Comment by justinator

Although I can see plastic gears being an alternative choice in some applications, their downsides seem to be many:

* Less load-carrying capacity compared to similarly sized metal gears.

* Molded gears cannot hold the same high tolerances that metal gears can.

* Plastic is less dimensionally stable when compared to metal and this results in dimensional variations due to temperature and humidity conditions.

* Material cost is significantly higher than that of base metals and can vary widely due to fluctuations in the cost of base chemicals.

* Difficulty in attaching plastic gears to metal shafts.

That last point seems especially problematic.