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Honeycomb Heroes: Making Composites for Aerospace

Ed Sinkora
By Ed Sinkora Contributing Editor, SME Media

There’s an old saw that if bumblebees were aeronautical engineers they would know they can’t fly. Quite apart from the miracle of their flight, bees also happen to make a lightweight structure of surprising strength, just the sort of thing you’d want if you were building aircraft: honeycomb.

Perhaps humbled by their supposed “mistake” regarding bumblebee flight, engineers have lately embraced the honeycomb for a number of critical aerospace structures. And nobody builds better honeycomb than a family owned multi-national called Euro-Composites  Group (EC).

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Euro-Composites creates an array of non-metallic honeycomb, each with its own mechanical properties, by varying the paper, cell size, cell shape, the number of resin dips, and the curing cycles

Euro-Composites is headquartered in Luxembourg, where it boasts a 652,400 sq ft production facility and a staff of roughly 830, including four PhDs. It also has a small production facility in Germany.

While the European facilities offer worldwide support, its main facility for serving the North American market is a 97,000 sq ft production facility in Culpepper, Virginia, and its team of 141, currently being expanded to 136,300 sq feet. The Culpepper plant specializes in non-metallic honeycomb for aerospace applications.

Paper, Glue, and Resin

Non-metallic honeycomb starts as paper. Literally paper. (Albeit with specialized DuPont NOMAX, KEVLAR, or fiberglass fibers.)

Euro-Composites’ Luxembourg plant uses what amounts to large printing presses to deposit lines of glue across wide sheets of this paper, put another sheet of paper on top, and deposit another set of glue lines between the position of the lines in the lower layer. A 3-4 inch 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 their own adhesive because it’s critical to the strength of each cell’s node bonds. Luxembourg ships the HOBEs to the Culpepper plant and things get more interesting.

Director of Production Steve Bjorkman explained, “We load the HOBE into an expansion tool, insert rods into pockets along the side, attach the rods to the expansion rack, and pull the paper layers apart using the rods. Now that three or four inch thick HOBE becomes something that’s 96 inches 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.”

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Gus Escudero inserts rods into foil loops attached to each side of the HOBE. He’ll then use these rods and the surrounding fixture to spread the HOBE, creating open cells.

At that point EC has what looks like a massive paper honeycomb, though for some applications they pull 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 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 x 4 x 3 ft, into a stabilization oven at 350°F 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 stabilization process stiffens the new form, but it’s still just paper.

“Like pulling a spoon from a jar of honey”

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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.

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 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 uneven coating and some cells would probably clog entirely. 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 possible 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, weight it, and dip it again, monitoring the density of the block after each step and making appropriate adjustments,” said Bjorkman. “Both the dipping time and the curing time are variable based on the application.”

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Getting multiple layers of resin to safely bond requires processing the blocks through a series of curing ovens, each capable of tightly controlling humidity, air flow, and temperature throughout.

By varying the thickness and makeup of the paper, the cell size, and the timing and temperature of the dip and cure cycles, EC can create a dizzying array of honeycomb. Each combination is a trade-off  between flexibility, weight, compression strength, and plate shear in each direction. Although EC documents and controls their process to a high degree, and can therefore  predict the results from each production run, they test the mechanical properties of each  each batch in an in-house lab.

Roughly half their product leaves the plant at this point, almost always to end users like Boeing or Lockheed. The rest is formed and/or machined into specific shapes for final assembly in various aircraft structures structures like vertical stabilizers or engine cowlings.

Senior Executive Vice President Alwin Heil explained that EC’s ability to make blocks up to 150 x 90 inches gives them has a significant advantage for certain parts. “In a traditional four by eight sheet, the glue lines run in the four foot direction, called the ribbon direction.

If you make a sixteen foot helicopter rotor blade and you need it to run in the ribbon direction, you would ordinarily have to splice four pieces together. That results in three splice lines, with additional weight and balance issues on a very critical item. We can provide eight foot ribbon direction, which cuts it down to one splice. In other applications we might be able to supply the part without any splice at all.”

Bending the Core

President & COO Michael Graham explained that “Honeycomb is at its strongest in resisting compression when the walls of the cells are perpendicular to the force. So for applications requiring both compressive strength and a curved surface, we bend the honeycomb to conform to the required shape, rather than machine the surface. An example might be the landing gear door for an airliner. It may have to be curved to match the aircraft’s profile and it will have to withstand tremendous air pressure when it’s opened during flight.”

