Send this page / Print this page Home / Composites News / Applications

16 Nov 2005  -  United States

North Coast RTM tooling innovations — A case study

The Companies of North Coast have proven time and again that cost-efficient resin transfer molding (RTM) is a viable manufacturing method for large complex, integrated aerospace structures given proper planning and tool design. A good example is the unitized and fastenerless all-composite vertical tail developed and manufactured by Lockheed Martin Aeronautics Co. Several years ago, as part of its self-funded Advanced Affordability Initiative, Lockheed decided to investigate RTM as a way to reduce costs for large parts like a tail structure, and built three demonstration parts. The project’s success was due in large part to North Coast Tool & Mold’s innovative tooling design concepts.
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 aluminum, slightly undersized to accommodate the reinforcement preform. Because aluminum has a higher coefficient of thermal expansion (CTE) than steel, North Coast counted on expansion of the mandrels against the 4140 mold 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 fiber 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.
But, how to place the braids on the mandrels? 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 fibers, 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 aluminum mandrels which were only held at one end and cantilevered in space. Provisions were designed into the handling device to account for the changing center 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 mold, 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 mold 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 mold deflection, and the possibility of fiber wash (fabric movement).”
The DRIV™ inserts are precisely fitted into machined pockets in the outer mold surfaces. They can virtually be any shape. Each one becomes a vent in the mold for resin flow management. These vents are custom engineered to suit the resin viscosity and permit a small amount of resin to flow through as a telltale, to positively indicate resin flow in that part of the mold. The inserts can be operated manually, with a technician opening each DRIV™ insert in turn, or they can be easily automated with microprocessors.
In this particular instance, DRIV™ inserts were positioned between the mandrels in the surface of the mold. 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 mold to overcome injection pressure which reduced overall tooling cost and made the molds 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 molds together but schedule delays forced a different solution. North Coast designed a straightforward and cost-effective clamp to secure the two mold halves as well as a simple rectangular steel I-beam frame to hold the assembled tool in its upright position. Then, aluminum wedges were inserted between the mold and frame. Again the CTE difference meant that during mold heating the aluminum 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.

Source : The Companies of North Coast