Ok I looked it up.
I am using delrin on my ERP. Its about twice as strong, twice as rigid, heavier, and about twice the price of HDPE. In fact, I used about $80 worth of delrin on my ERP (per version).
However, because its so much stronger, I can use my CNC and FEA to cut away large holes into all my parts to dramatically reduce weight without sacrificing strength. If I didn't have a CNC or money to spare, delrin would probably not be my new favorite material.