I’m Steyn Westbeek and I’m part of the Mechanics of Materials group at the Department of Mechanical Engineering. My work uses vat photopolymerization (VP) to enable the additive manufacturing (AM) of large-area ceramic parts without defects or geometrical inaccuracies. In order to obtain an improved understanding of the process and relevant phenomena, I use a simulation-based approach that focuses on modeling the AM step.
The best of three backgrounds
My research is matched with that of two other PhD students. Together, we have a nice mix of focus points for taking the VP process (for ceramics) to the next level. Whereas mine is on the fluid-to-solid transition that occurs in all added layers, Andrei Kozhevnikov is looking into modeling techniques to capture the deposition of resin/slurry layers using a variety of recoating techniques. The ideal deposited layer is fully flat, but different rheological aspects can cause significant deviations. The knowledge and insights which Andrei and I generate are valuable to Thomas Hafkamp, who works on integrating closed-loop control into the AM system using sensory systems and feed-forward approaches.
A new state-of-the-art
The current state-of-the-art in VP for ceramics allows for the successful printing of detailed and fine-featured parts. However, upon increasing product size and individual layer area, cracks and severe deformations start to emerge. From a modeling point of view, this AM process is highly multi-physical. This was already the case for the conventional VP process, in which a polymer part is printed by selectively irradiating a liquid photoreactive resin in a layer-by-layer fashion. The liquid-to-solid phase change coincides with exothermic heat generation and a drastic change in material properties. The thermal expanse exhibited by the material is often counteracted by polymerization-induced shrinkage.
To facilitate the AM of ceramics through VP, several pre- and post-processing steps are required. A large volume fraction of ceramic powder is mixed into the resin, which is now called a slurry and serves as input for the printer. The VP shaping process then produces a composite, the so-called ‘green’ part. Thermal post-processing converts this to the final ceramic. Even when limiting the scope to the AM process itself, the presence of solid-like material inclusions influences things. The mechanical and thermal properties of the resin and printed part change and a large amount of light scattering occurs during the irradiation step.
Achieving design freedom
A multi-physical and multi-scale finite element modeling framework is being developed to enable part scale process simulation. This also accounts for the presence of inclusions in the optical, thermal and mechanical properties of the composite material. Starting from the different constituents at the microscale and layer length scale, effective parameters are obtained through homogenization of both the light scattering and the properties. Together with printing process conditions, these serve as inputs for part scale process simulation. Using this multi-scale framework, residual stresses and deformations are simulated during printing and after releasing the (green) part from the base plate.
The improved process understanding that can be obtained through the simulation of various resin-ceramic compositions will lead to reduced development times and, eventually, to first-time-right printing. The ability to simulate different process conditions and part geometries is beneficial to optimization that minimizes deformation and residual stresses. The design freedom of the resulting large-area and high-quality ceramic parts will allow new applications in the high-tech and biomedical domains.