Conference Paper

Binder Jetting Additive Manufacturing of Ceramics: Feedstock Powder Preparation by Spray Freeze Granulation

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Abstract

Objective of this study is to prepare the binder jetting feedstock powder by spray freeze drying and study the effects of its parameters on the powder properties. Binder jetting additive manufacturing is a promising technology for fabricating ceramic parts with complex or customized geometries. However, this process is limited by the relatively low density of the fabricated parts even after sintering. The main cause comes from the contradicting requirements of the particle size of the feedstock powder: a large particle size (> 5 μm) is required for a high flowability while a small particle size (< 1 μm) for a high sinterability. For the first time, a novel technology for the feedstock material preparation, called spray freeze drying, is investigated to address this contradiction. Using raw alumina nanopowder (100 nm), a full factorial design at two levels for two factors (spraying pressure and slurry feed rate) was formed to study their effects on the properties (i.e., granule size, flowability, and sinterability) of the obtained granulated powder. Results show that high pressure and small feed rate lead to small granule size. Compared with the raw powder, the flowability of the granulated powders was significantly increased, and the high sinterability was also maintained. This study proves that spray freeze granulation is a promising technology for the feedstock powder preparation of binder jetting additive manufacturing.

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... Advantages of binder jetting include complimentary support and ease of debinding [2,3]. The feedstock powders used in binder jetting include nanopowder (or submicron powder) [4][5][6][7], micropowder [8][9][10][11][12][13][14], and granulated powder [7,[15][16][17][18][19][20][21][22][23]. Granulated powder is often prepared from nanopowder, and the granule size is usually in the micrometer range [21][22][23]. ...
... The feedstock powders used in binder jetting include nanopowder (or submicron powder) [4][5][6][7], micropowder [8][9][10][11][12][13][14], and granulated powder [7,[15][16][17][18][19][20][21][22][23]. Granulated powder is often prepared from nanopowder, and the granule size is usually in the micrometer range [21][22][23]. ...
... The effect of fraction of binder added to the granulated powder on its sinterability was studied by pressing and sintering. Using the same approach (i.e., pressing and sintering), the effects of granulation parameters and granule size on the sinterability of granulated powder prepared by spray freeze drying were studied [22]. However, sinterability has not been compared across different types of binder jetting feedstock powders, i.e., nanopowder, micropowder, and granulated powder. ...
Article
Feedstock powders used in binder jetting additive manufacturing include nanopowder, micropowder, and granulated powder. Two important characteristics of the feedstock powders are flowability and sinterability. This paper aims to compare the flowability and sinterability of different feedstock powders. Three powders were compared: nanopowder (with a particle size of ~100 nm), micropowder (with a particle size of 70 µm), and granulated powder (with a granule size of ~70 µm) made from the nanopowder by spray freeze drying. Flowability metrics measured included apparent density, tap density, volumetric flow rate, mass flow rate, Hausner ratio, Carr index, and repose angle. Sinterability metrics employed included sintered bulk density, volumetric shrinkage, and densification ratio. Results show that the granulated powder has a higher flowability than the nanopowder and a higher sinterability than the micropowder. Moreover, different flowability metric values of the granulated powder are either higher or lower than those of the micropowder, indicating these two powers have a comparably high flowability. Similarly, different sinterability metric values of the granulated powder are either higher or lower than those of the nanopowder, indicating these two powders have a comparably high sinterability.
... Differently, the present study aims to increase the density of printed and sintered parts. This aim could be better realized with an emerging granulation method, spray freeze drying [30]. Unlike the granules produced by grinding or spray drying that have an irregual shape or a dimpled structure, granules produced by spraying freeze drying have a spherical shape and a uniform structure [31]. ...
... The granule size and morphology could be controlled by process parameters such as spraying pressure and slurry feed rate. More details about the preparation of the granulated powder can be found elsewhere [30]. Fig. 3d is the structure of an individual granule, showing a smooth surface. ...
Article
Because of the high sinterability, nanopowder could be beneficial for ceramic binder jetting additive manufacturing to achieve a high density on printed and sintered parts. However, the flowability of the nanopowder is poor because of the large interparticle cohesion. This poor flowability prohibits the usage of nanopowder in ceramic binder jetting. In this study, to improve the flowability of nanopowder, alumina nanoparticles are granulated into micron-sized granules through spray freeze drying. The raw nanopowder and granulated powder are compared by characterizing their flowability and printability. Results show that the granulated powder has a much better flowability than the raw nanopowder. Because of the superior flowability, the granulated powder forms a denser and smoother powder bed than the nanopowder which results in the higher density and smoother surface of the printed and sintered samples. The improvement on the final part quality indicates that the printability of the nanoparticles was improved by granulation.
