Additive Manufacturing of Carbon Fibre Reinforced Polymer Composites

Additive manufacturing (AM) of carbon fibre reinforced thermoplastic composites can offer
advantages over traditional carbon fibre manufacture through improved design freedom and
reduction in production time and cost. However state-of-the-art fused deposition modelling
(FDM) approaches for the production of carbon fibre composites, generally possess high
porosity compared to conventionally manufactured advanced composites. On the other hand,
the addition of fibre to the polymer powder bed fusion processes such as selective laser
sintering (SLS) creates drawbacks such as poor fibre distribution in the powder feedstock,
high porosity and low strength in the final parts. The reason for low-performance composites
in AM processes is the lack of heat and pressure application. The traditional manufacturing
processes for advanced composites improve the consolidation of the layers with autoclave
machines or hot press processes, whereas in AM processes, compaction is not ideal to sustain
the complexity of final geometry.
In this study, the development of a novel alternative to the current composite AM based
on sheet lamination, named composite fibre additive manufacturing (CFAM), is presented
with the aim of reducing component porosity. This approach involves selectively inkjet
printing a binder and polymer powder onto discontinuous carbon fibre sheets which are
then compressed and heated to form net shape components. Using CFAM, complex-shaped
discontinuous carbon fibre reinforced nylon composite parts were successfully manufactured
for the first time.
First of all the effect of process parameters such as applied pressure level, compaction
time, and the volume of printed ink on the mechanical and microstructural properties of final
parts was investigated to benchmark the process. Further investigation focused on polymer
morphology to understand the effect of crystallinity on the CFAM final part properties. Additionally, a combination of hybrid layers involving continuous and discontinuous carbon fibre
was examined with the aim of improving the mechanical properties. Finally, complex-shaped
drone frames were manufactured to investigate the dimensional tolerances. The geometric
accuracy and the load-carrying performance of the final products produced with the CFAM and other AM processes are examined and compared.
The results demonstrate a correlation between the amount of pressure applied and the
percentage of porosity and fibre volume fraction in the final parts. By providing an optimum
level of pressure, the voids which can act as crack propagators, reduce significantly (1.5%).
Hence more consolidated parts with relatively higher tensile strength and stiffness were
obtained. Interaction between the crystallization process of thermoplastic matrix material in
CFAM and resultant part properties was established using a new approach based on elements
of DSC analysis, which provides a new level of understanding into the polymer behaviour
under the different processing conditions. Finally, the end-use product performance of
CFAM was found to be superior compared to the other composite AM techniques. The
innovative fabrication process established in this thesis achieved the rapid production of
high-performance discontinuous carbon fibre reinforced polymer parts with flexibility in
material composition.