Post-printing processing and aging effects on Polyjet materials intended for the fabrication of advanced surgical simulators.

Material Jetting (MJ) 3D printing technology is promising for the fabrication of highly realistic surgical simulators, however, the changes in the mechanical properties of MJ materials after post-printing treatments and over time remain quite unknown. In this study, we investigate the effect of different post-printing processes and aging on the mechanical properties of a white opaque and rigid MJ photopolymer, a white flexible MJ photopolymer and on a combination of them. Tensile and Shore hardness tests were conducted on homogeneous 3D-printed specimens: two different post-printing procedures for support removal (dry and water) and further surface treatment (with glycerol solution) were compared. The specimens were tested within 48 h from printing and after aging (30-180 days) in a controlled environment. All groups of specimens treated with different post-printing processes (dry, water, glycerol) exhibited a statistically significant difference in mechanical properties (i.e. elongation at break, elastic modulus, ultimate tensile strength). Particularly, the treatment with glycerol makes the flexible photopolymer more rigid, but then with aging the initial elongation of the material tends to be restored. For the rigid photopolymer, an increase in deformability was observed as a major effect of aging. The hardness tests on the printed specimens highlighted a significant overestimation of the Shore values declared by the manufacturer. The study findings are useful for guiding the material selection and post-printing processing techniques to manufacture realistic and durable models for surgical training.

High-resolution X-ray tomographic workflow to investigate the stress distribution in vitreous enamel steels.

Vitreous enamel steels (VES) are a class of metal-ceramic composite materials realised with a low carbon steel basement coated by an enamel layer. During the firing phase to adhere the enamel to the metal, several gas bubbles remain entrapped inside the enamel volume modifying its internal structure. In this work high-resolution X-ray computed tomography (micro-CT) was used to investigate these composite materials. The micro-CT reconstructions enabled a detailed investigation of VES minimising the metal artefacts. The tomograms were used to develop finite element models (FEM) of VES by means of a representative volume element (RVE) to evaluate the thermal residual stresses caused by the manufacturing process, as well as the effect of the 3D bubbles distribution on the internal stress patterns after the thermic gradient. The promising results from this study have the potential to inform further research on such composite materials by optimising manufacturing processes for targeted applications.

Vitreous enamel steels are a particular class of composite materials composed by a low carbon steel basement coated by a vitreous enamel layer. Throughout the firing process applied to fix the enamel on the steel substrate, several gas bubbles remain entrapped inside the internal volume of the enamel modifying its internal microstructure. The presence of these bubbles substantially modifies the internal mechanical state of the structure developing residual stresses both among the bubbles and between the enamel-metal surface. However, to date no methods are still available to properly investigate the 3D bubbles morphology, distribution and stress patterns inside these materials. For this reason, in the present study we developed for the first time a high-resolution X-ray computed tomography (micro-CT) protocol able to investigate the vitreous enamel steels full field structure and numerically study their mechanics when the thermal gradient is applied. The micro-CT scans reconstructions allowed the visualisation of the enamel coating structure minimising metal artefacts. Moreover, the scans were postprocessed developing unpreceded 3D reconstructions with which the distribution, the volume and the mean diameter of the bubbles were analysed and defined. Subsequently, full field finite element computational models able to evaluate the thermal residual stresses produced inside the enamel volume were developed. They permitted to investigate the effect of the bubbles distribution on the internal residual stress patterns due to the thermal gradient generated throughout the cooling phase. The promising results from this study have the potential to inform further research on such composite materials by optimising manufacturing processes for targeted applications.