Toughness - the property that is lacking in that part that just broke
In 1999, SRAM Corporation, makers of Grip Shift bicycle shifters, introduced a rear derailluer which featured structural parts that were injection molded using a version of nylon 6/6 with a reinforcing agent. While stiff and strong, these parts were subject to impact failure. This was the result of a combination of issues, including severe porosity (due to an undersized sprue and runner and a poor gate location), and an inappropriate material.
The parts were then completely re-engineered. Minor design changes were made to the part and the mold, including modifications to the sprue and a change in the size and location of the gate. A new material was used, a partially aromatic nylon with glass fiber reinforcement and a proprietary toughening agent. Finally, a series of processing changes were implemented. The final result was a rear derailleur that was light and durable, and the plastic parts had better structural integrity than the forged aluminum links.