Thank's for all your replys. Yes, the fault was always due to the slowly rising temperature (~1°C per 20s). The two heat mats are wrapped around a stainless container and are defined together as one tool (M563 P1 S"Extruder-Links" D0 H3:4 F-1 X3)
I had some troubles with auto tuning off the weak heater (5W). It failed due to a bad curve fit (A=17, C=231, D=1.7). The 30W heater was tuning fine at the first try.
After changing the heater model (M307) manually to the parameters of the failed auto tune, it finally succeeded the auto tuning procedure. The auto tuner algorithm then calculated M307 H2 A11.4 C129.7 D4.1 S1.00 V23.8 B0
I'm quite happy that I didn’t have to dive into my control theory lessons again to try it manually with Ziegler Nichols after a few years ^^
The Heater fault parameters are now set to M570 P3600 T100 and it's heating without any faults. Thanks for that hint
Despite the success, there's still a problem to solve. I'm using the chamber heater for the cooling compressor in bang bang mode. But the hysteresis is so small that it's restarting the compressor all the time and that's reducing it's lifetime drastically. Isn't it possible to adjust the hysteresis to +-1 degree instead of the 1/10 degree as default? Sure, I could mount the air sensor to a thermal mass but then it's not as responsive anymore.
Sadly, I'm not allowed to show you the printhead in detail at the moment but here is a picture of the idex research machine we're currently building and a youtube video of the predecessor single head machine. As soon as it's declassified I'll do a "my duet machine" blog post. Simplified it’s a melting chamber on top and some auger to extrude it. However, that’s where the magic happens
Thank you verry much for your support and keep going your exellent work! Makes me feel proud that that's all done with free soft- and hardware (big duet sticker on the controller side =P)