By Jan H. Schut
How do you extrude a functional, full-sized car body without a mold? That’s what Oak Ridge National Laboratory in Oak Ridge, TN (www.ornl.gov), set out to do when it developed the world’s first Big Area Additive Manufacturing (BAAM) machine in 2014. BAAM is a giant pellet-fed 3-D printer, which builds parts out of ribbons of nearly molten plastic from a conventional single screw extruder in an enormous build area of 20 feet x 7.75 feet x 6 feet. But given the size of BAAM parts, there were structural challenges in the new layered polymer technology.
ORNL had developed the BAAM machine rapidly working with Cincinnati Incorporated, Harrison, OH (www.e-ci.com), which adapted its commercial laser cutting machine. Cincinnati built the first BAAM prototype in less than three months using its existing gantry-style platform, but replacing the articulated laser cutter with a 1-inch extruder, fed pellets via a flexible pneumatic hose.
The first BAAM prototype had a smaller build area of 157.5 x 78.5 x 34 inches with an extruder that deposited 10 lb/hour. ORNL demonstrated the prototype by building a concept car body, called the Strati (which means “layers” in Italian) designed as part of a contest held by Local Motors, Phoenix, AZ (www.localmotors.com). When ORNL made the first Strati test parts out of 20% carbon-fiber-filled ABS, the beads delaminated in several places. It took three months of R&D and several more part iterations before ORNL had corrected the part design and successfully showed BAAM for the first time at the International Machine Tool Show 2014 in Chicago, dramatically building an entire Strati car during the show.
The Strati car body reportedly used about 1000 lbs of carbon-fiber ABS, so ORNL wanted a virtual way to simulate part designs to reduce physical test parts. In November 2014, ORNL partnered with AlphaSTAR Corp., Long Beach, CA (www.alphastarcorp.com), an aerospace and automotive software developer, to simulate crack initiation and propagation, residual stresses, and deformation in BAAM part designs.
The resulting “damage and fracture evolution” software was presented by ORNL research staff Vlastimil Kunc and AlphaSTAR chief scientist Frank Abdi at the Society of Plastics Engineer’s recent ANTEC conference in Orlando, FL (www.4spe.org), in March. “What is new is that (AlphaSTAR’s software) will predict if a given part geometry is producible and will be good quality in the end,” Kunc confirms. For people who didn’t attend ANTEC, all papers are available on CD from the SPE for $200 to members and $250 to nonmembers.
MODELING MICRO CRACKS BETWEEN BEADS
ORNL wanted software to be able to simulate an entire car build using the BAAM process in under 24 hours and needed the software by mid-January in time for the North American International Auto Show last January 12-15 in Detroit. “There are ten miles of bead in the Strati car, including many short segments because we need to stop and start printing depending on geometry,” Kunc notes. Extruded bead starts round and is tamped down to flatten it and make it adhere to the layer below. So there was a lot to simulate. AlphaSTAR met the deadline with software that can virtually build a car in only 12 hours on a personal computer.
To develop the new software, AlphaSTAR modified its mature generalized optimizer analyzer (GENOA) FEM-based software and material characterization and qualification (MCQ) code for virtual testing to the BAAM system. GENOA and MCQ had previously won awards from the National Aeronautic and Space Administration in 1999, R&D magazine in 2000, and the U.S. Small Business Administration in 2001 for its novel multi-scale failure prediction.
With BAAM’s layered extrusion process, the MCQ progressive failure-analysis software can simulate what happens to short-carbon-fiber-filled ABS down to micron-scale behavior of fibers, matrices, and their interphases including manufacturing defects like fiber waviness, agglomeration, resin rich areas, void shapes and sizes, and prediction of the degraded material’s ultimate strength and stiffness.
For example, “most fibers are oriented in the machine direction as hot bead is laid down, but orientation is never perfect,” Abdi notes. “There are also in-plane random and 3-D random fibers as well as oriented fibers. You have to figure out the effect of every one of them in damage evolution prediction. We had to include the damage evolution of every random fiber in each bead for the car – a total of 350,000 beads in a car – and propagate them to fracture and delamination between beads.”
Longer fibers are generally considered desirable, but if longer fibers are wavy, they can actually be weaker (in modulus and strength saturation) than shorter straight fibers. Longer fibers also create micro voids and porosity, which initiate cracks and can cause delamination.
In addition AlphaSTAR simulated the thermal behavior of BAAM parts, modeling how a multi-layer wall cools, while ORNL measured actual temperatures as a BAAM-layered wall was built. Simulation and testing both showed that as successive hot layers are laid down over cooler layers, the hot upper layers twist, initiating delamination.
Micro cracks, however, are sometimes useful. Micro scale simulation showed that “smaller threshold cracks are needed up to a point to relieve thermal stress accumulation and later disappear, so we had to determine threshold crack size,” explains Abdi. “MCQ determines what failures are caused by defects, environmental factors, and manufacturing, while GENOA predicts where, when and why damage occurs.” GENOA found, for instance, that putting a cover over the BAAM build area to keep heat in reduces delamination between layers, whereas heating the build platform did little to reduce cracking.
The new software starts with a stereo lithography standard tessellation language (STL) file of an object. Slicer software, developed by ORNL, takes the STL file and slices the object into horizontal layers to generate a machine “G code” or tool path for the 3-D printer to build the part. GENOA then makes a geometric FEM mesh of the part design, which MCQ software uses together with Abaqus solver software from Dassault Systemes, Velizy Villacoublay, France (www.3ds.com), to simulate and correct potential material failures in the part design. ORNL uses MCQ together with the Abacus solver, but AlphaSTAR says its MCQ solver can also be used alone for failure simulation.
“The GENOA, MCQ, and Abaqus software conduct progressive failure analysis and combine those results to predict structure/component behavior based on the physics and mechanics of the materials and the impact of the manufacturing process,” AlphaSTAR’s Abdi explains. “Finally, the software predicted the structural performance under service load – acceleration equal to the centripetal acceleration of the Strati car going 68 km/hour around a curve with a radius of 350 meters.”
MCQ’s multi-scale FEM software has simulated BAAM printed parts down to microscopic scale. Since 2006, MCQ software can also simulate material behavior even smaller down to nano scale, which AlphaSOFT says is the first “de-homogenized” or random damage simulation on a nano molecular scale, though ORNL hasn’t tested the BAAM process that small.
Cincinnati has reportedly sold four of the first size BAAM machines already, including ones to Local Motors, SABIC Innovative Plastics, Exton, PA (www.sabic-ip.com) for material design, and an aerospace company. An even bigger BAAM printing machine, expected to deposit up to 100 lb/hour, will be available for testing in ORNL’s laboratory by midyear. So additive manufacturing could well become a whole new way to build large car and aerospace parts faster, better, less expensively and with much less inventory than they are built today.