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composites


on top, and deposit another set of glue lines between the position of the lines in the lower layer. A 3–4" (76–102-mm) stack of such paper, with its staggered lines of glue, is called a “honeycomb before expansion,” or HOBE. The spacing of the glue lines in the paper will determine the cell size. EC makes its own adhesive because it’s critical to the strength of each cell’s node bonds. After Luxembourg ships the HOBEs to the Culpepper plant, things get more interesting. Steve Bjorkman, director of production, explained, “We load the HOBE into an expansion tool, insert rods into pock- ets along the side, attach the rods to the expansion rack, and pull the paper layers apart using the rods. Now that 3 or 4" thick HOBE becomes something that’s 96" [2438-mm] long.” But like a lot of things at EC, this is not a purely mechanical process. “There’s an art to making sure the cell alignment stays uniform.”


have a “memory” of their former flat state, and if they weren’t held in a fixture they would collapse. So the next step is to put the giant honeycomb, typically measuring 8 × 4 × 3' (2.4 × 1.2 × 0.9 m), into a stabilization oven at 350°F (176.7°C) or higher for about an hour. The oven blows air up from the base through the cells and maintains a constant temperature and humidity throughout the enclosure, all of which EC must test and certify to their customer base on a regular basis. This process stiffens the new form, but it’s still just paper.


An illustration of the basic steps in producing composite non-metallic honeycomb sheets. The term “foil” used here refers to the specialized materials like Kevlar, Nomex or fiberglass.


At that point, EC has what looks like a massive paper honeycomb, though for some applications the firm pulls the HOBE far enough to elongate the cells, making them more rectangular than the hexagonal shape we normally associate with honeycomb. Whether hexagonal or rectangular, the cells


“Like pulling a spoon from a jar of honey” William Jones, vice president of engineering, explained: “At this point we lower the whole block into a vat of phenolic resin. This is even more of an art than spreading the HOBE as the speed at which the block is raised has a big effect on the thickness and uniformity of the resin that remains on the cell walls. The slower you raise the block, the thinner the coating will be. Think of it like pulling a spoon from a jar of honey. If you pull it out quickly, you’ll get a lot of honey. If you pull it out very slowly you’ll get only a very thin layer of honey.” The required thickness of the resin depends on the application. But they can’t just lift the block out quickly to get a thick layer because doing so would create an uneven coating and some cells would probably clog en- tirely. To get thick, even layers of resin EC dips a block up to 16 times. The resin contains a significant amount of alcohol. So after each dipping the next step is a trip to a “purge oven” that heats the block to evaporate as much alcohol from the resin as pos- sible to prevent an explosion in later stages. Only then does the block go into a curing oven, which again heats the resin and (among other things)


causes the layers to bind chemically. “We cure it for a cycle, weigh it, and dip it again, monitoring the density of the block after each step and making appropri- ate adjustments,” said Bjorkman. “Both the dipping time and the curing time are variable based on the application.”


110 — Aerospace & Defense Manufacturing 2016


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