  Fused filament fabrication (FFF) is widely used to obtain polymer-based
composites with improved mechanical and thermal conduction properties. The
effective properties of the composites are sensitive to its structure including
the shape, distribution, and orientation of inclusions in the matrix and
FFF-specific defects. The present work aims to study the formation of composite
structure and relate it to the measured effective thermal conduction
properties. Polymer flow in the hot end and the structure of printed samples
are studied by metallography. The effective thermal diffusivity is measured by
the laser flash method and analyzed by the Maxwell Garnett theory. Fibers in a
polymer-matrix composite visualize polymer flow in the hot end of the FFF
printer. In the nozzle, fibers are generally oriented parallel to the flow
direction while gas bubbles may locally disturb flow and disorient fibers.
Printed composite Carbon fiber/TPU inherits preferential orientation of fibers
observed in the filament. Disorientation of fibers relative to the alignment
direction increases after printing. In the printed Al2O3/PA composite, there is
a lattice of triangle-based cylindrical pores parallel to the extrusion
direction. Al2O3 particles are of irregular shape and approximately equiaxed.
No segregation or agglomeration are observed. Significant anisotropy of thermal
diffusivity is observed in the printed samples of 5%vol. Carbon fiber/TPU
composite. The measured effective thermal diffusivity of printed Al2O3/PA
composites is essentially isotropic and significantly increases with the volume
fraction of inclusions. In the studied FFF-printed composites, the effective
thermal diffusivity is proportional to the thermal diffusivity of polymer
matrix, which is sensitive to its structure formed in non-equilibrium
conditions of rapid quenching from liquid phase.
