OK, we confess when we first heard about transporting freezers in space, we thought the story concerned getting Haagen Dazs up to the astronauts. But actually it’s much more scientific than that. Engineers and scientists at The University of Alabama Birmingham (UAB) Center for Biophysical Sciences & Engineering (CBSE) are working with NASA to develop freezer units capable of reaching temperatures as low as -160°C. Their latest project, the Polar Unit, is one of the first of their designs to incorporate 3D printed components produced in Stratasys’ FST-compliant ULTEM™ 9085 thermoplastic material. Unlike conventional methods, 3D printing offered optimal design solutions and saved significant interior space.
The primary purpose of the Polar Unit is to export biological samples to the International Space Station, with the intention of long-term testing aboard the ISS. With three current Polar freezers on deck, and a goal of 14 active units scheduled for the near future, maximizing space on board the ISS is critical to daily functions.
The freezer unit’s design is unique in that it required maximum interior space to carry samples while fitting carefully into a small locker unit. The team at UAB CBSE was challenged with the unit’s interior liner. Their initial concept using flat machined pieces of Polyether ether ketone (PEEK) proved to be insufficient due to its inability to achieve curvature design. Thermoforming the PEEK was also rejected because of inaccurate tolerances and varying levels of sheet thickness. Furthermore, thermoforming could not produce a composite design shell to account for additional electrical parts needed within the interior of the freezer. Additional jigs and fixtures would be necessary to join pieces, therefore compromising overall space.
Harnessing the power of 3D printing, the team collaborated with Stratasys Direct Manufacturing, an indirect subsidiary of Stratasys, to seamlessly produce the complex shell structure in one build.
“At first, we continued the line of thinking similar to thermoforming, in which many pieces would be 3D printed and joined together,” said Daniel Sealy, Mechanical Engineer at CBSE. “But after some research, we realized there were 3D printers with a large enough printing volume that the entire shell could be 3D printed as a single piece. This led us to where we are now. We had to go through learning about how to design for 3D printing and what types of geometry worked well, but it seems to be working great.”
The inner liner was produced with Fused Deposition Modeling (FDM) 3D printing technology on the Fortus 900mc 3D Production System by Stratasys in ULTEM 9085 material. Although traditional uses of ULTEM 9085 are often subjected to high temperature environments, the high-performance thermoplastic had passed the necessary testing under freezing conditions.
ULTEM 9085 is an ideal material for aerospace applications, because of its high heat and chemical resistance. Additionally, the engineering-grade thermoplastic delivers superior tensile strength, making it optimal against rocket vibrations during launch.
According to Sealy, the Polar Unit will test the long-term effects of zero gravity on biological samples, including human blood, cells and urine. Three Polar Units launched on January 10, 2015 aboard the SpaceX CR-5 Falcon 9.