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Title: Improved thermal control and mechanical property evaluation for multi-dimensional fused filament fabrication of sandwich cores
Author: Pollard, Dave
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2019
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Additive Manufacture (AM) is a technique capable of economically producing low volumes of complex components. Fused Filament Fabrication (FFF), a form of AM, successively deposits layers of extruded filament; the inter-layer bonds are typically the weakest regions the structure, with the strength determined by the deposition temperature and cooling rate. This thesis investigates its application to production of aerospace sandwich cores, through improved thermal control of the extruded temperature, the use of an industrial robotic arm for manipulation, and the mechanical properties of thin-walled components. An investigation into the high speed thermal dynamics of the FFF process monitored the filament temperature, identifying a relationship with the recent time history of the feed rate, and a thermistor embedded in the nozzle better reflected the extruded temperature than a conventional, block-mounted thermistor. The application of a feed-forward controller greatly reduced fluctuations in the nozzle and filament temperature. To overcome the "stacked" planes of inter-layer bonds produced by typical 3 axis FFF machines, the process was implemented on an 8 axis industrial robotic cell. This allowed for deposition of non-planar layers onto a cylinder and hemisphere, and the production of complex aerofoil cores. This system was then used to evaluate the inter-layer bond strength of components produced with different bed orientations, relative to the gravity vector, and nozzle orientations, relative to the print bed. Results showed there was little effect of bed orientation on bond strength, but a significant effect of the nozzle orientation. Additional testing identified the wall thickness affected the ductility of the failure under tension, with specimens of increasing wall thickness behaving closer to that expected of the bulk material. Similar results were observed for compressive testing of FFF cores, finding a similar specific compressive strength to the industrial-standard Nomex material, but a significantly higher compressive force.
Supervisor: Richards, Arthur ; Ward, Carwyn Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: FFF ; Arm-based FFF ; 3D Printing ; Thermal Control ; Core manufacture ; Sandwich structures