Like many nations, South Korea is introducing regulations to boost fuel efficiency in its vehicle fleet, requiring automakers to boost fuel economy by 20% between 2015 and 2020 to 20 km/l (47 mpg). Automakers there are adopting multipronged approaches to meet these targets, including low friction technologies, efficient or alternative powertrains, aerodynamics, and of course lightweighting.
At the recent JEC Asia Show in Seoul, Chi-hoon Choi, Research Fellow at the Polymer Research Laboratory of Hyundai Motor Group, detailed efforts underway there to meet these fuel efficiency targets.
Initially, the automaker investigated a compression high-pressure resin transfer molding (HP-RTM) that utilizes a fast cure epoxy resin thermoset system as a process for making lightweight hoods. The multifunctional epoxy, combined with an aromatic amine-based hardeners and an imidazole/benzylamine-based accelerator, cures in 2.5 minutes at 140 deg C. “We also investigated various reinforcing of carbon twill and non-crimp fabric (NCF) as well as the use of some glass fiber NCF to reduce costs,” says Choi. “A combination of twill and NCF delivered the best property balance.”
|A Hyundai prototype electric vehicle was the test bed for a carbon fiber composite hood.|
The carbon fiber-reinforced hood was 2-3 kg lighter than an aluminum hood, and 7-9 kg (63%) lighter than a steel hood, and also allowed a lower center of gravity to be achieved and a better distribution of weight between the front and the rear.
Another reason for selecting the hood as an initial development project is that it is easy to replace if damaged. Which brought the company to the issue of pedestrian safety. “Simulations showed that a 1.5-mm thick hood could not meet pedestrian safety standards as it was too stiff, so we designed a 1-mm-thick hood.”
3D preforming by automated fiber placement was also critical not only in streamlining the overall production process to a duration of six minutes, but to minimize wrinkles and waste, and ensure head impact collision safety standards were satisfied.
A unique flow simulation tool was also be developed for compression RTM, which assisted in optimizing placement of the injection port and vents, thereby reducing injection time by 8%. Processing was carried out on a 1250-tonne Dieffenbacher high pressure RTM press at the Korea Institute of Carbon Convergence Technology (KCTECH) in Gwangju.
“The big issue that remains is cost,” concedes Choi. “The carbon fiber composite hood was 5.8-times more expensive that steel, but we were able to reduce this to 4.9 times through automated fiber placement.” Choi says Hyundai must also closely look at aluminum parts, which are 45% lighter than steel, while cost is only slightly more than double. “This time we could not achieve our internal target as cost of carbon fiber is very expensive. And we, like every OEM, want make a carbon fiber part in one minute, which is comparable with steel stamping.”