05/15/2025
By Danielle Fretwell

The Francis College of Engineering, Department of Plastics Engineering, invites you to attend a Doctoral Dissertation defense by Juhyeong Lee on: "Understanding the processing-structure-property relationship in ABS: Focus on interlayer bonding in FFF."

Candidate Name: Juhyeong Lee
Degree: Doctoral
Defense Date: Thursday, May 22, 2025
Time: 12 - 2 p.m.
Location: ETIC 445

Committee:
Advisor: Jay Park, Associate Professor, Plastic Engineering, UMass Lowell

Committee Members

  • David Kazmer, Professor, Plastic Engineering, UMass Lowell
  • Amir Ameli, Associate Professor, Plastic Engineering, UMass Lowell
  • Christopher Hansen, Professor & SHAP3D Site Director, Mechanical and Industrial Engineering, UMass Lowell

Abstract:
Fused filament fabrication (FFF), a type of material extrusion additive manufacturing, offers distinct advantages in producing complex polymeric structures with high design flexibility and cost efficiency. Among various thermoplastics, acrylonitrile–butadiene–styrene (ABS) is widely used in FFF due to its favorable balance between mechanical and rheological properties for FFF as well as its accessibility. However, despite its frequent usage, a comprehensive understanding of the relationship between the microstructural characteristics of ABS and the interlayer bonding mechanisms intrinsic to the FFF process remains lacking. This dissertation aims to elucidate the processing–structure–property relationships that govern interlayer bonding in FFF using ABS, from both morphological and rheological perspectives.

The first study presents a fundamental investigation into how the polymerization method and resultant microstructure of four commercial ABS grades, one mass-polymerized and three emulsion-polymerized ABS grades, influence key rheological properties associated with interlayer bonding, such as zero-shear viscosity, reptation time, and number average relaxation time. The second study evaluates the practical implications of these findings by characterizing the interlayer bonding performance of each ABS grade in both as-printed and thermally annealed states. The results demonstrate the critical role of temperature-dependent reptation dynamics, rubber particle distribution, and interfacial adhesion between rubber particles and the SAN matrix in promoting healing of interlayer bonding and residual stress relaxation. The third study explores a novel application: the design of a dual-layer (DL) filament composed of a highly filled ABS/CaCO₃ composite core and a virgin ABS shell. This coextruded structure provides both structural and rheological advantages, resulting in significantly enhanced interlayer bonding and reduced anisotropy.

Collectively, this dissertation provides a comprehensive analysis of how morphology, rheology, and processing strategies interact to influence interlayer bonding performance in ABS-based FFF systems. The findings contribute not only to the mechanistic understanding of interfacial phenomena but also offer practical guidance for filament design and post-processing protocols to improve the structural integrity of FFF printed parts.