Processing-structure-property relationship in fused filament fabrication of Nylon-12
Keywords: Processing-Structure-Property Relationship
Crystallisation Kinetics
Fused Deposition Modelling
Solid Mechanics
Fused Filament Fabrication
Polymer Science
Additive Manufacturing

This thesis is devoted to the investigation of the structure-property-relationships in additive manufacturing (AM). General thermodynamic and kinetic aspects that are peculiar for this manufacturing mode are considered, and made specific and applied by selecting a particular flavour of AM (FFF) and material (Nylon-12).
Fused Filament Fabrication (FFF) is an extrusion-based additive manufacturing method for thermoplastic polymers. Nylon-12 is a semi-crystalline thermoplastic polymer that is one of the most commonly used materials in FFF. The FFF process results in a unique spatially dependent thermal history, which has a considerable effect on the resultant microstructure. Crystallisation within the polymer proceeds via temperature-dependent nucleation and growth. In turn, the crystal structure relates to and determines the mechanical properties in semi-crystalline polymers.
This relationship can be referred to as the processing-structure-property relationship. This thesis focuses on a specific area of this relationship in FFF Nylon-12. These focused areas are the effect of the thermal history on the development of crystal morphology in Nylon-12 and degree of crystallinity, and the subsequent effect of the degree of crystallinity on tensile properties.
Precise control has not yet been achieved over the properties of FFF samples, which has provided a considerable barrier to industry adoption. Additionally, there have been comparatively few studies on Nylon-12 in the context of FFF compared to other polymeric materials, despite its extensive use in FFF. The aim of this thesis, therefore, is to build a quantitative link between thermal history, degree of crystallinity and tensile properties in Nylon-12 which can be used to provide more precise control and the ability to tailor the properties of FFF samples.
In situ X-ray diffraction was performed on FFF Nylon-12 samples, revealing that the orientation resulting from extrusion leads to a particular temperature-dependent crystal structure. Slower cooling rates from the melt were shown to result in a higher degree of crystallinity and higher lamellar density. It was shown that the thermal conditions during FFF lead to formation of the γ-form crystal structure, but with crystal structure differing from cast, sintered or moulded Nylon-12. A model of structural evolution was proposed to explain these observations.
The thermal properties of Nylon-12 filament and FFF samples produced under different processing conditions were examined through Differential Scanning Calorimetry (DSC). It was found that Nylon-12 crystallises optimally at temperatures above 150 °C. Slower cooling rates and higher annealing temperatures were shown to result in a higher degree of crystallinity and more perfectly formed crystal structure. The processing-structure relationship between processing conditions and the degree of crystallinity was analysed. A model of the crystallisation behaviour was constructed based on this data. An alternative fit to model crystallisation behaviour was proposed, which has the ability to make quantitative predictions about evolution of crystallinity and rate of crystallisation based on thermal history.
Tensile testing of FFF-produced Nylon-12 samples revealed the relationships between processing conditions and tensile properties, specifically Young’s Modulus, ultimate tensile strength, yield point and elongation at break. These data sets were fitted with a quantitative relationship. Finally, governing equations for the processing-structure-property relationship for FFF Nylon-12 were proposed based on the collected body of data.
This thesis sheds light on the processing-structure-property relationship in FFF Nylon-12, providing a full link from processing conditions to the mechanical properties. Additionally, the analysis and modelling of crystallisation behaviour has allowed for the data to be well-fitted to a set of governing equations that allow the prediction of tensile behaviour across a wider set of processing conditions. The methodology proposed, developed and used in this thesis provides the basis for further investigation into other components of the processing-structure-property relationship in Nylon-12, or for building similar relationships for other semi-crystalline thermoplastic polymers. Moreover, the present framework can be extended to other AM modalities, e.g., laser-based fabrication. This new understanding can lead to tailoring of structural and mechanical properties via alteration of the thermal history, eliminating the need for trial and error in AM and improving its performance compared to other manufacturing methods.