11 November 2005
11 November 2005
The unitized and fastenerless all-composite vertical tail, developed and manufactured by Lockheed Martin as part of its self-funded Advanced Affordability Initiative, was developed with North Coast to explore RTM as a way to reduce costs.
The Lockheed design for the 13-ft long and 5-ft wide tail part involved two external skins unitized by a series of fourteen hollow torque tubes to handle structural loads. The challenge, says North Coast’s president and CEO Rich Petrovich, was designing a tool that could produce such a large and complex unitized part in one shot. “It came down to planning, and it involved not only the tool, but ancillary equipment used for loading the tool, and injecting the part, and a clamp to overcome the injection pressure. We think about preform loading, clamping, injection, disassembly, and handling. There’s a lot to be considered.”
The torque tubes, each with a unique shape and dimension, turned out to be a critical cost driver for the overall project. To create them, North Coast designed a series of interlocking and tapering mandrels using 6061T6 aluminium, slightly undersized to accommodate the reinforcement preform. Because aluminium has a higher coefficient of thermal expansion (CTE) than steel, North Coast counted on expansion of the mandrels against the 4140 mould steel to provide proper compaction and consolidation of the materials. Yet, the long, narrow mandrels, from 90” to 100” in length, proved problematic when it came to developing a layup approach. Dry broadgoods as well as prepreg had to be eliminated as too costly and complicated.
The solution was a triaxial braided sock preform, produced by A&P Technology (Cincinnati, Ohio). Because the mandrels’ root ends were several times the size of the tips, standard biaxial braid would have created too much fibre volume at the narrower ends. A&P was able to address this with the company’s trademarked Megabraider large braiding machine. Numerical controls adjusted the braid angles during preform fabrication to account for the progressively smaller diameters.
North Coast was able to develop an innovative ancillary device, essentially a long hook-shaped arm that accommodated easy preform loading. The device’s cantilevered arm bolted onto the root end of each mandrel, counterbalancing the mandrel’s weight. That left its entire length free for technicians to slip the required number of braids over it, align the fibres, and load it in the correct position in the tool. Petrovich explains that the ancillary device worked because the design department anticipated the awkwardness of handling the aluminium mandrels which were only held at one end and cantilevered in space. Provisions were designed into the handling device to account for the changing centre of gravity for each of the fourteen mandrels’ weights and lengths.
Tail skins were layed up with a combination of fabric and unidirectional tape material. The selected resin was Cytec Engineered Materials’ bismaleimide (BMI) resin. To address resin flow through all parts of the large mould, says Petrovich, a method was needed to ensure that the resin would flow and wet out both the skins and the torque tubes completely. While Lockheed Martin chose to undertake a detailed rheological study of resin flow fronts as part of the project, North Coast independently designed and built the mould with directed resin injection and venting (DRIV) inserts in the skin surfaces. This ensured that resin flow could be orchestrated, observed and verified.
“The inserts help take the ‘black art’ out of the RTM process, they reduce the risk. You don’t want to depend on uncontrolled flooding and you can’t raise injection pressures too high. Higher injection pressure means higher clamping pressures which translates into mould deflection, and the possibility of fibre wash (fabric movement).”
In this particular instance, DRIV inserts were positioned between the mandrels in the surface of the mould. Resin was introduced through machined troughs along the four edges of the two skin faces. Once resin reached each insert, indicated by a telltale emission of resin through the vent hole, that insert was turned off and the next row of inserts turned on.
“The inserts enabled better wetout at lower injection pressure,” states Petrovich. “That meant less material was needed for the mould to overcome injection pressure which reduced overall tooling cost and made the moulds easier to handle and heat.”
One additional innovation not only made part production easier but also significantly reduced the overall project cost. A large, multi-million dollar press had originally been specified to hold and clamp the two skin moulds together but schedule delays forced a different solution. North Coast designed a straightforward and cost-effective clamp to secure the two mould halves as well as a simple rectangular steel I-beam frame to hold the assembled tool in its upright position. Then, aluminium wedges were inserted between the mould and frame. Again the CTE difference meant that during mould heating the aluminium expanded more than the frame itself, which ensured continuous wedge pressure.
“The out-of-press design saved the project a lot of money,” says Petrovich. “Taking advantage of things like DRIV inserts, CTE differential, and simple tooling aids is just one example of how we use tooling to ensure project success.“
North Coast Tool & Mould, begun in 1976, produces high quality and highly accurate metal tooling for the aerospace composites industry, with RTM a key part of its business. North Coast Composites was begun in 2003 to produce parts for outside customers.
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