Epoxy-based blends printable in a Liquid Crystal Display (LCD) printer were studied. Diglycidyl ether of bisphenol A (DGEBA) mixed with Diethyltoluene diamine (DETDA) was used due to the easy ...processing in liquid form at room temperature and slower reactivity until heated over 150 ° C. The DGEBA/DETDA resin was mixed with a commercial daylight photocurable resin used for LCD screen 3D printing. Calorimetric, dynamic mechanical and rheology testing were carried out on the resulting blends. The daylight resins showed to be thermally curable. Resin’s processability in the LCD printer was evaluated for all the blends by rheology and by 3D printing trials. The best printing conditions were determined by a speed cure test. The use of a thermal post-curing cycle after the standard photocuring in the LCD printer enhanced the glass transition temperature T g of the daylight resin from 45 to 137 ° C when post-curing temperatures up to 180 ° C were used. The T g reached a value of 174 ° C mixing 50 wt% of DGEBA/DETDA resin with the photocurable resin when high temperature cure cycle was used.
The material properties and processing of investment casting patterns manufactured using conventional wax injection Molding and those manufactured by vat photopolymerization can be substantially ...different in terms of thermal expansion and mechanical properties, which can generate problems with dimensional accuracy and stability before and during ceramic shelling and shell failures during the burn-out of the 3D printed casting patterns. In this paper and for the first time, the monofunctional Acryloyl morpholine monomer was used for 3D printing of casting patterns, due to its thermoplastic-like behavior, e.g., softening by heat. However, the hydrophilic behavior of this polymer led to an incorporation of up to 60 wt% of Hexanediol diacrylate, to control the water absorption of the network, which to some extent, compromised the softening feature of Acryloyl morpholine. Addition of a powdered wax filler resulted in a delayed thermal decomposition of the polymer network, however, it helped to reduce the thermal expansion of the parts. The dimensional accuracy and stability of the wax-filled formulation indicated an excellent dimensional tolerance of less than ±130 µm. Finally, the 3D printed patterns successfully went through a burn out process with no damages to the ceramic shell.
In the vat photopolymerisation 3D printing technique, the properties of the printed parts are highly dependent on the degree of conversion of the monomers. The mechanisms and advantages of vat ...photopolymerisation at elevated temperatures, or so called “hot lithography”, were investigated in this paper. Two types of photoresins, commercially used as highly accurate castable resins, with different structural and diluent monomers, were employed in this study. Samples were printed at 25 °C, 40 °C, and 55 °C. The results show that hot lithography can significantly enhance the mechanical and dimensional properties of the printed parts and is more effective when there is a diluent with a network Tg close to the print temperature. When processed at 55 °C, Mixture A, which contains a diluent with a network Tg = 53 °C, was more readily impacted by heat compared to Mixture B, whose diluent had a network Tg = 105. As a result, a higher degree of conversion, followed by an increased Tg of the diluents, and improvements in the tensile strength and dimensional stability of the printed parts were observed, which enhanced the outcomes of the prints for the intended application in investment casting of complex components used in the aero and energy sectors. In conclusion, the effectiveness of the hot lithography process is contained by a correlation between the process temperature and the characteristics of the monomers in the mixture.
