Originally Posted by EricH

The angled part of the frame appears to be covered by Delrin, which is a rather slick type of plastic. The other option that I can think of, HDPE, is also somewhat slick, but Delrin is slicker. In block form, Delrin has been used as a chain tensioner in the past--stick the block in so the chain is slightly too tight and let it find its own tension by digging into the block.