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If I set temperature say 220 °C, printer heats up to it and it only varies +/- 0.5 °C under non operating condition. But if I start a print, there's a shift of +/- 15 °C.

I've already auto tuned PID parameters and When I run M503, printer shows the updated PID values.

Octoprint temperature graph showing hotend temperature drops

What could be the reason?

Electronics details:

  • Firmware used: Marlin
  • Controller board: Printrboard rev D
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  • $\begingroup$ The bed target is off, please explain why the bed temperature increased? E.g. did you shut of the bed prior to making the screenshot? $\endgroup$ – 0scar May 29 '18 at 18:11
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There could be a number of reasons for that behaviour:

Please check following items:

  1. part blower/fan cools down the nozzle - stop the fan
  2. material is extruded at high speed and takes the heat
  3. loose thermistor (when the move occurs as it moves a bit internally)
  4. the power supply unit voltage varies - so temperature reading varies as well - measure voltage and see if there are significant drops
  5. nozzle heater and thermistor cables can be loose as well (check screw terminals)
  6. bed heater connection cables having not a good connection (check screw terminals)
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  • $\begingroup$ I am not using any fan to cool the nozzle.Fan which came with the extruder cools the metal part which holds the nozzle.This is my extruder link. Also the heating coil and Thermistor is fastened to the nozzle using grub screws. Thing that confuses me is that, sometimes the hot end temperature stays constant during print (+/- 0.5), but most times it varies drastically. $\endgroup$ – Athul May 28 '18 at 16:54
  • $\begingroup$ Could be the issue of power supply. May be it can't supply enough current to motors and heating coil at the same time.But It's a 30A supply. I was thinking about checking it. Also the power supply s powered by an Inverter. $\endgroup$ – Athul May 28 '18 at 16:54
  • $\begingroup$ The point is that it could be broken wire (heater or thermistor) so it is not clear to see it. I'm case I replaced heater and got stable temperature $\endgroup$ – profesor79 May 28 '18 at 16:59
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Dropping hot end temperatures can be caused by the filament cooler blowing too much cold air onto the nozzle, or a too high of a print speed so that the nozzle is cooled by the filament (in combination with a heating element that cannot heat that fast to compensate).

To rule out the PSU you could try measuring the voltage of the PSU before and after powering the heat bed, if it drops dramatically when the heat bed starts change the PSU.

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  • $\begingroup$ nice timing Sir :) $\endgroup$ – profesor79 May 28 '18 at 14:17
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    $\begingroup$ Yours is more complete! But usually it is one of these. $\endgroup$ – 0scar May 28 '18 at 14:19
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Can you also show a "power output" graph (or something like that, which show PWD duty cycle output to the hot end)?

Have you noted any relation between temperature raise and fall and the printing position?

Looking at graph I can only suggest the heating is suddenly stops and then the same sudden resumes. It looks like you have a loose wire connection to the heater. When head is starting to move, it makes disturbance for the wiring, so heater occasionally stops to work. There are usual two weak points: first it is where cable is connected to the board. Check is connector heated? Are any noises/sparks from there? Another part: it is just at the point where wires going into the hot end. If wires are not secured to the printing head, then during printing those place experiences a lot of bending, which makes it weaker over the time.

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