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22.10.2026 | News
Durable Sealing Of Busbar Feedthroughs: Replacing Epoxy With Elastomer
Where copper wiring passes through housing walls, thermal stress can cause microcracks in conventional potting compounds. To prevent this, Freudenberg Sealing Technologies relies on bonding solutions that overcome this problem.
Chemically bonded seals for high-voltage feedthroughs in electric vehicles
When busbars are routed through housing walls in electric vehicles, the continual stress will cause microcracks in the conventionally used epoxy potting compounds. With the increasingly common use of rectangular busbars, this stress is concentrated in the corners. Freudenberg Sealing Technologies uses a different approach by chemically bonding elastomers to the busbar. Its application to rectangular cross-sections is patented.
Where copper is used, potting compounds crack
Busbars are rigid electrical conductors made of copper or aluminum. They connect the high-voltage components of the powertrain, such as the battery, junction box, inverter and electric motor. Metal and plastic meet at each housing feedthrough and expand at very different rates when exposed to heat. If the cross-sections are round, stress is distributed evenly and can be managed with an attached sealing element. In rectangular cross-sections, it is concentrated in the corners. The edges create an additional notch effect.
Potting compounds made of epoxy respond to this recurring stress by developing microcracks. These cracks make it possible for oil or coolants to enter areas that have to stay dry. Since the feedthrough also insulates the voltage-carrying conductor from the housing, this can result in a worst-case scenario of leakage currents or a short circuit. It is almost impossible to repair such an area at a later time.
A bond that moves
Freudenberg Sealing Technologies relies on a chemical bond between elastomers and busbars. During rigid overmolding, the metal surface is pretreated with a primer and coated with an elastomer layer before the component is overmolded with thermoplastic. This creates a chemically bonded connection. In the overmolded installed variant, the bonded elastomer seal is first applied to the busbar and only later installed in the housing. This compensates for manufacturing tolerances, misalignment and vibrations while enabling additional sealing functions.
In contrast to epoxy, the elastic bond accommodates relative movement between the busbar and the housing, regardless of whether they are caused by thermal expansion during operation or by the assembly. If the cross-sections are rectangular, a specially adapted sealing contour ensures uniform contact pressure even in the corners. Depending on the application, the company uses FKM, AEM, ACM, HNBR, NBR or EPDM, covering temperatures ranging from -40 °C to over 200 °C. The concept also offers advantages when it comes to manufacturing. The potting is often carried out toward the end of the production process – so if any defect occurs at this stage, the entire assembly may become unusable. Elastomer seals, by contrast, can be integrated at different stages of the manufacturing process. This can help reduce waste.
Tested on the finished component
Freudenberg Sealing Technologies has developed a dedicated testing program that covers not only material samples but also finished components. It is based on DIN EN 60068-2-2 for heat aging, DIN EN 60068-2-14 for temperature cycling and LV 214 for combined thermal shock. Applications in oil or refrigerants are also included. Installations in CO₂ air-conditioning (AC) compressors have particularly demanding requirements. CO₂ molecules can penetrate the elastomer and expand abruptly during rapid drops in pressure. Such an explosive decompression can damage the seal from the inside. For this reason, the bond must also withstand test pressures of over 100 bar.
Freudenberg Sealing Technologies first developed this process for busbar feedthroughs in oil-cooled electric motors, inverters and immersion-cooled batteries. Today it is also used in AC compressors, electric pumps, multi pin connectors and numerous other electrical applications. As voltages increase and installation spaces become more compact, the demands on reliable sealing continue to grow. This makes it essential to already consider the sealing concept as the busbar feedthrough is being developed.
More about sealing solutions for busbars: Busbar seals (https://www.fst.com/markets/automotive-truck-bus/busbars//)
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