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MK2.5S 2nd layer thermal runaway

Mar 17, 2019 3:11 pm


Ive been printing with my prusa for 2 years now and this is the first time ive run into this problem. I recently did the 2.5 upgrade which went well and was printing fine and last week prusa sent me the 2.5s parts. Ive installed the new X carriage and extruder and the first layer print relativity well. However, as it starts the second layer, the fan kicks in on 100% and the fan appears to be cooling the hotend instead of the PLA which is causing the hot end to drop in temperature extremely quickly which leads to a thermal runaway. I have tried to roll back the firmware to both only 2.5s and to the most recent 2.5 but neither of these make any difference. So, I assume its either the fan in spinning up to fast or something else. I dont know if anyone else has experienced this.

Has anyone got any ideas as im really stuck.

Thanks in advance


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Re: MK2.5S 2nd layer thermal runaway

Mar 19, 2019 5:56 pm

I am having the same problem. Printing PLA, on the 2nd layer the part cooling fan goes to 100%, and the extruder temperature promptly drops 10-15C and is unable to sustain the target hot end temp. I noticed it because the I heard the extruder skipping steps starting on the 2nd layer. This was due to the extruder not being hot enough to melt the PLA fast enough for the printing speed being used (100mm/s Linear Advance).

I've never noticed this problem prior to upgrading from MK2.5 to MK2.5S this past weekend. I suspect this is a side-effect of the new part cooling fan shroud.

This needs to be addressed in the slicer settings (although right now there is no printer-based limits on cooling fan speed... it's all in filament setup).... and/or the part cooling shroud needs to be redesigned. I'm surprised this wasn't discovered during testing.

I can reproduce this manually with the extruder at 0.2mm Z height, hot end temp set to 215, and part cooling fan set to 255. Hot end cannot maintain 215. (Yes, I did PID calibration). If I raise the Z up to 2mm, then the hot end is able to maintain 215 (barely), and it seems to get easier the higher the Z axis is away from the print bed.

I've read others reporting the same problem in this forum. Some suggest using a silicone sock on the hot end. I don't have one of these available to test with.

I'm curious to know what Prusa's recommendation is. Has anyone contacted support about this yet?

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Re: MK2.5S 2nd layer thermal runaway

Mar 19, 2019 6:30 pm

Interesting.... in the knowledge base, under thermal runaway it says:
"Make sure the fan power settings aren't set too high in your slicer of choice. Also, check the material temperature settings. (100% fan speed is only possible with nozzle temperatures around 200°C with the V6.1 hotend.)"

If 100% fan speed is possible only with nozzle temperatures below 200C, then why does the Prusa PLA filament setting call for a 100% fan speed with a 210C temperature after the 2nd layer? This seems contradictory.

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Re: MK2.5S 2nd layer thermal runaway

Mar 22, 2019 3:54 pm

I had this already with MK2.5, when I printed the new hotend type. It is simple cooling too much. I made two things. Replaced the heater (2.5 years old, it was already quite week not heating so fast) and bought the silicon socks. Nothing else helped, tested several custom designs from forum, but still the same.
Now ok again.

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