Recent issues with abnormal wear of safety critical components in bogies and resulting derailments have caused further investigations of the non-metallic parts.
The shocking discovery was that cheap plastic components, neither specified on the drawings nor approved, were used in these cases. Not complying with specifications and using non-original materials was the leading cause of many unfortunate accidents.
These cheap plastic materials, also called thermoplastics or thermosoftening plastics, are polymers which become pliable or mouldable above a specific temperature and return to a solid state upon cooling.
While this allows for low-cost manufacturing of the parts, it also causes the finished product to be rubbery and dimensionally unstable.
Plastic materials are not reinforced, and are severely affected by changes in atmospheric moisture, heat and high loads. Obviously the wrong material for use in the arduous and high-load environments of any railway application.
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