... In addition to density and microstructure control, shell printing has intrinsic advantages, such as lower binder usage, short debinding time, and less binder residual. Furthermore, shell printing is compatible with other methods for density and microstructure control, such as powder granulation [16,[28][29][30], mixing powders of different sizes [17,31], powder bed compaction [32], and infiltration [33][34][35]. It is possible to combine shell printing with these methods. ...
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Shell printing is an advantageous binder jetting technique that prints only a thin shell of the intended object to enclose the loose powder in the core. In this study, powder packing in the shell and core was investigated for the first time. By examining the density and microstructure of the printed samples, powder packing was found to be different between the shell and core. In addition, the powder particle size and layer thickness were found to affect the powder packing in the shell and core differently. At a 200 µm layer thickness, for the 10 µm and 20 µm powders, the core was less dense than the shell and had a layered microstructure. At a 200 µm layer thickness, for the 70 µm powder, the core was denser and had a homogeneous microstructure. For the 20 µm powder, by reducing the layer thickness from 200 µm to 70 µm, the core became denser than the shell, and the microstructure of the core became homogeneous. The different results could be attributed to the different scenarios of particle rearrangement between the shell and core for powders of different particle sizes and at different layer thicknesses. Considering that the core was denser and more homogeneous than the shell when the proper layer thickness and powder particle size were selected, shell printing could be a promising method to tailor density and reduce anisotropy.
... When high homogeneity granules (multi-material granules) are required, as for nuclear applications [20] and transparent ceramics [21], freeze granulation is recommended. Freeze granulation could be useful for 3D printing techniques which used powder bed like binder jetting [22,23]. In addition, for hazardous powders, freeze granulation is a better option than spray drying because it is a dust-free process [24]. ...
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This paper proposes a comparative study of two techniques of granulation of a submicronic alumina powder with high binder content slurries, by spray drying and freeze granulation for the preparation of mixed granules. First, the viscosity and flow index of the suspensions are given as a function of dispersant, solid and binder contents versus alumina content, and the data are analysed in order to give a predictive model in a wide range. Suspensions with varying viscosities (7–208 mPa.s), densities (1.31–1.76) and surface tensions (23–40 mN/m) were then granulated. The first observations reveal the importance of the content and the molar mass of the binder: when they are too high, the freeze granulation fails and filaments are produced instead of granules due to extensive stretching of the molecular chains of the binder during spraying. Then through a theoretical analysis of the phenomena leading to granulation, an original dimensionless number is proposed to describe the evolution of the granule size as a function of suspension formulation. This number is related to the Reynolds and Weber number and is able to predict the granule size over a wide range (20–500 μm for freeze granulation, and 5–30 μm for spray drying). Spray drying leads to smaller granules with various shapes, from full shape to hollow or donut-like, whereas freeze granulation leads to bigger but spherical granules with a microporosity, and a size easier to predict as no drying shrinkage is observed.
... Because of the inherent porosity and large particle size with binder jetting, it is even a challenge to sinter oxide ceramics to full density [62,432]. Recently, spray freeze drying has been applied to ceramic powder to prepare granules from angular powders to improve powder flowability and sinterability [206,748]. For ceramic filters, sensors, and solid oxide fuel cells, the porosity is a benefit for liquid and gas permeability [749][750][751], so making porous filters and other devices out of oxide ceramics is beneficial. ...
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As a non-beam-based additive manufacturing (AM) method, binder jet 3D printing (BJ3DP) is a process where a liquid binder is jetted on layers of powdered materials, selectively joined and then followed by densification process. Among AM technologies, binder jetting holds distinctive promise due to possibility of rapid production of complex structures to achieve isotropic properties in the 3D printed samples. By taking advantage of traditional powder metallurgy, BJ3DP machines can produce prototypes in which material properties and surface finish are similar to those attained with traditional powder metallurgy. Various powdered materials have been 3D printed, however, a typical challenge during BJ3DP is developing printing and post-processing methods that maximize part performance. Therefore, a detailed review of the physical processes during 3D printing and the fundamental science of densification after sintering and post heat treatment steps are provided to understand the microstructural evolution and properties of binder jetted parts. Further, to determine the effects of the binder jetting process on metallurgical properties, the role of powder characteristics (e.g. morphology, mean size, and distribution), printing process parameters (e.g. layer thickness, print orientation, binder saturation, print speed, and drying time), sintering (e.g. temperature and holding time) and post-processing are discussed. With the development of AM technologies and need for post-processing in 3D printed parts, it is necessary to understand the microstructural evolution during densification process and here, processing steps are explained. Finally, opportunities for future advancement are addressed.
