Additive Manufacturing and Mechanics of Multiscale Architected Flexible Syntactic Foams

Abstract

Polymer syntactic foam is a lightweight composite consisting of hollow particles, like Glass Micro-Balloons (GMBs) or cenospheres, reinforced in a continuous polymer matrix. Due to their inherent weight-saving characteristics and enhanced mechanical properties, these foams are attractive for use in aerospace and marine industries. Recent advances in additive manufacturing (AM) techniques have enabled the development of complex-shaped parts of syntactic foams and circumvents the need for advanced highcost equipment to produce these parts. Selective Laser Sintering (SLS) is a widely adopted powder-based AM technique used to manufacture 3D parts by sintering polymer powder, and unlike other 3D printing methods, SLS does not require support structures. SLS has been reported to generate a segregated matrix system when used with Thermoplastic Urethane (TPU) in a standalone manner. However, the introduction of GMBs to this manufacturing method has thus far not been extensively studied. Consequently, the influence of GMB parameters on the mechanical response of syntactic foam with a segregated matrix is not fully understood. In this work, we use SLS to fabricate and further investigate the mechanical performance of segregated TPU matrix syntactic foam with different grades and volume fractions of (GMBs). We show for the first time that GMB size drives internal microscale architecture within syntactic foams that manifest as counterintuitive macroscale mechanical responses. That is, GMBs with a diameter larger than gaps between the cell walls of the segregated matrix get lodged between the cell walls while those smaller tend to get lodged inside the cell walls of the segregated matrix. Because of this, larger particles increase the stiffness of the syntactic foams while smaller ones do not contribute to this significantly. On the other hand, larger particles with their lower crushing strength reduce the densification stress of the foam, whereas the foam with smaller particles with higher crushing strength behaved similar to pure TPU but with significantly reduced weight. Overall, we show that coupling hollow particle parameters with print parameters can enable the fabrication of 3D printed syntactic foams with hierarchical tailored architectures and functional properties. These findings can be adapted to the development and design of cores for lightweight sandwich structures in the marine and aerospace industries.