Enhancing inlet gas temperature in aero/gas turbines to reduce their carbon-footprint, has led to a strive for better performing inlet cooling mechanism of the turbine blades. The internal cooling of ...the blades is made by ceramic cores in their casting process, but conventional ceramic molding has long reached its maximum possible geometrical complexity, hence shedding light on 3D printing of these cores. The objective of this study is to develop low-viscous, fully stabilized, commercially viable ink for vat-photopolymerization of silica-based ceramics. This paper investigates the best dispersion type and amount for different formulated monomer mixtures, and explains the best correlation between viscosity, solid loading, binders, dispersants, peeling forces and mechanical properties, and offers an optimized mixture to avoid the common ceramic printing issue, namely crack propagation of cores during sintering. Among five dispersant agents, the SOL20, SOL24 and FA4611 exhibited better performance than other dispersion agents, and the optimum concentration level for each binder and dispersant agent was ensured through sedimentation test. Their dispersion capability and long-term stability were further investigated to designate the best dispersion agent for each binder system. Further verification was made by sedimentation study of the samples at 40 °C for 40 days and reducing the superficial area of the used powder mixture. According to the result of the rheology analysis, the best dispersions were achieved using SOL20 for the loaded binder mixtures of M1 and M4, SOL24 for M3and FA4611 for M2. The instability of M1 and M2 with their respective dispersant agent was coordinated through the thixotropic agent of TX/2, and complete stabilization and near-Newtonian behavior were achieved. However, the research showed that the addition of TX/2 to fully stabilized M4 and M2 suspensions negatively impacts the mixtures’ rheological behavior from near-Newtonian to shear-thickening. In the final stage of this study, peeling forces, sintering and three-point bending tests were conducted to determine the final formulated suspension to print ceramic core components. M4 and SOL20 combination was selected for SiO2-ZrSiO4 loading and dispersing, respectively. The impact of solid loading between the range of 58 and 65 vol% on the rheological behavior of the final suspension and the mechanical properties of sintered bodies were investigated to assign an optimum solid rate. The adequate strength on sintered and degree of viscosity for ceramic vat-polymerization processing was achieved at 58 vol%. Lastly, a validation study is conducted by printing a complex ceramic core model by a commercial LCD hobby printer. This validation shows the significance of this study to scale up the manufacturing of complex-shaped ceramic cores and to revolutionize the sector, by printing inexpensive and readily available irregular-shaped (non-atomized) ceramic powder, using the most cost-effective LCD printers (non-specialized expensive ceramic printers).
•This paper summarises part of a bigger study on LCD-DLP 3D printing high temperature ceramics.•A range of binder formulations for high loading and micron-size silica, alumina, and zircon ceramic ...powders in commercial particle shapes, for the use in vat-polymerisation 3D printing.•Characterisation involves: (1) Curing performance (2) Rheology of different ceramic loaded mixtures; and (3) The green body mechanical properties.•All shortlisted binders in this study provide an adequate cure depth for commercial 3D printing, even at high ceramic loadings, while maintaining their printable viscosity.•To our knowledge this is the first study on the binder design for silica, alumina and zircon based ceramic suspensions used in LCD-based Vat-polymerisation.
The primary objective of this study is to demonstrate the possibility of developing silica, alumina, and zircon-based photocurable ceramic suspensions that can be used for visible light photopolymerization (> 450 nm) and to optimise the binder formulations for the purpose of LCD-based ceramic 3D printing applications. Reference ceramic components for this work are ceramic cores employed in the investment casting of high-pressure turbine blades and vanes. Arguably, one of the most critical steps in photoinduced ceramic 3D printing is developing suitable ceramic suspensions, having high ceramic loading, low viscosity, and short curing times. Ceramic suspensions with four different novel binder formulations and commercial ceramic powders used in core manufacturing (SiO2, Al2O3 and ZrSiO4) were investigated to achieve the best trade-off between: (1) their curing performance (cure depth and curing speed), (2) rheological properties of the binder mixtures at the solid loadings of 60 vol.% for SiO2, 55 vol.% for ZrSiO4, and 45 vol.% for Al2O3; and (3) the green body mechanical properties of the mixtures after printing. The effect of ceramic particles on the selected binders was examined individually, and the correlation between cure depth (Cd), volumetric loading, and curing speed are evaluated. The results show all binders designed in this study provide an adequate cure depth, even at high ceramic loadings. When the curing behaviour of all unloaded binder mixtures from the previous study 1 compared with the 10 vol.% SiO2 loaded mixtures, the cure depth of all formulated binder mixtures increased 50–55 % and the curing thickness of 60 vol.% SiO2 loaded suspensions were still slightly higher than their unloaded counterparts. The rheology outcomes indicate that lower viscosity binders always result in lower viscosity of the ceramic loaded inks, even without taking the effect of dispersants into account. Besides, the addition of N-Vinyl-2-Pyrrolidone (NVP) monofunctional monomer to the binder mixtures significantly reduces the viscosity and changes the normally linear relationship of the mix viscosity and its silica loading content. Among the binder formulations loaded with 60 vol.% of SiO2, the formulation providing the lowest viscosity and highest mechanical property consists of 5 wt.% of NVP, 45 wt.% of HDDA and 50 wt.% of Photocentric 34 resin. Although this binder mixture showed the highest green flexural strength when loaded by 55 vol.% ZrSiO4, all other mixtures loaded with zircon flour also demonstrated a near-fluid behaviour, below 200 s−1. In Al2O3 loaded mixtures, the HDDA di-functional binder formulations present lowest viscosity and the di- and multifunctional monomer blends (HDDA-Photocentric27) showed the highest mechanical properties when used in a 50/50 ratio. This work summarises the best binder choices for silica, alumina and zircon based ceramic suspensions used in core printing for investment casting applications through LCD screen printing.