... These two issues of spray-dried granules can be potentially resolved by another granulation technique called spray freeze drying [218][219][220]. The granules from spray freeze drying can be soft, making the particles loosely bonded and easily breakable. ...
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The objective of this review paper is to summarize the current status, identify the knowledge gaps, and provide future directions of research on density in ceramic binder jetting additive manufacturing. This paper begins with the process overview, material considerations, and process parameters. Low density or high porosity is identified as the most challenging issue in this literature survey, therefore different aspects of density are reviewed, including various terminologies, measurement methods, and achieved values. The techniques to increase the part density are reviewed in detail in two categories: material preparation techniques (powder granulation, mixing powders of different sizes, using slurry feedstock, mixing different materials, and applying sintering additives) and post-processing techniques (sintering, chemical reaction, infiltration, and isostatic pressing). Finally, the knowledge gaps in the literature are analyzed and the corresponding future research directions are suggested, including investigations of powder spreading and powder granulation.
... These two issues of spray-dried granules can be potentially resolved by another granulation technique called spray freeze drying [218][219][220]. The granules from spray freeze drying can be soft, making the particles loosely bonded and easily breakable. ...
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This study reports the manufacturing process of 3D interconnected macroporous tricalcium phosphate (TCP) scaffolds with controlled internal architecture by direct 3D printing (3DP), and high mechanical strength obtained by microwave sintering. TCP scaffolds with 27%, 35% and 41% designed macroporosity with pore sizes of 500 μm, 750 μm and 1000 μm, respectively, were manufactured by direct 3DP. These scaffolds were then sintered at 1150 °C and 1250 °C in conventional electric muffle and microwave furnaces, respectively. Total open porosity between 42% and 63% was obtained in the sintered scaffolds due to the presence of intrinsic micropores along with designed pores. A significant increase in compressive strength between 46% and 69% was achieved by microwave compared to conventional sintering as a result of efficient densification. Maximum compressive strengths of 10.95 ± 1.28 MPa and 6.62 ± 0.67 MPa were achieved for scaffolds with 500 μm designed pores (~ 400 μm after sintering) sintered in microwave and conventional furnaces, respectively. An increase in cell density with a decrease in macropore size was observed during in vitro cell-material interactions using human osteoblast cells. Histomorphological analysis revealed that the presence of both micro- and macropores facilitated osteoid-like new bone formation when tested in femoral defects of Sprague–Dawley rats. Our results show that bioresorbable 3D-printed TCP scaffolds have great potential in tissue engineering applications for bone tissue repair and regeneration. Copyright
Article
Solid freeform fabrication (SFF) enables the fabrication of anatomically shaped porous components required for formation of tissue engineered implants. This article reports on the characterization of a three-dimensional-printing method, as a powder-based SFF technique, to create reproducible porous structures composed of calcium polyphosphate (CPP). CPP powder of 75-150 microm was mixed with 10 wt % polyvinyl alcohol (PVA) polymeric binder, and used in the SFF machine with appropriate settings for powder mesh size. The PVA binder was eliminated during the annealing procedure used to sinter the CPP particles. The porous SFF fabricated components were characterized using scanning electron microscopy, micro-CT scanning, X-ray diffraction, and mercury intrusion porosimetry. In addition, mechanical testing was conducted to determine the compressive strength of the CPP cylinders. The 35 vol % porous structures displayed compressive strength on average of 33.86 MPa, a value 57% higher than CPP of equivalent volume percent porosity made through conventional gravity sintering. Dimensional deviation and shrinkage analysis was conducted to identify anisotropic factors required for dimensional compensation during SFF sample formation and subsequent sintering. Cell culture studies showed that the substrate supported cartilage formation in vitro, which was integrated with the top surface of the porous CPP similar to that observed when chondrocytes were grown on CPP formed by conventional gravity sintering methods as determined histologically and biochemically.
