According to the comments, “PETG for strength and durability, polyethylene terephthalate (think plastic bottles) and the G means it’s been glycol modified for extra durability. PLA for speed and a little extra quality. It’s good stuff. PEI is an ideal print surface for PETG; it adheres well but doesn’t fuse as it does to glass. The advantage of PETG is minimal warping, so you don’t need a heated chamber and virtually no smell with PETG.”.