Cutaway model of Morgan Polymer’s design with ITW for GM’s Cadillac.

The Challenge

General Motors (GM) tasked Morgan Polymer Seals to collaborate with Illinois Tool Works (ITW) to design a mechanical seal for a new capless refueling system in the Cadillac CT6. The custom seal system had to reduce hydrocarbon emissions, seal on the fuel fill-nozzle, and remain flexible at a temperature as low as -40°C. To avoid a larger redesign of the car’s body, the sealing system also had to fit within the sedan’s existing size constraints.

Background

For years, automakers have equipped their cars with capless refueling systems to give drivers the ability to refuel their car without the use of a gas cap. Most capless systems enable the driver to insert the fuel fill-nozzle through a series of redundant plastic doors with integrated seals to ensure a closed and sealed fuel system. When a driver removes the fuel fill-nozzle, the spring-loaded door seals snap back into place and keep the gas vapors inside the fuel tank.

While capless refueling systems are prevalent in today’s automobiles, they are not necessarily the standard. So when GM set out to convert the existing refueling system on the CT6 to a capless system, they needed a design that would effectively seal gas and vapor, without making any changes to the car’s body size and shape. To that end, the existing carbon canister could not be resized. In addition, they wanted the capless refueling system to operate effectively at temperatures down to -40°C, so the rubber components of the system needed to be flexible and seal at that extreme temperature.

GM looked to Morgan Polymer Seals and ITW to develop a capless refueling solution that could meet their requirements.

The Solution

Morgan Polymer and ITW collaborated on a specialized capless refueling system, which featured a dual-component mechanical seal made with fluorocarbon-based materials (FKM) that could integrate into an existing module. (Figure 1)

Figure 1: Capless refueling system designed for the Cadillac CT6

Converting to a Capless Refueling System

The resulting design was a capless refueling system made up of an over-molded plastic carrier with three unique seals: a spring-loaded overflow door seal, a nozzle seal, and an outer diameter seal which seals the system to the inner diameter of the capless refueling module.

The mechanical seal (Figure 2) was the key component in the new design because it made it possible for the system to seal on the fuel fill-nozzle. By creating an absolute seal around the nozzle, gas vapors are forced to go through the vehicle’s carbon canister to be absorbed, which eliminates hydrocarbon emissions during refueling.

Creating an air-tight seal around the gas nozzle presented another challenge that the mechanical seal needed to address: what to do in the case of over-fueling. The solution was to design a proprietary, ramped overflow door seal that would act as an escape hatch for excess fuel while still maintaining the integrity of the seal around the fuel-fill nozzle.

The overflow feature (Figure 3), enables fuel to escape the capless system in the event of a system blockage or back pressure during refueling. This seal has a unique ramped designed since it must create an airtight seal against a spring-loaded plastic door.

Figure 3: Model drawing of overflow feature

The second component of the capless refueling system is the double-lip seal (Figure 4) that is designed to create a seal on the fuel fill-nozzle. For this component, engineers designed a unique seal for the Cadillac CT6 that could seal on the variety of fuel fill-nozzles used worldwide and still meet the insertion/removal force specified by GM. Double lip seals by themselves can be difficult to mold due to the undercut of the seal design that traps the part in the tool, making removal without tears or defects from the tool a challenge. Having to accommodate the bypass door only added to the seal’s design complexity.

Figure 4: Model drawing of double-lip seal

The third component of the mechanical seal is an over-molded outside diameter (OD) seal that had to be designed to fit into the capless refueling module, which is pressed into the fuel-fill tube. The capless refueling module shown below is sealed by two pipe body seals.

View of Mechanical Seal installed in system

Combined, this proprietary carrier gasket is then pressed into the capless fueling module, becoming part of the entire sealing system.

Now that the engineers had solved the mechanical challenge of how to convert the CT6’s fuel system into a capless one, the next challenge was how to design a system that could operate in extreme cold temperatures.

Designing for Extreme Cold Temperatures

GM wanted the capless refueling system in the Cadillac CT6 to function anywhere in the world, including areas with extreme cold temperatures. The design engineers specified that the rubber components had to be made with FKM fluorocarbon-based materials that remain flexible at temperatures down to -40°C. This would ensure that the insertion force of the fuel-fill nozzle through the double lip seal would not be an issue for drivers in extremely cold climates.

Rubber commonly becomes less elastic as the temperature drops. At a certain temperature, known as the glass transition temperature (Tg), the rubber becomes glass-like and loses its ability to flex. Beyond this point, the rubber can become brittle and tear.

FKM materials by nature have a higher coefficient of friction, and as the temperature drops, this resistance can be problematic for an application like the mechanical seal. Most FKM materials used for fuel systems have a Tg of -15°C, 25 degrees shy of GM’s requirement. To accommodate GM's requirements for the CT6, Morgan Polymer developed a proprietary FKM compound with a Tg of -40°C.

Creating High-Precision Parts with Minimal Waste

The next challenge was to injection mold the seal using the proprietary FKM compound at a low scrap rate. Waste reduction when molding with specialized FKM is especially important since the cost of the material exceeds $129/lb.

Injection over-molding is a manufacturing process for producing parts by injecting pre-heated material into a mold using a screw and piston, and once inside the heated mold cavity, the material will start to cure. (For more detailed information on injection molding, click here to read our “Rubber Injection Molding” white paper.) It is often used to manufacture parts with more complex and tighter tolerance geometries.

Such was the case with the carrier gasket used in the capless refueling system. First, Morgan Polymer manufactured the plastic carrier and then over-molded the FKM material to create the mechanical seal. The part required high-precision and low waste, so injection molding was the ideal solution. That process, however, introduced another challenge.

FKM can be difficult to injection mold due to the high viscosity of the material - almost like trying to push concrete into the tool. As a result, using FKM for a complex injection over-mold application can potentially generate a lot of waste and scrap if the tooling is not designed correctly. In addition, the FKM compound created to meet the -40°C Tg requirement for the capless refueling system was significantly more costly than typical FKM materials. Due to the high cost of this material, Morgan Polymer’s quality team had to ensure the part ran at a very low scrap and waste rate.

Morgan Polymer Seals: An Extension of Your Design Team

GM presented a tough design challenge to Morgan Polymer Seals and ITW: create a capless refueling system that would function within the existing architecture of the Cadillac CT6 and hold up in extreme cold temperatures. Together, the technical teams of all three companies co-developed an elegant solution that perfectly suited the application.

Since 1997, Morgan Polymer Seals has been a reliable full-service supplier to OEMs in North America, Europe, and China. To learn more about how Morgan Polymer can be an extension of your engineering team, click the button below to contact a product specialist.

Contact Us

Customer Story: Capless Refueling Seal for GM's Cadillac CT6