Materials Update 19977

Some of the best applications of injection molded parts are the ones that barely get noticed. Take, for instance, the engine cover on Piaggio motorcycles. It's nylon, withstands high heat, saves weight, and dampens sound. When it works, the rider shouldn't notice it. Yet, if it fails to do any of these jobs, it sticks out like a sore thumb. The goal, then, was to make a cover that basks in obscurity.

To do the job, motorcycle manufacturer Piaggio of Pontedera, Italy selected DuPont Zytel HTN high-performance nylon 6/6 for the covers of its liquid coolant Low Emission ADvanced Engine (Leader) family.

Design director of Piaggio's Engine Div., Paolo Nesti, says, "We selected Zytel HTN for the Leader covers since it displayed the best combination of mechanical characteristics and processing ease of any polymer we tested." The mechanical characteristics of DuPont's high-performance polyamide that most appealed to Piaggio were its dimensional stability and resistance to constant high temperatures (more than 100C).

Nesti continues, "During our testing, we saw that motorcycles with engine covers of Zytel HTN display far better sound and vibration dampening characteristics than engine covers made of similar polymers. Production benefits resulting from our switch from aluminum to DuPont's high-performance polyamide include a cost savings of around 25 percent and a overall weight savings of 40 percent."

Piaggio's Leader family, launched in March 1999 to meet European environmental requirements, originally featured aluminum covers. Yet even as the Leader range was being announced to the world, Piaggio was already in talks with DuPont Italiana regarding the substitution of metal with engineering polymers to simplify production, save weight, and gain design freedom through injection molding. The first molding trials took place remarkably quickly, and the engine covers of Zytel HTN were in mass production by February 2000. The Zytel HTN engine covers were used on the 2001 models of Piaggio's Hexagon, Runner, and X9 motorcycles (four-valve, 125-cu-cm and 180-cu-cm liquid coolant configurations).

Nesti confirms that this is one of the first times, to his knowledge, that thermoplastic engine covers have been commercialized on four-stroke scooter engines. "Today's high performance engines mean higher oil temperatures leading to constant overall engine temperatures of around 140C," he explains. "DuPont's Italian technical center in Cologno Monzese provided us with feasibility studies that confirmed that 35 percent glass-reinforced Zytel HTN would be suitable for the Leader liquid coolant covers, even taking account of this tough specification. We were comfortable with this recommendation since we have been working with DuPont since 1998 to substitute other metal and rubber engine components with thermoplastics."

DuPont specialists recommended Zytel HTN because of the resin's high temperature resistance and increased flex modulus (compared to standard nylon), dimensional stability, rigidity and tensile strength, good heat aging characteristics, and high glass transition temperature performance.

From February 1999, Piaggio spent a year subjecting the application to a range of rigorous testing. In Nesti's words, "We confirmed that the high temperature performance of this polymer was excellent." Piaggio found that Zytel HTN resisted constant exposure to gasoline and engine oils at 140C with ease. It retained its dimensional stability over a range of impact and burst resistance tests. Whereas other similar polymers had suffered distortion and warpage problems during testing, Zytel HTN did not. During molding trials, Piaggio and DuPont worked with molder Istamp, located near Turin.

Piaggio's Leader features four-stroke SOHC engines from 125 to 180 cu cm, in two- or four-valve configurations, liquid or air cooled. The engines were expressly designed to provide customers with high performance and maximum environmental acceptability. They comply with the 1999 European Multi-directive (Euro1) with particular reference to gas and noise emissions.