Competition between bead boundary fusion and crystallization kinetics in material extrusion-based additive manufacturing
Résumé
Assembling polymer beads into well-defined 3D geometries through a layer by layer deposition process, such as
material extrusion additive manufacturing, remains challenging due to the complex interplay between the
several kinetics involved (flow, inter-beads diffusion/fusion, cooling, solidification, etc). Here, we explore the
influence of physical parameters like bead cross section geometry, partial bead fusion (also called bead coalescence
or bead welding) and crystallization kinetics on the 3D printing of isotactic polypropylene as a model
semi-crystalline polymer. New methods for the characterization of printed stacks cross sections, interlayer
cohesion and viscoelastic coalescence are introduced. Our results demonstrate how printing conditions, through
input parameters like layer height and nozzle temperature, can greatly affect interlayer cohesion in relation to
polymer interdiffusion at the interface. We show that the relevant timescale for fusion is mainly driven by the
polymer’s rheology, and that fusion is many-fold faster under Hertzian compression, i.e. due to the effect of
contact pressure. Quantitative description of the influence of the extrusion nozzle and build platform temperatures
on the crystallization time of printed beads highlights how the processability window is defined by the
competition between fusion and crystallization. Our approach therefore provides a framework for the optimization
of process parameters based on the physical processes involved during the production run of semicrystalline
polymers.