Too cool!!  

Re: Too cool!!

Yeah, but it should not cool the heat block/thermistor.

As it happens, my current model has plenty of what you describe and I don't see any thermal runaway (I use 100% part fan all the time). I wonder if it is a slicer issue (or even a build issue)?

Peter

Re: Too cool!!

Not sure. The air stream is over the nozzle. Wouldn't this cool the heat block too?

Either that or I have a sub-standard heating element!

Re: Too cool!!

I've seen this problem on the Mk1. I've also dialed back the bridge fan speed, but that's not a permanent solution. Are you using Slic3r? Can you run a test print with higher infill? At higher infill I believe the cooling is disabled (based on the Slic3r bug reports, haven't delved into the code). It might be that the circulation from the fan is blowing back up towards the nozzle (essentially bouncing back up from the model). Honestly I haven't spent much time looking at the fan outlet orientation to see if this is really what's happening.

Re: Too cool!!

Now that could be the issue.

I used 10% in fill and the print would 'whistle' every now and then indicating that the air stream was affected by the honeycomb infill.

Almost like blowing across the top of a bottle.

Perhaps the air was being derflected onto the block as you say.

I will experiment when doing other prints to see what happens. At the moment, a lower bridge fan has cured the problem.

Re: Too cool!!

Ok Guys.

I've done some experimentation and got some interesting results.

Step 1. Auto home the extruder. Move extruder to middle of the bed keeping Z height at the home setting. (Nozzle just above the bed.)
Step 2. Menu -> Settings ->Temperature -> Nozzle -> 240 C
Step 3.Settings -> Temperature -> Fan Speed -> 100

Result: Nozzle temp holds indefinitely.

Step 4. Raise Fan Speed to -> 200

Result: Nozzle temp holds indefinitely.

Step 5. Raise Fan Speed to -> 225

Result: Nozzle temp starts to drop to 237 after 3 minutes then holds indefinitely.

Step 6. Raise Fan Speed to -> 255

Result: Nozzle temp starts to drop to 232 after 3 minutes 25 seconds then holds indefinitely.

So it seems that to maintain a good 240 C nozzle temp when bridging, the max fan speed should not be above 78% (200 fan speed) and 88% MAX (225 fan speed)

Additional Notes: A nozzle temp of 210 C can be maintained indefinitely with a fan speed of 255.

Peter - in a previous reply, you said that you could run your fan at 100% all day long. Would this be at higher nozzle temps or PLA type nozzle temps?

My question would be - What temperature drop and duration would causes a thermal runaway error?

Also, if somebody could repeat the experiment, it would be interesting to see.

Thanks.

Re: Too cool!!

Adrian

From the firmware source code:

/*================== Thermal Runaway Protection ==============================
This is a feature to protect your printer from burn up in flames if it has
a thermistor coming off place (this happened to a friend of mine recently and
motivated me writing this feature).

The issue: If a thermistor come off, it will read a lower temperature than actual.
The system will turn the heater on forever, burning up the filament and anything
else around.

After the temperature reaches the target for the first time, this feature will
start measuring for how long the current temperature stays below the target
minus _HYSTERESIS (set_temperature - THERMAL_RUNAWAY_PROTECTION_HYSTERESIS).

If it stays longer than _PERIOD, it means the thermistor temperature
cannot catch up with the target, so something *may be* wrong. Then, to be on the
safe side, the system will he halt.

Bear in mind the count down will just start AFTER the first time the
thermistor temperature is over the target, so you will have no problem if
your extruder heater takes 2 minutes to hit the target on heating.

*/

and

// Parameters for all extruder heaters
//#define THERMAL_RUNAWAY_PROTECTION_PERIOD 40 //in seconds
//#define THERMAL_RUNAWAY_PROTECTION_HYSTERESIS 4 // in degree Celsius

Although the parameters are commented out, they will probably be set elsewhere.

This is the first time, you have mentioned specific temperature; I use PLA at temperatures between 185 and 205 degrees (with first layers higher).

The problem you mention was quite prevalent with the Mk1 at one point, especially with PLA (which was at the time the material which required the part fan to be tunning).

The Mk1 did have a few design issues to cause this problem (orientation of the heat block, nozzle outlet design, low powered cartridges) and PR ended up shipping out 40W heater cartridges with the Mk1.

This issue was supposed to have been resolved with the Mk2 (and it appears to have been for PLA...), but with different materials requiring higher temps and the use of the part fan, it has now returned.

First thing is to accurately measure the resistance of the heater cartridge and the voltage of the 12V rail so the power can be calculated. It may be that you have a cartridge which is well outside the 30W specification.

Peter

Re: Too cool!!

Well, the heating element comes in at 5.2 Ohms and the rail is 12V dead on.

By my reckoning, that gives me 27.69 W

This is 92.3% performance compared to a full 30 Watts.

Could this be an issue?

Re: Too cool!!

Adrian

There will be about 0.5V drop from the 12V due to the output stage in the RAMBo, The 27W is OK for the 30W cartridge; some were significantly less.

So it looks as though you have a few choices:

Peter

Re: Too cool!!

I've the same problem. For some reason I have to print pla at 240 (still diagnosing temp issues) and I find that with the Adalinda the moment the fan comes on at the second layer the temp plummets from 240 down to 214. If I tune the fan speed down to ~100 the temp will be held.

Waiting on a multimeter to test the voltage.

Re: Too cool!!

I've the same problem. For some reason I have to print pla at 240 (still diagnosing temp issues) and I find that with the Adalinda the moment the fan comes on at the second layer the temp plummets from 240 down to 214. If I tune the fan speed down to ~100 the temp will be held.

5.4 Ohms at the heater, and 12V on the rail, for 26.6W.

Why would my heater not be able to recover when everyone else's seems to be fine?

Re: Too cool!!

OK guys, I have run a test on my printer.

Here is the graph:

Fan turned on at -4 min and off at -1 min.

My heater is approximately 27W power, ambient about 26 degrees.

Peter

Re: Too cool!!

Peter, if I'm interpreting correctly, your chart shows that your system is able to quickly recover from changes in temp caused by the part heater.

How can I reproduce your test with mine? What tools did you use to measure temp and graph results?

Re: Too cool!!

Curtis

Yes, the graph does show an exceedingly stable extruder temperature and quick recovery when the part fan is turned on/off.

As stated, this is almost a worst case scenario in that the nozzle was in the middle of the bed only 1mm away from the print surface (at that temp, I didn't want the nozzle any lower than that for a longish period of time).

The graph is a standard temperature graph from OctoPrint. It is one of OctoPrint's less interesting features.

Peter

Re: Too cool!!

I've had a couple of thermal runaways on the MK2. I think they happened because Slic3r turned the fan on 100% and blasted cold air at a solid surface. I've noticed (on the MK1 as well) that the extruder is generally unable to surpass 230C when the fan is on 100%. I only learned that by watching the Octoprint temperature graph. (Octoprint on a Raspberry Pi 3 is awesome!)

The ambient temperature is usually 19-20C. Also, I'm about 4600 feet above sea level, high enough to change the effect of fans compared with lower altitudes.

I limit the fan to 50% except on layers that take less than 30 seconds. 50% lets me have the extra precision afforded by a fan while being able to print stably at high temperatures.