Thermal analyses and characterisation of fused filament fabrication

Fused filament fabrication (FFF) is the most accessible 3D printing or additive manufacturing technology to the general public and professional customers. It has been increasingly used in daily life, as well as primary, secondary and higher education, and industries such as automotive, aerospace and medicine.

FFF employs a thermoplastic filament to fabricate a 3D part in an additive manner, typically strand-by-strand and then layer-by-layer. During the process, the filament feedstock is softened inside a hot metal block and then extruded through a nozzle by the pressure from the filament itself from the cold end. A motion control system then selectively dispenses the extrudate to attach to an existing anchor, such as a supportive build plate or previously deposited material. Different deposited strands then fuse together to form a part, driven by the thermal energy of the material.

The thermal process and thermal history in the printed part have been widely investigated since the invention of this technology. However, many aspects have not been thoroughly understood. For example, the extrudate temperature at the nozzle outlet, the significance of radiant heat transfers between the printed part and the hot-end, or the weak bond between layers. These unknowns or doubts are related to the fundamental characteristics of FFF, including the changing geometry during the additive deposition, the small heat transfer dimensions (typical order of magnitude for the layer thickness 10^2 µm), and rapid phase changes from glass to visco-elastic states then back to glass state within typically 10^1 or 10^2 s. A comprehensive description of the entire thermal process from the filament to the printed part is still missing.

This thesis investigates the thermal process in FFF through thermal monitoring and modelling in order to achieve better process control, part performance and higher production efficiency. It combines experimental monitoring by infra-red thermography with numerical and analytical analyses to study the heat transfers inside and outside the hot-end, particularly the temperature fields and their variations during the printing.

An open-access numerical model, T4F3: Temperature for Fused Filament Fabrication, is developed to calculate the transient and steady-state temperature of the printed part when the user specifies the material, geometry and process conditions. Comprehensive model validations have been done against experimental datasets gathered from the literature and collected in-house. They cover different machines, geometries, materials, process conditions, temperature measurement methods, etc. The influence of thermal conduction (e.g., intra-layer and inter-layer reheating, conduction anisotropy), thermal convection (the role of a closed chamber, printing in vacuum) and thermal radiation (between printed part and the far environment or the hot-end) are investigated in detail. The model has been successfully applied to simulate printing on the Moon, to develop adaptive printing strategies to avoid meltdown, to design in-situ radiant pre-heating and post-heating to eliminate weak bonds and to design printing paths that lead to location-independent bond quality.

The model is openly available at https://iiw.kuleuven.be/onderzoek/aml/technologyoffer/FFFthermalsimulation/T4F3_original. Name the FFF process, it gives all temperature information on the printed part.