Some savvy firms are responding to customer needs and marketplace trends by making advancements to the core materials of composites.
Airline passengers often get peanuts or pretzels, but designers of those planes prefer sandwiches.
Composite sandwich panel constructions, typically made from synthetic foam cores or honeycomb materials bonded to carbon-epoxy face sheets, have mechanical properties that make them ideal for cabin flooring, stowage bins, galleys and lavatories. Honeycomb is lightweight and offers high mechanical strength, durability and corrosion resistance.
So when a three-member team at M.C. Gill Corporation met recently to discuss ideas for improving core materials used in the aerospace industry, it faced an interesting question: How could they improve upon material that was already well accepted in the market? Should they even bother discussing a new approach?
“Yes” was the answer, and an innovative core material was the result.
A New Way to Fill Honeycomb with Foam
The team at M.C. Gill, which has been manufacturing core materials since the 1980s, realized that hollow cells in a conventional honeycomb’s core accommodate air through which heat, noise and vibration can be transferred. They believed that filling those cells with a dampening material would provide better acoustic and thermal insulation properties, and could improve resistance to compressive and shear loads in service.
One conventional way to fill honeycomb cells – pressing low-density foam into the empty cells – limited the process to very-low-density phenolic foams, and the process generates unwanted dust as the foam breaks down, says Matthew Lowry, director of research and development at M.C. Gill. The dust typically interferes with the bonding of the facing to the core, and can contaminate other materials in the sandwich layup process, he says. Another option – filling honeycomb cells with non-foamed material and allowing it to foam – is messy and requires additional steps to produce a slice of core ready for making a part.
“We thought, ‘What if we could put a secondary coating on the core that would foam when the core was used to make a sandwich construction?’” Lowry says. “This method meant the end user wouldn’t have to handle foam. We could control the density and properties of the foam and the material could be processed just like a regular piece of core. That’s when the project went from ‘what-if’ to ‘a-ha.’”
M.C. Gill’s ingenuity led to the formation of a patented technology called GillFISTS, a system of coating and filling the honeycomb core with material that expands under heat and pressure. The company’s scientists conducted a series of tests to analyze the technology’s thermal conductivity and insulation, acoustic management and damage tolerance (using high-impact-resistant sandwich structures with and without GillFISTS, exposed to impact from a 1-inch-diameter spherical dart). Those tests confirmed the technology’s ability to thrive in applications requiring heat resistance and noise control.