Produced from a single sheet of SymaLite in a common low-pressure mold, the underbody shield of the latest BMW 5 Series soon may be followed by roof panels, hoods and deck lids made of the same material.
According to the folks from Quadrant Plastic Composites (Lenzberg, Switzerland), the underside of BMW’s latest 5 Series is as compelling as the rest of the vehicle. Made from SymaLite, the company’s latest Glass Mat Thermoplastic (GMT), the four-part underbody shield is molded from a single sheet of the product in a common tool under low pressure. “This low-pressure stamping or thermoforming process,” says Quadrant’s Harri Dittmar, Manager Market Development Composites, “makes this material amenable to both paint films and polymer skins.” It also makes it ideal for use with roll-coated sheet aluminum for drop-in roof units and body panels.
Unlike conventional GMT, SymaLite is produced using a modified textile process that homogeneously mixes the glass and thermoplastic fibers into a high-loft fleece that is formed and needled. (Needling gives control over the strand orientation and loft by arranging the fibers in a non-woven pattern.) “The use of long–75 mm–fibers,” says Dittmar, “increases the homogeneity of the mix because the fibers don’t ‘slip’ during forming.” Which means greater mat coverage in corners, folds, and around indentations. A heat source (infrared, contact ovens or hot air) warms the fleece to the point where it “lofts” (expands) up to six times its original thickness, and the heated blanks are transferred to a water-cooled molding press. There the fleece is compressed with the outer layer in a single step, eliminating the need for binders or glue.
In order to create a thicker section with SymaLite, it’s not necessary to add more material as with a conventional GMT. “To get a thicker section,” says Dittmar, “you construct the tool in such a way that it doesn’t compact the material as much in the chosen areas. Areas that need high tensile strength are pressed thinner during molding, while areas that need high stiffness are not compressed to the same degree.” A fully compressed fleece part can have a glass content of 20% to 60%, and weights from 600 g/m2 to 3,000 g/m2. The higher the glass content, the greater the loft of the material.
“Another advantage of SymaLite is its low molding pressure versus traditional GMT.” It’s <0.2 MPa vs. 14 to 17 MPa.” Fast, low-cost prototype production can be done using the final material and wood or plastic tools, and because there is no material flow in the tool, knit lines are not a problem. Holes can be stamped or cut using mechanical cutting tools, water jets, or a laser. In the case of stamping, the area around the hole is thinned in the mold prior to it being stamped to provide the greatest dimensional stability. The lack of knit lines also means the holes can be located closer to part edges. “The low pressure also means multiple parts can be formed at the same time in a single tool,” says Dittmar. In the case of the BMW 5 Series undertray, the SymaLite sheet is held vertically in a multi-cavity tool to avoid sagging across its width, with the press acting along its horizontal axis.
Roof modules that use SymaLite behind a Class A surface material (either coil-coated aluminum, engineered thermoplastic, or polypropylene film) are under study at Quadrant’s headquarters in Switzerland. Production steel roof modules weighing 7 to 9 kg/m2 will, Dittmar believes, be replaced within the next five years by SymaLite panels weighing 3 kg/m2 and glued to the body structure. “The coefficient of expansion is similar for the materials,” he says, “and our longer fibers means there can be a greater panel curvature before there is any tearing.” However, there are problems to be overcome. For example, the volume and color choices for coil-coated aluminum must be increased to meet the auto industry’s production requirements. And the different coefficients of expansion between the paint film and thermoplastics can lead to delamination from the heat cycling that takes place during the molding cycle. These areas currently are under study. Once the roof panel issues are under control, hood and trunk lids are next.
“It takes much longer to get a new material into the vehicle structure,” says Dittmar, “because a lot of education must take place, you have to overcome a ‘steel first’ mentality, and the technology must be proven at the start of a program.” Obviously, Quadrant is shooting for inclusion on the next generation of vehicles.
Although advanced composites aren’t likely to become the dominant material for automotive applications in the near future (or even the far future, we suspect), according to David White, chairman of the Automotive Composites Alliance (ACA)—a trade group consisting of molders, material suppliers, and toolmakers who deal with thermoset composites—there have been some recent breakthroughs that will lead to greater body panel applications for the materials. “The biggest hindrance the industry has faced over the last few years for Class A parts has been paint pops,” he says. White is also vice president-Sales for Meridian Automotive Systems, a supplier of composite components (and even various parts and structures made from metal), so he has more than theoretical knowledge of this issue. He maintains that this problem has pretty much been put to rest thanks to two developments. The first is a UV-cured primer sealant (he specifically cites DynaSeal from BASF) that is applied to the surface of a sheet molded composite part to prevent the outgassing problem during painting. The second development relates to the basic material chemistry: the resin used for composites panels is formulated to be more elastic so that when parts are handled during processing, the knocking around that they invariably receive is less likely to lead to microcracking, which can lead to paint defects.
White says that a considerable amount of work is presently underway to make SMC more competitive with aluminum. He notes that presently, the specific gravity of aluminum is 1.3 but it is 1.9 for typical SMC. There is a low-density SMC material that’s more in line with aluminum. He notes, for example, that the Corvette hood inner panel is made with this low-density SMC (the outer is standard). The issue that they’re working with is improving the surface finish of the low-density material because as the formulation is changed to reduce the density (e.g., reducing the amount of calcium carbonate filler; replacing long glass fibers with glass spheres), it “degrades the surface appearance.” White believes that given the amount of knowledge that they’re developing, this should be resolved.
In the meantime, White and his colleagues will continue to note the tooling cost savings that can be realized by using composites rather than metal for lower volume applications and the ability to consolidate parts (he cites the front fender support on the Dodge Viper, which he says went from more than 30 stampings to a single composite part).—GSV
The Dodge Viper is essentially a study in advanced materials, as RIM or SMC panels are used for the windshield frame, cowl panel, rear quarter panels, front fenders, doors, hood, deck lid, and front and rear fascia. David White of the ACA says that the front fender support has gone from being an assembly of some 30 stampings to a single composite component.