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A new track for Zytel®

By Josep Ros and Jaume Ventayol
DuPont Engineering Polymers, Spain

Railway fastenings fulfill a number of functions. Beyond their obvious tasks of keeping the rail attached to the sleeper and maintaining the longitudinal direction, gauge and inclination of the rails, one of their principal roles is to spread, absorb and transmit the dynamic loads from the wheels of the moving train to the sleeper and track bed. It was in relation to performance in this key area that Mondragón Soluciones, a Spanish company with more than 50 years of experience in producing injection-molded parts for telecommunications and railroad industries, sought an alternative to the steel base plates commonly used in railway fastenings around the world. “We wanted to replace steel with nylon in this extremely critical application – which takes the full impact of the train wheel – in order to gain performance benefits that were previously unobtainable due to the rigidity of steel,” explains Juan Vives Clavel, director of R&D at Mondragón Soluciones. “We chose a 35% glass-fiber reinforced grade of DuPont™ Zytel® as the replacement material – a nylon resin we were already familiar with in terms of its high dimensional stability and ease of processing.”

A range of benefits

Initial trials and prototype testing of base plates molded from Zytel®70G35HSL demonstrated that the material possesses not only a high modulus of elasticity, required to withstand the impact of train wheels, but also sufficient elongation at break. As a result, the nylon plate is capable of absorbing energy generated by the passing train, thereby reducing the impact and vibration forces responsible for track deterioration. The dielectric strength of the material, necessary to electrically-isolate the tracks from the ground and from each other, also far exceeded customers’ requirements. “To further ensure that both the design and the material met all our customers’ expectations, we conducted additional tests which enabled us to confidently predict that the base plate molded from Zytel® would retain its mechanical properties in the long-term. This is due, in large part, to the nylon’s high chemical resistance to products such as oils, greases and herbicides that are occasionally used during track maintenance,” adds Juan Vives Clavel.

Familiar with the injection-molding of Zytel® from previous developments, the processing of the nylon base plates presented few problems to the technicians at Mondragón Soluciones. Demolding was facilitated by the use of a small draft angle, despite the part’s high wall thickness, while sink marks at the intersection of the ribbing and the walls of the molded part were avoided by optimizing the ribbing’s thickness. “As with previous projects, the support and advice that DuPont provides has given us additional confidence in our choice of material, and we value it highly,” adds Juan Vives Clavel.

Further confirmation of Zytel® nylon resin as a suitable alternative to steel for the application soon followed. The Mondragón Soluciones’ design has since been certified as conforming to the Spanish standard UNE-EN 13146 by the Laboratory of Science and Engineering Materials department at the University of Cantabria. Those handling material on-site have also welcomed the new product, which is transported manually to each sleeper, as it is approximately a sixth of the weight of its steel equivalent.

A perfect partner: Hytrel® rail pads

Similar to other suppliers of railway fastenings from around the world (see ED 2006-1 PDF), Mondragón Soluciones uses rail pads made of DuPont™ Hytrel® thermoplastic polyester elastomer in its own systems (colored light gray in exploded view, above). The pads complement the base plates made of Zytel® in terms of their performance, providing the ideal combination of flexibility and durability required to perform under high load over an extended period. The chemical resistance of Hytrel® was a further key requirement.

The new fastenings were first installed on a suburban metro line in Mexico City during the first half of 2008, and most recently for a tram system in Blankenberge, Belgium. “Our experience to date has confirmed our decision to develop this new type of polymer product. It combines the required strength for the application with the right elasticity, which is essential when seeking to reduce impact and vibrations in the track structure. In fact I would go as far as to say that it has destroyed the myth that steel must be used for this type of rail fastening,” concludes Juan Vives Clavel.