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Extruder overheating --- causes?

Posted: Tue Mar 24, 2015 12:19 am
by WillAdams
Okay, yesterday my last print was blobby and shiny --- didn't think too much about it, figured I'd troubleshoot today.

Reprinting the same file (which has been printed okay a bunch of times) had almost the same result.

Noticed that the hot end was getting much hotter than usual, but stupidly kept going, so naturally it overheated to the point of beginning to fall apart --- at least I had the sense to pull the plug at that point.

Thoughts on what might have caused this?

Should I have a fan just for the hot end?

Re: Extruder overheating --- causes?

Posted: Tue Mar 24, 2015 2:29 am
by Dave Durant
If I had to guess, I'd guess it's not reading the correct temperature or you told the slicer to use the wrong temperature.

Does it say it's getting really hot or just behave like it?

M104 and M109 gcodes look correct in the file you're printing?

Re: Extruder overheating --- causes?

Posted: Tue Mar 24, 2015 2:36 am
by WillAdams
It was a file I'd printed successfully before, and the temperature being displayed in repetierhost was how I puzzled out that it was too hot.

Trying to convince myself I should buy a Pico hot end from B3....

Re: Extruder overheating --- causes?

Posted: Tue Mar 24, 2015 2:38 am
by Will Winder
I believe the fan on the hot end is usually for cooling ABS quicker after it extrudes. Hot end temperature should be regulated by the firmware using whatever you set the temperature to in the slicer software.

Did your hot end come with a thermistor? Usually the manufacturer will tell you how the thermistor needs to be configured for the firmware.

Of course make sure the thermistor is mounted in the correct location, and tape/thermal paste are applied as necessary. The control software should give you a temperature graph and the temp should show that the temp never go far above what it's set to.

Re: Extruder overheating --- causes?

Posted: Tue Mar 24, 2015 3:05 am
by WillAdams
Looked up the thermistor code (it was from a J-head) and had input it, and had the thermistor taped into the receptacle hole --- I'd run the PID autotune command previously and had entered those values into the firmware --- does that need to be re-done periodically?

Glad to know that a fan isn’t assumed or absolutely necessary.

I guess I’ll sleep on it, and make a decision tomorrow sometime after. At least I don't have to worry about doing the library demo for a while now.

Theory on reddit was a partial clog which caused the overheating.

Re: Extruder overheating --- causes?

Posted: Tue Mar 24, 2015 3:30 am
by Dave Durant
Fans are usually used to cool down freshly-extruded PLA. It may help ABS but PLA is the one that really benefits from it. It's also used to cool the area right above the extruder hot zone so heat doesn't move up the system - if it does, PLA will soften and form a plug and block the whole thing, which is a real drag (this is also why you see fins right above the hot zone on some extruders.. to dissipate heat). You can also use a fan to cool down the hot zone itself to improve retraction but I don't think many people do that.
the temperature being displayed in repetierhost was how I puzzled out that it was too hot.
If repetier is telling you it's too hot and it's behaving like it's too hot and you're sure the temp is set to the right value.. uh.. that should not happen.

The firmware knows what value you want. If it's reading sane values from the extruder but things are still getting hotter.. Maybe some sort of electrical problem? Moving plastic through the system will suck out a lot of heat. Things won't get hotter unless something pushes more energy into the system, which it should not be doing unless the set temperature is above the current temperature.

TL;DR: I have no idea. Sorry.

Re: Extruder overheating --- causes?

Posted: Tue Mar 24, 2015 3:44 am
by cvoinescu
The usual cause of a hot-end overheating is a loose thermistor, so that the firmware thinks the hot end is cooler than it really is.

But that's not the case here: you say that you saw the actual temperature reading in the host program, not a bogus low value, so the thermistor was working and sensing the temperature of the hot end. It was either commanded to go that hot by the G-code (not likely), or there was a short-circuit that had the heater powered continuously (a burned out transistor will sometimes do that too). One wire of the heater goes straight to the positive pole of the supply, and the other is connected via a transistor (MOSFET) to ground (negative pole). If the second wire shorts to ground, the heater will be on even when the transistor is off.

Re: Extruder overheating --- causes?

Posted: Tue Mar 24, 2015 4:02 am
by WillAdams
Okay, how’s this sound for a shopping list?

Fan --- to cool extruder
Digital infrared thermometer w/ laser pointer --- to verify and spot-check temperatures
Pico hot-end --- all metal, should survive any overly high temperatures
Metal mounting plate --- the laser cut wood just doesn’t look right on the machine

First print will be a mounting / shroud for the fan --- anything else I’m forgetting?

Re: Extruder overheating --- causes?

Posted: Tue Mar 24, 2015 11:05 am
by WillAdams
The weird thing is, the heater was still switching on / off, the temperature would spike high, then cool, then the heater would turn on again.

How often should one run the PID autotune?

Re: Extruder overheating --- causes?

Posted: Tue Mar 24, 2015 12:09 pm
by cvoinescu
Once. And again if you make thermal changes, e.g. if you insulate the heater block.

Maybe PID was way off for some reason. I'd try to find out what's going on, before replacing the hot end. (When your chair keeps catching on fire, the solution is not shopping for asbestos underpants.)