There’s also another reason to go the thermoplastic route – an environmental one. “The thermoforming nature of thermoplastics also makes them more highly recyclable than their thermoset counterparts,” Ive says.

The NCC built prototype doors using polyetherimide (PEI) glass thermoplastic for the skins and internal structure. The doors also feature a lightweight thermoplastic-based glazing. The NCC used a 10-meter high-temperature autoclave for the larger parts.

This demonstration subway door, made of PEI glass thermoplastic skins and internal structure, weighs only half as much as existing metallic doors.

This demonstration subway door, made of PEI glass thermoplastic skins and internal
structure, weighs only half as much as existing metallic doors.

Ive notes that the high processing temperature required by the PEI composite was a challenge. Standard vacuum bagging consumables can’t handle the 300 C temperatures required for part of the processing, while those vacuum bag consumables that could take the heat were not suitable for handling vacuum at lower temperatures. The NCC, therefore, explored using several different techniques and products.

“The design of tooling and use of appropriate materials throughout is of great importance when cycling a material through such a large temperature range,” says Ive. “In the work done around press forming, we found a more robust process through eliminating the requirement for these consumables.”

Some of the intricate internal elements were made using thermoset phenolic composites, but Ive anticipates the door could be completely thermoplastic in full production.

Once the prototype was built, it was static load tested and subjected to fatigue testing. This had two purposes. One was to prove that the door could meet the subway system’s requirements. The second was to validate the finite element analysis of the demonstration door done by consortium member Atkins, a London-based design, engineering and project management consultancy. Finally, the prototype was tested to destruction.