Nozzle Thermal Runaway on Layer 2  

RE: Nozzle Thermal Runaway on Layer 2

Posted by: @nomarch

Ive had my Mk2 for over 4 years and never had a problem with Temp runaway.  I have printed at 240 with 100% fan and 80C heatbed and never had a problem.  I upgraded to the 2.5s a few days ago and yesterday it started dropping by over 15 degress and throwing a preheat error and killing the print when the cooling fan kicks in on layer 2.

Even at 210C and 40% fan it drops about 10 degress and takes forever to come back up again.  (Up to ten minutes or it just sits on 200C indefinitely)

I have checked all the connections to the Rambo and theres no sign of overheating or melted plastic.  Im now thinking maybe its a broken wire on the fan?

Thanks

So first I would check the heating element and the thermistor, also the voltage of the power supply (12V).

Nozzle heating element resistance (12V/40watts) -> about 3.6 ohms; (12V/30watts) -> about 4.8 ohms

Thermistor (Semitec-104GT) resistance -> about 100kohms at 25degree C.

Temperaturtabelle für Thermistor Semitec 104GT

RE: Nozzle Thermal Runaway on Layer 2

Also If I just run the fan on its own you can hear the speed fluctuating which I don't remember happening before.

I have a multimeter here but I dont know what readings I should be looking for to see if theres a problem with either the nozzle heater , fan or temp sensor.

RE: Nozzle Thermal Runaway on Layer 2

@karl-herbert

Thanks I will give that a go and get back to you.

RE: Nozzle Thermal Runaway on Layer 2

I've just connected the old nozzle cooling fan to the rambo and the thermal drop has disappeared.  I just need to workout whether its the wire or the part cooling fan itself now.

Thanks for you help!

RE: Nozzle Thermal Runaway on Layer 2

After replacing the fan I have managed to get rid of the speed fluctuations but I am still getting the big temp over/underruns.

If I am printing PLA at 215C HB 60C when it gets to layer 2 and the fan kicks in at 100% the temp drops to 197C and will sit there indefinitely.  I have to reduce the fan speed to 50% to get it to climb back up to 210C.  Even at 60% fan speed it doesn't get past 208C.

So, marginally better than before I put the new fan on.

Also when I turn the fan off completely the temp ramps up +10 degress before it starts falling back down.  Apparently, according to Prusa, this is somewhat normal.  I did see this behaviour before I bothered to run the Temp Sensor tuning but once done it has religiously stayed with a couple of degress of set temperature for years now.

I am going to order a new Temp Sensor and Heater and hopefully that will do the trick.

If anyone has any other suggestions in the meantime though I would love to hear them!

RE: Nozzle Thermal Runaway on Layer 2

@nomarch

You can reduce the temperature fluctuations if you would use a silicone sock. Renewed pidtuning in the problematic temperature zones would not be bad either.

RE: Nozzle Thermal Runaway on Layer 2

@karl-herbert

I had this same issue. I checked my heater resistance and it was 5.7 ohms - consistent with a 25W heater. It seems that the original heaters just can't keep up with the stronger blower. Thanks for posting this.

I'm going to order a 40W heater cartridge and hopefully that will fix it. For now I am limping along with a reduced fan speed and low nozzle temp (200-205C FOR PLA)

RE: Nozzle Thermal Runaway on Layer 2

Try my nozzle fan shroud fix

https://www.prusaprinters.org/prints/56604-mk25-fan-shroud-fix

RE:

I had a similar issue - print failed on first or second layer depending on the size of the print.  Always received a Thermal Runaway Error.

Attempted all typical solutions, checked heater, thermistor all connections - nothing was out of expectations. 

The actual failure was the hot end was losing (or lost) power during the first or second layer print.  (very consistent)

However, when the printer was static, no motor running it would stay hot for 1/2 hour or more, I tried moving the X/Y axis to see if a connection was loose.  Temp remained at a steady 215C. 

I formed a theory that the power supply might be failing. The theory was that if the heater bed and hot end were on, motor(s) off, the power supply had enough to keep these heated. This is why it stayed heated in my static test.  But when the motors are active the power supply starts to fail due to higher current requirements.

First (easy) test.  Start the print, leave the bed temperature off. (28C)  That print did not fail for thermal runaway but did start having some other issues like the bed jumped later on in the print.  This confirmed (to me) that the power supply was suspect.  I happened to have another 12 volt p.s. from another project, (280 watts), removed the Prusa Supply and replaced it temporarily with the other supply.

I did have to run the calibration process - for some reason the layer jump in the Y axis continued. ????

I have made several new prints, no issues.  Put a power supply on order to make my printer 'pretty' again.

Hope this helps some one out there.

RE: Nozzle Thermal Runaway on Layer 2

It's hard to know exactly what's happening here, but it sounds like you went 15 under temp.  I had this happen to mine when I was printing large broad swaths of flat area.  It turned out that my fan at 100% was over cooling my hot end and drove my nozzle temps 15 below temp which was a cutoff.  My initial solution was to reduce my fan  speeds on these kinds of prints, but my long term solution was to buy a silicon sock to place over the heat block and reduce the cooling effects of the fan.  This solution turned out to be a winner.  For safeties sake I did re-run pid tuning with the sock and it keeps temp well.

-Bob

RE: Nozzle Thermal Runaway on Layer 2

Bob,

Thanks for the reply.  My failure was not related to long flat prints - the print I was trying to print was a shell 20 x 20 x 20 mm with 1.0mm walls.  The failure happened on the 2nd layer, very repeatable.  I have about 10 failures for thermal runaway.  Also, the cool down was not what I would expect where the heater wasn't keeping up - in my case it was like the power was removed from the heater element.

Anyway, I received my new power supply today, installed and the print that initiated this thread printed without fail.  The only thing that I changed was the power supply.

I'll post again if this doesn't fix the issue long term.