Rheology, crystallization, and process conditions: The effect on interlayer properties in three-dimensional printing

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. Furthermore, 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 the 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 the 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.

Thermo-Mechanical Recyclability of Additively Manufactured Polypropylene and Polylactic Acid Parts and Polypropylene Support Structures

“…They identified the raster deposition angle, nozzle temperature, printing speed, and infill density significantly impact the mechanical properties of 3D-printed PLA parts. The effect on interlayer properties caused by the rheology, crystallization, and processing of PLA and PP thermoplastics was investigated by Arit et al [ 48 ].…”

Mechanical characterization of additively manufactured polymer composites: A state‐of‐the‐art review and future scope

“…[ 190 ] The print size and the printing orientation significantly affect the thermal flow‐induced crystallization in the FDM printed parts, depending on the print size, the rates of cooling and solidification of the polymer melted during printing can change, which may impact the crystallization level. [ 191 ] Modifying the print size or layer height is one technique for controlling the flow‐induced crystallization effects. Slower cooling rates may come from thicker layers or higher print sizes, which could enhance crystallization.…”

Influence of FFF Process Conditions on the Thermal, Mechanical, and Rheological Properties of Poly(hydroxybutyrate-co-hydroxy Hexanoate)

“…On the other hand, interlayer welding can also be affected. Models [73,[82][83][84] predict that the development of oriented polymer chains results in a shear-induced disentanglement process that decreases entanglement densities. This could negatively impact welding energy for printing conditions where entanglement does not have sufficient time to recover during cooling, as the degree of chain stretching during shear is determined by the competition between shear deformation and chain relaxation in the tube.…”

“…The flow strength can be quantified by dimensionless numbers, such as the Deborah number, De, and the Weissenberg number, Wi. Following the analysis proposed by McIlroy and Olmsted [84,88], the description of the 3D printing process can result in flowinduced crystallization under the following conditions: (1) The flow is sufficiently strong to stretch polymer chains, a condition that can be quantified by the Rouse Weisenberg number (W iR = . γτ R ) that governs polymer stretching, greater than 1.…”