"Typically, we machine parts like these out of aluminum, steel, or titanium," said Kobie Boykins, a member of the technical staff at JPL's Mechanical Engineering Section. "Unfortunately, this design had thin walls and tiny, complex features that ruled out both metals and machining. It would have been exorbitantly expensive to create the part that way."

JPL's engineers were able to surmount several design obstacles by using the Sinterstation 2500 system from DTM Corporation of Austin, TX. Using the selective laser sintering (SLS) process, they were able to produce a satisfactory science cup from DuraForm, a plastic SLS material also supplied by DTM.

Boykins and his NASA colleagues had considered other options for creating the part, including stereolithography (SLA). They chose SLS, in part, because NASA already had a DTM Sinterstation system in-house. Engineers were also drawn to the strength of the DuraForm material. "The published numbers for DuraForm were better than those of the SLA materials," said Boykins. "We were also concerned about parts changing form, since there was a time lapse from the time the parts would be produced to actual assembly and takeoff."

The engineers also felt the DuraForm part would have a better chance of holding the required part tolerances of ±0.125mm. When NASA tested the DuraForm SLS parts produced on the Sinterstation system, engineers' strength calculations proved correct. The parts were strong enough for the application, and tests showed that a #0-80 UNF threaded insert could hold approximately 89 Newtons of force. "From what we could tell," said Boykins, "these pieces held up well to all kinds of abuse, handling, and machining. We also didn't notice any dimensional changes."

The Sinterstation Process

The DTM Sinterstation method used by the JPL team creates solid, 3D objects -- layer by layer -- from plastic, metal, or ceramic powders that are "sintered" or fused using CO₂ laser energy. The steps of the process are as follows:

• Step 1: The user inputs 3D STL CAD data.
• Step 2: A thin layer of powder is spread across the part-build area via a roller mechanism. CO₂ laser energy is used to selectively "draw" the object on the powder layer. The laser energy heats the powder to a temperature above its softening or melting point, sintering the particles into a solid mass. Laser power is modulated so that only the powder described by the object's geometry is fused. This entire process is repeated until the object is complete.
• Step 3: The object is removed from the part-build chamber, and any loose powder is brushed or blown away. The object may be undergo further treatment, such as sanding or annealing, before final use.

NASA's Final Results

Using the SLS method to produce the science cups saved NASA time and money. Boykins estimated that it would have cost from $3,000 to $5,000 to fabricate the parts using machining and other traditional methods and materials. The SLS DuraForm parts cost about $300. This method also made it easier to fine-tune the part design. "What's nice about the SLS parts is we could make small changes very rapidly, with machining or sanding, or by simply changing the computer file and generating another set of SLS parts," said Boykins, who produced four SLS parts and more than 30 manual modifications to these parts.

On February 11, NASA sent four FFMs, each equipped with a DuraForm science cup, into space. Each held more than six instruments, including laser beacons, sun sensors, a magnetometer, batteries, a transmitter, and an antenna. All instruments, except the batteries, were glued into the science cups using a space-grade epoxy. The elements formed an assembly that slid into a graphite epoxy shell that was screwed together, forming a protective case. The tiny spacecraft were ejected from a rocket and fell back to Earth, measuring minute variations in the aurora's magnetic fields. Scientists at NASA and the University of New Hampshire (UNH) say that these fluctuations may be the cause of the mysterious light show.

"Our scientists and principal investigators at JPL and UNH are very happy with the results," said Boykins. "They are excited about the SLS technology and how we can use it to produce small, self-contained spacecraft. The opportunities are immense and could include missions to other planetary bodies."

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