Additive manufacturing or 3D printing is an alternative to the traditional shaping/forming processes currently used in the ceramic manufacturing industry. Between all the different technologies available, we will focus today on Vat Photopolymerization and related ceramic formulations.
Initially, the ceramic powder is homogeneously dispersed in a photo-curable organic binder system. A green body shape is built layer by layer via selective exposure of this suspension to UV light. The production of dense materials structures requires suspensions with a uniform distribution of the ceramic particles and a high solid loading. This can be achieved using the right dispersant into the formulation. Dispersants not only help to achieve final mechanical strength but they also play a key role in the printing process. They are used to wet and disperse fine powders in the photo curable resin, and for rheology control. Dispersants can help to decrease viscosity and help to achieve a narrow particle size distribution by reducing agglomeration.
Lubrizol has developed dispersant technology under the Solsperse™ AC Hyperdispersants brand that are adapted to the new manufacturing processes in advanced ceramics, including additive manufacturing.
The following is an edited transcript of a conversation with Dr. Baschar Ozkan, scientist at The Additive Manufacturing Centre of Excellence (AM-COE) in the United Kingdom. AM-COE has used Solsperse™ AC dispersants to develop new ceramic formulations for stereolithography printing of ceramic.
Q. Could you please tell us a bit more about AM-COE and the projects you're working on?
AM-COE aims to elevate the use of vat-photopolymerization 3D printing in high-value-manufacturing industries, where complex geometries, advanced material, sustainability and durability, design and performance, and concept-to-production cycle-time are industry drives and enablers. Enjoying both high-end research facilities as well as manufacturing capabilities, AM-COE offers material design and ink development for different applications, a wide range of high-end and affordable DLP 3D printers for metal and ceramic manufacturing for mass production and research, and professional services, including 3D printing ceramics, metals, and polymers, metrology and surface measurement, X-ray and NDT services, material characterization, and engineering/design services. We are strategically located in Derby, close to Rolls Royce and Bombardier facilities and other clients and suppliers, within three hours' distance to all major cities, manufacturing centers, and airports in England.
We mix, 3D print, debinde, sinter, and impregnate most types of ceramics and composite mixtures, but our main focus is on silica, zircon, mullite, alumina, quartz, and other high-temperature ceramics. We mainly work on 3D printing of intrinsic and complex ceramic cores for equiaxed, DS, and SX superalloy casting, all at commercial specs and quality. In addition, sacrificial casting patterns and ceramic cores for the investment casting process are our other main specialties. We 3D print wax-loaded and highly accurate casting patterns compatible with aerospace and IGT applications with the highest dimensional stability.
I am a 3D-printing scientist with expertise in technical and high-temperature ceramics. I am mainly working on the material development of highly loaded photocurable ceramic suspensions for various applications and sectors, like aerospace, medical, energy, automotive, and oil & gas. My other responsibilities at AM-COE are designing the entire manufacturing process of ceramic components from pre-printing to post-printing optimization and from development projects to mass manufacturing setups. I am also looking forward to starting to work on 3D-printed batteries very soon.
Q. What are you looking for when selecting a dispersant for additive manufacturing ceramic formulations?
Generally, we aim to achieve at least 50 vol.% in our ceramic suspensions to avoid cracks and delamination during post-processing, but even higher loadings are desired in our processes and consolidation of ceramic parts. However, for lithography-based ceramic printing applications, achieving the right balance between viscosity and solid loading in the mixture is often very challenging. On the one side, a higher volume fraction of solid particles is essential for less shrinkage and higher density (i.e., mechanical strength after sintering). On the other side, higher solid content significantly impacts the viscosity of the mixture and printing process itself. That leads to segregation of the solid content, raising in printing time, and promoting convex mound on the surface of the printing body and high peeling forces, leading to cracks on the parts. Ceramic suspensions in our applications must be highly mobile, uniform, stable and homogenous to facilitate self-levelling and recoating of the printlayer (i.e., the viscosity of resin should be as low as possible, preferably less than 5000 mPa·s with no pronounced yield point that interferes with achieving uniform thin layers).
Dispersants are essential compounds improving the processability of our highly loaded ceramic suspensions in our printing processes. Ceramic materials used for most of our applications are oxide powders and typically have hydroxy groups on their surface, accounting for their hydrophilic nature. Therefore, we usually look for the compatibility of dispersants with our photopolymerizable (meth)acrylate or epoxy-based binder systems and ceramic-powder mixtures. Identifying the correct type of dispersant and dosage level has a vital impact on our photocurable ceramic suspensions' rheological behavior. Increasing the chain size of dispersant above its adsorption limit causes either an increase in viscosity or a reduction in the stability of the mixtures, due to collapses in the resin layer. Therefore, a suitable dispersant chain size should be considered based on particle size and solid fraction while designing suspension formulation.
