Design and fabrication of a new micro-extruder for 3D printing of biopolymers

The goal of this thesis is providing solutions to fabricate, via Fused Deposition Modelling, scaffolds for the regeneration of long bones, based on biopolymeric blends or polymer-ceramic composites. The background of the thesis is the EU-funded GIOTTO project, which was conceived to fight the consequences of osteoporotic fractures by developing advanced tissue engineering solutions. Both the direct and the indirect 3D printing of blends, starting from pellets of the pure components, were considered. The main part of the thesis focused on the direct strategy, with the design of a novel kind of ‘micro-extruder’, which brings to a miniaturized scale the process of Hot Melt Extrusion (HME) of thermoplastics. The device that can be installed on a Fused Deposition Modelling (FDM) 3D printer in place of the conventional filament-based hotend. The micro-extruder design was carried out using a mathematical model of the polymer conveyance and extrusion process, with particular focus on maximizing multiple materials mixing, and minimizing their thermal degradation. The mechanical components were verified for extrusion flow rates and material viscosities commonly found in FDM printing, and for a nozzle diameter down to 0.6 mm. The heating system was designed with the help of finite elements simulations of the heat diffusion and convection. A value of fictitious filament diameter was found, so that the extrusion happens at the correct flow rate according to the model, when the device is controlled by conventional FDM slicing software. This simplifies dramatically the print control. A prototype version of the micro-extruder was assembled, mounted on a pedestal, and successfully tested for warming up to 220-250 °C, and extrusion of a continuous filament from pellets with a diameter of 1.75 mm. On the other hand, for the indirect strategy, it was designed a quantitative characterization method of the geometry of the feedstock filaments for subsequent conventional FDM. These are fabricated by conventional HME starting from pellets, and have stringent requirements of size and shape to allow a proper printing