PERFORMANCE ATTRIBUTES: PLASTIC VERSUS STEEL
Manufacturability
Terne plate holds a materials cost advantage over HDPE: $0.66-0.79/kg vs. $0.86-1.08/kg.13 The cost of the material is not the only driver; consideration includes the net cost of the fabricated tank and its reliability within the total fuel system of the vehicle, including the tank, filler tube, level control, baffles, the housing reservoir for the sending unit, and other assorted tubing, fittings, and seals from the tank to the engine. All of these components must function properly with the various fuel types and for the life of the car. Unforeseen corrosion can easily contaminate the fuel delivery system and cause costly repairs.
Manufacturing costs for either tank material seem conflicting, depending on the source. Nevertheless, due to the invested capital of OEMs on stamping, welding, and assembly equipment for metal tanks, their cost structure indicates a lower cost per piece on steel tanks versus plastic ones, with the latter usually being outsourced (except for some Ford models).
Plastic tanks are formed by blowing a thick continuous tube of HDPE within a mold that determines the final shape of the virtually seamless part, which could include the filler neck. The blowing molds are cast from aluminum and cost considerably less than machined steel dies used to stamp steel tanks. In general, four or more molds are integrated into one rotary style blow-molding machine to achieve the desired productivity (i.e., one station blows while the other one cools). Typically, the OEMs outsource the plastic tanks to various suppliers who bid for the business. The plastic tank manufacturer also has to either chlorinate or fluorinate the plastic to retard permeation, and both processes can be highly toxic if mishandled. This represents additional OSHA requirements, which add to the cost of the tank.
Design Features and Weight
Plastic tanks have the ability to meet packaging constraints with complex shapes, and design engineers have greater flexibility in the car design and styling without having to worry about fitting the gas tank. The plastic tank could virtually be made to fit whatever cavities are left by the design. Other attachments to and within the plastic tank require gasketed mechanical joints. However, plastic swells with constant exposure to organic liquids and vapor, thus making the joints very critical in the event of repairing the tank hardware.
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| Chrysler Chrysler made the decision to outsource plastic tanks in 1988 and, given recent developments in multilayer HDPE technology, they remain committed to plastic tanks. The listed advantages of plastic over steel according to Chrysler include lack of corrosion concerns, easier packaging, ability to design thickness where needed, and lower weight. Whether all of these attributes offset the higher unit cost of plastic tanks remains uncertain. Their plans indicate that all of their tanks will be made of plastic by the end of 1996. Eventually, all plastic HDPE or sulfonated HDPE tanks will have to be switched to the more expensive and slower-to-process coextruded ethylene-vinyl alcohol/HDPE layered tanks—a must for the tanks to pass stricter permeability standards. This mandatory switch represents an opportunity for steel tanks, which will be more cost competitive against the coextruded plastic tanks. Ford General Motors
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The average gas tank for a compact automobile (e.g., Nissan Altima) can boast weight savings of up to 30% versus a similar steel tank.14 However, Cadillac claims that although their plastic tanks allow design flexibility with increased safety, they do not achieve any weight savings over steel tanks.15 These two examples seem to contradict the general view that plastic's weight advantage increases with the size of the tank. On the other hand, the new permeability requirement is expected to diminish the weight advantage of plastics.
Safety
One critical part of the performance criteria of the tank is its ability to meet crash requirements. Generally, plastic tanks are considered safer in crashes because they are seamless and, thus, not prone to failures in the vulnerable seam areas. They are not a source of sparks. Also, plastic tanks deform and have some ability to rebound back to shape. When steel tanks absorb energy and deform, the pressure within the tank increases as the volume decreases. This makes them vulnerable at welded or clamped areas where failure can potentially occur.
The thermal properties of the chosen material are also an issue, especially due to the proliferation of injector fuel delivery systems, where a portion of the unused fuel delivered by the gas pump is returned to the gas tank at "engine-hot" temperatures. At the same time, the tank must withstand extreme temperatures in North America from -40°C to 79°C in-tank temperatures. The 79°C temperature not only exceeds the boiling point of the alcohol fuels, but also creates sagging problems for plastic (especially under the weight of a filled tank) while the extreme cold introduces potential cracking problems. As a result, OEMs resort to heavier gauge plastic, negating at least some of the weight advantage, and must also use support brackets and special shields against the heat of local sources like an inferior or perforated muffler or tailpipe. High ambient temperatures underneath the car remain a consideration.
Plastic acts as an insulator to retard heat transfer to the fuel when compared to a steel tank. In the case of an under-car fire, plastic tanks will retard the rise in fuel temperature, but they will soften, sag, and eventually release the fuel. A steel tank does not sag in a fire; however, the fuel temperature may rise rapidly, perhaps resulting in over pressurization and release of fuel through a mechanical fitting. The American Iron and Steel Institute reports16that a series of more than 75 tests undertaken by the National Fire Prevention Research Foundation and Factory Mutual Research Corporation indicated that plastic containers storing flammable or combustible liquids in general purpose warehouses fail abruptly when exposed to a small fire. This failure results in a rapidly developing spill fire that overpowers conventional sprinkler systems. The same tests conducted with flammable and combustible liquids stored in steel containers resulted in no spill fire, no excessive temperatures, no content involvement, and no significant loss of visibility due to smoke. The fires involving the steel containers extinguished themselves. These findings have led to a return to steel containers from plastics for safety and fire insurance cost reasons. It is not known if tests have been conducted by OEMs to compare the performance of steel and plastic tanks in under-car fire situations.
Corrosion
Corrosion is a well-known concern on both the inside and outside surfaces of tanks. The outside surfaces and supporting structure are exposed to road chemicals, salt, mud, and gravel. The corrosion issue is critical with zinc-coated products that replace terne plate because of their sacrificial nature, which puts an even higher demand on the quality of the barrier film for both inside and outside surfaces. In contrast, the HDPE gas tanks are inert to the corrosive environments inside and outside the tank.
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