Does anybody know what is this white thing? It’s embedded in the print, impossible to pull out, very strong, doesn’t seem to be filament dependent. It just started showing up, no matter which color I print with.
It could be teflon/PTFE tape. Which setup (printer, hot-end) are you using?
I have printrbot simple with Ubis hot end
Definitely looks like a bit of Teflon tape.
I’ve never seen that, maybe @Printrbot (@Brook_Drumm ) know of the issue!
I agree that it looks like teflon tape but it’s hard to imagine that the tape wouldn’t clog the print head or even quite frankly could make its way through the nozzle. You’d think it’d get a bit balled up and not be perfectly stretched out. Crazy.
Also, which filament are you using? (PLA/ABS, supplier, which colors)
sometimes it clogs the nozzle, I just pull it out of the nozzle then
I don’t think that’s teflon. I think it’s fibers from an insulating sleeve like Makergear uses: https://cdn.shopify.com/s/files/1/0030/7372/files/hot_end_7.jpg?1291089808
Have you got one of those on there? The fibers are extremely strong.
Looks more like Kevlar fiber or Tyvek.
If its clogging the nozzle then its either teflon tape from the PEEK barrel or your filament is very low quality. 
… My guess is that its the teflon tape.
Now I can’t print at all. The hotend is clogged and I can’t unclog it, even if I unclog it, it’s clogged again after 5 minutes. I bought the hotend just a month ago to replace one I bough two months ago, which failed for a different reason. I hate printrbot so much. I wish I have bought a real 3D printer.