To begin, the team developed an out-of-oven (OoO) process that uses carbon nanotube film to heat and cure out-of-autoclave prepreg materials. In the early years, MIT grew vertically aligned carbon nanotubes and “knocked them down” to create the horizontally aligned CNTs this process requires. Now, they are commercially purchased in large format sheets. Researchers then created a NPN that provides capillary pressure to enable curing traditional autoclave-required graphite epoxy prepreg materials. The NPN film is made of 8-nanometer diameter, multi-walled, vertically aligned carbon nanotubes that are grown at MIT. While the largest part cured so far is a x 30-centimeter flat panel, Estelle Cohen, lead researcher for the collaboration at Metis Design, plans to make a 60 x 60-centimeter flat panel and a curved part by the fall, with customers talking about scaling to much larger and complex parts early next year.
The technique has many advantages. Foremost are reduced energy costs. This resistive heating technique uses just 1% of the energy that would be required to cure large composite parts in an autoclave. “That’s a step change in energy efficiency,” says Jeonyoon Lee, a postdoctoral researcher leading the project at MIT.
The process is also faster. Seth Kessler, president of Metis Design, says that because CNTs heat more rapidly and uniformly than an oven or autoclave, the technique can be used to cure parts in roughly half the time. Most recently, the team documented a 90-minute reduction in cure time, which is good news for throughput. “That’s going to translate into making more parts per hour, per day, which is obviously going to translate into increased margins,” says Kessler.
CNT-based resistive curing could significantly reduce barriers to entry for companies that can’t afford an autoclave. “All you need is enough shop floor space for the tool,” says Kessler. “That means that any medium or even small-sized business that can afford to get a warehouse would possibly be able to make large composite parts. This technology really has the opportunity to revolutionize the way that composites enter non-traditional markets.”
Tools will also be far cheaper – perhaps a fraction of the cost of a traditional mold – spurring further market entry and faster innovation. “Right now it is very difficult to iteratively design and test composite parts, especially after production begins because traditional molds can cost millions of dollar, take six months to fabricate and can be virtually impossible – or at least financially impractical – to re-work,” Kessler explains. “Pressure-free curing outside of an autoclave allows you to rapidly prototype parts and go through design iterations much more easily.”