Carbon composite replaces steel in a panoramic sun roof frame in the 2015 Kia Sorento. The number of parts is reduced from 33 to four and weight is cut by 50 percent, about 7 kg.
It’s described by Kia as the first mass-produced (more than 100,000 units per year) vehicle that uses carbon fiber that does not dramatically increase costs.
Localized steel reinforcement is used to increase part performance while optimizing geometry and thickness, according to Joel Myers, senior engineer at Hyundai America Technical Center, Superior Township, Michigan.
A steel insert injection molding process is used by Inalfa Roof Systems, Gyeonggi-do, Korea.
Mechanical properties of the carbon composite are improved by using a new compounding process in which fibers are twisted after being pulled from a cooling bath. The twisting results in longer fiber length in the pellet and better resin impregnation, according to Myers. The matrix resin is polyamide 6 with 20 percent carbon fiber from GS Caltex, a South Korean oil refiner that has expanded downstream into plastics.
A glass fiber composite frame has also been tried for the Sorento sun roof. Use of carbon fiber reduces weight and improves stiffness at a reasonable weight, says Myers. The glass fiber system (30 per cent glass) used an alloy of PBT-type polyester and ASA styrenic alloy. The CFRP system provides a lower center of gravity, improving ride and overall handling performance. There is also increased visual area through the sun roof while reducing cross section of the frame.
Tooling for the part was made captively by Hyundai, which owns the Kia brand. Special effort is required to minimize warpage after cooling.
The part won the exterior category in the Society of Plastics Engineers’ Automotive Division’s 44th-annual Automotive Innovation Awards Competition.