Plastic/metal hybrid technology encapsulates thin, sheet-metal stampings in a glass-filled polyamide. The result is a single integrated structural part that out-performs comparable versions in either plastic or metal alone. The technique uses the injection-molding process to create rib structures made out of 30% glass-reinforced PA 6 that are mechanically interlocked inside a steel U-shaped channel. This structure bears heavier loads and weighs less than ordinary equivalents. With this technology, features that are expensive to form in sheet metal are easily integrated via plastic overmolding and tight dimensional tolerances are possible in mass production.
The first commercial part using the technology was the front-end module (FEM) carrier developed and manufactured by Faurecia for the 1998 Audi A6. Compared to a conventional GMT (glass-mat thermoplastic) design the metal/plastic hybrid FEM weighed 10% less while lowering investment and direct costs by 25 and 10%, respectively. Ford Motor Co. also chose plastic/metal parts for the grille opening reinforcement (GOR) on the 2000 Focus. Here, the new component weighed 40% less and investment costs were half those of a pure metal design. Seventeen different functional features were integrated into the part using injection molding.
With plastic/metal hybrid technology multiple steel stampings join together during molding in a process called in-mold assembly. This lets steel go only where needed. Stampings join together by specially designed "button" connections that transfer the loading from one steel stamping directly into the adjacent stamping.
The first step in producing a plastic/metal hybrid component is to place a deep-drawn, stamped, thin-walled metal profile in the injection-molding tool. The tool closes and then fills with Bayer Polymer's Durethan BKV130-PA 6 resin, as in the standard injection-molding process. During the fill cycle, Durethan resin flows through the stamped, perforated openings and surrounds the edges of the metal profile. Solidification of the plastic creates a mechanical, interlocked connection between both materials, producing a single, unified component. Once cooled, the part ejects from the tool as a complete assembly needing no additional secondary operations.
In many applications, teaming PA 6 with steel boosts the performance of each. That's because thin-walled, steel structures alone tend to buckle under load before reaching their theoretical load-bearing capacity. Buckling is due to the low geometric stability of the thin-walled structures. The achievable load level, however, can be significantly higher if the structure geometry is supported. Thermoplastics such as PA 6 are ideal support materials because stabilization forces required to prevent buckling are relatively small. Delaying the onset of buckling helps fully realize the mechanical properties of the sheet metal.
The material properties that steel brings to the union are largely independent of temperature and include a high Young's modulus, low coefficient of thermal expansion, and a combination of high strength and high elongation at break.
Glass-reinforced Durethan PA 6, on the other hand, possesses good fatigue resistance under dynamic load, a coefficient of linear thermal expansion that is compatible with steel, good chemical, impact, and scratch resistance, good resistance to heat aging, and a low density. It also provides complex design freedom and gives a good-looking visible surface that can be colored. Its ability to stress relax lets it equilibrate the stress induced by the steel insert during injection molding. Glass reinforcement is needed to boost dimensional stability, heat deflection temperature, and impact resistance.
The resulting benefits of plastic/ metal hybrids include:
The injection-molding process opens up numerous possibilities for integration of functional elements. Savings result from the reduced number of components and elimination of additional assembly steps. Success in the automotive area opens the door for the hybrid technology to branch into other areas that benefit from structural integrity, cost and weight savings, as well as parts consolidation. Potential nonautomotive applications include information technology (laptops, handheld devices), sporting goods, appliances, aircraft, motorcycles, lawn and garden equipment, and recreation/agriculture vehicles.
Bayer Polymers, 100 Bayer Rd., Pittsburgh, PA 15205, (412) 777-2000, www.bayerpolymers.com
Fundamental proof testing
A series of comparative studies helps illustrate the benefits of hybrid structures. Three variations of a 340-mm-long beam were tested for stiffness and strength in bending, axial, and torsional loading. The beam profiles tested were a closed-U (box) steel structure made from 0.7-mm-thick steel, an open-U steel profile also made from 0.7-mm-thick steel, and a plastic/metal hybrid made from Durethan BKV130 30% glass-reinforced, impact-modified PA 6 and 0.7-mm-thick steel.
Under a torsional load, the closed box profile had the highest stiffness, followed by the hybrid profile. The open U-shaped design, however, quickly reached its stability limit and failed much earlier under all three types of loading. This effect would be even greater in profiles with thinner sheet metal or a larger cross-sectional area.
The advantages of the hybrid technology are better illustrated, however, by correlating mechanical performance to profile weight. Normalizing the data to the lowest, open-U profile, gives the following results:
The closed U-profile has a bending strength-to-weight ratio of 1.1 and an axial buckling strength-to-weight ratio of 1. This means the closed-U profile has 1.1 times more bending strength and the same axial strength per pound of material as the open steel U-profile.
The plastic/metal hybrid profile, likewise, has a strength-to-weight ratio at bending and axial loads of 1.8. This gives the hybrid profile 1.8 times more bending strength per pound of material than the open-U profile.
For the torsional test, the closed-U and hybrid profiles had stiffness-to-weight ratios of 8.5 and 13, respectively, which translates to 8.5 and 13 times more torsional stiffness per pound of material than their open-U shaped counterpart.
Weight-adjusted stiffness factors
|---------- Load type *----------|
|Hybrid with X ribbing, PA-GF 30%, steel thickness||1.8||1.8||13|
|Closed-U steel profile, 0.7-mm thick||1.1||1||8.5|
|Open-U steel profile, 0.7-mm thick||1||1||1|
Typical structural applications for plastic/metal hybrids involve simultaneous bending, axial, and torsional loads. Plastic/metal hybrid structures are more weight efficient for all three cases. The superior strength-to-weight ratio makes them ideal candidates for lightweight, high-performance structures.* Data normalized to the open-U steel profile.