I have rigged my Snapmaker 2.0 with dual extruder to print using Octoprint running on a Raspberry Pi connected via USB. This worked fine until I tried printing a small object the first layer of which was contained within around a 20mm square of the center of the bed. The print would stop with a “thermal runaway” alarm about two minutes into the print. The detail in the log showed this was due to under temperature, not over (presumably this is intended to detect a faulty temperature sensor).
I tried printing the same object using the Snapmaker Touch Screen. It printed fine, but I noticed that the bed temperature dropped from the set point of 65c to about 58c, exactly the same under temperature that triggered the stop in Octoprint. This drop lasted about two minutes and then it started rising again remaining within one degree of the set point for the rest of the print.
I conclude that the problem is the fans on the dual extruder cooling the bed faster than the heater can heat it. For a small print the head ‘lurks’ around the middle of the bed where the sensor is located. For a larger print, the head spends less time over any given location spreading out the cooling effect. Later in the print, the head has risen higher from the bed and more of the bed is insulated by already printed layers.
I experimented with a cardboard deflector plate on the bottom of the extruder head to deflect the fan airflow upward. The temperature droop still occurred, but limited to around three degrees which was not enough to trigger the thermal runaway alarm. I am now thinking about how to make a permanent deflector, but this seems like a design fault with the dual extruder?
But an additional disturbing discovery is that the touch screen seems to have been programmed to ignore the thermal runaway alarm rather than to fix the root cause of the problem. Is this safe if a real thermal runaway occurs or the temperature sensor fails? Is this substantial temperature droop going to impact the quality of prints in some cases?
I hope these observations are helpful to others and would welcome any insight or experience others may offer, or comments from Snapmaker Support.
** I should mention that I have fitted the replacement parts kit to the dual extruder, including the replacement door with different vent outlets.
Thanks for sharing this observation, very helpful! Just one thought: Instead of tinkering with a deflector: Why not just print the object a bit deplaced from the center of the bed, away from the temperature sensor? Since the problem only occurs for small objects, this should not be an issue in any way.
Thanks for this clever piece of lateral thinking! Yes, it works, particularly moving towards the front of the bed since the back side of the extruder is the only side that air does not blow from. However, I note that the temperature of the bed at the worksite will still be drooping, it is just that the sensor cannot see it, whereas a deflector actually stops the droop (or at least lessens it). Maybe it would be worth Luban being modified to place small objects off center by default?
Never observed that the tool head fan could cool the heated bed in a significant way.
Not even with more cooling power and additional fans, my bed is able to heat it up anyway (maybe a few seconds temperature drop when turning the additional fans on but not problematic).
Are you sure about the setpoints in your gcode?
Maybe 1st layer more hot than the 2nd?- If this is the case I could imagine that the bed falls even further than the setpoint till heated up again.