Thermal runaway

Question

I have a bit of a headache with my Anycubic i3 Mega which originally had a Trigorila 32-bit board.

I decided to swap it because of loud A4988 drivers and got myself an MKS Robin Nano 1.2.

This printer is heavily modded. All the hot-end is swapped to MT2.

Now, MT2 mod was done with Trigorilla, so no problems back then. Also, I know that Anycubic locks their max fan speed at ~80 %, so with that in mind done tests with 100/80/50/30 % cooling fan and the result is the same; temperature drops, and I get "Thermal Runaway".

Now, the strangest part is: If I set manually 200 °C, wait for it to climb, and then blast 255 °C on the cooling fan, it drops to 199 °C and then back to 200 °C.

I have added a picture below, it has been like this already for about 15-20 min, no probs.

PID autotune was performed numerous times, with values saved, PID repeated, new values stored, etc.

• Anycubic i3 Mega
• MKS Robin Nano 1.2
• E3D V6 original.
• Marlin 2.0.5.3

And snippets from Marlin cfg:

#define TEMP_SENSOR_0 5

// Comment the following line to disable PID and enable bang-bang.
#define PIDTEMP
#define BANG_MAX 255     // Limits current to nozzle while in bang-bang mode; 255=full current
#define PID_MAX BANG_MAX // Limits current to nozzle while PID is active (see PID_FUNCTIONAL_RANGE below); 255=full current
#define PID_K1 0.95      // Smoothing factor within any PID loop

// Ai3 mega
  #define DEFAULT_Kp 27.69
  #define DEFAULT_Ki 2.02
  #define DEFAULT_Kd 94.89

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Additional information (1):

I have XYZ cube and fan on from layer 3 G-code. It always seems to fail on layer 3, so turned manually fan from layer one, was printing fine till layer 3 and then thermal runaway.

Done a test, sent G-code to see if M106 is an issue.

M104 S210 T0
M109 S210 T0
M106 S204

But no, printer kept temperature steady... Makes me crazy, please help

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Additional information (2):

I updated all the images:

Answer 1

Thank you for updating the pictures (twice) and giving us the info to work with!

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At the moment I see two things:

• First, the printhead doesn't reach the 210 °C set into it in the last timeframe. After some time of printing, this forces TRP (Thermal runaway protection) as the board thinks: "HEY! my thermistor might be loose, I try to HEAT here!"
• Second, your graph shows that in the first area, your printhead actually is able to reach the temperature. But something does change which prevents it from holding the temperature.

Then, there's the Anecdotal note:

running same print without cooling fan and its going steady

Those three together tell us exactly what is going on: The airstream from the part cooling fan must somehow brush over the hotend, cooling the heater block and throwing off the measurements.

Fixing in

There are basically 2 things how you can deal with that:

• isolate the printhead from the airstream. There are a lot of solutions for that:
  • Silicone sock to keep away the airstream
  • Kepton Tape/Tinfoil to try to imitate a silicone sock
  • different position or geometry of the fan duct to not hit the heater block
• turn off/tune down the fan