Extrusion-based AM of ceramic materials for biomedical applications

The rise of the digital interconnection with physical manufacturing in the current Fourth Industrial Revolution has fueled the ability and market for mass personalization. In the last 10 years, additive manufacturing technology has also been of great relevance globally for its time-efficient prototyping capacity and for its ability to produce geometrical features that cannot be realized by traditional subtractive manufacturing. The technology has even become commercially available at a price point affordable to the common household. Furthermore, the biomedical industry continues to expand to accommodate the increasing population and the vast amount of unfortunate health issues around the world. Various biomedical devices have the potential to drastically improve the quality of life of patients by restoring important functions. Some ceramic materials are of particular interest for their excellent biocompatibility. Additive manufacturing enables biomedical implants to be tailored to individual patients allowing for increased performance and decreased discomfort. AM also enables the fabrication of porous structures in implants to improve osseointegration. The aim of this work is to carry out experimental research to develop an understanding of the limitations and capabilities for Extrusion Additive Manufacturing of ceramic material with the intent of developing a real biomedical application case study. The study was realized by EFeSTO, a specially developed 3 degree parallel kinematic machine, with two off-the-shelf pelletized ceramic feedstocks: Alumina and Zirconia. A combination of CAD, µ-CT, mesh refining and slicing software and technology are used to develop and optimize the resultant G-Code that is fed to the controller of the 5-axis machine tool for table movement and material extrusion. A variety of parts were printed, including unique AM feature tests. To achieve final form, the created parts undergo a two-step debinding process and a one-step sintering process. Global dimensional variation can be reduced by measuring the resultant deviation of green state parts relative to reference files and sintered parts relative to their former green state, to optimize related process parameters. SEM technology facilitated detailed qualitative analysis of the exterior, while µ-CT scanning technology with image analysis techniques facilitated a quantitative evolution of porosity study. Validation of the experimental results is supported by multiple samples and repeated measurements. Experimental results are expansive, and a comprehensive background considering EFeSTO and ceramic materials was formed for future use. From the deviation analysis, it was found that orientation and material have prominent effects on dimensional accuracy of a print. Zirconia in particular experiences large deviations in Z due to swelling of the binders in the feedstock. Tapering X-Y cross-section with respect to increasing Z was also observed. Absolute deviation analysis identified an average deviation for Green state relative to reference of -0.34 mm ± 0.25 mm and for Sintered state relative to intended part of -0.22 mm ± 0.19 mm. The wall thickness test determined 1 or 2 road minimum wall thicknesses were best achieved through continuous processes where the start and end of each layer were at the same point. The other 3D feature tests determined 40° overhang to be the limit, and that further refinement is necessary for bridge building. The determined volumetric shrinkage for Alumina and Zirconia was 35% and 48%, respectively. The total mass loss for Alumina parts through the post process steps averaged 16%. Lower part modulus (Volume relative to Surface Area), was confirmed to increase debinding and shrinkage. The SEM micro-graphs provide qualitative insight into the small and large cracks experienced by both materials. Relative to Alumina, Zirconia is considerably more difficult to print with, and more susceptible to brittle fracture, due to its higher viscosity, low thermal conductivity and absorption, and high thermal diffusivity. The porosity analysis indicates a transition of increased closed to open-porosity. A negative gradient porosity distribution was identified with respect to increasing Z for the first 3 states, and was then homogenized from the sintering process. Average Sphericity and average Equivalent Diameter both progressively increase through all of the processes. Gained knowledge regarding optimization of the G-code for EFeSTO and the refinement of process parameters for EAM production of ceramics enabled the development of a successful case study. Together with µ-CT scanning technology and EFeSTO, an Orthodontic Root Analogue Implant was effectively created out of Zirconia ceramic to replicate a unique human tooth. A business case was developed in accompaniment to emphasize the cost-effective opportunity EAM technology can provide.