Abstract

Semicrystalline polymers are an attractive feedstock choice for material extrusion
(MatEx)-based three-dimensional printing processes. However, the printed parts often exhibit poor
mechanical properties due to weak interlayer strength thereby limiting the widespread adoption of
MatEx. Improved interlayer strength in the printed parts can be achieved through a combination
of process parameter selection and material modification but a physics-based understanding of the
underlying mechanism is not well understood. Further, the localized thermal history experienced
by the prints can significantly influence the strength of the interlayer welds. In this work, a
combined experimental and modeling approach has been employed to highlight the relative impact
of rheology, non-isothermal crystallization kinetics, and print geometry on the interlayer strength
of printed parts of two semicrystalline polymers, namely, polylactic acid (PLA) and polypropylene
(PP). Specifically, the print properties have been characterized as a function of print temperature
and print speed. In case of single road width wall (SRWW) PLA prints the total crystalline fraction
increases due to the broadening of the crystallization window at higher print temperatures and
lower print speeds. The results are substantiated by the constitutive modeling results that account
for the effects of quiescent crystallization. However, SRWW PP prints display a reduction in the
interlayer properties with temperature likely due to significant flow-induced crystallization effects,
as suggested by the model. Interestingly, in case of multilayer PP prints, the repeated
heating/cooling cycles encountered during printing counteracts the flow-induced effects leading
to an increase in mechanical properties with print temperature consistent with SRWW PLA prints.