DEVELOPMENT AND CHARACTERIZATION OF DIW 3D PRINTED EPOXY SYNTACTIC FOAM FOR LIGHTWEIGHT STRUCTURAL APPLICATIONS

Abstract

The development of lightweight materials with high specific strength and energy absorption capability is critical for advancing structural applications in aerospace, automotive, and defense industries. Syntactic foams, composed of hollow glass microballoons (GMBs) embedded within a polymer matrix, have emerged as promising candidates for such applications due to their tunable mechanical properties and low density. However, conventional fabrication methods such as casting present significant challenges when processing highly viscous, particle-filled epoxy systems, often leading to defects, voids, and limited geometric control.This research focuses on the development, processing, and mechanical characterization of 3D printable epoxy-based syntactic foam inks reinforced with hollow glass microballoons using Direct Ink Writing (DIW) technology. Three different types of GMBs from 3M: S60, iM16K, and iM30K, were incorporated into EPON 815C epoxy resin with Epikure 3230 curing agent. Rheological modifiers, including fumed silica and thixotropic additives, were used to tailor the ink’s shear-thinning behavior and yield stress for printability. Rheological characterization using a Discovery HR-30 Rheometer confirmed that all inks exhibited desirable shear-thinning behavior, while yield stress values varied depending on microballoon type. The iM16K ink exhibited the highest yield stress due to its highest volume fraction of microballoons resulting from its lower density and smaller particle size. Mechanical characterization was performed on both cast and 3D printed specimens. Tensile testing of 3D printed dogbone specimens in 0°, 45°, and 90° orientations revealed print orientation-dependent mechanical behavior, with the 0° orientation generally producing the highest tensile modulus, while strength varied depending on defect sensitivity and interlayer bonding. Compressive testing was conducted on cube specimens printed in both unidirectional and bidirectional print patterns and tested in the X- and Y-directions to evaluate anisotropy. Printed samples exhibited more consistent mechanical performance compared to cast specimens, which suffered from voids and inconsistencies due to the challenges of casting highly viscous inks. Ultimately, hybrid ink formulations were developed by combining iM16K and iM30K microballoons in different ratios (3:1, 2:2, and 1:3) to optimize specific strength and specific modulus. The iM30K_2 iM16K_2 hybrid ink emerged as the optimal formulation, exhibiting the highest balance of specific mechanical properties and superior crashworthiness parameters, including energy absorption (EA), specific energy absorption (SEA), mean crushing force (MCF), and peak crushing force (PCF). Finally, the optimized hybrid ink was used to design, and 3D print hexagonal honeycomb core structures with 30% infill density. In-plane compression testing revealed direction-dependent behavior, with the Y-direction exhibiting higher peak compressive strength due to loading along inclined walls, while the X-direction showed slightly higher stiffness. These results validated the capability of DIW 3D printing to fabricate architected epoxy syntactic foam structures with tailored mechanical performance suitable for lightweight, energy-absorbing applications. This research provides a comprehensive framework for the formulation, processing, and evaluation of DIW 3D printed syntactic foams, contributing to the advancement of lightweight structural materials for future engineering applications.