Microstructures of two different foam materials. At left, a traditional open-cell stochastic foam, and a right, an additively manufactured (AM) foam with the face-centered-tetragonal (FCT) lattice structure, the diameter of each cylindrical strut being 250 μm. Source: LLNL. Click to enlarge.

Traditionally, foams are created by processes that lead to a highly non-uniform structure with significant dispersion in size, shape, thickness, connectedness and topology of its constituent cells.

As an improved alternative, scientists at the additive manufacturing lab at LLNL recently demonstrated the feasibility of 3D printing of uniform foam structures through a process called direct-ink-write. The material is built up layer-by-layer, with each layer consisting of equally-spaced parallel cylinders of the same uniform diameter.

However, since 3D printing requires the use of polymers of certain properties, it is important to understand the long-term mechanical stability of such printed materials before they can be commercialized. This is especially vital in applications such as support cushions, where the foam material is subjected to long-term mechanical stresses.

To address the stability question, the LLNL team performed accelerated aging experiments in which samples of both traditional stochastic foam and 3D-printed materials were subjected to a set of elevated temperatures under constant compressive strain. The stress condition, mechanical response and permanent structural deformation of each sample were monitored for a period of one year and, in some cases, even longer. A method called time-temperature-superposition was then used to quantitatively model the evolution of such properties over a period of decades under ambient conditions.

This study convincingly demonstrated that 3D-printed materials age slowly—i.e., they better retain their mechanical and structural characteristics—as compared to their traditional counterparts. Interestingly, native rubber (i.e. elastomer) comprising each foam showed exactly the opposite effect—i.e., the rubber in the printed material aged faster than the corresponding rubber used in the traditional foam.

To gain further insight into why the printed cellular material displayed superior long-term stability, the team imaged the 3D micro-structure of each foam sample with X-ray computed tomography, and performed finite-element analysis of the stress distribution within each micro-structure. They found that there is a much wider variation in local stresses within the stochastic foam, with points of extreme stress significantly higher than the maximum stress points within the more uniform 3D-printed foam.

This paper represents the very first study of this nature. Here the long-term mechanical characteristics of a 3D printed polymer foam is carefully compared with that of a traditional stochastic foam through the analysis of multi-year-long accelerated aging data using a time-temperature-superposition procedure based on geometric arc-length minimization. The resulting master curves predict clearly superior long-term performance of the AM foam, both in terms of compression set and load retention. This result is remarkable given that the AM foam is created out of rubber with three times the stronger propensity for permanent deformation as compared to the rubber constituting the stochastic foam.

To gain insight, we have imaged the microstructures of both foams with X-ray computed tomography and carried out Finite-element analysis of stress distribution. Such analysis leads us to conclude that the superior long-term behavior of the AM foam is due to a more uniform local stress distribution pattern relative to the stochastic foam, which develops more extreme stress points within its microstructures. The latter is likely responsible for irreversible damage to the foam structure including pore collapse, strut fracture, and permanent deformation of the cell wall.

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An open-access paper on the research appears in the journal Scientific Reports.

3D printing of foams offers tremendous flexibility in creating programmable architectures, customizable shapes and tunable mechanical response. Now that our work strongly indicates superior long-term stability and performance of the printed material, there is no reason not to consider replacing traditional foam with appropriately designed 3D-printed foam in specific future applications.

—lead author Amitesh Maiti

Other Livermore co-authors include Ward Small, James Lewicki, Todd Weisgraber, Eric Duoss, Sarah Chinn, Mark Pearson, Christopher Spadaccini, Robert Maxwell and Thomas Wilson.

The group also acknowledges contributions from the National Security Campus, Missouri (formerly Kansas City Plant) where some of the early aging experiments were performed.

Resources

• A. Maiti, W. Small, J. P. Lewicki, T. H. Weisgraber, E. B. Duoss, S. C. Chinn, M. A. Pearson, C. M. Spadaccini, R. S. Maxwell & T. S. Wilson (2016) “3D printed cellular solid outperforms traditional stochastic foam in long-term mechanical response” Scientific Reports 6, Article number: 24871 doi: 10.1038/srep24871