Out-of-autoclave manufacturing: The green solution

One of the arguments against moving parts from autoclave curing to out-of-autoclave (OOA) curing is that energy consumption is a small portion of total costs, perhaps only 5 percent. The implication is that the requalification costs for an existing part well exceed the potential savings in reduced energy costs and the benefits of lean processing vs. batch autoclave curing. But if the volumes are high enough, and if one can reduce energy consumption by 50 percent or more, this adds up to 2.5 percent of sales to the bottom line. If a manufacturer is fortunate enough to earn a return of 10 percent on sales, this results in a 25 percent increase in profits. That’s something not to be ignored.

Selling OOA processing solely from the standpoint of reduced energy in manufacturing overlooks the most significant reason to pursue such technologies. OOA processing not only reduces energy consumption, it also shortens cycle times and cuts both capital and tooling investments, enabling more composites to earn their way onto aircraft because they are more cost-effective. Further, and most importantly, OOA processing has implications for the next wave of commercial aircraft.

The Boeing 787 and the Airbus A350 already have demonstrated that lighter and more aerodynamic carbon fiber wings, combined with new engines, can deliver upwards of 20 percent less fuel consumption for airline customers on long-haul routes. Although these wings are made via conventional autoclave curing, the net result is much greener airplanes, especially when the lifetime of the aircraft is considered. These twin-aisle aircraft are manufactured in relatively low volumes, so going the autoclave route made sense, considering the maturity of various materials and processes at the time the designs were finalized.
The holy grail of the advanced composites industry is similar adoption of carbon fiber on the next full redesigns of Boeing and Airbus single-aisle aircraft,  the 737 and the A320. Due to increasing demand for air travel, build rates are forecasted to reach 45 to 50 aircraft per month for each OEM within the next 10 years. There has been considerable speculation that these aircraft eventually will have carbon composite wings, but the business case is not as attractive as it is for long-haul aircraft like the 787 and A350 because single-aisle planes tend to make numerous takeoffs and landings each day and spend less time at cruising altitudes. From an environmental standpoint, even if efficiencies were improved only 10 percent, the reduction in greenhouse gases from lower fuel consumption would still be significant simply due to fleet sizes.

The key to making the business case viable is to drive the costs of composites manufacturing well below current autoclave practice. Delivering 100 aircraft per month requires manufacturing 400 wingskins, each weighing roughly 2,000 lb (0.91 metric tonnes). By some estimates, this would require the addition of 20 or more very large autoclaves, with installed costs of up to $40 million to $50 million each. The power needed to heat these autoclaves and deliver nitrogen at 100 psi (7 bar) is substantial.

OOA processing is maturing quickly and is to the point where it merits serious consideration for these future opportunities. Within five to seven years, and with adequate investment, the technology should reach the technical readiness level (TRL) for incorporation onto commercial aircraft. The simplest option is replacement of autoclave-qualified prepregs with the evolving range of vacuum-bag only (VBO) prepregs, which are edging closer to meeting the mechanical performance requirements of aircraft primary structures. Long debulk times (to remove entrapped air) and slow oven cures need to be overcome for VBO to succeed in high-volume production.

A second route is resin infusion (VARTM, for example), which reduces material costs compared to prepreg. Issues here include achieving sufficient toughness and fiber volume to match existing autoclave prepreg performance. The first is being tackled with various methods of putting the thermoplastic toughener in the preform. I have seen fiber volumes of 62 percent attained with pressure as low as 7 psi/0.5 bar for multiaxial infused laminates. That opens the door for low-tonnage clamping frames to be employed.

There are several technologies under development that could be employed in concert with these options. These include integral liquid heating/cooling (Quickstep Composites LLC, for example), multiple-frequency microwave curing, directed hot-air impingement, and induction heating. These technologies have the virtue of being not only out-of-autoclave, but out-of-oven as well. And all offer much shorter cycle times.

OOA methods offer multiple shades of green. Fewer greenhouse gases are generated during manufacturing and throughout the part’s in-service life. That puts more cash in the pockets of shareholders and consumers. That’s a win-win and an effort well worth undertaking.