Thermal Runaway definition

I don't know what it means exactly but to avoid it:

a) make sure fan comes on very gradually (examine gcode for benchy);

b) make sure fan is low (like a couple of mm above the nozzle tip);

My understanding it "thermal runaway" is when Marlin thinks the temperature readings no longer fit with what it expects given the amount of heat it's applying to the block. So if it's heating but the block is cooling it thinks "uh oh" and cuts out.

 

Yes, this is a protection built into the firmware, if the system detects it has sent power to heat the extruder and it hasn't heated up enough this is a fail safe.
It was built to cover the instance of the sensor falling off or disconnecting and the heater kept heating up and running away. Printers have melted themselves this way from too much heat!

The issue with the bigbox is the fans are too good! Because they are cooling so well the printer thinks it's not heated up enough.
Two solutions, first the fans on the side of the extruders need to be as low as possible, also I set my max fan speed to 60% for pla which helps as well.

 

I agree with Pierce. The standard Benchy script defaults to 100% fan speed at a certain layer height (you can hear the fans speeding up when this happens). When you are quick enough you can manually lower the speed before the thermal runaway kicks in.

 

A thermal runaway occurs when the system believes the temperature is escaping its control. I think the kill point is when the temperature is 20 degrees above or below where it should be. On the pre-supplied benchy print, the fan comes on at layer 3 and increases 10% per layer until it hits 100%. Layers are quite small at the bottom of the benchy and finish quickly so the fan escalates quickly. The heater pushes all it can give but fails to keep up. Two possible solutions

• Tune your PID, this might be enough, and I'd recommend this anyway.
  • https://forum.e3d-online.com/index.php?threads/pid-tuning.731/#post-5738
• Reslice with a less agressive fan profile.
  • e.g.

 

@Kiwimaddog I don't know Repetier Host. It seems like something missing from the firmware, so I've entered a suggestion against Marlin. Someone else thinks it's an interesting project too:

https://github.com/MarlinFirmware/Marlin/issues/3135#issuecomment-198916945

 

I noticed that RichRap has wrapped his hot end with Kapton tape to protect it from the fan blast. I have used a strip of fibre glass mat to achieve the same thing but on a larger hot end. A new PID may have to be run to compensate but I did not need to do that.

 

Under the "Manual Control" section of Repetier you can send commands with the Gcode dialogue box + send.

 

I have not tried this with BB as I am still waiting for mine but it does work with my current printer. If you cannot control the printer from the "Manual Control" screen, you are not connected correctly so check your settings.

 

To answer the question on thermal runaway. The theory is that if a thermistor should come loose from the heater block the temperature will start to drop. When this happens the system will try and ramp up the heater power to "bring it back to temp" eventually heating to the point of dangerous temperatures.

Same can hold true if the heater circuit gets stuck open (for whatever reason).

It looks for a range of drop over a certain time.

Running the PID tuning at a higher temp (say 280) will help to have better response at lower temperatures.

 

Any of your favorite control software will do it. Supposedly they added PID tuning to the LCD for Marlin 1.1.0 RC4