Article
Hydroxyapatite (HAP) and tricalcium phosphate (TCP) are two very common ceramic materials for bone replacement. However, in general HAP and TCP scaffolds are not tailored to the exact dimensions of the defect site and are mainly used as granules or beads. Some scaffolds are available as ordinary blocks, but cannot be customized for individual perfect fit. Using computer-assisted 3D printing, an emerging rapid prototyping technique, individual three-dimensional ceramic scaffolds can be built up from TCP or HAP powder layer by layer with subsequent sintering. These scaffolds have precise dimensions and highly defined and regular internal characteristics such as pore size. External shape and internal characteristics such as pore size can be fabricated using Computer Assisted Design (CAD) based on individual patient data. Thus, these scaffolds could be designed as perfect fit replacements to reconstruct the patient's skeleton. Before their use as bone replacement materials in vivo, in vitro testing of these scaffolds is necessary. In this study, the behavior of human osteoblasts on HAP and TCP scaffolds was investigated. The commonly used bone replacement material BioOss(R) served as control. Biocompatibility was assessed by scanning electron microscopy (SEM), fluorescence microscopy after staining for cell vitality with fluorescin diacetate (FDA) and propidium iodide (PI) and the MTT, LDH, and WST biocompatibility tests. Both versions were colonised by human osteoblasts, however more cells were seen on HAP scaffolds than TCP scaffolds. Cell vitality staining and MTT, LDH, and WST tests showed superior biocompatibility of HAP scaffolds to BioOss, while BioOss was more compatible than TCP. Further experiments are necessary to determine biocompatibility in vivo. Future modifications of 3D printed scaffolds offer advantageous features for Tissue Engineering. The integration of channels could allow for vascular and nerve ingrowth into the scaffold. Also the complex shapes of convex and concave articulating joint surfaces maybe realized with these rapid prototyping techniques.
Article
The aim of this study was to evaluate the ability of computer assisted designed (CAD) synthetic hydroxyapatite and tricalciumphosphate blocks to serve as precise scaffolds for intramuscular bone induction in a rat model. A central channel to allow for vessel pedicle or nerve integration was added. Natural bovine hydroxyapatite blocks served as controls to evaluate and compare biocompatibility of the new matrices. Individually designed 3D-printed rounded and porous hydroxyapatite (HA) and tricalcium phosphate (TCP) blocks were placed in pouches in the Musculus latissimus dorsi in 12 Lewis rats bilaterally. Bovine hydroxyapatite blocks with and without a central channel served as controls. Simultaneously, 200 microg rhBMP-2 in 1 ml sodium chloride was injected on both sides. For 8 weeks, bone generation was monitored by computer tomography and fluorescence labeling. The increase rates of bone density in CT examinations were higher in the HA groups (184-220 HU 8 weeks after implantation) compared to the TCP group (18 HU; p<0.0001). Microradiography and fluorescence microscopy 8 weeks after implantation showed new bone formation for all materials tested. For all scaffolds, toluidine staining revealed vital bone directly on the scaffold materials but also in the gaps between. It can be concluded from our data that the specially shaped hydroxyapatite and tricalcium phosphate blocks tested against the bovine hydroxyapatite blocks showed good biocompatibility and osteoinductivity in vivo. Further studies should explore if the stability of the individually designed blocks is sufficient to cultivate larger replacements without an external matrix for support.
Article
Hydroxyapatite/bis-GMA composites were processed by new technique which comprised three dimensional printing (3DP) of hydroxyapatite and then impregnation by bis-GMA based resin. Two types of composites which used either as-fabricated green 3DP samples or 1,300 degrees C sintered 3DP samples were studied. It was found that both 3DP composites have higher flexural modulus, strength and strain at break than the initial 3DP hydroxyapatite sample. Composites produced from sintered sample has higher hydroxyapatite content, higher density and greater modulus, but lower strength and strain at break than composite produced from green 3DP sample. In vitro toxicity shows that 3DP hydroxyapatite/bis-GMA based composites are non-toxic. Osteoblast cells were observed to attach and attain normal morphology on the surface of composites.
Advanced Technical Ceramics) -Determination of Density and Apparent Porosity
  • Iso
ISO, 2013, "ISO 18754:2013 Fine Ceramics (Advanced Ceramics, Advanced Technical Ceramics) -Determination of Density and Apparent Porosity."