In August 2015, Lawrence Livermore National Laboratory (LLNL) and Autodesk teamed up for a project that could have a profound impact on the gear that everyone from football players to soldiers to construction workers wear to protect their heads.
The lab, which has already performed some exciting work around the development of metamaterials through 3D printing microscale structures, is pairing their expertise with Autodesk’s Project Dream Catcher generative design software to create a new generation of helmets with 3D-printed cushions. The 18-month study is well underway and LLNL has just released some research results suggesting that 3D-printed foam is actually more durable and maintains better long-term mechanical performance than standard foam.
Tear apart the padding in a bike helmet, and one sees that the foam material has a nonuniform makeup, with the pockets of air and material varying greatly in terms of size, shape and thickness. As a result, the foam material found ubiquitously throughout the aerospace, defense, automotive fields and just about every other industry may not be optimized for a given application, and the long-term stability of such a material might be difficult to predict. LLNL has developed its own solution with a 3D printing process capable of controlling the physical properties of an object through the precise arrangement of its microstructure.
Using the lab’s direct-ink writing technology, LLNL scientists are able to 3D print features as fine as 200 nanometers. A continuous filament is fed into the device, allowing the nozzle of the machine to extrude plastics and conductive inks at speeds of up to 10 cm/sec. By changing the 3D-printed pattern at the layer level, the team demonstrated the ability to alter the energy absorption properties of 3D-printed silicon foam. The research led the lab to determine that some microarchitectures were capable of greater stiffness, but would buckle under compression, while others created softer objects with more give.
The next step was for LLNL to understand these materials over time. In turn, LLNL researchers set about determining whether or not 3D-printed foam would have greater longevity than traditional foam materials.
To research the behavior of foam over time and therefore pave the way for the use of 3D-printed foams in commercial applications, the LLNL team performed accelerated aging experiments with standard and 3D-printed rubber foam samples. The samples were exposed to elevated temperatures and subjected to constant compression over the course of a year or longer, with the researchers studying the mechanical response and permanent deformation of the materials.
Though both the 3D-printed and traditional foam samples were made from rubber, the LLNL team determined that the 3D-printed objects maintained their mechanical and structural characteristics better than standard foam. At the same time, the material itself, at the microlevel, actually exhibited the opposite behavior. The rubber comprising the 3D-printed object aged quickly, while the rubber that made up the standard sample aged more slowly. Upon performing X-ray computed tomography and finite element analysis, the researchers attributed this variation in longevity of the microstructure to the greater variation of stress distribution throughout the standard foam. In other words, portions of the rubber within the traditional material held up longer because the material was more varied in terms of shape, size and thickness throughout, while the stress to the 3D-printed foam was uniform throughout.
Nevertheless, the 3D-printed foam held up overall, leading LLNL to champion its use over traditional foam, with lead author on the study, AmiteshMaiti, reporting, “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.”
The full report has been published in the April 27, 2016, edition of Scientific Reports.