[thefro526]

As others have said so far, Delrin (Acetal), Lexan (Polycarbonate), and ABS are all pretty common plastics that are fairly easy to work with. Composites like Fiberglass, Garolite, etc are also well within the range of most FRC teams but require some special considerations when being machined and used, specifically ventilation, etc.

My personal method for deciding whether or not a part can be made out of plastic is based on a bit of previous experience and some guessing. One of the biggest considerations is the purpose of the part and how strong it needs to be. If it's something that needs to hold relatively precise geometry, but doesn't necessarily need to be strong, Plastics are a pretty good option and can be easier to work with than Aluminum. The common trade off is that you're going to need more (thicker) material to get a part that is strong enough, which means that you could get into situations where a plastic part is the same weight as a part made of Aluminum and is as strong.

One of the other nice things about plastic is that they're really useful when something needs to be a specific thickness for one reason or another. We've done bearing blocks and things along those lines from 1/4" Delrin before since they didn't need to be strong, but did need to keep the bearing flush with a surface.

There's also the added bonus of elasticity, should you need to have a part that has a known amount of 'give' or impact resistance before it permanently yields. Both lexan and delrin are pretty handy for this purpose, since they're fairly good at taking a beating if they're outside of the frame perimeter.