EC uses a set of proprietary fixtures and methods for doing this and guards their secrets like a sentry bee as the speed with which they can form honeycomb is another thing that sets them apart from their competition.

Machining Air and Paper

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EC machines honeycomb on large 5-axis routers, creating contours, cut-outs, and chamfered edges. Fixturing is a challenge and tolerances can be “metalworking tight.” Here the part is being held by vacuum.

The cured honeycomb that stays in-house now goes into a bandsaw that cuts it horizontally into sheets as thin as 1/16 of an inch or as thick as 6 inches. The saw blade is high speed steel (HSS) and produces a fine dust as it cuts, which an overhead vacuum simply sucks up like in a woodworking shop.

After this, the sheets are cut into near-net shapes for final machining and/or assembly. In the latter case, EC may glue several pieces together and then machine the construction as a whole. Much of this cutting and gluing is done by hand and EC employs a number of former cabinet makers, a fine example of repurposing traditional skills.

Machining is generally used to produce rather subtle contours, chamfers along the edges, or cutouts. The bulk of this machining takes place on large 5-axis CNC routers with a capacity of up to 20 x 30 x 5 ft. Fixturing is a challenge because the parts can’t be clamped without damaging them and since they’re non-ferrous, a magnetic table is useless.

One method is to simply cover the sheet with double-sided tape and then tape it to the fixture. Another method is to use single sided tape to hold a sheet of plastic across the entire surface of the part and then use suction from below to hold the part to the table.

The fixtures show the operator where to place the near-net shape and the machines orient themselves to the fixture. Since every edge of the part will be machined, it’s not necessary to know its precise position in the fixture. Feed rates are comparable to many metalworking applications (e.g. 50-100 inches per minute), depending on the stability of the material as well as its geometry.

Tolerances are generally in the range of a few thousandths, but can sometimes be so tight that the entire profile must be checked with a CMM or even a laser tracker. Jones quiped, “We’re machining 80% air with a little paper, glue, and resin, yet we have to meet metalworking tolerances.”

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C.J. Johnson measures the contoured surface of a block of honeycomb with a laser tracker. A significant portion of EC’s product must meet the kind of tight tolerances that demand such methods

If it’s a tight tolerance part with a complex surface, EC might have to measure every surface at two inch intervals. While the CMM functions automatically after programming, the laser tracker can’t be as it requires moving a reflective ball from point to point across the surface.

So it takes a skilled operator and significant time. But it’s extremely accurate and portable, enabling EC to use it both for measuring parts during production and in the lab, in addition to using it for machine calibration.

Environmental Control is Especially Important for QA in this Field

“The aerospace industry requires very large parts,” explained Heil, “but because non-metallic honeycomb is hygroscopic, the bigger the part the harder it is to control form accuracy. So we simulate our customers’ layup environment by controlling the temperature and humidity of our entire machining area and QA lab. Then we seal the parts in barrier bags with desiccant before they leave the plant. That delivers an end item to our customers that’s usable without any significant dimensional changes. It’s a huge expense but it differentiates us from everyone else in the industry.”

The need to control the climate of their production area was actually a requirement for EC’s work on the Joint Strike Fighter program. But now that they’ve made the investment it benefits their other customers as well. As Heil said, “It often goes like this. The military has the highest demands, they drive new capabilities, and the rest benefit.

“A lot of these composite products need to be handled like babies. They cannot get too hot. They cannot get too cold. They have to stay dry. And we test every product that goes into the shop for shear, compression, heat resistance and adhesion. Everything must be logged and tagged. Even the raw materials get their own ‘passport’ when they come in and we have to track any time they spend outside of refrigerated storage as there are strict limits after which the product must be disposed of.”

Heroic Lead Times

Heil said he recently “read a study that said 65% of all aerospace products are delivered late. Our owner got into this business when he wanted non-metallic honeycomb for a racing application and was quoted twelve month delivery. There is a strong demand for much faster deliveries and we offer significantly shorter lead times to meet this need.”

He added that their Center of Excellence encourages big companies like Boeing, Airbus, and DuPont to partner with EC on any new product that they think might be doable in honeycomb. “For example, we are the one and only company qualified by Boeing on their non-metallic honeycomb spec for their Class 8 material, which is the next generation of KEVLAR honeycomb. We developed it, and we’re currently developing a glass fiber polyamide coated honeycomb forvery high temperature applications and a quartz glass cyanate ester honeycomb core for space applications.”

As Graham said, “When you consider the speciality of what we’re producing and the skill we’re bringing to it, many of the people here can say they are among the best in the world at what they do.”

They’re honeycomb heroes.

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