Sunroof Frame Goes Natural

When you see the name “international” in a company’s name, you can sometimes take it with a grain of salt, Salz, sel, or what-have-you. But that’s most certainly not the case with International Automotive Components (IAC; iacgroup.com), as the Tier One supplier, which is headquartered in Luxembourg, has 85 manufacturing facilities in 20 countries, and if you add in its design, technical and commercial centers, that puts IAC in 22 countries. Yes, that’s what “international” means.

But this is not about grains of salt.

Rather, it is about fibers, natural fibers, a combination of a hemp from European sources and kenaf from Bangladesh.

Researchers at an IAC technology center In Ebersberg, Germany, not far from Munich, spent some four years working on the right combination of fibers and binder to create a material that they could apply to the construction of what is considered to be the first roof frame made with natural fibers.

Michael Behnke, product manager for Overhead Systems at IAC’s Advanced Development Europe facility, explains that while there are some sunroof frames made with ABS, possibly including glass fibers, for the most part, the frames are produced with 0.7-mm steel, which requires painting or zinc chromating to protect it from the environment.

And Fritz Schweindl, who is the director of Advanced Engineering for IAC, points out that sunroofs—both large and small—are finding increased application in cars and crossovers, so they thought it important to find the ways and means to create an alternative, one that is significantly lighter than the steel incumbent. As in up to 50 percent lighter than a conventional metal-reinforced steel sunroof frame.

And so they created the IAC “EcoMatHot” fiber mat made with the aforementioned fibers, a mat that is treated with a water-based acrylic binder, Acrodur 950 L from BASF Corporation (basf.com). All in, the binder accounts, Behnke says, for 28 percent to 30 percent of the finished part, with the natural fibers constituting the rest.

IAC has been working with natural fibers for some 20 years, so they have developed a good understanding of how the mats need to be put together for various applications (they’re involved in several applications, from trunk trims to door arm rests to glove box lids, although these applications use a thermoplastic binder, not a thermoset, as in the case of the roof frame).

Another factor in the selection of hemp and kenaf—both of which are very fine fibers with the requisite properties including elasticity, tensile strength and flexural modulus—for this application is economic: as these are fibers that are grown, not pulled out of a vat, environmental variations can have a consequence on the market prices of the materials—changes on the order of 20 to 30 percent can be seen. The use of the two fibers helps protect them on the cost front.

To produce the frames, strips of material are cut from the mat and then placed in the mold so that their edges overlap. This approach minimizes scrap (consider if they simply used a sheet: they would need to cut a large hole out of the center of it to get the frame shape).

The material is subjected to both heat (approximately 200°C) and pressure. It is necessary to open the mold a couple of times during the cycle in order to let the steam dissipate. The entire cycle time, which includes both trimming and the molding in of fasteners, requires about a minute.

Behnke says that the development program was largely driven by trying to reduce weight. The headliner sandwich has been subjected to mass reduction, so they looked to talking weight out of the sunroof frame. 

While there are, as mentioned, applications of ABS glass-filled frames, Schweindl points out that (1) those frames don’t save a whole lot of mass compared to the steel unit because of the need to have thick plastic sections in order to achieve the required rigidity and (2) the coefficient of thermal expansion is sufficiently different from that of the headliner material so that warpage and wrinkling occurs at about 100°C. Which is not an issue with the fiber-based frame. (While the coefficient of thermal expansion of the fibers in the thermoset binder is similar to that of steel, the expansion of aluminum is greater than steel, which is one of the reasons why the light metal is not used in the application.)

In addition to which, the natural fibers are not as abrasive as the glass fibers, which is better for tool life.

Tools are another advantage of the fiber frame, but in this case compared with a steel frame. While the fiber frame can be made with a single tool, it may be necessary to have as many as five for the steel component. This is not only costly, but it is also time consuming.

In fact, one of the key reasons why the natural fiber frame is cost-neutral compared with the traditional steel relates to the reduced tooling costs.

The IAC fiber frame is being used for the 2017 Mercedes E-Class. Well, at least the models that are not sold in the U.S. Realize that whereas steel is homogeneous and so its properties can be simulated, that’s not the case with the non-oriented fibers in the EcoMatHot mat. While IAC engineers have developed the CAE data defining the properties of the fiber mat, when this work was proceeding three years ago, they didn’t have it and so they were unable to perform the FMVSS (Federal Motor Vehicle Safety Standards) head impact tests in time to make the production launch, so they tooled the part for the U.S. vehicles in steel.