CompositesWorld

JAN 2016

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CompositesWorld.com 25 NEWS N E W S N S N E W S E N W S W Composite Spiral Staircase Clear Carbon has become known for developing new processes for composites projects that have hit roadblocks in manufacturing. "A lot of what we do is deciphering how to make composites produc- tion possible," says Dunham. With the architect's fnalized digital design fles in hand, Clear Carbon began devising its plan for how to build a 4m tall, one- piece CFRP spiral spar. Enabling a spiral tool and fxture "Te one-piece spar was key," says Dunham, "not only for aesthetics, but also because it would be so structurally inefcient to break apart the unidirectional reinforcements specifed in the design." So the team focused on how to enable a one-piece layup process. Clear Carbon's engineering manager Ben Quay invented a fxturing system that comprised a central tower to which fve hori- zontal rods or "halos" (named after the fxture used for neurosur- gery) reached out to locate and secure a spiral foam core preform (think mandrel which does not get removed). Tis preform would serve as the surface onto which the CFRP spar was layed up and cured (see Step 1, p. 26). First, a compass rose, or reference dial, was CNC-machined into a fat, level base plate to provide a true zero from which to build the foam preform. Tis would enable technicians to check true position and conformance to digital design drawings throughout fabrication using a variety of tools, including a laser plumb bob. Over the compass rose, Quay built the central fxturing tower out of approximately 50 CNC-machined pieces of medium-density fberboard (MDF) — roughly 2.4m long by 0.4m wide and 15.2 cm thick — which ft together like a puzzle. Next, fve "halo" bars were attached, recalls Dunham. "Tese projected out like arms to position the foam preform in mid-air," he explains, "and also to position the stairs at the various heights and angles that they needed to be." (Step 2, p. 26). Because the layup preform would not have to carry struc- tural load, it was made from 13 pieces of easy-to-machine rigid urethane foam sourced from Poly Cel (Stow, MA, US) and cut using Clear Carbon's 3-axis CNC mill (see Fig. 1, above). Dunham says the 3D modeling and CNC machining for the fxturing tower and preform was the part of the project that went the fastest, completed in less than two days. Holes were machined into each foam preform component, enabling them to be assembled vertically, glued and then held together with dowels until the adhesive set. Te foam preform was then "rigidized" using three layers of Sigmatex (Benicia, CA and Cameron, SC, US) 400-g/m 2 double- bias carbon fber fabric, supplied by Core Composites (Bristol, RI, US), which was wet laminated with room-temperature cure PRO-SET epoxy resin (Gougeon Brothers, Bay City, MI, US). Clean lines, multi-step process Although the cured preform could now support layup, it frst had to be sanded to achieve the clean, sharp lines essential to the stair design. Next, a prepreg resin flm was applied to provide a chemical bonding layer for the spar's structural layup: 10 plies of 600-g/m 2 Gurit (Bristol, RI, US) carbon fber/ epoxy unidirectional prepreg. Because there was no tool surface to which tech- nicians could tacky-tape the vacuum bag, the entire vertical layup was enveloped in vacuum bag flm (a technique known as "envelope bagging") for intermediate debulks and fnal cure. Te spar layup was debulked every few plies and, for cure, it was enclosed in a temporary oven made using a 3M (Minneapolis, MN, US) metallized Mylar flm, which had a much higher temperature rating than typically used visqueen polyeth- ylene sheeting (Step 3, p. 26). Tis was key, because the spar's 91°C cure temperature exceeded visqueen's 82°C maximum service temperature. Digital controllers used temperature data from thermocouples applied prior to vacuum bagging to cycle propane heaters on and of as needed without overheating during the 6-hour cure. After cure, the spar was debagged and sanded before applica- tion of two layers of double-bias carbon fabric, prepregged with the Fig. 1 Modeling & Machining Clear Carbon underscores that its digital modeling and CNC machining capabilities were keys to project success. Here, the 13 pieces designed for the foam spar preform are shown nested with the stair wedge foam cores for CNC machining. Source | Clear Carbon and Components Clear Carbon has become known for developing new processes for projects that have hit roadblocks ....

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