MT Magazine March/April 2024

THE ADDITIVE ISSUE

16

rapidly.

A Cautionary Tale Remember the 20% carbon fiber loading of the material? “We needed at least 20% to give us the stiffness needed to constrain the residual stress and prevent distortion,” Love says. But, he recalls, about an hour into the build, two ORNL colleagues came to him and said the car “was curling up on us.” They determined that the problem was that someone had changed the loading from 20% to 15% carbon fiber-reinforced ABS because they could get it for free. Apparently, the engineer who made the decision thought that a 5% difference wouldn’t be that much, but as Love points out, “In reality, that’s 25% less reinforcement.” They continued to deposit the material. “We decided to forge ahead, believing that if we could get far enough along in the print, the structure would increase in stiffness and stop distorting.” It did. But it made machining the structure more difficult. Being economical doesn’t necessarily result in savings. The machine ran around the clock. In the early hours of Tuesday, Sept. 9, 2014, the 3D printing was complete. Making It Move — and Then Not Not everything in the Strati was 3D printed, including things like the seats, steering wheel, instrumentation, tires, and windshield. And not the propulsion system. Rogers says that he and his colleagues looked around the global auto industry to find an electric propulsion system that could be used for the Strati. They concluded that the Renault Twizy, which is categorized in Europe as a “quadracycle” rather than “automobile,” had a 13-kW motor and 6.1-kWh lithium-ion battery pack that would fit the requirements. So they set about to get a Twizy that they would deconstruct to get the necessary components. Renault, of course, is based in France. However, as Rogers and his colleagues soon discovered, European vehicle manufacturers build for the European market to meet European standards set by the United Nations Economic Commission for Europe – and build for the U.S. market to meet the standards set by the Federal Motor Vehicle Safety Standards. That meant they couldn’t go to Paris, buy a Twizy, and ship it back to Local Motors in Phoenix, Arizona. They thought about going to Europe to buy the propulsion system. They couldn’t do that, either. Eventually they were able to work with U.S. government regulators to buy two Twizys (Rogers said they didn’t want to be in a situation where something didn’t work on one of them without a backup). But it came with a hitch: Once they had used the Twizy system for the Strati, they had to destroy it as well as the two vehicles – and prove it to the

And as for the material they were using, they sourced an

ABS material from chemical manufacturer SABIC* that was reinforced with 20% chopped carbon

fiber. Neff says that not only does the carbon fiber contribute strength to the structure, but because of the way the fibers orient as a result of extrusion, “When it cools down, it lowers the coefficient of thermal expansion along the length of the extrusion by about an order of magnitude, so it lowers the stresses in the part and helps keep the part from warping.” What they didn’t want was delamination. “We discovered a ‘Goldilocks’ effect,” Love says of the process. “For a given flow rate on the extruder, if you went too fast, the part got too hot and wouldn’t hold its shape. If you went too slow, it got too cold and would crack.” Two of his ORNL colleagues, Vlastimil Kunc and Brett Compton, developed the models that would help get the printing “just right.” But there was still the question of the deposition rate. Love says they worked with Tim Womer, who at the time was with Xaloy, a developer of screws, barrels, and other components for extruders and injection molding machines. They developed a new screw for the extruder that would increase throughput to 40 pounds per hour. “We took delivery of the screw at ORNL the week before IMTS,” Love says. They put it on the ORNL machine on the Thursday of that week and verified its performance. On Friday, they flew the screw to Chicago. “Saturday, ORNL had a team install the screw and calibrate the system, then started printing Sunday morning at 7 a.m.,” Love says. In Addition to Additive While the Strati structure was printed, some areas still required machining, such as assembly points for the electric drivetrain. Neff notes that the machining was tricky because the structure was still hot when it was being milled and was consequently shrinking as it cooled, so they had to chase the tolerances – literally. The machining was done on a machine from Thermwood Corp., a company that sold mainly into the wood and plastics industries at the time. (Thermwood would go on to add the Large Scale Additive Manufacturing (LSAM) line of equipment to its offerings.)

*Cincinnati Inc. had sold the machine to SABIC, which recognized that additive manufacturing had a solid future. The gantry machine made its stop at IMTS 2014 before being sent on to a SABIC facility.