I read that PTFE starts to deteriorate past 260 °C. Does that mean heating to 250 °C is no problem at all, or will that destroy the PTFE material over time to?
Degradation starts at 260 °C and shifts towards full blown decomposition towards 350 °C. 250 °C is technically fine, but you should keep in mind that you've got little to no wiggle room for error at that temperature. Your thermistor and board may not be accurate enough to guarantee you'll never overshoot that temperature, and the way 3D printers often handle temperature adjustment exacerbates that risk. You can print at 250 °C, just be aware you've got basically no margin for error.
2$\begingroup$ how fast does ptfe detoriorate at 260°? is that something that takes seconds or days? $\endgroup$ Jun 3, 2019 at 11:29
3$\begingroup$ More like weeks or months. $\endgroup$– DavoJun 3, 2019 at 11:41
$\begingroup$ yeah if you're exactly at the temperature it's going to take an extended amount of time. I still wouldn't try printing at those temperatures though, because PTFE breaking down isn't an issue because it's wearing out, it's an issue because of the highly toxic and corrosive compounds that teflon degenerates into. Check this link out: ewg.org/research/canaries-kitchen/teflon-offgas-studies $\endgroup$– Nach0zJun 3, 2019 at 14:16
3$\begingroup$ "Highly toxic" is meaningless without specifying concentration and exposure. Non-stick pans regularly get around or above 250°C in cooking use. I'd be a lot more concerned about the fumes from the materials being printed at these temperatures than from the PTFE. $\endgroup$ Jun 3, 2019 at 15:58
$\begingroup$ Also, the PTFE end is not supposed to be getting that hot - if it were, the filament would be melting inside it. Assuming your cooling is working right (and if it's not, you'll have catastrophic failure regardless of what your hotend temp is set to), it should be at least a few degrees, and ideally a few tens of degrees, cooler than the set temperature. $\endgroup$ Jun 3, 2019 at 16:02
High temperature rated PTFE tape is rated for up to 288°C (550°F).