Rheological and Thermal Insights into Solidification Dynamics in Extrusion-Based Additive Manufacturing of Polymer Composites

Abstract

Polymer extrusion-based additive manufacturing (EB-AM) is a processing technique that allows building 3D parts by depositing liquified material layer by layer, which solidifies to form a solid structure. Its key advantages over traditional methods like injection molding include design flexibility, the ability to create complex geometries (e.g., lattices, internal channels), and faster prototyping due to the absence of mold fabrication. However, EB-AM also presents challenges, where each layer must solidify sufficiently to retain its deposited shape and support subsequent layers, requiring understanding of the viscoelastic properties of the material and the use of solidification mechanisms to ensure structural integrity during printing. While solidification favors shape retention, it hinders interlayer chain diffusion, compromising interlayer adhesion and mechanical integrity. Achieving adequate EB-AM processing requires understanding solidification to ensure structural fidelity without hindering interlayer adhesion. This balance is influenced by printer format, with small-format EB-AM promoting rapid cooling and more anisotropy due to higher shear rates, while medium and large-format systems retain more heat, enhancing adhesion at the cost of increased likelihood of shape distortion, especially at high printing speeds (short layer times). To address the differences in processing at different size scales and printing speeds, a literature review in Chapter 2 explores the differences between process physics in EB-AM across size (from small to large-format printers) and time scales (the effect of printing speeds and layer time) on the printing process, focusing on semicrystalline polymers as feedstock. We explore how polymer physics control process-structure-property relationships at different length- and time scales, focusing on how disparities in shear rates, rheological behavior, and heat retention affect crystallization kinetics and polymer chain mobility, relating it to the development of printed microstructure and interlayer adhesion. Strategies to minimize common issues in printing semicrystalline polymers are also included, for example through material design and processing modifications to reduce volumetric shrinkage and warpage. Lastly, fundamental research gaps in processing semicrystalline polymers in EB-AM are included, focusing on the importance of formulating the next-generation of materials and process monitoring tools to enable facilitated implementation of EB-AM across different size scales. To complement the thorough discussion of EB-AM of semicrystalline polymers at different size/time scales in Chapter 2.1, a brief review of fundamental aspects related to EB-AM processing of hydrogels through small-format direct ink writing (DIW) is presented in Chapter 2.2. The key rheological requirements for effective processing are highlighted, and a brief explanation of chitosan hydrogels is included, as this is the only non-semicrystalline polymer used in this dissertation. Chapter 3 continues on EB-AM processing of chitosan with DIW by exploring the rheological predictors of shape retention when printing without the immediate application of a solidification mechanism. This study investigates the rheological behavior of chitosan (CS)-based hydrogels incorporating graphene and titanium dioxide (TiO₂) as functional fillers. By correlating rheological properties with printed bead morphology, we identify tan delta as a key predictor of shape retention, where formulations exhibiting a predominant solid-like behavior (tan delta ≤ 1 at low angular frequencies) demonstrated adequate structural integrity post-deposition. The yield stress of formulations did not correlate with extrusion reliability, which was instead influenced by particle loading. Lastly, the timescale required for formulations to change from predominant solid-like to liquid-like was studied, correlating it with the ability to successfully print a multilayer part. These findings provide valuable insights into rheology-driven design strategies for hydrogel-based inks in DIW, enhancing the effectiveness of polymer composite printing. In Chapters 4 and 5, the focus changes from chitosan hydrogels to semicrystalline polymer composites, and from small-format EB-AM through DIW to medium-format fused granular fabrication EB-AM, exploring intricacies of structure-process-properties relationships in the solidification behavior of polypropylene (PP) composites and how their properties impact high-speed printing (short layer times). In Chapter 4, the focus is on the solidification behavior of PP filled with graphite (GR) and carbon fiber (CF), contrasting the effect of medium-format EB-AM processing and fillers on affecting crystallization and physical gelation of the system. We show that fillers aid the physical gelation process by increasing interconnectivity of crystal clusters, especially in CF due to its longer aspect ratio. However, even though fillers facilitate physical gelation, crystallization studies showed that the addition of fillers to the 3D printing grade of PP used in this work slows down the nucleation process, especially GR. Processing history was also shown to negatively affect the ability of the material to crystallize due to thermal degradation. In addition, the effect of medium-format EB-AM processing on the alignment of polymer crystals is studied, indicating less alignment than literature values for small-format EB-AM due to smaller shear rates associated with the process. The results shown in this chapter provide substantial insights into the effect of fillers on the solidification and physical gelation of PP for additive manufacturing, highlighting the importance of filler considerations when designing formulations for EB-AM. Lastly, Chapter 5 continues to build up the current understanding of medium-format EB-AM of semicrystalline polymers by investigating material requirements necessary to optimize high-speed printing (short layer times) of PP composites. One of the drawbacks of EB-AM of polymer parts is due to long printing times caused by low printing speeds (typically 50–70 mm/s), primarily due to heat accumulation at higher speeds that compromises shape fidelity. This study investigates the influence of CF and GR fillers on the rheological properties and thermal diffusivity of a 3D printing grade of PP, correlating these with shape fidelity at high-speed printing conditions (333 mm/s, 1.1 s/layer). Enhanced shape fidelity in filled samples is attributed to improved heat dissipation caused by higher thermal diffusivities. However, the size of the deposited bead is largely dependent on filler type, being more impactful on heat retention than improvements on thermal diffusivity caused by the inclusion of fillers. In samples prone to more heat retention, improvements in shape fidelity are dependent on larger storage modulus, enabling better shape retention for parts that stay hotter for longer. The findings highlight the critical role of material composition and bead geometry in overcoming speed-related limitations in EB-AM.