The primary focus of this study is to offer guidelines for developing binder formulations for visible light photoinduced ceramic 3D printing applications. Different monomer combinations with a ...variety of functional groups were mixed at various ratios to systematically study their rheological and curing behaviour, and mechanical properties (ultimate tensile strength, elongation at break, Young’s modulus). The optimum ratios of monomer blends were investigated by adjusting the balance of difunctional monomers (HDDA and Photocentric 27) and multifunctional monomers (tri- and tetrafunctional - PPTTA, Photocentric 34 and PE(EO)nTTA) at the weight rates of 20:80, 40:60, 50:50, 60:40, 70:30 and 80:20. The key curing parameters (penetration depth Dp and critical exposure energy Ec) were calculated by fitting values based on curing thickness (Cd), using Jacobs' Basic Working Curve Equation, to estimate optimum printing exposure time for each mixture. Although the curing performance of the mixtures was enhanced by increasing the functionality and molecular weight of the monomers in the mixture, the best viscosity and mechanical properties were obtained when difunctional monomers were mixed with a trifunctional (meth)acrylates. Two types of difunctional monomers were used in the mixtures and showed a similar performance. The best ratio between di- and trifunctional monomers were 50:50 and 40:60 wt ratios, yielding the best mechanical properties and minimum viscosity required for ceramic slurry mixtures. Moreover, the effect of vinyl-based monofunctional monomers as reactive diluents as well as hexa-functional monomers as cross-linkers on the shortlisted formulations was studied. This work summarises the best binder base choices for the daylight curable ceramic suspensions, used in LCD screen printing applications, and further works mcust be done to find the best debinding cycles.
An enhanced-temperature vat photo-polymerisation of ceramic-loaded mixtures, or so called hot ceramic lithography, has been presented in this paper, for 3D printing silica-based ceramic cores used in ...the investment casting of hot section parts in aero and industrial turbines. The pre and post-sintering properties of the 3D printed ceramic parts highly depend on the polymerisation degree of the base resin binders and the rheological behaviour of ceramic slurry, and hot lithography can play a significant role both. First, the paper aimed to better understand how different printing temperatures (25,35,45 and 55 °C) would affect the properties of the base binder mixtures before loading ceramic particles, by conducting tensile, DMA and FTIR analysis on binder-only prints. The impact of elevated printing temperature on the mechanical properties of ceramic loaded binder mixtures was further investigated. The highest mechanical properties for the base binder and the ceramic loaded mixtures were achieved when the process temperature was set closer to the binder mixtures' initial glass transition temperature (when the binder is cured at room-temperature). This printing temperature is yet lower than the level causing heat-induced cross-linking yet high to enhance the mobility of monomers and active species, when compared with conventional room-temperature vat photopolymerisation printing. The results indicate that printing temperature, if set as described, can significantly impact the mechanical property of parts and conversion rate of the base binders for a successful print process. The second part of this study investigates the impact of temperature on the rheological behaviour and peeling forces of highly loaded ceramic-loaded mixtures (61.2 vol% of SiO2-ZrSiO4). The suspension showed shear thickening behaviour at 25 °C, hindering the replenishment of the slurry during printing, while a high transition from shear-thickening to shear-thinning behaviour, ideal for flawless ceramic 3D printing, was observed at an optimum process temperature of 35 °C. When print temperature increases to 45 °C and then 55 °C, the viscosity of the mixture at very low shear rate (when the material is static during printing) increases, hence affecting peeling forces. Comparing peeling force measurements and FTIR results showed the higher cross-linking density, and the reactivity of the base binder, leads to a stronger adhesion between the cured layers and the printer's transparent film, hence higher separation/peeling forces. To sum up, hot ceramic lithography at the right printing temperature can resolve many print and post print issues, such as deformation and cracking, leading to a stronger and more dimensionally-accurate ceramic components.
