meet the requirements of stereolithography, effects of temperature, additive content and ball-milling time on the viscosity of silica slurry were investigated, and properties of silica ceramics sintered at different temperature were also researched.The results show that increasing temperatures strongly decrease the viscosity unless when the temperature is above 70°C. The minimum of viscosity was observed for an appropriate addition of dispersant, which is corresponding to the best dispersion state of silica particles in the photopolymerizable monomer. And optimizing ball-milling time showed the lowest viscosity suitable for the stereolithography process. The appropriate temperature, additive content and ball-milling time facilitating stereolithography was 70°C, 2% and 60min respectively. The prepared ceramics sintered at 1220°C showed a density and flexural strength of 1.57g/cm3 and 13.31MPa respectively.

A hybrid exposure concept for lithography-based additive manufacturing

Lithography-based Additive Manufacturing Technologies (L-AMT) exploit the curing of photosensitive materials upon light exposure. We developed a hybrid exposure concept. This system is able to overcome the challenge of providing good surface qualities and excellent feature resolution as well as a throughput similar to dynamic mask-based L-AMT systems by combining two light sources. A Digital Light Processing (DLP®) Light Engine (LE) with a building area of 144 x 90 mm² offers a pixelsize of 56 μm. In order to further improve the achievable resolution, a continuous laser-exposed contour line (spot size 20 μm) on the outside of the projected envelope can be written with an additional scanning laser-system. The matching of the DLP® projection mask and the laser-contour is crucial for accurate printing. Therefore a calibration tool was developed, which facilitates the alignment of the two light sources. A dichroic coated mirror enables a perpendicular alignment of the DLP® light beam and the laser beam.

Visible Light Photoinitiator for 3D-Printing of Tough Methacrylate Resins.

Lithography-based additive manufacturing was introduced in the 1980s, and is still the method of choice for printing accurate plastic parts with high surface quality. Recent progress in this field has made tough photopolymer resins and cheap LED light engines available. This study presents the influence of photoinitiator selection and post-processing on the thermomechanical properties of various tough photopolymers. The influence of three photoinitiators (Ivocerin, BAPO, and TPO-L) on the double-bond conversion and mechanical properties was investigated by mid infrared spectroscopy, dynamic mechanical analysis and tensile tests. It was found that 1.18 wt % TPO-L would provide the best overall results in terms of double-bond conversion and mechanical properties. A correlation between double-bond conversion, yield strength, and glass transition temperature was found. Elongation at break remained high after post-curing at about 80–100%, and was not influenced by higher photoinitiator concentration. Finally, functional parts with 41 MPa tensile strength, 82% elongation at break, and 112 °C glass transition temperature were printed on a 405 nm DLP (digital light processing) printer.

Producing parts with multiple layer thicknesses by projection stereolithography

We present a novel method to increase the manufacturing speed and versatility in lithography-based additive manufacturing processes. Usually, a part made with additive manufacturing consists of equally thick layers that are fabricated upon one another. We use a projection stereolithography (PSL) system to manufacture parts containing several layer thicknesses, which reduces the manufacturing time and adds controllability to the system. The method is based on the possibility to control the cure depth by adjusting the operation wavelength. To demonstrate the applicability of the PSL system for this purpose, we used two different operation wavelengths to produce parts that include 50- and 300-µm layers. The method presented in this paper can be used in several PSL techniques to increase their performance by offering a new tool to control the layer thickness without affecting the manufacturing resolution.

Biocompatibility of hydroxyapatite scaffolds processed by lithography-based additive manufacturing.

The fabrication of hydroxyapatite scaffolds for bone tissue engineering applications by using lithography-based additive manufacturing techniques has been introduced due to the abilities to control porous structures with suitable resolutions. In this research, the use of hydroxyapatite cellular structures, which are processed by lithography-based additive manufacturing machine, as a bone tissue engineering scaffold was investigated. The utilization of digital light processing system for additive manufacturing machine in laboratory scale was performed in order to fabricate the hydroxyapatite scaffold, of which biocompatibilities were eventually evaluated by direct contact and cell-culturing tests. In addition, the density and compressive strength of the scaffolds were also characterized. The results show that the hydroxyapatite scaffold at 77% of porosity with 91% of theoretical density and 0.36 MPa of the compressive strength are able to be processed. In comparison with a conventionally sintered hydroxyapatite, the scaffold did not present any cytotoxic signs while the viability of cells at 95.1% was reported. After 14 days of cell-culturing tests, the scaffold was able to be attached by pre-osteoblasts (MC3T3-E1) leading to cell proliferation and differentiation. The hydroxyapatite scaffold for bone tissue engineering was able to be processed by the lithography-based additive manufacturing machine while the biocompatibilities were also confirmed.

Light curing strategies for lithography-based additive manufacturing of customized ceramics

Lithography-based additive manufacturing (AM) is increasingly becoming the technology of choice for the small series or single unit production. At the TU Vienna a digital light processing (DLP) system was developed for the fabrication of complex technical ceramics, requiring high levels of detail and accuracy. The DLP-system used in this study creates a ceramic green part by stacking up layers of a photo-curable resin with a solid loading of around 45vol.% zirconia. After a thermal debinding and sintering step the part turns into a dense ceramic and gains its final properties. The native resolution of the DLP process depends on the light engine's DMD (digital mirror device) chip and the optics employed. Currently it is possible to print 3D-structures with a spatial resolution down to 40μm. A modification of the light source allows for the customization of the light curing strategy for each pixel of the exposed layers. This work presents methods to improve the geometrical accuracy as well as the structural properties of the final 3D-printed ceramic part by using the full capabilities of the light source. On the one hand, the feasibility to control the dimensional overgrowth to gain resolution below the native resolution of the light engine—a sub-pixel resolution—was evaluated. Overgrowth occurs due to light scattering and was found to be sensitive to both exposure time and exposed area. On the other hand, different light curing strategies (LCSs) and depths of cure (Cd) were used for the 3D-printing of ceramic green parts and their influence on cracks in the final ceramic was evaluated. It was concluded that softstart LCSs, as well as higher values for Cd, reduce cracks in the final ceramic. Applying these findings within the 3D-printing process may be another step toward flawless and highly accurate ceramic parts.

Processing of 45S5 Bioglass® by lithography-based additive manufacturing

Lithography-based Additive Manufacturing Technologies (AMTs) were used for fabricating cellular structures made of 45S5 Bioglass®. Using AMTs it is possible to design and fabricate cellular structures with a resolution of around 40μm and wall thicknesses down to 200μm. The presented process relies on selectively polymerizing a photosensitive, ceramic-filled resin with a dynamic mask. The dynamic mask is based on a digital mirror array which projects blue light (light emitting diodes with a wavelength of 460nm) onto the resin. The mechanical properties of sintered bulk samples (biaxial strength) and cylindrical cellular structures (compressive strength) were determined. The biaxial strength of the manufactured samples was 40MPa, and the compressive strength of the cellular structure was 0.33MPa, which is slightly lower than the strength of porous Bioglass® structures made by the foam replica method. Lithography-based AMT offers an excellent alternative to existing bone implant and scaffold fabrication methods providing accurate control of 3D morphology and pore architecture.