Plastic, especially high density "waxy" plastics are inherently self lubricating. Plus, they are inherently softer than the lead screw, which means that they, and not the far more expensive screw, will be the part to show wear.
You can actually use any plastic, but make sure it's hard, resilient, and somewhat slippery...those are the desirable traits.
Basically, it comes down to the fact that plastics are easy to come by, cheap, and easily workable without a machine shop. And, they actually have a few benefits, like their slipperiness! No lubricants needed, in most cases.
-- Chuck Knight |