Objective:Efforts to prevent depression, the leading cause of disability worldwide, have focused on a limited number of candidate factors. Using phenotypic and genomic data from over 100,000 UK ...Biobank participants, the authors sought to systematically screen and validate a wide range of potential modifiable factors for depression.Methods:Baseline data were extracted for 106 modifiable factors, including lifestyle (e.g., exercise, sleep, media, diet), social (e.g., support, engagement), and environmental (e.g., green space, pollution) variables. Incident depression was defined as minimal depressive symptoms at baseline and clinically significant depression at follow-up. At-risk individuals for incident depression were identified by polygenic risk scores or by reported traumatic life events. An exposure-wide association scan was conducted to identify factors associated with incident depression in the full sample and among at-risk individuals. Two-sample Mendelian randomization was then used to validate potentially causal relationships between identified factors and depression.Results:Numerous factors across social, sleep, media, dietary, and exercise-related domains were prospectively associated with depression, even among at-risk individuals. However, only a subset of factors was supported by Mendelian randomization evidence, including confiding in others (odds ratio=0.76, 95% CI=0.67, 0.86), television watching time (odds ratio=1.09, 95% CI=1.05, 1.13), and daytime napping (odds ratio=1.34, 95% CI=1.17, 1.53).Conclusions:Using a two-stage approach, this study validates several actionable targets for preventing depression. It also demonstrates that not all factors associated with depression in observational research may translate into robust targets for prevention. A large-scale exposure-wide approach combined with genetically informed methods for causal inference may help prioritize strategies for multimodal prevention in psychiatry.
During previous viral pandemics, reported co-infection rates and implicated pathogens have varied. In the 1918 influenza pandemic, a large proportion of severe illness and death was complicated by ...bacterial co-infection, predominantly
and
.
A better understanding of the incidence of co-infection in patients with COVID-19 infection and the pathogens involved is necessary for effective antimicrobial stewardship.
To describe the incidence and nature of co-infection in critically ill adults with COVID-19 infection in England.
A retrospective cohort study of adults with COVID-19 admitted to seven intensive care units (ICUs) in England up to 18 May 2020, was performed. Patients with completed ICU stays were included. The proportion and type of organisms were determined at <48 and >48 h following hospital admission, corresponding to community and hospital-acquired co-infections.
Of 254 patients studied (median age 59 years (IQR 49-69); 64.6 % male), 139 clinically significant organisms were identified from 83 (32.7 %) patients. Bacterial co-infections/ co-colonisation were identified within 48 h of admission in 14 (5.5 %) patients; the commonest pathogens were
(four patients) and
(two patients). The proportion of pathogens detected increased with duration of ICU stay, consisting largely of Gram-negative bacteria, particularly
and
. The co-infection/ co-colonisation rate >48 h after admission was 27/1000 person-days (95 % CI 21.3-34.1). Patients with co-infections/ co-colonisation were more likely to die in ICU (crude OR 1.78,95 % CI 1.03-3.08,
=0.04) compared to those without co-infections/ co-colonisation.
We found limited evidence for community-acquired bacterial co-infection in hospitalised adults with COVID-19, but a high rate of Gram-negative infection acquired during ICU stay.
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