Lawrence Livermore National Laboratory (LLNL) material scientists have found that 3D-printed foam is more durable and maintains its mechanical performance characteristics longer than standard cellular materials.

Foams, also known as cellular solids, are an important class of materials, with applications ranging from thermal insulation and shock-absorbing support cushions to lightweight structural and flotation components. These materials are essential to a large number of industries, including automotive, aerospace, electronics, marine, biomedical, packaging and defense.

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 the 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.

Microstructure of a traditional open-cell stochastic foam (left) and a 3D-printed foam (right). Image credit: LLNL.Microstructure of a traditional open-cell stochastic foam (left) and a 3D-printed foam (right). Image credit: LLNL.However, since 3D printing requires the use of polymers of certain properties, it was important to determine the long-term mechanical stability of such printed materials before they can be commercialized. This is especially important 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 one year, and in some cases 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.

The experiments convincingly demonstrated that 3D-printed materials age more slowly—i.e., they retain their mechanical and structural characteristics longer—than standard cellular materials. Interestingly, native rubber (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.

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