【DESIGN NEWS】Winning Composites
by: DN
Staff, Materials & Assembly,Automotive,December 13, 2004
If you
think that carbon-fiber composites belong only in the air and on racetracks,
think again. The 34th annual Society of Plastics Engineers' Innovation Awards,
which honor the year's best automotive plastics applications, not only featured
innovations in injection-molded thermoplastic parts (see sidebar on page 66)
but also highlighted three interesting composites applications.
Two of them show the promise of carbon-fiber
composites on low- to mid-volume production vehicles. The third involves a
patented processing breakthrough that expands the design possibilities for
compression-molded composites.
2005 Porsche Carrera
Composite Engine Frame
With its 605 hp engine and a top speed greater than
200 mph, the 2005 Porsche Carrera GT has plenty in common with a racecar. And
the similarities don't come down to power and speed alone. The car features an
open-top, rolling-chassis design that's more LeMans than Livonia. At the same
time, though, this car still takes to the road with safety performance that
meets or beats international standards. "Think of it as a race car with a
road license," says Walter Schaupensteiner, team leader for interior body
engineering.
To build a car that
combines racecar looks and performance with safety, Porsche engineers made
liberal use of carbon-fiber composite technology throughout the car's monocoque
chassis. For example, the Carrera features a carbon-fiber-reinforced epoxy
passenger tub. And for the first time in a production car, it has a brand new
composite engine frame, which won the SPE's Engineering Excellence Award.
Weighing in at 45 kg,
the frame supports both the engine and gearbox. And along with the passenger
tub, it acts as the car's chief structural member. Porsche engineers designed
the frame in composites normally used in aerospace applications-toughened epoxy
prepreg (Cytec 997), carbon fiber, and some Nomex. The company's composites
supplier, ATR of Italy, makes the parts at a rate of three per day using
hand-lay-up techniques and an autoclave process.
According to Schaupensteiner,
the use of aerospace composites resulted in important weight and stiffness
benefits. He estimates that the composite engine frame weighs about 40 percent
less than designs based on steel tubes. At the same time, he says, these stiff
composites contributed to improvements in the car's torsional and flexural
stiffness, which "enhances the driving dynamics."
The composite parts
also offered a huge parts integration advantage. Schaupensteiner notes that the
engine frame has more than 200 attachment points to accommodate the drivetrain
and crash structures. And from a quality standpoint, he adds that Porsche
attaches components to the frame with a group tolerance of just plus or minus 4
mm. This need to precisely attach other components was one reason that Porsche
engineers ruled out an aluminum space frame design. Schaupensteiner says that
the aluminum design would have required many heavy brackets, which would have
been tough to locate on the frame and would have offset any possible weight
savings. What's more, aluminum had unacceptable CTE differences with some of
the attached components, potentially threatening Porsche's tight manufacturing
tolerances.
Porsche engineers had to overcome
three key challenges to reap the benefits of composites. First, they had to
find a materials system that would work in difficult environmental conditions.
The engine frame, which essentially forms the engine compartment, commonly
experiences temperatures as high as 180C. And heat isn't the only environmental
threat. "These materials are exposed to heat, oil, dust, moisture and salt
for 20 years," Schaupensteiner says.
And with the structural importance of the engine frame, Porsche
engineers finally had to worry about the possibility delamination of the
composite sandwich in high-stress areas. Schaupensteiner reports that the
engineering team used FEA tools to optimize the location of all the attachment
points and the direction of the fiber reinforcements.
2005 Ford GT
Carbon-Fiber Decklid
Porsche isn't the only company to adopt carbon-fiber-reinforced
epoxy composites in a production vehicle. Ford's 2005 GT uses a composite
system-in this case, one based on a Toray quick cure epoxy-prepreg-for the
inner panel that forms the structural backbone of the car's decklid assembly.
Here, too, weight savings, torsional rigidity, dimensional stability, and parts
integration represented the drivers for composites use.
Adrian Elliot, senior technical specialist at the Ford Research
Laboratory, reports that the 6.4 kg decklid weighs about 50 percent less than a
comparable aluminum part and 75 percent less than steel. He adds that the
carbon-fiber part, which was one of the Innovation Award finalists, helped Ford
meet critical strength and stiffness goals. And the design freedom imparted by
composites also helped enable creation of one large part that integrates hinges
and closure hardware. After a hand lay-up and autoclave molding process, the
decklid inner is trimmed with a CNC router and ultimately adhesively bonded and
hemmed to the vehicle's aluminum outer panels.
The ability of composites to shave off the pounds while offering
parts integration opportunities may not come as a surprise. What may is that
carbon-fiber composites actually represented a low-cost alternative. For
example, Ford considered the use of superformed aluminum for the decklid inner.
But aside from weighing more, these aluminum-based designs would have required
multiple parts-and thus multiple tools. Elliot estimates that the one-piece,
one-tool carbon fiber decklid will cost about 32 percent less than aluminum
over the life of the project. Mostly the savings come from avoiding tooling
cost-by dropping from four tools for a multipart design to one Invar tool for
the carbon fiber part. "Recognizing full amortization, carbon fiber is
considerably less expensive than the alternatives," Elliot says.
Because the decklid structure combines
carbon fiber composite with aluminum, Ford also had to find ways to avoid
galvanic corrosion and CTE mismatches between the inner and outer materials.
Elliot reports that the company developed a patented method for dealing with
both problems. "We came up with an innovative way to isolate the carbon
fiber from the aluminum," he says. Ford's method involves adhesively
bonding the inner and outer panels so that they can move relative to one
another. The adhesive also physically separates the two materials. For the
hemmed portions of the decklid structure, Ford puts a glass scrim between the
composite and rolled aluminum edge.
Cequent Compression-Molded Towing Tray
Changing gears from thermoset
composites to those based on thermoplastics, an aftermarket towing tray made
from a 25 percent glass-filled, compression-molded polypropylene took the top
honors in the Performance and Customization category. These platforms extend
from the back of a truck or SUV to provide extra space for hauling gear.
Usually they're fabricated from steel or aluminum, but Cequent Towing instead
took advantage of a new gas-assisted composite molding process developed by
Composite Technologies and Alliance Gas Systems.
This patented compression molding
process uses a reciprocal gas pin to inject air into a closed
compression-molding cavity during the molding process-so that the air
selectively hollows out wall sections. The technology is similar to the gas
assist systems used in injection molding for years. "But this is the first
time it's been applied to compression molding," says Maria Ciliberti, vice
president of Composite Technologies.
Like gas assist for injection molding,
the compression-molding version offers compelling manufacturing and design
advantages. The partially hollow parts weigh less and cool faster. "Cycle
time reduction is the primary driver for the technology," says Ciliberti.
And from a design standpoint, she adds, hollow sections allow engineers to
stiffen parts without increasing wall thickness.
In Cequent's case, all these
advantages came into play. At just 14.5 lb, the two-piece plastic tray assembly
weighs 25 percent less than a comparable steel grate and 15 percent less than
aluminum. It still resists deflection, though, and has a capacity up to 500
lbs. Thanks to gas assist, the plastic part has 50 percent less wall stock in
the lip that runs around its edge. And Ciliberti says this material removal
reduced part weight by 5 percent compared to solid plastic. It also improved
cycle times by 40 percent for a $1.00/part savings. Compared to the cost of
metal parts, the gassed plastic cost 25 percent less than steel and 30 percent
less than aluminum, according to Ciliberti.
Though this job for Cequent involved a
thermoplastic composite, Ciliberti says the technology could be applied to
thermoset composites, too. "Gas assist changes the economics of
compression molding," she says.
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