Boeing is currently using both thermoset and thermoplastic OOA prepreg materials, as well as resin infusion OOA processes. It is working closely with material suppliers both to qualify current generation OOA composites and to develop the next generation of OOA prepregs. Bringing costs down is a significant focus of this work, Bond says.
Advances in resin chemistry and a better understanding of lay-up processes made OOA technology feasible. For example, resins are more latent in OOA processes. “You could have them at a 250 F temperature, where the resin viscosity is low, for up to six to eight hours,” says Cichon. “During that time, under vacuum bag pressure, air will naturally start to find its way out if you have a really robust process.” Manufacturers can manipulate several variables, adding breather plies, glass tape and fabric, and varying the frequency of debulking to make those processes more robust.
There are tradeoffs when using OOA instead of an autoclave. OOA requires more process study, more parts fabrication work, more investigation to ensure that the resins work properly and often more nondestructive (NDT) inspection on critical parts.
“OOA, especially VBO, is generally a longer lay-up process before you get to the cure, and the cure cycle can be longer to get all of the air out,” adds Sean Johnson, thermoset project manager at TenCate. “So you save a lot of operating expense, but labor time is money as well. If you increase labor on the front end, you may not save as much money as you think.”
Improvements in OOA technology tend to be more evolutionary than revolutionary. TenCate is working to make its resins more robust and tougher. “With some of our latest technologies, the resins have very low moisture absorption so that they retain their strength under hot/wet conditioning. Other developments involve formulating resins that have a longer tack life and a tool life/out time of 25 to 30 days to facilitate larger parts,” Cichon says.
TenCate will introduce a new prepreg system (TenCate TC380), whose combination of hot/wet properties can produce a composite with good mechanical strength and high compression and open-hole compression strength. “These are the key allowables that you have to address when you’re designing aircraft or aerospace structural parts,” Johnson says. The resin must also have good handling and manufacturability characteristics.