Q. You've recently tested Solsperse™ Hyperdispersants from Lubrizol. What results have you obtained?
Generally, Solsperse™ Hyperdispersants with our photocurable (meth)acrylate monomers are great for achieving higher dispersion performance with a wide range of ceramic particles. Thanks to the Lubrizol products, we can increase the concentration of various ceramics to produce highly stable and low viscosity ceramic suspensions up to 60-65 vol.%. Besides the dispersion capability and rheological behavior, the formulated suspensions must have high stability (good homogeneity) that can be used and stored over a long period. Compared to typical traditional dispersants, the Lubrizol polymeric dispersants provide us with high stability and reliability for long 3D-printing operations. I can confirm that they demonstrated high purity and low ash content in our burn-out and sintering processes.
Figure 1 below demonstrates the dispersion performance of some Solsperse™ Hyperdispersants for highly loaded silica-loaded ceramic suspensions with 60 vol.%. As shown in Figure 1, Solsperse™ AC 1 and Solsperse™ AC 2 products are suitable for the dispersion of small particle sizes to achieve high density and purity on the final products. In contrast, the Solsperse™ AC 3 and Solsperse™ AC 4 are ideal for controlling the stability of large particle-sized ceramics where high porosity is demanded, such as in ceramic core applications. Each Lubrizol dispersion formulation has a unique impact on our formulations, thus delivering the required material and part properties depending on the type of ceramic and binder used in our printing applications. Figure 3 and 4 shows an example of how Lubrizol products impact the viscosity of our 60 vol.% loaded alumina and zirconia suspensions, respectively. We can even achieve higher solid loadings such as 65-66 vol.% with the dispersant available in the Lubrizol product range.
Q. How do Solsperse™ AC Hyperdispersants compare with other dispersants available on the market? What benefits do they bring?
Generally, the dispersion capacity of other products on the market is insufficient to achieve high solid loading, and they are limited to up to 50 vol.% in our formulations. However, even with such solid loading, the viscosity of our formulated suspensions with these dispersants was considerably higher than Lubrizol products, thus leading to high peeling forces during printing, resulting in cracks and delamination on the final parts. In addition, the non-linearity of our suspensions dispersed with other products increases significantly with the increase of solid loading; the suspensions above 55 vol.% solid fraction exhibit shear-thickening behavior with a sharp viscosity increase when the shear rate grows. Such behavior inhibits the uniformity of new spreading layers and must be avoided, especially for ceramic-printing applications where high-processing time and accuracy are needed. We also experienced viscoelastic behavior in other traditional products, where the suspension possesses both viscous and elastic properties. The shrinkage of the dispersion layer after replenishing a new layer by the recoating blade is not desired due to the viscoelastic phenomenon that implies the part deformation or hinders the layer cohesion (degradation of polymerized layers surface). Figure 5 demonstrates a comparison study on the performance of Lubrizol products and other dispersants in the market we tested for our applications, showing the benefits of reduced viscosity, improved rheology and homogeneity, and capability for high-solid loading.
Q. How do you see the future of additive manufacturing for advanced ceramics?
Advance ceramics application is soaring in aerospace and many other sectors, where they are becoming the material of choice thanks to their light weight, durability and sustainability plus other high-performance properties, such as high-temperature resistance, thermal shock resistance and wear resistance. Compared to polymer or metal additive manufacturing, ceramic-3D printing was less popular and more employed for prototyping and R&D purposes. However, in recent years, ceramic additive manufacturing has attracted significant attention in the aerospace, defense, dental and medical sectors. These industries are also interested in producing small series parts.
This technology is getting more popular today because it allows the creation of tools and components that are much less expensive and more complex. Conventional techniques such as injection molding, hot isostatic pressing or extrusion have long been used in the ceramics industry. Still, the associated expenses remain high with the geometrical restrictions on the finished parts and long lead terms. This is where ceramic-additive manufacturing comes in, like metal and polymer markets. Although there are many unknowns on ceramic-3D printing applications, and it is still a young technology, the 10-year forecasts show that ceramic-additive manufacturing has a bright future ahead of it. The impact of additive manufacturing on advanced ceramics for the fabricated components with fewer design restrictions and the science behind it is relatively unexplored, which will be a big game-changer for the entire manufacturing industry.
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