07/02/2025
By Danielle Fretwell

The Francis College of Engineering, Department of Plastics Engineering, invites you to attend a Doctoral Dissertation defense by Nikhil Patil on "Additive Manufacturing of Elastomeric Composites: Systematic Mechanical Characterization and Material Structure Property Relationship."

Candidate Name: Nikhil Patil
Degree: Doctoral
Defense Date: Wednesday, July 16, 2025
Time: 10 a.m. - noon
Location: ETIC 245

Committee:

  • Advisor: Jay Hoon Park, Ph.D., Associate Professor, Department of Plastics Engineering, University of Massachusetts Lowell
  • Amir Ameli, Ph.D., Associate Professor, Department of Plastics Engineering, University of Massachusetts Lowell
  • Alireza Amirkhizi, Ph.D., Associate Professor, Department of Mechanical and Industrial Engineering, UMass Lowell
  • Eric D Wetzel, Ph.D., Team Leader Strategic Polymers Additive Manufacturing, US Army Research Laboratory, Maryland

Abstract:
Additive manufacturing of elastomeric materials has gained significant importance due to their unique flexible structures, extensibility, and customizable geometries. This has led to the widespread use of such materials in functional components, soft robotics, and automotive applications. Filament-based flexible material extrusion additive manufacturing is highly desirable, yet it faces critical challenges such as buckling, limited print resolution and repeatability.

Chapter 3 focuses on enhancing the printability of a thermoplastic elastomer (TPE) using a series of core–shell filaments composed of a TPE shell (Shore hardness 75A) and a rigid ABS core, with ABS volume fractions ranging from 11% to 78%. The presence of the ABS core imparts rigidity to the filament, inhibiting buckling and enabling successful, high-fidelity 3D printing. Rheological characterization of TPE and ABS using capillary and parallel-plate viscometry was conducted to determine optimal coextrusion parameters and improve printability and interfacial wettability. Izod impact, three-point bending, and tensile tests revealed tunable mechanical properties for parts printed in the Z-direction using ABS+TPE filaments. Lower ABS content resulted in higher flexibility and impact resistance, while higher ABS content provided increased stiffness and tensile strength. Notably, parts printed with 30–40% ABS demonstrated higher toughness and impact strength than specimens made from ABS or TPE alone. A coil spring printed with an ABS+TPE filament showed reversible extension up to 120 mm, more than twice that of a comparable ABS-only printed spring.

Chapter 4 explores the mechanical tunability of ABS+TPE core–shell composite filaments by investigating the effects of the filament core/shell ratio and raster orientation on controlling mechanical anisotropy in 3D-printed structures. Load frame experiments demonstrated control over a wide range of tensile modulus (50 MPa to 2200 MPa) and flexural modulus (50 MPa to 2600 MPa) by varying the number and sequence of raster orientations (0°, ±45°, and 90° plies) within 16-layer test coupons. Asymmetric ply stacks (with respect to the sample mid-plane) exhibited bending responses sensitive to the direction of bending. Segmented designs, in which print orientation varied along the coupon length, displayed localized bending behavior. Analytical composite laminate theory and finite element simulations captured the broad trends in mechanical response for these soft 3D-printed composites. However, these models overpredicted structural stiffness as ABS content increased, primarily due to strain localization and softening in the TPE phase.

Chapter 5 investigates the mechano-rheological behavior of silicone rubber-based thermoplastic elastomers (TPSiV) for material extrusion additive manufacturing using core–shell filament-based strategy. By combining ABS with two grades of dynamically vulcanized TPEs TPSiV 75A and TPSiV 50A we examined how molecular architecture and phase morphology influence extrusion performance, printability, and mechanical behavior. Rheological and thermal analyses (DMA) revealed that TPE50A has a softer and more heterogeneous structure, with a higher molecular weight between crosslinks (Mc = 315.31 g/mol) compared to TPE75A (Mc = 83.08 g/mol). AFM imaging showed contrasting phase morphologies: TPE50A displayed a rubber-rich matrix, whereas TPE75A exhibited dispersed rubber domains within a rigid thermoplastic matrix. These differences in material architecture directly influenced flow characteristics and printability of the core–shell filaments. Mechanical testing of printed structures confirmed that the core–shell design enables tunable mechanical anisotropy showcasing significantly enhanced directional mechanical behavior, achieving an E₀/E₉₀ ratio greater than 15× for ABS+TPE50A at a 30:70 ratio. Compression tests further demonstrated improved compliance and energy absorption in high-TPE-content structures.