Interlaminar morphology and nonisothermal healing in fused filament fabrication
Date
2021
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
Fused filament fabrication (FFF), sometimes called material extrusion (ME) offers an alternative option to traditional polymer manufacturing techniques to allow the fabrication of objects without the need of a mold or template. However, these parts are limited in the degree to which the welding interface is eliminated (healed) post deposition, resulting in a decrease in the interlaminar fracture toughness relative to the bulk material (Z-strength). Here reptation theory under nonisothermal conditions is utilized to predict the development of healing over time, from the rheological and thermal properties of Acrylonitrile-Butadiene-Styrene (ABS). ABS is rheologically complex and acts as a gel and as such considerations had to be made for the relaxation time of the matrix which is important in predicting the degree of interfacial healing. The nonsiothermal healing model developed is then successfully compared to experimental interlaminar single edge notched bend (SENB) fracture experiments at variable printing temperatures, allowing future optimization of the process to make stronger parts. ☐ Additionally, we demonstrate a method to rapidly optimize print processing conditions to maximize both weld strength as well as the surface contact developed between layers using a modified ASTM D1938-19 trouser tear experiment. This creates a processing map, consisting of both interlayer morphology and fracture toughness, giving 3D printing operators a guide to picking processing conditions that maximize the part strength. Here we once again are using ABS, a complex structured copolymer that has been shown to exhibit gel-like behavior. ☐ In our previous studies, we focused on one grade of ABS, limiting us to a narrow understanding of potential variations in material behavior. ABS is a blend of grafted butadiene particles, and a copolymer matrix, both have respective variable molecular architectures and characteristics. This complex incompatible blend, both exhibits gel-like characteristics, and can further gelate at elevated temperatures (butadiene particle agglomeration). Here, we study six different grades of ABS. Each grade of ABS is rigorously characterized rheologically, including the strength and relaxation of the gel-like behavior. The exact mechanisms describing the gel-like behavior aren't fully understood, but characteristics about the gel-like content such as insoluble gel fraction, and the particle size distributions are provided to give context to the origin of the gel-like behavior. We provide an experimental framework for characterizing filled and unfilled polymeric materials, allowing conclusions to be made about the suitability of a material for printing or fiber drawing from the molecular characteristics. ☐ Again utilizing the processing map approach, the interlaminar morphology and welding trends for polylactic acid (PLA) are characterized and compared to the results for ABS. Despite large differences in the thermal and viscoelastic fluid properties of the two materials, the surface contact developed between single layers remains identical for printed parts. As hypothesized, we demonstrate that the relaxation time (of molecular diffusion) is a key metric in the assessment of a material, for the maximization of fracture toughness. ☐ Finally, composites consisting of talc blended in PLA were processed to exploit the role of talc as a nucleating agent for crystallization in PLA. The talc was unsuccessful at inducing crystallization at the high cooling rates experienced in FFF. However, we characterize the welding behavior of the as printed (amorphous) samples, thus isolating the role of talc on the fracture toughness. Then the welding behaviour of samples annealed to achieve full crystallization is compared to the amorphous samples, demonstrating the isolated effect of crystallization on fracture toughness of FFF specimens. We observe that higher crystallization may be undesirable as it lowers fracture toughness considerably, with possible implications for other semicrystalline polymers for use in FFF.
Description
Keywords
3D printing, Additive manufacturing, Fracture toughness, Interlaminar morphology, Welding