Thermal Runaway

Discussion in 'Calibration, Help, and Troubleshooting' started by Old_Tafr, Apr 4, 2016.

  1. Old_Tafr

    Old_Tafr Well-Known Member

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    I have printed the "benchy" little ship first time, with minimal lift off the glass plate at the bows, and an odd vertical (ish) line up the starboard side, essentially good for a very first print ever.

    Then printed two wheels small dia 30 and 20 mm both fine, both on rafts, this raft being the only change to simplify3D settings.

    Then tried three little wheels side by side (configured in Simplify3D) which failed, the raft didn't stick to glass, again no change the simplify3D settings.

    Then a single 20mm wheel, got as far as the raft then "Thermal Runaway" which I assume means too low extruder temp?

    Several things may be relevant.....


    On using Octoprint I noticed that the extruder temp set straight away at 190oC without manual intervention but the bed was initially set at 0. I tried to input the bed at 60oC manually in Octoprint it would not accept this or selecting PLA, then suddenly there it was there set at 60 so not sure if this was going to happen anyway.

    The fans that clip on I have sitting a small distance (lower by say 4mm ) from the X-Carriage so they will be blowing on the threaded heat break between the heat sink and the Al block holding the nozzle. I will try lifting them as high as this may have an adverse cooling effect on the nozzle.

    The clip on fans both come on at 40oC as expected, but after a while...may have been just before the "thermal runaway" message (which crashes the Rumba) one or both of the other fans come on (I assumed so as the noise increased) This is strange as according to the temp graph in Octoprint the temp was oscillating around 190oC just fine, but then dipped... hence the runaway message? Can't quite see the temp but I suspect it was 177oC.

    I will try moving the two front fans upward as far as they will go and try another print. I won't manually add the bed temp and see what happens, i.e. if it just appears a minute or two later then the extruder temp.

    Also could someone explain which fans can be set to come on a different temp (are we talking about the default 40oC setting?) or is the setting to run the (one or both?) fans at a different speed depending on temp? Think this was answered as the print cooling fans can be set for a speed or temp.

    Finally if I remember correctly the bency test piece had a manually set temp of 200 or 210 not the 190 for PLA odd as this is higher but worked with the PLA supplied.

    =============
    Sameo sameo..... a minute or less after the print starts and before the raft is finished the print cooling fans start, the temp drops from around 190 to about 177 and the print fails with "thermal runaway" Still had to set the bed temp manually.

    I will take a look at the settings for temp in simplify3D as I didn't really follow the other posts as to whether the settings were fan speed related (pwm control?) or temp related or a bit of both. But it seems silly that they start (no changed from default whatever that is) soon after the print starts. I had move the heat sink fans up but that made no difference.
     
    #1 Old_Tafr, Apr 4, 2016
    Last edited: Apr 4, 2016
  2. R Design

    R Design Well-Known Member

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    Distinguish between the "Extruder Fan" (the one that is clipped to the heat sink) and the "Parts Fan" (the one that points at the nozzle).

    The Extruder fan comes on automatically at 40C and should not be touched. Prevents the filament from melting all the way up into the heat break...

    The Parts fan is controlled by settings in the Cooling tab of S3d.

    General wisdom is that it comes on at layer 3 (after base layers are well stuck to bed) at, say, 10% and then creeps up over the coming layers (not too quickly, taking into account that if it's a small layer the layers go by very quickly....)

    VERY IMPORTANT the parts fan should be very LOW... i.e. just above the tip of the nozzle. I set mine to the same height as the IR sensor.

    If any higher the nozzle gets too much wind and that sets you up for thermal runaway.
     
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  3. Old_Tafr

    Old_Tafr Well-Known Member

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    Not sure where I got the position of the fans from, they are set level with the bottom of the Al block holding the nozzle so could be moved at least 5mm lower. I will try that as reducing the parts fan speed to 50% made no difference.
     
  4. Old_Tafr

    Old_Tafr Well-Known Member

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    With the parts fans lower 2mm to 3mm above the print already further than the last two attempts. Looking closely to the fan side of the nozzles it's easy to see a duct that directs air away from the nozzle. Unless the fan is lowered to within a few mm of the end of the nozzle then air will be directed at the nozzle not the print itself.

    Even so there is a dramatic (more than 10oC) drop in nozzle temp as soon as the parts fans come on which now the print continues moves smoothly back to 190oC

    As yet I didn't see the setting for when the fans first start as nothing set by default to anything like 10%.
     
  5. R Design

    R Design Well-Known Member

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    During setup with the rubber belt we set the IR sensor to be 1.5mm above the tip of the nozzle (the thickness of the belt).

    No need to put the fan any higher than that.

    The parts fan settings are in the "Cooling" tab of Simplify3D and are programmed on a layer by layer basis.

    Typically you might have

    Layer 1 Fan 0

    Layer 3 Fan 10%
    Layer 4 Fan 20%....

    etc. up to say 60%

    If the layers are small / fast then leave a larger number of layers between fan speed increments to let Marlin get used to the new thermal conditions. Or panic will set in aka Thermal Runaway.
     
  6. elmoret

    elmoret Administrator

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    Thermal runaway kicks in as a safety feature when heat is being requested but the temperature is not rising. The firmware views this as a fault condition, such as maybe the PT100 sensor has pulled out of the block. It is not "crashing the RUMBA", but rather hard stopping the firmware and powering off all outputs until reset by power cycling. This is to grab your attention that something potentially bad has occurred.

    If the fan is blowing on the heater block, it will trip this. If the fan is ramped up too quickly, it will cool the heater block so fast that the heater cannot respond fast enough, resulting in a trip as well.

    Solution:

    Don't point fans at heaters
    Ramp the fan up slowly. It should be at zero for the first layer or two anyway, ramp it up say 10% every few layers.
     

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